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A/C overcurrent protection

Posted By: JERRY0129

A/C overcurrent protection - 07/12/02 04:46 PM

A Residential AC unit was installed with the following nameplate info
Minimum Circuit Ampacity: 22 amps
Minimum Fuse/Breaker: 35 amps
Maximum Fuse/Breaker: 35 amps
My question is whatis the minimum size conductors that can be used. The circuit is only 10-12' long. A code section would also help. Thanks
Posted By: Bjarney

Re: A/C overcurrent protection - 07/12/02 05:05 PM

See Art.440--primariy 440-6b. Use 10AWG conductors, and they'll not be a problem.
Posted By: Joe Tedesco

Re: A/C overcurrent protection - 07/12/02 05:33 PM

Look at the following link:

[This message has been edited by Joe Tedesco (edited 07-29-2002).]
Posted By: MikeW

Re: A/C overcurrent protection - 07/13/02 04:31 PM

Hello Jerry,

Based on 1999 NEC I’m going to give my opinion and if anyone sees I’m wrong please step in.

Section 440-4(b) says the manufacturer shall provide a nameplate with the minimum supply circuit conductor ampacity, along with other info. 110-3(b) requires equipment to be installed to this minimum.

Since you say the nameplate info given is, “Minimum Circuit Ampacity: 22 amps” I would use #12 AWG CU and I believe this would be code compliant with (310-16).

[This message has been edited by MikeW (edited 07-13-2002).]
Posted By: tdhorne

Re: A/C overcurrent protection - 07/14/02 10:58 PM

With a circuit selection current of 22 amperes and the branch circuit serving only one motor compressor the branch circuit conductors must be sized to 125% of that ampacity or the motor compressor rated load current whichever is greater. 125% of 22 amperes is 127.5 amperes which brings us back to #10 AWG Cu conductors.

440.32 Single Motor-Compressor.
Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater.
Posted By: caselec

Re: A/C overcurrent protection - 07/15/02 02:17 AM

If he is looking on the name plate of the compressor only then you would need to follow 440.32 but I would guess that his is looking at the name plate on the "box" sitting outside which would be a multimotor load (fan and compressor) so the conductors need to be sized based on the minimum circuit ampacity on the name plate. #12 wire would be sufficient size for the short distance your going but I would recommend using at least #10 from the branch breaker to the disconnect to prevent excessive voltage drop if this circuit was much longer.
Posted By: MikeW

Re: A/C overcurrent protection - 07/15/02 12:58 PM

Please, can you give us all of the nameplate information?

Caselec & tdhorne,
Yes, I was talking about the visible nameplate on the outside of the entire A/C unit. Which on the new units I have seen list the FLA of the fan motor along with Min. Circuit Amps, etc. And I thought this would then require you to use 440-4(b) of 1999 NEC.

I agree #10 is fine but Jerry asked for the minimum size wire allowed and I believe it to be #12.

Anyone else have any thoughts on this subject? [Linked Image]

Thanks, MikeW
Posted By: resqcapt19

Re: A/C overcurrent protection - 07/15/02 02:47 PM

I agree that the NEC permits #12 copper for this application. I am assuming that this is a multimotor (condenser fan and compressor) unit and the nameplate markings are per 440.4(B). If so the minimum circuit ampacity marked on the nameplate can be used directly size the conductors. See 440.35.
Posted By: wolfdog

Re: A/C overcurrent protection - 07/15/02 07:04 PM

I agree with Don. The minimum amps marked on A/C units includes the 125%.
Posted By: JERRY0129

Re: A/C overcurrent protection - 07/15/02 07:23 PM

I don't recall ever seeing the "Minimum Fuse/HACR Breaker" designated.I think that is what raised my original question. My thinking was if the manufacturing has a Min and a max of 35 amps, does he want the wire sized based on 310-16?
Posted By: JERRY0129

Re: A/C overcurrent protection - 07/15/02 07:25 PM

Thanks a lot for all your input. Great BB.
I can probably get a digital image of the nameplate but I'm not sure how to attach or forward to this forum
Posted By: George

Re: A/C overcurrent protection - 07/15/02 07:49 PM

There is a difference between what is "to code" and what is "good practice."

#12 is the MINIMUM "to code" perhaps.

This is a residence. In a residence I regard it to be "poor practice" to place a 35amp breaker on #12 wire. I would use #8 wire.

The 1999 ICC 1&2 Family Dwelling Code (4205.7) limits the overcurrent protection device of #10 wire to 30amps. This would require #8 wire. (This may be a NEC requirement also. I don't know.)

There is a reasonable rational for requiring #8 wire: When the A/C is replaced by something else not governed by the A/C exception (perhaps even a different A/C), the circuit is still NEC compliant.
Posted By: Reel-Break

Re: A/C overcurrent protection - 07/15/02 08:19 PM

I personally would not use the 12 because we all know the lights will dim and the home owner will be very concerned.If it`s a matter of code vs a better installation go with the 10. Remember code is the minimum..
Posted By: resqcapt19

Re: A/C overcurrent protection - 07/15/02 08:36 PM

This is a residence. In a residence I regard it to be "poor practice" to place a 35amp breaker on #12 wire. I would use #8 wire.
Why do you say this is a "poor practice"? The breaker is only provding short circuit and ground fault protection for this circuit. The overload protection is provided by the AC unit. Under a short circuit or ground fault, there will be no difference in the trip times or the current required to open the breaker for all breaker sizes between 15 and 60 amps.
The basic NEC rules would require a #8 on a 35 amp OCPD, but motor, refirgeration and some other circuits are not required to comply wiht the basic rule. Also I don't think the #12 would increase the flicker or dimming effect on the lights. It may increase the lenght of time that they dim, but should also reduce the amount of dimming.
Dimming is caused by the voltage drop on the service conducotors. Smaller wire will limit the current and reduce the voltage drop.
Yes the code is a minimum and you can exceed the minimum. The original question asked for the mimimum conductors that are "required" for the circuit.
Posted By: MikeW

Re: A/C overcurrent protection - 07/16/02 01:22 AM

I also agree with what Don says and he says it a lot clearer then I ever could.

We are lucky to have such a great forum - Thanks to all!!
Posted By: lynnbar

Re: A/C overcurrent protection - 07/30/02 02:36 AM

I always use 30 amp breaker for household AC units. when i think 30 amp I think #10, like a kitchen circurt 20 amp, when i see 20 amp I think #12. and so on. depends on the lenth of run as well. 40 amp ele stove # 6.
just my 2 cents worth
Posted By: George

Re: A/C overcurrent protection - 07/30/02 02:50 AM

I understand the issues.

You asked about my "poor practice to use #10 wire" comment.

At some time in the future it is possible to look at the 35 (or 40)amp breaker and install some device that will draw 35 (or 40) amps.
Now we have the possibility of #10 wire carrying more than 30amp. I believe that is not to code.

I would rather avoid these types of situations.

In a non-residential setting I would have no problem using #10 wire.
Posted By: C.Urch

Re: A/C overcurrent protection - 07/30/02 05:46 PM


I believe the minimum and maximum breaker size of 35 amps is for motor start up while the full load running amps of the unit is 22 amps. The oversized breaker is so the start up current does not trip the device. # 10 wire is what I would use to feed the unit, since it would not demand more than 22 amps of continuous load during operation.
Posted By: sparky

Re: A/C overcurrent protection - 07/30/02 09:37 PM

with all due respect George,
i'm really not big on having more than one electrical code, especially if they conflict.....

nor do i wish to wire for future changes via DYI caliber hamheads who would not recognize the original circuits function/usage.....
Posted By: George

Re: A/C overcurrent protection - 08/02/02 11:04 PM

C.Urch ---

I believe that the code also allows the use of #8 wire in this situation. If you want to use #10 do so. If I had to hire an electrician to do this work, the spec would call for #8 wire NM-B.

sparky ---

I do very simple electrical work in residences. I try to avoid traps that "DYI caliber hamheads" might fall into. I try to be considerate to the poor HVAC man who installs a replacement for the unit.
Posted By: John in Jersey

Re: A/C overcurrent protection - 08/24/02 03:48 PM

Hi I am new to this forum.

We do a lot of work with a local HVAC company. For the above A/C installation we would run 8 wire with a 40 amp breaker, then fuse it down to 35 amps. In our experience in dealing with the service department with this company doing this has reduced service calls. For this A/C unit 10 wire with a 30 amp breaker will work for the distance mentioned, but if there is a small deviation in voltage drop the breaker WILL trip. The manufacturer of the condenser (york) even reccommends running the max size mentioned in the nameplate and fusing it to the proper size.
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