ac unit conductor sizing

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chateau mike

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i have a unit with a minimum circuit ampacity of 23 amps, and maximum fuse of 35 amps. i come up with 28.75 amps (23x1.25). based on that calculation, #10 wire with a 30 amp disconnect and a 30 amp breaker should suffice. my supervisor believes a 60 amp switch with 35 amp fuses 30amp breaker and #8 wire should be used. this appears to be a gray area with un clear interpretations from the code book and training. :-?
 
Take a look at 440.32 for sizing branch-circuit conductors for a single motor-compressor, and 440.22 for sizing of overcurrent devices for A/C units.

With that said, you can use the branch-circuit selection current marked on the nameplate of the A/C unit to size your branch circuit conductors without using the 125% adjustment. So from you example you could use #12 THHN for your branch-circuit conductors (Table 310.16 60 or 75 degree column).

Also you can use up to a 35 amp fuse to protect these conductors in accordance with 440.22(C)

Hope this helps,

Chris
 
chateau mike said:
i come up with 28.75 amps (23x1.25). based on that calculation, #10 wire with a 30 amp disconnect and a 30 amp breaker should suffice.
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This calculation is not necessary. It is already factored in on the manufacturer's nameplate.

A good rule of thumb is this:

Size the conductor based on the minimum circuit ampacity.
Size the OCPD based on the minimum or maximum namplate size.
 
Is there a cut-off on the size of HACR CB's? (after which you have ask specifically for an HACR CB)
 
Always size your OCPD to the max fuse or breaker size to eliminate call backs and nuisance tripping. Air conditioning compressors are notoriously hard starting beasts as they age. They're trying to give you a benefit, so take advantage of it. Don't fuss with your calculations... somebody already did that for the particular piece of equipment when the made the data plate.
 
chateau mike said:
with that said, the maw fuse size is 35 amps therefore a 60 amp disconnect with #8 is the best bet in your opinion.
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#8 is certainly okay, but #10 would be more than adequate. As mentioned above, even #12 would be legal.

It's a waste of money to use #8, just like using a higher octane of gasoline than required to prevent knock.
 
chateau mike said:
with that said, the maw fuse size is 35 amps therefore a 60 amp disconnect with #8 is the best bet in your opinion.
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Yes about the 60 amp disconnect and the 35 amp fuses.

The wire size is strictly a design issue, use 12 AWG or use 6 AWG either will be safe and NEC compliant.

The one real issue could be voltage drop at start up.

I would not use 12 AWG for anything but a short run.
 
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