# MOC & MCA Conductor sizing



## Easy (Oct 18, 2017)

I'm not real sure what this means. 
"MOC, which is the maximum over current protection, or the maximum amps you want to give to your machine. The MCA is the minimum circuit amperage, or the minimum amount of amps you need to supply your machine. The MOC and MCA are what we use to determine the breaker size for your machine, having it be between these two numbers." 
I guess feed with a 50 amp OCPD and size my fuses at the disconnect to ? Amps ?


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## emtnut (Mar 1, 2015)

Easy said:


> I'm not real sure what this means.
> "MOC, which is the maximum over current protection, or the maximum amps you want to give to your machine. The MCA is the minimum circuit amperage, or the minimum amount of amps you need to supply your machine. The MOC and MCA are what we use to determine the breaker size for your machine, having it be between these two numbers."
> I guess feed with a 50 amp OCPD and size my fuses at the disconnect to ? Amps ?


MCA is minimum circuit ampacity ... size conductors according to this

MOC is the the maximum breaker you can use ... So a 50A breaker is too high.


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## HertzHound (Jan 22, 2019)

The minimum circuit amps is the full load amps *125%
That is the smallest wire size you can run to the equipment. 

Maximum overcurrent protection is the full load amps *175%
That is the largest size size overcurrent device. 

When there are multiple loads with the piece of equipment, these multiplier values are for the largest loads, and then the other loads are added in for the MCA and MOC.


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## HertzHound (Jan 22, 2019)

By putting those values on the equipment, they did all the math for you.


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## Awg-Dawg (Jan 23, 2007)

Easy said:


> The MOC and MCA are what we use to determine the breaker size for your machine, having it be between these two numbers."
> I guess feed with a 50 amp OCPD and size my fuses at the disconnect to ?]


 If you use a fused disconnect, the conductors from the breaker to the disconnect have to be sized to the breaker. 

I’d you use a non fused, the conductors can be sized to the MCA.


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## HertzHound (Jan 22, 2019)

Awg-Dawg said:


> If you use a fused disconnect, the conductors from the breaker to the disconnect have to be sized to the breaker.
> 
> I’d you use a non fused, the conductors can be sized to the MCA.


I don’t think I ever heard that. Do you have a code section?


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## Awg-Dawg (Jan 23, 2007)

HertzHound said:


> I don’t think I ever heard that. Do you have a code section?


 The rule in 440 applies to branch circuits, by putting a fused disconnect in, the conductors between breaker and disco become a feeder. The feeder has to be sized to 310.15


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## HertzHound (Jan 22, 2019)

Awg-Dawg said:


> The rule in 440 applies to branch circuits, by putting a fused disconnect in, the conductors between breaker and disco become a feeder. The feeder has to be sized to 310.15


That’s a stretch, and not how it is enforced around here. Going by the wording in definitions it would sound correct. But to me it is a branch circuit to a safety switch, not a feeder. 

This months code FAQs in the Electrical Contractor has a residential AC compressor question in it. The guy that writes the article threw me for a loop. He is saying that the small conductor wire size 14,12 and 10 that is restricted to 15,20 and 30 amps by article 240 is not overridden by article 440. WTF? That’s a first for me. I never heard that before?


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## Awg-Dawg (Jan 23, 2007)

HertzHound said:


> That’s a stretch, and not how it is enforced around here. Going by the wording in definitions it would sound correct. But to me it is a branch circuit to a safety switch, not a feeder.
> 
> This months code FAQs in the Electrical Contractor has a residential AC compressor question in it. The guy that writes the article threw me for a loop. He is saying that the small conductor wire size 14,12 and 10 that is restricted to 15,20 and 30 amps by article 240 is not overridden by article 440. WTF? That’s a first for me. I never heard that before?


 By definition, it’s a feeder and would be a violation. I see no harm in it, but an EI would have a basis for failing it.


I’ve never heard of the small conductors not allowed to be used for 440.

If they were using NM , it’s possible since it’s only good at 60*, which would match the small conductor amperage?


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## Easy (Oct 18, 2017)

Thanks for all your help. I "overkill" everything I do. Most all of the equipment I have worked with involved actual "fused disconnects" or buss duct "plug in Fused disco's". The fuse were always sized to protect the equipment.

IDK 24.5 amps at 125% = 30.625 Amps = #8 wire and should be able 2 fit it all in a 3/4" run. I may have to compensate for voltage drop depending on where the chiller is placed. 

I visited a brewery today that passed inspection 3 years ago and it was wired in surface mounted EMT with *set screw connectors & couplings*. It seamed strange. No rain-tight fittings or even a compression fitting anywhere. Seriously ... I didn't drink anything It just seemed strange.


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## Easy (Oct 18, 2017)

Went for a tour of a brewery to see how it's done.


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## Easy (Oct 18, 2017)

emtnut said:


> MCA is minimum circuit ampacity ... size conductors according to this
> 
> MOC is the the maximum breaker you can use ... So a 50A breaker is too high.


Good deal as I don't have access to a hydraulic bender. I'm to weak to bend 1" with a hand bender. :crying:


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## Awg-Dawg (Jan 23, 2007)

Easy said:


> IDK 24.5 amps at 125% = 30.625 Amps = #8 wire and should be able 2 fit it all in a 3/4" run. e.



You don’t need to multiply the 125% to the 24.5 , it’s already done n the nameplate.

Going by the nameplate, 10 thwn and a 45a breaker to a non fused disconnect should be code compliant.


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## Easy (Oct 18, 2017)

Awg-Dawg said:


> By definition, it’s a feeder and would be a violation. I see no harm in it, but an EI would have a basis for failing it.
> 
> 
> I’ve never heard of the small conductors not allowed to be used for 440.
> ...


You can wire "commonly" to as low as 12 AWG for branch circuits, motor feeds or what ever you want at 480 volts. I'm not talking lighting ballast wires or nothing like that. I think Awg-Dawg meant something entirely different and we both missed it. I better go back and read but I trust his judgment. He knows things.. :smile:


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## Easy (Oct 18, 2017)

Awg-Dawg said:


> You don’t need to multiply the 125% to the 24.5 , it’s already done n the nameplate.
> 
> Going by the nameplate, 10 thwn and a 45a breaker to a non fused disconnect should be code compliant.


Thanks Awg-Dawg .. Sometimes it pays to conserve. Why not put in a 35 amp breaker on the # 10s? Do you think the inrush current might be an issue?


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## Easy (Oct 18, 2017)

"MOC, which is the maximum over current protection. Of what? The equipment of course. Logical then why not meet or exceed the "MOC" ratting. After all why even have the ratting at all?


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## HertzHound (Jan 22, 2019)

> Thanks for all your help. I "overkill" everything I do. Most all of the equipment I have worked with involved actual "fused disconnects" or buss duct "plug in Fused disco's". The fuse were always sized to protect the equipment.
> 
> IDK 24.5 amps at 125% = 30.625 Amps = #8 wire and should be able 2 fit it all in a 3/4" run. I may have to compensate for voltage drop depending on where the chiller is placed.



24.5 amps is the total loads added together. That is not what you want to do your calculation if it didn't do it for you already.


You have four loads. 

1. one compressor @17.3
2. two fan motors @ 1.9 apiece
3. one pump motor @3.4


The compressor is the largest load so that is the full load amps you want to multiply by 1.25


(17.3 x 1.25) + 1.9 + 1.9 + 3.4 = 28.8 



28.8 amps would require #10 ga. and you would fuse it up to 45 amps


MOC= (17.3 x 1.75) + 1.9 + 1.9 + 3.4
MOC = 46.1


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## Awg-Dawg (Jan 23, 2007)

HertzHound said:


> This months code FAQs in the Electrical Contractor has a residential AC compressor question in it. The guy that writes the article threw me for a loop. He is saying that the small conductor wire size 14,12 and 10 that is restricted to 15,20 and 30 amps by article 240 is not overridden by article 440. WTF? That’s a first for me. I never heard that before?


I found that article, it doesn’t make any sense to me. 

He said 240.4 (g) applies , then it doesn’t.

He does mention that NM can’t be used in a wet location, other than that, I believe he is wrong.


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## Awg-Dawg (Jan 23, 2007)

Easy said:


> Thanks Awg-Dawg .. Sometimes it pays to conserve. Why not put in a 35 amp breaker on the # 10s? Do you think the inrush current might be an issue?


 Yes, that’s the reason the breaker/fuse is sized higher than the conductor.

The possibility of it tripping on startup is the reason it’s higher.


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## HertzHound (Jan 22, 2019)

Easy said:


> "MOC, which is the maximum over current protection. Of what? The equipment of course. Logical then why not meet or exceed the "MOC" ratting. After all why even have the ratting at all?



The rating is there for inrush current. They allowed you to go 175%. Not to exceed, so 45A would be the max breaker size and you would be allowed to put it on a 10ga wire (because of your MCA), even though 240.4 (D) (7) says that normally 10ga can only go on a 30a breaker.


Now If you read this months EC mag, you cant do it.


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## Easy (Oct 18, 2017)

Awg-Dawg said:


> Yes, that’s the reason the breaker/fuse is sized higher than the conductor.
> 
> The possibility of it tripping on startup is the reason it’s higher.


I sure feel like a dummy. Ok I have heard it from 2 members I trust and I will remember it. I especially like how HertzHound laid it out like a school teacher. Easy to understand.


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## Awg-Dawg (Jan 23, 2007)

Easy said:


> I sure feel like a dummy. Ok I have heard it from 2 members I trust and I will remember it. I especially like how HertzHound laid it out like a school teacher. Easy to understand.


 Never feel like a dummy, the fact you are here shows you’re learning.

Yeah, Hertzhound laid it out nice, it makes it easier when you see the formula.

Here’s a video I that’s worth watching also. 

https://www.google.com/url?sa=t&rct...=9lPDYEAgWD4&usg=AOvVaw3bslm7O8WLVAFZuarDrMv-


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## Easy (Oct 18, 2017)

HertzHound said:


> The rating is there for inrush current. They allowed you to go 175%. Not to exceed, so 45A would be the max breaker size and you would be allowed to put it on a 10ga wire (because of your MCA), even though 240.4 (D) (7) says that normally 10ga can only go on a 30a breaker.
> 
> 
> Now If you read this months EC mag, you cant do it.


I better read EC mag  I am preying that the EE puts in all the specks for me and stamps it. I think I get a copy of the plans tomorrow. I imagine that the chiller will be about 75 to 100 feet from the service so voltage drop issues possibly but I might be fine. I think this Chiller can operate slightly under 480v I will call the manufacture if needed. Most of the construction work I did was as a Maintenance Electrician and the EE's always detailed conduit and conductor size. That's why I'm so fuzzy headed on construction.


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## Easy (Oct 18, 2017)

Awg-Dawg said:


> Never feel like a dummy, the fact you are here shows you’re learning.
> 
> Yeah, Hertzhound laid it out nice, it makes it easier when you see the formula.
> 
> ...


Great Video .. First time I have seen this one by M.H. inc ... He has excellent information. This has been a very valuable learning experiences for me and I appropriate it greatly.


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## HertzHound (Jan 22, 2019)

It's good to do the calculations even when they are done already. Everyone makes mistakes, even the A&E. 



The job I'm laying out now has an I beam through the middle of the gear. It's early enough for a re-design. Actually a re-re-design. the fix they proposed was to split the gear into two sections and turn the one piece 90 degrees. So now instead of going through the gear, it's just in the work clearance. you will need a stubby screwdriver to get the top screws of the cover out. They are looking for a solution that won't cost extra money. Eventually they will figure out that they will need to lower the floor and footings by a foot. Nothing is even dug out yet, but they are impacting the schedule. In the days of 3D modeling this stuff shouldn't be happening.



A couple of jobs ago all the parallel grounds were the wrong size in all the feeders. They only sized them for the wire in each conduit instead of the full breaker size. all caught before it cost big bucks. but it did get past the engineers and plan review.


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## Easy (Oct 18, 2017)

Well I'm signing off for tonight. Thanks again for the help. Good luck with your project HertzHound. and yes some engineers mess up badly and it's good for us to know the code and make simple calculations. Doing things twice is never fun. Inspectors are also not always well versed either and as an electrician it's to our advantage to know at least as much as they do. Oh yes and check local enhanced codes or what ever they call them. Some cities have strange requirements but I think conductor sizing is probably not disputable. Who knows .. Maybe they will ask for #8 out of ignorance but at least now I have a good line of defense by knowing these rules.


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## CoolWill (Jan 5, 2019)

There are articles in EC&M? I only ever see ads.


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## Dennis Alwon (May 9, 2009)

Awg-Dawg said:


> The rule in 440 applies to branch circuits, by putting a fused disconnect in, the conductors between breaker and disco become a feeder. The feeder has to be sized to 310.15


That rule comes from the definition of feeders vs branch circuit. I had one cmp member agree with me that the feeder should be considered a branch circuit in this case however, when I wrote a code proposal to change the definition it got nixed. It is a ridiculous rule and I don't believe that was the intent to consider one as a feeder and the other as a bc.


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## Awg-Dawg (Jan 23, 2007)

Dennis Alwon said:


> That rule comes from the definition of feeders vs branch circuit. I had one cmp member agree with me that the feeder should be considered a branch circuit in this case however, when I wrote a code proposal to change the definition it got nixed. It is a ridiculous rule and I don't believe that was the intent to consider one as a feeder and the other as a bc.





I agree , I’ve never heard of anyone failing for it either though.

Do you still have the proposal or do i need to look it up?

I’d like to read it.


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## JamesA (1 mo ago)

Sorry to bump a 3yo thread but I just wanted to verify since this particular was not brought up.
I have a condenser unit with mca 20.8 & mop 35A
We ran #12 mc cable to a nonfused disconnect because #12 is good for 25A. I'm not limited to the #14,12,10 to 15,20,30A or am I?


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## Almost Retired (Sep 14, 2021)

JamesA said:


> Sorry to bump a 3yo thread but I just wanted to verify since this particular was not brought up.
> I have a condenser unit with mca 20.8 & mop 35A
> We ran #12 mc cable to a nonfused disconnect because #12 is good for 25A. I'm not limited to the #14,12,10 to 15,20,30A or am I?


NEC article 440 .... just a few pages .... look it up for yourself and you will never need to ask again
not to mention you can show a lot of electricians where they are wrong
as well as the uneducated inspectors you see from time to time

oh ..... you dont have an nec? order online
you cant do this work properly without one


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## JamesA (1 mo ago)

OK This is what I got. 
440.6 The size of the conductors ..... Shall be selected from 310.15(B)16-19 or calc from 310.15. The req ampacity and rating det from 440.6(A-B)
I guess I'm asking about the ** asterisk on T310.15(B)16 that refers to 240.4(D) Small conductors ...... 
This leads to (G) where it mentions Air Conditioning & Ref 440 parts 3 & 6
So back to 440 part 3
440.21 In addition to amendatory of article 240
So there it is if anyone else needs to find it. You are limited to 20A on #12 wire for the MCA rating. This means I'll need to run #10 wire for my AC MCA 20.8A MOP 35A. If that was wrong please advise thanks.


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## Awg-Dawg (Jan 23, 2007)

JamesA said:


> OK This is what I got.
> 440.6 The size of the conductors ..... Shall be selected from 310.15(B)16-19 or calc from 310.15. The req ampacity and rating det from 440.6(A-B)
> I guess I'm asking about the ** asterisk on T310.15(B)16 that refers to 240.4(D) Small conductors ......
> This leads to (G) where it mentions Air Conditioning & Ref 440 parts 3 & 6
> ...


. 

This is what you need to read. This allows the MC on a 25a.

240.4(D) (D) Small Conductors. UNLESS SPECIFICALLY PERMITTED IN 240.4(E) OR (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have. 240.4[G) is going to send you to 440.xx hermetic motors, which is what ACs use.


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## Almost Retired (Sep 14, 2021)

JamesA said:


> OK This is what I got.
> 440.6 The size of the conductors ..... Shall be selected from 310.15(B)16-19 or calc from 310.15. The req ampacity and rating det from 440.6(A-B)
> I guess I'm asking about the ** asterisk on T310.15(B)16 that refers to 240.4(D) Small conductors ......
> This leads to (G) where it mentions Air Conditioning & Ref 440 parts 3 & 6
> ...


try this link and scroll down to post #18
watch the video
its a full explanation









why fail?


i wired the conduits with 14awg and the inspector failed me for it, what gives?




www.electriciantalk.com


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