# 80% rule for hvac units?



## bm4branes (May 8, 2010)

I'm the mechanical contractor for a project and of course my equipment needs power. I give the sparky the equipment spec sheet and point to the appropriate figure to size from and he blurts out that the branch circuits for my _listed_ package equipment need to be de-rated to 80% of capacity. We agreed that the fuse rating should be as per nameplate.

I may have misunderstood his comment but kinda doubt it. I don't want to tell the guy how to do his job, prove him wrong, or do the job myself, I just want the correct interpretation of the nec.

My understanding is that HVAC package units-typical rooftop, are considered NON-CONTINIOUS (210)and therefore, wiring is @100% of nameplate rating. section 440 is more specific about hvac but does not conflict with 210. Like I stated earlier, package unit, listed, with multiple motor loads inside.

I don't mind the guy blindly overdoing the wiring so long as the protection is correct.


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## jwjrw (Jan 14, 2010)

bm4branes said:


> I'm the mechanical contractor for a project and of course my equipment needs power. I give the sparky the equipment spec sheet and point to the appropriate figure to size from and he blurts out that the branch circuits for my _listed_ package equipment need to be de-rated to 80% of capacity. We agreed that the fuse rating should be as per nameplate.
> 
> I may have misunderstood his comment but kinda doubt it. I don't want to tell the guy how to do his job, prove him wrong, or do the job myself, I just want the correct interpretation of the nec.
> 
> ...


 
The nameplate will have max fuse or breaker which is the max breaker size allowed. Your min circuit ampacity is what you use to size your wire to the unit based off 310.16 I believe


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## McClary’s Electrical (Feb 21, 2009)

Why would you consider it non continuous. What about a really cold day,,,or really hot day.

It's continous, and that's why your sparky said that.


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## jwjrw (Jan 14, 2010)

mcclary's electrical said:


> Why would you consider it non continuous. What about a really cold day,,,or really hot day.
> 
> It's continous, and that's why your sparky said that.


 
What his sparky said makes no sense. You can load a breaker to 80% but when you have a continuous load you figure it at 125%. That said most hvac equipment is already figured at 125% on the nameplate.


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## william1978 (Sep 21, 2008)

jwjrw said:


> Your min circuit ampacity is what you use to size your wire to the unit based off 310.16 I believe


What degree column do you use 60,75,90?


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## Bob Badger (Apr 19, 2009)

mcclary's electrical said:


> Why would you consider it non continuous. What about a really cold day,,,or really hot day.
> 
> It's continous, and that's why your sparky said that.


I would not consider an HVAC load continuous per the definition in Article 100.

I would use the units tagged MCA for service / feeder sizing.


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## McClary’s Electrical (Feb 21, 2009)

jwjrw said:


> What his sparky said makes no sense. You can load a breaker to 80% but when you have a continuous load you figure it at 125%. That said most hvac equipment is already figured at 125% on the nameplate.


 

The MCA has the 125% for continuous already built into the figure.

Did he mention MCA?,,,No

For giggles,,,load is 50 amps continous X 1.25 =58.6

Another way,,,,his electricians way,,,is derate the cable. 

50 amp load you could use #8's,,,,but hold the phone derate it to .80=guess what ?#8's will no longer work,,,so you jump up to #6'

#6's are 65 amps,,,,,,x .80= 52 amps,,,,,

see,,,it works fine,,,,,80% deration of cables gives same result as 125% increase in the circuit.

It is simply a different way to arrive at the same wire size


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## jwjrw (Jan 14, 2010)

william1978 said:


> What degree column do you use 60,75,90?


 75 because the equipment terminals are rated for 75 correct


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## jwjrw (Jan 14, 2010)

mcclary's electrical said:


> The MCA has the 125% for continuous already built into the figure.
> 
> Did he mention MCA?,,,No
> 
> ...


 
Yea they are reciprocals. Just the way he said it confused me. It dosen't take much to do that!:laughing:


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## william1978 (Sep 21, 2008)

jwjrw said:


> Depends on what type of wire you use correct?


 No, #14 romex is good for 25amps when wiring a ac unit.


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## jwjrw (Jan 14, 2010)

william1978 said:


> No, #14 romex is good for 25amps when wiring a ac unit.


 
I always run thhn and size off 75 deg column.
Romex to ac? Wouldn't that be a wet location?


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## william1978 (Sep 21, 2008)

jwjrw said:


> I always run thhn and size off 75 deg column.
> Romex to ac? Wouldn't that be a wet location?


 Romex to the disconnect and come in the back of the disconnect and then thhn in the flex to the unit itself. From the disconnect to the unit is a wet location.


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## jwjrw (Jan 14, 2010)

william1978 said:


> Romex to the disconnect and come in the back of the disconnect and then thhn in the flex to the unit itself. From the disconnect to the unit is a wet location.


 
Ah I see. I got commercial on the brain lately. I thought you meant from the disco. Either 60 or 75 says #14 is good for 20 amps in 310.16. What exception says 25 for an ac?


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## william1978 (Sep 21, 2008)

jwjrw said:


> Ah I see. I got commercial on the brain lately. I thought you meant from the disco. Either 60 or 75 says #14 is good for 20 amps in 310.16. What exception says 25 for an ac?


 Let me look to find the exact code section that allows that. I think it is in 240 or 210.


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## jwjrw (Jan 14, 2010)

william1978 said:


> Let me look to find the exact code section that allows that. I think it is in 240 or 210.


 
I saw an asteric that said see 240.4(d) I think


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## 480sparky (Sep 20, 2007)

jwjrw said:


> I saw an asteric that said see 240.4(d) I think



240.4(D) is the 'small conductor' rule.

You're looking for 240.4(G).


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## jwjrw (Jan 14, 2010)

480sparky said:


> 240.4(D) is the 'small conductor' rule.
> 
> You're looking for 240.4(G).


Ok but where does it say a #14 is good for 25 amps when used to wire an ac? Maybe I'm just missing it. 
I see 240.4(g) says for ac go to 440 parts III and VI. Maybe I'm confused again:laughing:
Can anyone post it for me?


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## randomkiller (Sep 28, 2007)

william1978 said:


> No, #14 romex is good for 25amps when wiring a ac unit.


 
Not on any job I have ever been on.


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## 480sparky (Sep 20, 2007)

jwjrw said:


> Ok but where does it say a #14 is good for 25 amps when used to wire an ac? Maybe I'm just missing it.
> I see 240.4(g) says for ac go to 440 parts III and VI. Maybe I'm confused again:laughing:
> Can anyone post it for me?



OK.............. so did you go to 440 parts III and VI?


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## jwjrw (Jan 14, 2010)

480sparky said:


> OK.............. so did you go to 440 parts III and VI?


Ummmm yes I did. And I still am having trouble finding it. Which art in part III or VI is it I'm missing 480? I can be a little slow so help me out please.


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## 480sparky (Sep 20, 2007)

440.52(A)(3) maybe.


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## jwjrw (Jan 14, 2010)

480sparky said:


> 440.52(A)(3) maybe.


 

2008? Maybe? Guess I'll stick to using the 75 degree column and thhn.
I know william knows his stuff but could he be mistaken? Are you William?. :blink:


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## 480sparky (Sep 20, 2007)

jwjrw said:


> 2008? Maybe? Guess I'll stick to using the 75 degree column and thhn.
> I know william knows his stuff but could he be mistaken? Are you William?. :blink:



I guess I don't know what you're looking for.

The phrase "14 THHN is good for 25 amps when used for an air conditioner"?

And who's William?


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## Bob Badger (Apr 19, 2009)

:scooter:


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## jwjrw (Jan 14, 2010)

480sparky said:


> I guess I don't know what you're looking for.
> 
> The phrase "14 THHN is good for 25 amps when used for an air conditioner"?
> 
> And who's William?


 
William1978 and see his post #10.


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## bm4branes (May 8, 2010)

bm4branes said:


> I'm the mechanical contractor for a project and of course my equipment needs power. I give the sparky the equipment spec sheet and point to the appropriate figure to size from and he blurts out that the branch circuits for my _listed_ package equipment need to be de-rated to 80% of capacity. We agreed that the fuse rating should be as per nameplate.
> 
> I may have misunderstood his comment but kinda doubt it. I don't want to tell the guy how to do his job, prove him wrong, or do the job myself, I just want the correct interpretation of the nec.
> 
> ...


Thanks for the responses

so a continuous load is one that runs for 3 hours, correct? *Properly sized/operating* hvac equipment is designed to cycle 3-4 times an hour, on for 5min-off for 10 min during design conditions based on 75f indoor and outdoor based on 99% occurrence. 

MCA includes the 125% for sure?

Also, equipment in question has MCA of 27 and MOCP 0f 35 and lets say the disco is fused at 30A, 10 awg fly?

Thanks again


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## jwjrw (Jan 14, 2010)

bm4branes said:


> Thanks for the responses
> 
> so a continuous load is one that runs for 3 hours, correct? *Properly sized/operating* hvac equipment is designed to cycle 3-4 times an hour, on for 5min-off for 10 min during design conditions based on 75f indoor and outdoor based on 99% occurrence.
> 
> ...


 
Yes 125% is already figured in and yes #10 is good for a minimum circuit ampacity of 27a


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## 480sparky (Sep 20, 2007)

jwjrw said:


> William1978 and see his post #10.



So you think we're the same person?

And I said the same thing he did..... #14 THHN is good for 25 amps.


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## jwjrw (Jan 14, 2010)

480sparky said:


> So you think we're the same person?
> 
> And I said the same thing he did..... #14 THHN is good for 25 amps.


 
Well no you are 480(tool) william1978 is an ex member of the dark side here.( county insp)


I never saw you actually say #14 is good for 25 amps on an ac unit. I was looking for the exact wording that allows that. You know how you post and highlight the article in a lot of threads. Im a little slow you know that so stop shaking your head back and forth like this:no:! Thanks 480


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## 480sparky (Sep 20, 2007)

jwjrw said:


> Well no you are 480(tool) william1978 is an ex member of the dark side here.(meck county insp)
> 
> 
> I never saw you actually say #14 is good for 25 amps on an ac unit. I was looking for the exact wording that allows that. You know how you post and highlight the article in a lot of threads. Im a little slow you know that so stop shaking your head back and forth like this:no:! Thanks 480


You should know better than expect the NEC to state something that simple. They need to spread it out across 40 pages and 3 articles.


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## Shado (Jan 1, 2009)

480sparky said:


> You should know better than expect the NEC to state something that simple. *They need to spread it out across 40 pages and 3 articles*.


 
And leave you scratching your heading wondering where to look next.....then you get to come on here and argue and debate the findings....:laughing::whistling2:


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## BuzzKill (Oct 27, 2008)

480sparky said:


> #14 THHN is good for 25 amps.


okay got a problem here: throughout my time in this trade, the 90C column has been used for derating only, so I think the 75C (20amp) column should be followed here instead?


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## 480sparky (Sep 20, 2007)

BuzzKill said:


> okay got a problem here: throughout my time in this trade, the 90C column has been used for derating only, so I think the 75C (20amp) column should be followed here instead?



So derate it. What conditions do you have that require derating?


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## BuzzKill (Oct 27, 2008)

hmmmm...I want more opinions on this. So #14 will carry 25A according to 310.16. (taps fingers....)


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## 480sparky (Sep 20, 2007)

Lemme throw this out:

310.15(A) says you can either use the tables specified in 310.15(B), or you can us engineering supervision.

So let's use the Tables mentioned in 310.15(B).

310.17 doesn't apply.
310.18 doesn't apply.
310.19 doesn't apply.

Guess that leaves 310.16.


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## BuzzKill (Oct 27, 2008)

little vague, although credible..I need more than that.


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## 480sparky (Sep 20, 2007)

BuzzKill said:


> little vague, although credible..I need more than that.



What more do you need than a table (the one your codebook probably flops open to if you drop it, no less!) that says "AWG 14........25 amps"?


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## BuzzKill (Oct 27, 2008)

It's one of those things that is beat into your brain and you can't shake it loose.


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## jwjrw (Jan 14, 2010)

480sparky said:


> What more do you need than a table (the one your codebook probably flops open to if you drop it, no less!) that says "AWG 14........25 amps"?


 
Ok where I am confused is that 310.16 #14 at 90 deg is 25 amp but you only use that for derating. Since the hvac equipment terminals are most likely rated at 75 deg do you not have to use the 75 deg column which says #14 is only good for 20 amps?


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## jwjrw (Jan 14, 2010)

480sparky said:


> You should know better than expect the NEC to state something that simple. They need to spread it out across 40 pages and 3 articles.


 
Yea and my little mind has trouble following what they are trying to say a lot of the time.


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## jwjrw (Jan 14, 2010)

480sparky said:


> What more do you need than a table (the one your codebook probably flops open to if you drop it, no less!) that says "AWG 14........25 amps"?


 

480 are you saying that a #14 used for a motor can be fused off the 90 deg column? Maybe that is why I'm getting confused. I've always thought you could not exceed the temp of the terminals(75 degree)


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## jwjrw (Jan 14, 2010)

BuzzKill said:


> okay got a problem here: throughout my time in this trade, the 90C column has been used for derating only, so I think the 75C (20amp) column should be followed here instead?


 

I'm glad I'm not the only one who thinks that.


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## McClary’s Electrical (Feb 21, 2009)

jwjrw said:


> Ok where I am confused is that 310.16 #14 at 90 deg is 25 amp but you only use that for derating. Since the hvac equipment terminals are most likely rated at 75 deg do you not have to use the 75 deg column which says #14 is only good for 20 amps?


 
You guys really need to read the A/C section. It would not be uncommon at all to see a 14 guage on a 30 amp breaker for an a/c unit


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## Bob Badger (Apr 19, 2009)

jwjrw said:


> 480 are you saying that a #14 used for a motor can be fused off the 90 deg column? Maybe that is why I'm getting confused. I've always thought you could not exceed the temp of the terminals(75 degree)


No you cannot size from the 90 C column except for when derating or all the terminals are rated 90C which is a very unusual situation.

14 AWG is rated 20 amps at 60 C or 75 C and that is the max we can load 14 AWG to.

BUT .... that still does not matter.

If I have a HVAC unit that the label says; 

_Minimum Circuit ampacity: 19 amps

Maximum Overcurrent Protection: 40 amps_

I would be perfectly safe and NEC compliant to use 14 AWG with a 40 amp fuse or breaker.


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## william1978 (Sep 21, 2008)

I was wrong by saying 14 was good for 25 amps. As others have stated it is good for 20a, 12--25a, 10--35a, 8--50a. Use the 75 degree column.


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## JohnR (Apr 12, 2010)

I have been taught for the last 18 or so years that you have to look at art. 110.14 C 1 a 1-4 for the terminal temp ratings.(0-100A @60Deg.) 

which are the qualifying codes for finding which conductor temp you should use in T310.16 and others. 

Also to use 110.14 C 1 b 1-2 for Terminal temp ratings for (75 Deg for 100A and up) 

The 90Deg scale on 310.16 and others is for the derating of cond. ampacity when the cond. goes through a hotter area and you are using a cond. rated at 90 Deg. like THHN.


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## Bob Badger (Apr 19, 2009)

JohnR said:


> I have been taught for the last 18 or so years that you have to look at art. 110.14 C 1 a 1-4 for the terminal temp ratings.(0-100A @60Deg.)


That is 100% correct unless the terminals are marked otherwise, many terminals under 100 amps are labeled 75C


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## Mr. Sparkle (Jan 27, 2009)

Bob Badger said:


> If I have a HVAC unit that the label says;
> 
> _Minimum Circuit ampacity: 19 amps
> 
> ...


I am not a code guru by any means but it amazes me how many inspectors do not even remotely understand this.

It is like arguing with a brick wall trying to explain this one.


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## NolaTigaBait (Oct 19, 2008)

They have the #12 is good for 20 amps no matter what stamped in their brain....I 've try arguing this before....I think it's pretty cut and dry.


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## jwjrw (Jan 14, 2010)

Bob Badger said:


> No you cannot size from the 90 C column except for when derating or all the terminals are rated 90C which is a very unusual situation.
> 
> 14 AWG is rated 20 amps at 60 C or 75 C and that is the max we can load 14 AWG to.
> 
> ...


 


Yes because #14 is good for 20amps. I got confused thinking there was an exception saying #14 was good for 25amps because William said he thought there was. I even said in my first post that you size your wire off the mca.Its easy for me to get confused.:thumbsup:


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## william1978 (Sep 21, 2008)

jwjrw said:


> Its easy for me to get confused.:thumbsup:


 Hey it is easy for all of us to get confused in the code book. The book is wrote so that it takes a lawyer to read it.


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## 480sparky (Sep 20, 2007)

william1978 said:


> Hey it is easy for all of us to get confused in the code book. The book is wrote so that it takes a lawyer to read it.


I learned a long time ago that the NEC is a legal document.


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## william1978 (Sep 21, 2008)

480sparky said:


> I learned a long time ago that the NEC is a legal document.


 Yea, Me too.


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## RIVETER (Sep 26, 2009)

480sparky said:


> I learned a long time ago that the NEC is a legal document.


Yes, it is.


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## jwjrw (Jan 14, 2010)

william1978 said:


> Hey it is easy for all of us to get confused in the code book. The book is wrote so that it takes a lawyer to read it.


 

And I bet even a lawyer is scratching his head more often than not trying to understand what the heck they are trying to say. Why can't they just say you can do this but not that unless your wife gives you permission or something like that!:laughing:


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## Michael1976 (Jun 16, 2010)

well , im kinda having similar problem , im installing two 5 ton cooling units and having a little trouble sizing conductors. with a deration at 80 percent , and unit calls for a double pole 50 , will 8 awg be the right size with the combined fla of 34.8 amps each.


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## Dennis Alwon (May 9, 2009)

Michael1976 said:


> well , im kinda having similar problem , im installing two 5 ton cooling units and having a little trouble sizing conductors. with a deration at 80 percent , and unit calls for a double pole 50 , will 8 awg be the right size with the combined fla of 34.8 amps each.


 What is the minimum cir. ampacity of the unit. Are you sure the FLA is 34.8? The unit should be marked with the MCA, if not it should be the fla times 125%.

Thus 34.8 * 1.25 = 43.5. Table 310.16 allows #8 if you are not using nm, se cable or any other wiring method that requires 60C rating. If so you must use #6.

Now if the MCA is 34.8 you can use #10 wire again if the wiring method is not a 60C method. If you use a 75c or 90C wire then #10 is sufficient.

You, of course, should always consider VD if that enters into the equation. In the case of VD, bigger is better.


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## oldtimer (Jun 10, 2010)

Dennis; I like the way you explain things. Keep up the good work. Oh yes, and I like Ray Charles too. :thumbup:


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## Dennis Alwon (May 9, 2009)

oldtimer said:


> Dennis; I like the way you explain things. Keep up the good work. Oh yes, and I like Ray Charles too. :thumbup:


Thank you-- I wish my wife would agree with you.


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## jwjrw (Jan 14, 2010)

Dennis Alwon said:


> Thank you-- I wish my wife would agree with you.


 
Mine would agree but would add that although I explained it well IM WRONG! Ill feel your pain.


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