Wire and Breaker Sizes

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steve66

steve66

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Illinois
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Engineer
A friend asked me to size the wire and breaker for a geothermal heat pump. Here are the specs:

240V, 1 phase; 27.7 Full Load Amps; Min. circuit ampacity 32.2 Amps; Max. HACR breaker 50 Amps.

Can he install a 50 amp HACR breaker and run #8 wire (40 Amps at 60 deg.), or is #6 wire required (55 amps at 60 deg.) for a 50A breaker?
Steve
 
Re: Wire and Breaker Sizes

#8 is in compliance with the code on a 50 amp breaker for this application. If the equipment and terminations on both ends are suitable for 75?C, then you could use #10.
Don
 
Re: Wire and Breaker Sizes

Originally posted by don_resqcapt19:
#8 is in compliance with the code on a 50 amp breaker for this application. If the equipment and terminations on both ends are suitable for 75?C, then you could use #10.
Don
Click to expand...
Unless the wiring method is type NM cable, which would require a #8 regardless of the temperature, due to 334.80 :)
 
Re: Wire and Breaker Sizes

Steve Just to add a little understanding why this is allowed, as it through me for a loop when I heard it the first time.
The breaker is only for short circuit and ground faults protection. If you ever run a #6 NM cable you would notice that it only has a #10 ground wire this is because the #10 will trip the breaker fast enough to protect the wire. The wire is also protected from over loading by a overload protector that is part of the AC unit or compressor motor. This is the minimum ampacity that the AC unit requires for this overload to protect. It sounds strange at first but its ok. I still have a few inspectors that won't allow it because they don't understand this.
 
Re: Wire and Breaker Sizes

Thanks, and thanks for not giving me a hard time about asking a question that's pretty basic.

I think all the Square D circuit breakers are rated for 75 deg. C, but I am still reluctant to start using the 75 deg. column of table 310.16 for anything under 100 amps.

Steve
 
Re: Wire and Breaker Sizes

I understand how the specs allow for an 8AWG copper conductor to be protected by a 50 amp rated circuit breaker.
Are you saying that a 10 AWG copper conductor can also be protected by the same 50 amp rated circuit breaker for this application?
OR
A 10 AWG copper conductor can be protected by a 30 amp rated circuit breaker for this application?
 
Re: Wire and Breaker Sizes

The 10 awg on the 50 is code compliant. The motor overloads in the unit protect the conductors from overload.

With a listed minimum circuit ampacity of 32 amps the smallest breaker we could use is a 35 amp breaker

[ September 08, 2004, 11:04 PM: Message edited by: iwire ]
 
Re: Wire and Breaker Sizes

Pierre
Think about it all romex from 10awg to 6awg will have only a #10awg ground in it. doesn't this breaker (60 amps) protect this ground wire if the hot were to short to an EGC on the load end?

Now look at table 250.122 at 30 amp, 40 amp, 60 amp. then look at what size grounding wire is required.

So yes a #10 will be protected by a 60 amp breaker from a short circuit. Now this is not an over load by no means but that is what the motor over load is for it will not allow enough current on the circuit wires to cause them to over heat.
 
Re: Wire and Breaker Sizes

Originally posted by ryan_618:
Originally posted by don_resqcapt19:
#8 is in compliance with the code on a 50 amp breaker for this application. If the equipment and terminations on both ends are suitable for 75?C, then you could use #10.
Don
Click to expand...
Unless the wiring method is type NM cable, which would require a #8 regardless of the temperature, due to 334.80 :)
Click to expand...
That isn't right, is it? Article 334.80 says, "The ampacity shall be in accordance with the 60C conductor temperature rating." Table 310.16 says #8 for a 40A, and #6 for a 55A.
 
Re: Wire and Breaker Sizes

royta,
The required conductor ampacity for this application is 32.2 amps per the original post. The condutor that has an ampacity equal to or greater than 32.2 amps can be connected to the 50 amp breaker for this equipment.
Don
 
Re: Wire and Breaker Sizes

OK, I understand. I got scared there for a minute. I thought I had oversized the wire I just bought for a customer's kitchen remodel. They need a 50A appliance circuit, so I bought #6 NM-B.

Thanks.

Roy
 
Re: Wire and Breaker Sizes

royta,

There is nothing wrong with oversizing a wire, other than wasting money and natural resources.

It is the o.c.p.d. that determines the ampere rating of the branch circuit, not the wire.
 
Re: Wire and Breaker Sizes

Originally posted by steve66:

I think all the Square D circuit breakers are rated for 75 deg. C, but I am still reluctant to start using the 75 deg. column of table 310.16 for anything under 100 amps.

Steve
Click to expand...
I was taught that way too, but apparently, my supervisor corrected me as soon as I came to work at this office. He says use 75 deg. no matter what. He told me not to use the 60 deg.

I know it's determined by the terminal of this device, but does anyone else do this?


Lady :)
 
Re: Wire and Breaker Sizes

Originally posted by peter d:
royta,

There is nothing wrong with oversizing a wire, other than wasting money and natural resources.

It is the o.c.p.d. that determines the ampere rating of the branch circuit, not the wire.
Click to expand...
As per homeowner, so 6-3 it is.

O.C.P.D. ??? What does this acronym stand for?
 
Re: Wire and Breaker Sizes

Wire for min.,fuse for max.in this senario.the data name plate tells you what to do.
 
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