air conditioning sizing conductors

[brother]

i havent done much sizing of conductors for AC/heating units, but reading 440.2 definitions of 'branch circuit selection current' I thought there was another chart like they had for motors in sizing conductors, but apparently not. You just follow the nameplate rating and mulitply 125%. Correct me if im wrong please. and if this is the case why couldnt they just worded it 'simplier' in the NEC. Basically you can have a #12 awg on a 40 amp breaker if the numbers are right!!

I had to correct a guy when he was getting upset about an AC unit that had #12 awg on a 30 amp.

Thanks.

 

[augie47]

If I understand Art 440 correctly, 440.6 advises to select a conductor by its ampacity from 310.16-19 with that ampacity requirement designated by 440.4 and displayed by the manufacturer.
I do not see any requirement for a 125% multiplier.
Likewise 440.21(C) limits the maximum breach circuit protective device based on manufacturers data.

 

[brother]

If I understand Art 440 correctly, 440.6 advises to select a conductor by its ampacity from 310.16-19 with that ampacity requirement designated by 440.4 and displayed by the manufacturer.
I do not see any requirement for a 125% multiplier.
Likewise 440.21(C) limits the maximum breach circuit protective device based on manufacturers data.

The 125% comes from 440.32

 

[curt swartz]

440.32 doesn?t apply to typical A/C equipment. 440.32 is for a single motor compressor not multi motor equipment. The manufacture has already the 125% to the MCA marked on the unit.

 

[brother]

440.32 doesn?t apply to typical A/C equipment. 440.32 is for a single motor compressor not multi motor equipment. The manufacture has already the 125% to the MCA marked on the unit.

this unit is a single motor compressor, other than the fan motor, its a condensor.

 

[iMuse97]

this unit is a single motor compressor, other than the fan motor, its a condensor.

I think what C. Swartz is saying: a typical A/C compressor/condenser unit has two motors, and has a minimum circuit ampacity listed on the nameplate which is used to size the wire to the unit. You do not multiply this number by 1.25; you just use wire that will carry that ampacity, with any derating or correction factors figured in.

In addition, I would add that on any newer equipment the nameplate will also list maximum breaker size.

 

[billsnuff]

In addition, I would add that on any newer equipment the nameplate will also list maximum breaker size.

I seem to recall breaker or fuse, depending what is spec'd on the unit tag, or am I all wet. ( i.e. if it says fuse you can't use breaker?)

 

[dcspector]

I seem to recall breaker or fuse, depending what is spec'd on the unit tag, or am I all wet. ( i.e. if it says fuse you can't use breaker?)

Most list either or however some Mitsubishi units show fuse only and or different fuse size versus breaker size.? I am guessing here must be due to differing AIC ratings between the two (fuse vs. breaker)

 

[abode electric]

The name plate gives you 125% of largest motor and 100% of all others. The min circuit ampacity is your wire size (you can use #14 for 20amps #12 for 25 amps) Then the max fuse or circuit breaker is marked on the name plate. I have seen some units that you could wire with a #14 and protect with a 30 amp breaker.

 

[Dennis Alwon]

Things are changing fast. I have noticed that the higher seers rated a/c seem to have a small difference bewteen MCA and MOCP. My last unit called for a MCA of 12 amps with a max OCP of 15 amps.

Those older units would allow a MOCP of 20 or 25 amps.