# Minimum Circuit Ampacity



## Jlarson (Jun 28, 2009)

Tell them to go **** off.


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## 99cents (Aug 20, 2012)

Get it inspected. You did pull a permit, didn't you?


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## ponyboy (Nov 18, 2012)

Jlarson said:


> Tell them to go **** off.


This x2


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## brian john (Mar 11, 2007)

99cents said:


> Get it inspected. You did pull a permit, didn't you?


A what?????:blink:


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## 99cents (Aug 20, 2012)

brian john said:


> A what?????:blink:


Well, pull the permit now and get it inspected. Otherwise, it's just a pi$$ing contest and telling someone to shove it won't solve the issue.

What is minimum circuit ampacity anyway? I'm just asking. I don't know...
Never mind, I just figured that out. Brain fart.


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## asphalt (Jun 2, 2011)

I guess I'm lucky. I am the electrician for an HVAC company and do all the electrical work. The installers just look at me with a blank stare when I start talking wire. I prefer it that way. :thumbup:


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## rewire (Jul 2, 2013)

Ask the manufacturer for breaker and wire specs. We installed a ptac and it was in the manual minimum circuit ampacity 30 minimum wire size #12 HVAC guy just said oh I guess you are right. 
Why do they always claim it will void the warranty:no:


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## Pete m. (Nov 19, 2011)

99cents said:


> Well, pull the permit now and get it inspected. Otherwise, it's just a pi$$ing contest and telling someone to shove it won't solve the issue.
> 
> What is minimum circuit ampacity anyway? I'm just asking. I don't know.


I agree that telling someone to go pound sand, even though that would probably make you feel better, isn't the answer.

Unless the manufacturers spec's call for a certain wire size you are permitted, by the NEC, to size your wire to the minimum circuit ampacity listed on the unit nameplate and then provide a OCPD device sized to the maximum permitted.

Pete


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## Dennis Alwon (May 9, 2009)

I would do two things. Firstly I would call the homeowner and explain why it is perfectly okay to do what you did and let her know that you could arrange a conversation with the electrical inspector if she wishes.
Secondly I would call the hvac guys boss and explain to them that it is inappropriate to bad you to the customer and it is twice as bad when they are wrong.

I am assuming vd is not an issue here.


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## 99cents (Aug 20, 2012)

Dennis Alwon said:


> I would do two things. Firstly I would call the homeowner and explain why it is perfectly okay to do what you did and let her know that you could arrange a conversation with the electrical inspector if she wishes.
> Secondly I would call the hvac guys boss and explain to them that it is inappropriate to bad you to the customer and it is twice as bad when they are wrong.
> 
> I am assuming vd is not an issue here.


I like that idea. Suggesting that a conversation with an inspector can be arranged is a way of saying you have nothing to hide. I find that technical explanations, particularly with home owners, can lead to blank stares very quickly. I asked an inspector a question one time and he simply said to me, "If you wouldn't do it in your own home then don't do it in someone else's". That stuck with me and it is something I sometimes tell clients.


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## Dennis Alwon (May 9, 2009)

99cents said:


> I like that idea. Suggesting that a conversation with an inspector can be arranged is a way of saying you have nothing to hide. I find that technical explanations, particularly with home owners, can lead to blank stares very quickly. I asked an inspector a question one time and he simply said to me, "If you wouldn't do it in your own home then don't do it in someone else's". That stuck with me and it is something I sometimes tell clients.


The blank stares are fine and I don't expect them to understand however if you sound like you know what you are talking about it looks good to the home owner. 

I bet I can explain it well enough that most home owners would understand.


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## 99cents (Aug 20, 2012)

Dennis Alwon said:


> The blank stares are fine and I don't expect them to understand however if you sound like you know what you are talking about it looks good to the home owner.
> 
> I bet I can explain it well enough that most home owners would understand.


...and I assume that VD never becomes an issue either


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## CraigV (May 12, 2011)

Voltage drop, bundling more than 2', and/or excessive temperature (any of this running in an attic?) are the possible reasons you'd need to upsize.

I agree with talking to the HVAC's management regarding badmouthing you in front of the customer. But then, it might be management's idea to cut down on warranty work by immediately disavowing the warranty due to a BS excuse.


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## Meadow (Jan 14, 2011)

Joefixit2 said:


> I'm getting tired of defending my HVAC circuits to HVAC installers who do not understand the meaning of MCA. Last week it was a mini split with a FLA of 9.3 and an MCA of 15. We ran #12s. The installer refused to connect it, saying it had to be #10 and he was not going to be responsible for damage to the unit or a fire.
> Today another installer told a homeowner I should have ran #8s to his condenser with an MCA of 26 (30-40 foot run). We ran #10s. He told her I violated the NEC and the wire could start a fire because it is on a 40 amp breaker (per the nameplate). He said the warranty is void on the unit thanks to me. I haven't had a chance to speak to this guy yet.
> Whaddya do?


 
Tell the HVAC guy there is a reason hes not an electrician.


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## erics37 (May 7, 2009)

I've gotten into this argument several times with one HVAC contractor in particular. I wound up faxing over 440.6 to their shop


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## Joefixit2 (Nov 16, 2007)

No vd issues, the mini split circuit is already bigger than the nameplate requires anyway and the 26 amp unit is about 50 feet away. No permits by the way, owners did not want to get them. If I pull one just for my elec everyone else gets busted. Whuddya do? I'm not going to turn work away. Then again if the hvac guys give me grief I like the idea of mentioning let's call in the inspector!

No ambient factor either.....crawl space under house. Are you saying maybe the hvac guys are thinking ambient? Ha


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## Magnettica (Jan 23, 2007)

It's never a good idea to bad mouth another contractor because Karma's a bitch. I would boast to the customer of my experience and knowledge required to obtain the electrical license and simply explain as clearly as possible that code allows for this installation. The trouble with getting the electrical inspector involved is that half of them don't even know the HVAC and motor sizing rules for conductor, disconnects, and overcurrent protection so that could make you look even worse.


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## ponyboy (Nov 18, 2012)

Magnettica said:


> The trouble with getting the electrical inspector involved is that half of them don't even know the HVAC and motor sizing rules for conductor, disconnects, and overcurrent protection so that could make you look even worse.


I can't relate to this statement. I feel like grasping the concept of circuit ampacity and it's relation to motor loads is a very basic electrical concept and can't see how an electrical inspector could obtain his position without knowing all that


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## Magnettica (Jan 23, 2007)

ponyboy said:


> I can't relate to this statement. I feel like grasping the concept of circuit ampacity and it's relation to motor loads is a very basic electrical concept and can't see how an electrical inspector could obtain his position without knowing all that


You must not be from New Jersey :whistling2:

There's plenty of brain dead inspectors out there who got their positions only God knows how. I don't know how else to explain it. Maybe it's an NJ thing, I dunno.


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## Deep Cover (Dec 8, 2012)

Joefixit2 said:


> I'm getting tired of defending my HVAC circuits to HVAC installers who do not understand the meaning of MCA. Last week it was a mini split with a FLA of 9.3 and an MCA of 15. We ran #12s. The installer refused to connect it, saying it had to be #10 and he was not going to be responsible for damage to the unit or a fire.
> Today another installer told a homeowner I should have ran #8s to his condenser with an MCA of 26 (30-40 foot run). We ran #10s. He told her I violated the NEC and the wire could start a fire because it is on a 40 amp breaker (per the nameplate). He said the warranty is void on the unit thanks to me. I haven't had a chance to speak to this guy yet.
> Whaddya do?


I'm assuming you used a 2P switch on the first one, but did you use a fused disco or non fused on the condenser? If you used a fused disco on the condenser, the 10's would have to be protected at (max) 30A.


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## hardworkingstiff (Jan 22, 2007)

Deep Cover said:


> I'm assuming you used a 2P switch on the first one, but did you use a fused disco or non fused on the condenser? If you used a fused disco on the condenser, the 10's would have to be protected at (max) 30A.


I'm not arguing with you, but what is the code reason for that statement?

Thanks


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## Dennis Alwon (May 9, 2009)

hardworkingstiff said:


> I'm not arguing with you, but what is the code reason for that statement?
> 
> Thanks


I think because it is a feeder and not a branch circuit. Here is one place where I think the code needs a change


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## SmithBuilt (Jan 9, 2009)

Dennis Alwon said:


> I think because it is a feeder and not a branch circuit. Here is one place where I think the code needs a change


Fuses in the disconnect make it a feeder?

I've done this a few times and never really thought about it.


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## Dennis Alwon (May 9, 2009)

SmithBuilt said:


> Fuses in the disconnect make it a feeder?
> 
> I've done this a few times and never really thought about it.


Look at the definition of feeder and branch circuit



> Branch Circuit. The circuit conductors between the* final
> overcurrent device* protecting the circuit and the outlet(s).
> 
> Feeder. All circuit conductors between the service equipment,
> ...


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## Dennis Alwon (May 9, 2009)

I started writing a proposal a few years ago for the 2014. I thought I sent it in but apparently I did not


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## sbrn33 (Mar 15, 2007)

Joefixit2 said:


> No vd issues, the mini split circuit is already bigger than the nameplate requires anyway and the 26 amp unit is about 50 feet away. No permits by the way, owners did not want to get them. If I pull one just for my elec everyone else gets busted. Whuddya do? I'm not going to turn work away. Then again if the hvac guys give me grief I like the idea of mentioning let's call in the inspector!
> 
> No ambient factor either.....crawl space under house. Are you saying maybe the hvac guys are thinking ambient? Ha


Kinda off subject but I never let the customer dictate whether or not a permit gets pulled. By letting him tell you not to take a permit you are already saying that you will do whatever he wants you to do, even if it doesn't meet code.
IMO it just makes us look unprofessional as a trade.


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## robnj772 (Jan 15, 2008)

SmithBuilt said:


> Fuses in the disconnect make it a feeder? I've done this a few times and never really thought about it.


So by theory if you didn't use fuses( I never do) and just use a straight up non fused disconnect and the unit had a minimum of 10 amps and a max of 25 I could run 14/2 to the unit and put it on a 25 amp breaker?


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## MTW (Aug 28, 2013)

robnj772 said:


> So by theory if you didn't use fuses( I never do) and just use a straight up non fused disconnect and the unit had a minimum of 10 amps and a max of 25 I could run 14/2 to the unit and put it on a 25 amp breaker?


Yes.


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## sbrn33 (Mar 15, 2007)

robnj772 said:


> So by theory if you didn't use fuses( I never do) and just use a straight up non fused disconnect and the unit had a minimum of 10 amps and a max of 25 I could run 14/2 to the unit and put it on a 25 amp breaker?


This is what we do. Every single day. Wire is sized off MCA and the breaker is sized off of max breaker size. Non fused almost every time. Sometimes I will use a 2 circuit can so I can "fuse" it down if the panel is crap or obsolete because 80% of the new units require a smaller breaker.


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## hardworkingstiff (Jan 22, 2007)

robnj772 said:


> So by theory if you didn't use fuses( I never do) and just use a straight up non fused disconnect and the unit had a minimum of 10 amps and a max of 25 I could run 14/2 to the unit and put it on a 25 amp breaker?


That's the way I understand it.

Edit: Let me add that I doubt the numbers you are posting in your example would not exist in real life (due to 440.22).


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## BBQ (Nov 16, 2010)

robnj772 said:


> So by theory if you didn't use fuses( I never do) and just use a straight up non fused disconnect and the unit had a minimum of 10 amps and a max of 25 I could run 14/2 to the unit and put it on a 25 amp breaker?





MTW said:


> Yes.





hardworkingstiff said:


> That's the way I understand it.
> 
> Edit: Let me add that I doubt the numbers you are posting in your example would not exist in real life (due to 440.22).


I also agree.


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## Incubus311 (Jan 10, 2014)

Joefixit2 said:


> He told her I violated the NEC and the wire could start a fire because it is on a 40 amp breaker (per the nameplate).


I mean #10 wire is rated for 30 amps. If its on a 40 amp breaker the wire would burn up before the breaker trips...


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## electricmanscott (Feb 11, 2010)

Incubus311 said:


> I mean #10 wire is rated for 30 amps. If its on a 40 amp breaker the wire would burn up before the breaker trips...


And babies would die


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## electricmanscott (Feb 11, 2010)

Magnettica said:


> The trouble with getting the electrical inspector involved is that half of them don't even know the HVAC and motor sizing rules for conductor, disconnects, and overcurrent protection so that could make you look even worse.





ponyboy said:


> I can't relate to this statement. I feel like grasping the concept of circuit ampacity and it's relation to motor loads is a very basic electrical concept and can't see how an electrical inspector could obtain his position without knowing all that





Magnettica said:


> You must not be from New Jersey :whistling2:
> 
> There's plenty of brain dead inspectors out there who got their positions only God knows how. I don't know how else to explain it. Maybe it's an NJ thing, I dunno.



It's certainly not just Jersey unfortunately.


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## hardworkingstiff (Jan 22, 2007)

Incubus311 said:


> I mean #10 wire is rated for 30 amps. If its on a 40 amp breaker the wire would burn up before the breaker trips...


In the example for this thread, the circuit is dedicated to a specific load. It won't see more than that load (except maybe on start up) unless there is a short circuit. The circuit breaker is providing short circuit protection.


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## Incubus311 (Jan 10, 2014)

hardworkingstiff said:


> In the example for this thread, the circuit is dedicated to a specific load. It won't see more than that load (except maybe on start up) unless there is a short circuit. The circuit breaker is providing short circuit protection.


The breaker is meant to protect the wire. The wire should not be smaller then the breaker


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## MTW (Aug 28, 2013)

Stuff like this is why I dislike trade work. Let's face it - most tradesmen are not intelligent, so trying to explain why the installation is NEC compliant is an exercise in futility.


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## hardworkingstiff (Jan 22, 2007)

Incubus311 said:


> The breaker is meant to protect the wire. The wire should not be smaller then the breaker


Break out your code book and read 440.22

While you are there, you should review 440


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## Incubus311 (Jan 10, 2014)

hardworkingstiff said:


> Break out your code book and read 440.22 While you are there, you should review 440


So that wire is rated for 30 amps and the breaker is 40. Say the unit has an issue and runs at 35 amps....breaker wont trip will it


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## erics37 (May 7, 2009)

hardworkingstiff said:


> Break out your code book and read 440.22
> 
> While you are there, you should review 440


I would add 240.4(G) to the list.



Incubus311 said:


> So that wire is rated for 30 amps and the breaker is 40. Say the unit has an issue and runs at 35 amps....breaker wont trip will it




Small conductors on bigger breakers is a very common wiring application. Ever wired a motor with, say, #14s on a 30 amp breaker? Depending on the motor it could be perfectly legit.


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## Incubus311 (Jan 10, 2014)

erics37 said:


> I would add 240.4(G) to the list.  Small conductors on bigger breakers is a very common wiring application. Ever wired a motor with, say, #14s on a 30 amp breaker? Depending on the motor it could be perfectly legit.


Yes with a disconnect and the right size fuses. I just don't get why you wouldn't match the breaker to the wire


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## MTW (Aug 28, 2013)

Incubus311 said:


> Yes with a disconnect and the right size fuses. I just don't get why you wouldn't match the breaker to the wire


Because the NEC doesn't require it and it's a waste of material and resources to do so.


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## erics37 (May 7, 2009)

Incubus311 said:


> Yes with a disconnect and the right size fuses. I just don't get why you wouldn't match the breaker to the wire


I don't get why you would run bigger wire than is necessary when the equipment nameplate clearly indicates what wire size and what overcurrent device you should use.

Read 240.4(G), it's a table of all the specific conductor applications where the standard "#10, 30 amps; #12, 20 amps, etc" convention is not adhered to. Refrigeration and air conditioning is included on that list, and is therefore excepted from the normal 240.4(D) wire sizing rules.


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## Incubus311 (Jan 10, 2014)

erics37 said:


> I don't get why you would run bigger wire than is necessary when the equipment nameplate clearly indicates what wire size and what overcurrent device you should use.


I never said run bigger wire. Just match the breaker to the wire its supplying. Leaving a 40a breaker supplying 10 ga wire just doesn't seem smart to me.


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## hardworkingstiff (Jan 22, 2007)

Incubus311 said:


> Yes with a disconnect and the right size fuses. I just don't get why you wouldn't match the breaker to the wire


Mostly because an electrician doesn't want to come back for a breaker tripping due to starting currents of the compressor.


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## hardworkingstiff (Jan 22, 2007)

Incubus311 said:


> So that wire is rated for 30 amps and the breaker is 40. Say the unit has an issue and runs at 35 amps....breaker wont trip will it


No, it won't. The authors of the NEC don't seem to have the same concerns as you. I guess they can't imagine how that might happen.

How do you think it could happen?


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## erics37 (May 7, 2009)

Incubus311 said:


> I never said run bigger wire. Just match the breaker to the wire its supplying. Leaving a 40a breaker supplying 10 ga wire just doesn't seem smart to me.


*facepalm*

It *is* matched to the breaker in compliance with 240.4(G) and Article 440.

You need to get the idea out of your head that the convention of #10 wire on a 30 amp breaker applies to everything.


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## Ultrafault (Dec 16, 2012)

Incubus311 said:


> So that wire is rated for 30 amps and the breaker is 40. Say the unit has an issue and runs at 35 amps....breaker wont trip will it


There is a device called a thermal overload protector that will trip if the current exceeds the the nameplate value. Your example is impossible. Think and read before you argue.


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## hardworkingstiff (Jan 22, 2007)

Ultrafault said:


> There is a device called a thermal overload protector that will trip if the current exceeds the the nameplate value. Your example is impossible. Think and read before you argue.


I didn't take it as arguing, mostly just doesn't know and is asking questions.

Now he does seem to not want to listen too well, :laughing:


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## Dennis Alwon (May 9, 2009)

Incubus311 said:


> I never said run bigger wire. Just match the breaker to the wire its supplying. Leaving a 40a breaker supplying 10 ga wire just doesn't seem smart to me.


That is partly because you may not be understanding what the purpose of the breaker is in this situation. The breaker in the panel is only there for short circuit and ground fault problems, The #10 wire is protected because the a/c or heat pump has overload protection that will protect the wire at it's rated capacity or a lower rated depending on the unit.


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## Shockdoc (Mar 4, 2010)

I had the same problem this past summer with a HVAC assclowns, I had a w/p panel on the side of the house for pool, outdoor and AC equipment. The panel is 10' from the AC compressors, the AC contractor told the HO my installation was not up to code because there were no pull out disconnects installed and he was trying to void his warrantee over my electrical work. He called his electrician over to inspect my work citing not having a pull out is dangerous . I hope his guy charged him a service call to set him straight. The breaker , clearly labeled to each compressor is a suitable means of disconnect.


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## Incubus311 (Jan 10, 2014)

Ultrafault said:


> There is a device called a thermal overload protector that will trip if the current exceeds the the nameplate value. Your example is impossible. Think and read before you argue.


Ha I was not arguing. I know what a thermal overload protector is lol. I also said before I would have a disconnect with the right size fuses, overload etc. Isn't there special breakers that allow for in rush current? I know there are time delayed fuses, but I thought there was breakers as well..


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## azgard (Nov 25, 2011)

And you have been told multiple reason's why according to NEC and in technical term's yet you still refuse to accept it. The wire is sufficiently protected from being overloaded by the device itself and the breaker is sized to protect the wire from a short whilst preventing nuisance tripping from motor start's what are you missing?


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## electricmanscott (Feb 11, 2010)

MTW said:


> Stuff like this is why I dislike trade work. Let's face it - most tradesmen are not intelligent, so trying to explain why the installation is NEC compliant is an exercise in futility.


That's unfortunately very accurate. The majority of people in the trades, from my experience, are kind of dumbo dirtbags. 


This particular example is always misunderstood. A friend of mine was wiring an a/c system and was asking me about wire size. I forget the exact details but I explained what size wire and why. He asked an HVAC guy and he said you have to match the wire size to the breaker or something like that. I'm pretty sure he asked the inspector and he too was clueless. So he spent a bunch more money on materials than he had too.


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## Incubus311 (Jan 10, 2014)

azgard said:


> And you have been told multiple reason's why according to NEC and in technical term's yet you still refuse to accept it. The wire is sufficiently protected from being overloaded by the device itself and the breaker is sized to protect the wire from a short whilst preventing nuisance tripping from motor start's what are you missing?


I got it chief


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## electricmanscott (Feb 11, 2010)

Incubus311 said:


> I got it chief


I don't think he's a chief, looks like he's still just a gard. Your az is safe when he's around. :thumbsup:


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## Elephante (Nov 16, 2011)

Why aren't you allowed to go higher than the maximum breaker on hvac unit? What would happen if I use a hundred amp breaker instead of a forty ?


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## markore (Dec 7, 2011)

Elephante said:


> Why aren't you allowed to go higher than the maximum breaker on hvac unit? What would happen if I use a hundred amp breaker instead of a forty ?


Its not higher than the maximum breaker on the HVAC unit. you use the breaker and wire size specified in the manual and/or nameplate supplied with the units. With motors and compressors there is usually in inrush current which is higher then the continuous current rating of the wire.


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## azgard (Nov 25, 2011)

markore said:


> Its not higher than the maximum breaker on the HVAC unit. you use the breaker and wire size specified in the manual and/or nameplate supplied with the units. With motors and compressors there is usually in inrush current which is higher then the continuous current rating of the wire.


He's asking why you can't exceed the plate. Other then violating the listing, I have no clue.


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## papaotis (Jun 8, 2013)

Shockdoc said:


> I had the same problem this past summer with a HVAC assclowns, I had a w/p panel on the side of the house for pool, outdoor and AC equipment. The panel is 10' from the AC compressors, the AC contractor told the HO my installation was not up to code because there were no pull out disconnects installed and he was trying to void his warrantee over my electrical work. He called his electrician over to inspect my work citing not having a pull out is dangerous . I hope his guy charged him a service call to set him straight. The breaker , clearly labeled to each compressor is a suitable means of disconnect.


try reading 440.14 'within sight of'


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## markore (Dec 7, 2011)

azgard said:


> He's asking why you can't exceed the plate. Other then violating the listing, I have no clue.


Thanks, if that's the case, I misread. A reasonable amount of protection must be on the line side of feeders since a short across the load would bypass the integral overcurrent device. The overcurrent rating for the breaker listed on the plate is derived _*similarly*_ to engineering supervision procedure.

*110.22 Identification of Disconnecting Means.*
(A) General. Each disconnecting means shall be legibly
marked to indicate its purpose unless located and arranged
so the purpose is evident. The marking shall be of sufficient
durability to withstand the environment involved.
(B) Engineered Series Combination Systems. Where circuit
breakers or fuses are applied in compliance with series
combination ratings selected under engineering supervision
and marked on the equipment as directed by the engineer,
the equipment enclosure(s) shall be legibly marked in the
field to indicate the equipment has been applied with a
series combination rating. The marking shall be readily visible
and state the following:
CAUTION — ENGINEERED SERIES COMBINATION
SYSTEM RATED _______ AMPERES. IDENTIFIED
REPLACEMENT COMPONENTS REQUIRED.
FPN: See 240.86(A) for engineered series combination
systems.
(C) Tested Series Combination Systems. Where circuit
breakers or fuses are applied in compliance with the series
combination ratings marked on the equipment by the manufacturer,
the equipment enclosure(s) shall be legibly
marked in the field to indicate the equipment has been
applied with a series combination rating. The marking shall
be readily visible and state the following:
CAUTION — SERIES COMBINATION SYSTEM
RATED ____ AMPERES. IDENTIFIED REPLACEMENT
COMPONENTS REQUIRED.
FPN: See 240.86(B) for tested series combination systems.


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## markore (Dec 7, 2011)

*210.20 Overcurrent Protection. Branch-circuit conductors*
and equipment shall be protected by overcurrent protective
devices that have a rating or setting that complies
with 210.20(A) through (D).
*(A) Continuous and Noncontinuous Loads. *Where a
branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the rating of
the overcurrent device shall not be less than the noncontinuous
load plus 125 percent of the continuous load.
Exception: Where the assembly, including the overcurrent
devices protecting the branch circuit(s), is listed for operation
at 100 percent of its rating, the ampere rating of the
overcurrent device shall be permitted to be not less than the
sum of the continuous load plus the noncontinuous load.
*(B) Conductor Protection. *Conductors shall be protected
in accordance with 240.4. Flexible cords and fixture wires
shall be protected in accordance with 240.5.
*(C) Equipment. *The rating or setting of the overcurrent
protective device shall not exceed that specified in the applicable
articles referenced in Table 240.3 for equipment.


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## erics37 (May 7, 2009)

Elephante said:


> Why aren't you allowed to go higher than the maximum breaker on hvac unit? What would happen if I use a hundred amp breaker instead of a forty ?


Huh, I never thought of that.

Excellent idea. That's an ideal way to prevent nuisance trips.

From now on every house I wire is getting a 1000 amp main breaker.


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## hardworkingstiff (Jan 22, 2007)

Elephante said:


> Why aren't you allowed to go higher than the maximum breaker on hvac unit? What would happen if I use a hundred amp breaker instead of a forty ?


You would be in violation of the NEC. Most of us try to be in compliance with the NEC.


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## Meadow (Jan 14, 2011)

Elephante said:


> Why aren't you allowed to go higher than the maximum breaker on hvac unit? What would happen if I use a hundred amp breaker instead of a forty ?


 The theory is the breaker provides short circuit protection only.


A 100 amp breaker would have an instantaneous magnetic trip that might take to long to be met. In other words it might not be reached. Yes the breaker would trip eventually, but in a run of #10 of say 200 feet the wire impedance may not pull enough current to get to the magnetic trip. Breaker will probably go on thermal trip, which of course would trip the breaker quickly but the wire could become substantially overheated.

Instantaneous (magnetic) trip is usually 8 to 12 times the handle rating. A 40 amp would be about 360 amps, however a 100 amp would be about 1000amps.


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## Elephante (Nov 16, 2011)

meadow said:


> The theory is the breaker provides short circuit protection only. A 100 amp breaker would have an instantaneous magnetic trip that might take to long to be met. In other words it might not be reached. Yes the breaker would trip eventually, but in a run of #10 of say 200 feet the wire impedance may not pull enough current to get to the magnetic trip. Breaker will probably go on thermal trip, which of course would trip the breaker quickly but the wire could become substantially overheated. Instantaneous (magnetic) trip is usually 8 to 12 times the handle rating. A 40 amp would be about 360 amps, however a 100 amp would be about 1000amps.


 oh ok I got ya.. The magnetic trips on different breakers are set at different levels..it's crazy all the testing that goes on..


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## Meadow (Jan 14, 2011)

Elephante said:


> oh ok I got ya.. The magnetic trips on different breakers are set at different levels..it's crazy all the testing that goes on..


The scary part is FPE breakers and bulldog panels don't have magnetic trip Short 2 wires together and it wont trip until the wire is actually warm, of course in the case of FPE your assuming the trip curves are as factory intended.


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## Dennis Alwon (May 9, 2009)

hardworkingstiff said:


> You would be in violation of the NEC. Most of us try to be in compliance with the NEC.


In most cases yes but the NEC allows us to go 225% in some cases-- 440.22 Take a 40 amp a/c unit and multiply by 225%-- 90 amps


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## hardworkingstiff (Jan 22, 2007)

Dennis Alwon said:


> In most cases yes but the NEC allows us to go 225% in some cases-- 440.22 Take a 40 amp a/c unit and multiply by 225%-- 90 amps


Wouldn't it be incumbent on you to prove that 175% would not hold the start up current?

I don't think you can just go straight to 225% unless it's on the nameplate.


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## hardworkingstiff (Jan 22, 2007)

Dennis Alwon said:


> In most cases yes but the NEC allows us to go 225% in some cases-- 440.22 Take a 40 amp a/c unit and multiply by 225%-- 90 amps


If you have a unit that has a FLA of 18-amps and the data plate says the maximum OCPD is 35-amps, are you saying you can use 440.22 to install a 40-amp breaker?


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## Meadow (Jan 14, 2011)

hardworkingstiff said:


> If you have a unit that has a FLA of 18-amps and the data plate says the maximum OCPD is 35-amps, are you saying you can use 440.22 to install a 40-amp breaker?


 
I would think you would add 35 amp fuses to the disconnect. I know that when a nameplate says 25 amps max fuse size or the like if there is no "breaker" written after it has to be a fuse. 

I think.


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## Dennis Alwon (May 9, 2009)

hardworkingstiff said:


> If you have a unit that has a FLA of 18-amps and the data plate says the maximum OCPD is 35-amps, are you saying you can use 440.22 to install a 40-amp breaker?


It appears that way to me if done under the conditions given


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## markore (Dec 7, 2011)

Dennis Alwon said:


> It appears that way to me if done under the conditions given


Agree. Mike Holt specifically cites this example in his video series, just as described: 40A for 35A nameplate. You use the smallest breaker size the manufacturer has that accommodates at least the rated current, no lower.


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## hardworkingstiff (Jan 22, 2007)

markore said:


> Agree. Mike Holt specifically cites this example in his video series, just as described: 40A for 35A nameplate. You use the smallest breaker size the manufacturer has that accommodates at least the rated current, no lower.


If you look at 240.6, a 35-amp breaker is a standard breaker (that is exactly why I used it in my example).


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## markore (Dec 7, 2011)

meadow said:


> I would think you would add 35 amp fuses to the disconnect. I know that when a nameplate says 25 amps max fuse size or the like if there is no "breaker" written after it has to be a fuse.
> 
> I think.


You could do this separately to protect from motor overload if you knew the motor did not have internal protection and was subject to overloading, *but it is not required by the NEC.*

Mike Holt described a scenario where birds landing on his boat launch controls would burn up the windings in the stalled motor because no air would circulate to cool them, feeder connectors were fine. He *added* a *time delay* fuse like you are describing. Circuit breakers are not intended to account for this special case.


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## markore (Dec 7, 2011)

hardworkingstiff said:


> If you look at 240.6, a 35-amp breaker is a standard breaker (that is exactly why I used it in my example).


If 35A is standard and available for your panel that's what you use.
Inspector still approves all materials based on what's market available.


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## hardworkingstiff (Jan 22, 2007)

markore said:


> If 35A is standard and available for your panel that's what you use.
> Inspector still approves all materials based on what's market available.


I don't believe an inspector would make a determination on what is market available in this case. With shipping options, you can have that breaker tomorrow.


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## markore (Dec 7, 2011)

hardworkingstiff said:


> I don't believe an inspector would make a determination on what is market available in this case. With shipping options, you can have that breaker tomorrow.


I agree with you, as in, if its on the market and still being manufactured and listed for use in the US, its available.

This comes into play when codes require devices that are not available on the market yet because no one has started production or finished listing.


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