Is a 50amp breaker on a 10awg wire safe for an AC?

[tkurland]

I'm buying a house. It has an outdoor AC compressor on a #10 wire with a 50amp breaker.

My inspector says it should be a 30amp breaker.

The electrician who installed it says its within the NEC code based on these facts:

1) 310.16 says up to 40amp is ok on a #10 wire.

2) 440.22 says the breaker can be 175%, up to 225%, of the rated load current of the compressor.

3) At 175%, a 30amp wire can support a 50amp breaker ( 30 * 1.75 = 52.5 ).

It sounds like he is mistakenly using the rated WIRE current instead of the rated LOAD current.

And so a 50amp breaker would only be allowed on a device with 28.5 RLC (or 22.5 RLC at 225%).

If the RLC of the AC compressor really was a full 28.5amp, would anyone use a #10 wire, even though 'technically' it appears to be legal?

Or do most AC installations require the 225% rule? And if so, would you still put a 22.5+ RLC on a #10? When do you cut over to a #8?

I wish I had the running current but I don't close until Wednesday and can then get the spec's on the compressor.This is a home AC compressor cooling 2800 sq feet.

My main concern - does this sound safe?

Thanks for your help!

Todd

 

[ryan_618]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

Todd. There are three aspects to overcurrent protection:
1) Short circuit protection
2) Ground fault protection
3) Overload protection.

The code simply states that you must allow the motor to start. This means that the 225% rule is not mandatory, but rather permissive.

Short circuit and ground fault protection are provided by the overcurrent protection device feeding the circuit (ie, the breaker), whereas overload protection is being provided by the heaters inside of the motor. The size of conductor need not be as large as the breaker, but it must be as large as the running load amps of the motor/compressor.

In a nut shell, the breaker will open the circuit in thecase of a grounbd fault or short circuit, and the heater will open the circuit in the case of an overload.

This sounds like a compliant installation to me.

Anyone agree?

 

[iwire]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

You do not even need to worry about 175% 250% ect.

For a unit like this you go directly off the nameplate of the unit.

First the unit tag should say something like "Minimum Circuit Ampacity"

This is just what it sounds like, the minimum allowed.

For the sake of this I will say the tag indicates 30 amps minimum circuit ampacity.

We could still use 10 THHN for this load as it is rated 30 amps at 60 C

Also the unit tag will say maximum breaker (or fuse) size.

For a 30 amp MCA unit this might be 40 amps or more.

What ever this tag says you can not exceed it.

The wire size will not match the breaker size.

So this could very well be a perfectly fine installation.

To give a firm answer you need to tell us exactly what the unit tag says as that is what determines the wire and breaker size.

[ February 22, 2004, 05:06 PM: Message edited by: iwire ]

 

[ryan_618]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

Good point Bob. I should have mentioned 440.22(C)

 

[ccha9219]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

I gotta go with iwire on this one
The a/c compressor doesn't have heaters like motor starters do (in my exp. I have not seen one)
As a rule go with the nameplate rating. At the very least if it does burn up the manufacture will replace it not you

 

[iwire]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

Originally posted by ccha9219:
The a/c compressor doesn't have heaters like motor starters do (in my exp. I have not seen one)

Any motor-compressors (the typical sealed units) I can remember have internal thermal overloads that take the place of the "heaters" we see on other motor loads.

Try short cycling one of these units and you will hear the motor try to start but not be able to do so, after a time you will hear the power cut off as the thermal opens.

All motors are required to have some type of overload protection by part III of 430.

Separate Overload Device.

Thermal Protector.

Integral with Motor.

Impedance-Protected.

 

[tom baker]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

tkurland: Your inspector is wrong. Its a general rule that 10 AWG is to be protected at no more than 30 amperes, but Art 430 on overcurrent protection (240.3) states that that other equipment shall be protected per table 240.3, which lists Art 440-Air Conditioning. Typically the AC will list the minimum circuit ampacity, and size of overcurrrent protection required. Take a look at the name plate and see what it states.

 

[aceofdiamond]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

this is a reprint from mike holt's.doc
Q19. A 230 volts central air conditioning specifies a maximum 45-ampere fuse or HACR breaker, with a minimum circuit ampacity of 27.8 amps. Can I use No. 10 THHN with a 40-ampere HACR Breaker?

A19. Yes. Figure 7.
Conductors Sizing Section 440-33. Conductors supplying a motor-compressor with additional load(s) shall have an ampacity not less 125 percent of the highest motor or motor-compressor rating in the group, plus the other loads.

Example: A/C with the following loads: Compressor 18.7 amperes, OD Motor 1.8 amperes, ID Motor 2.6 amperes.
Conductor Nameplate Size = 18.7 amperes X 1.25 + 1.8 amperes + 2. 6 amperes
Conductor Nameplate Size = 27.8 amperes

Protection Sizing Section 440-22(b). The protective device having a rating or setting not exceeding 175% of the largest motor-compressor rated-load current, plus the other loads, shall be permitted. Where the protection at 175% is not sufficient for the starting current of the motor, the protection rating can be increased to 225% of the largest motor-compressor rated-load current, plus the other loads.

Example: A/C with the following loads: Compressor 18.7 amperes, OD Motor 1.8 amperes, ID Motor 2.6 amperes.
Protection Nameplate Size = 18.7 amperes X 2.25 + 1.8 amperes + 2. 6 amperes
Protection Nameplate Size = 46.48 amperes, next size down = 45 amperes

So you can have a 50Amp breaker on #10 wire. the 30Amp and #10 is for resistive loads like water heater and dryer. the inspector doesn't know what he is talking about. he seems like the ones that memorized the book and passed the test, no field experience.

 

[tkurland]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

Great! Thanks for the replies, everyone. You've reassured me and educated me. I'll check the nameplate when we move in. I think I understand better now. Motors just let the current flow without the extra resistance that a water heater would have, so overheating the wire is less of an issue and the breaker is then larger to allow for the startup requirements. Great posts. Thanks again, Todd

 

[iwire]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

Originally posted by tkurland:
Great! Thanks for the replies, everyone. You've reassured me and educated me. I'll check the nameplate when we move in.

Your welcome.

All you need to be sure this is correct.

The wire has to be rated at least as high as the listed minimum circuit ampacity.

The over current device is not larger than the listed maximum overcurrent device.

It is possible that the unit tag will only list a maximum fuse size. If that is the case (no breaker size listed) you will need to use fuses in a fuseable disconnect.

In order for the wiring inside the unit to be properly protected as approved by UL, you must not use a higher rated overcurrent device than listed or a different type of overcurrent device listed on the tag.

As Ryan pointed out this is addressed in 440.22(C) and 440.35

Good luck.

 

[a.wayne3@verizon.net]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

The over current device is not larger than the listed maximum overcurrent device.
This is the key,Max breaker is just that.No smaller than and no larger than listed on data plate

 

[abode electric]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

tkurland,
If this AC is wired in nonmetallic cable (romex) it is only good for 30amps. Nonmetallic cable can only use the 60c column. It still may be able to be fused at 50.

 

[tkurland]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

Followup... we moved in today and I checked the nameplate on the AC. 36amp minimum wire and 50amp maximum breaker. The wire is a flat white #10 wire - 'Colonial Type NM-B 600Volt'. I suspect this is in spec; but would I be safer if I put in a #8 wire? Thank you, Todd

 

[caj1962]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

NM-B would not be acceptable in an outdoor application. And yes you should go with a #8 if the unit says 36 amp circuit apmacity.

 

[a.wayne3@verizon.net]

Re: Is a 50amp breaker on a 10awg wire safe for an AC?

36 amp min.,then it has to be wired in #8 as a min.This never should have been passed on an inspection if it ever was ???????