AC Inspection ???

[Mtripp]

Nameplate on air handler Min circuit-ampacity 31, Max-OCPD 35.
#10 NM cable run to AH in attic from panel box located in garage, landed in a 30 amp breaker.

My assessment is cable is under

 

[Dennis Alwon]

The cable may or may not be undersized depending on if it is installed at the 75C column of the table. The breaker is undersized
see -240.4(G) reference by the double asterisk in the ampacity table

 

[kwired]

The cable may or may not be undersized depending on if it is installed at the 75C column of the table. The breaker is undersized
see -240.4(G) reference by the double asterisk in the ampacity table

If it is NM cable it must be sized per 60C column.

Unless it has a heat strip in it, that is a big air handler.

 

[infinity]

If it is NM cable it must be sized per 60C column.

I agree change it to MC cable and you're good.

 

[drcampbell]

I hate to ask about the engineer(s) who designed this air handler, approved it, and manufactured it.

Just what sort of ivory tower are they working in that they didn't contemplate how it would be installed & used? Couldn't they reduce its MCA by 3% -- maybe by making the blower 3% narrower, or improving the internal aerodynamic drag by 3% -- to facilitate installation in the real world, where 30-amp breakers and 10AWG wire are riding around in the back of every service truck in the country?

Full disclosure: I am an engineer.
Even-fuller disclosure: That's why short-sighted engineering work like this triggers my rants.

 

[Dennis Alwon]

Yes, one amp can change the entire setup. Those 10 kw furnaces are the same. They calculate, with fan, to 56 amps instead of 55 amps which changes things in a dwelling

 

[Jraef]

I hate to ask about the engineer(s) who designed this air handler, approved it, and manufactured it.

Just what sort of ivory tower are they working in that they didn't contemplate how it would be installed & used? Couldn't they reduce its MCA by 3% -- maybe by making the blower 3% narrower, or improving the internal aerodynamic drag by 3% -- to facilitate installation in the real world, where 30-amp breakers and 10AWG wire are riding around in the back of every service truck in the country?

Full disclosure: I am an engineer.
Even-fuller disclosure: That's why short-sighted engineering work like this triggers my rants.

1A could have been a rounding error in the MCA calculations IMHO... so I agree, seems a bit short sighted.

My guess; the unit was designed and manufactured overseas based upon other voltages/frequencies and when adjusted for installation here the numbers ended up that way, i.e. the fan speed would be lower at 50Hz, so the current draw would be lower too compared to what it will be at 60Hz. But the foreign supplier doesn't really care about the consequences to our electricians.

 

[Mtripp]

If it is NM cable it must be sized per 60C column.

Unless it has a heat strip in it, that is a big air handler.

Yes it does have a heat strip.
My take this installation should have if NM used a # 8 cable and a 35 amp breaker?

 

[Dennis Alwon]

Yes it does have a heat strip.
My take this installation should have if NM used a # 8 cable and a 35 amp breaker?

That would be correct...

 

[Eddie702]

It depends what's in the ahu. If its a motor load only #10 is ok but the breaker must be a 35a. The wire sizing table shows #10 good for 40 amps if it is a motor load. If you have electric heat or other loads that are not motor loads the you need #8s with a 35. NM cable would have to be #8

 

[Mtripp]

Thanks ALL for the assistance and confirmation!

 

[kwired]

It depends what's in the ahu. If its a motor load only #10 is ok but the breaker must be a 35a. The wire sizing table shows #10 good for 40 amps if it is a motor load. If you have electric heat or other loads that are not motor loads the you need #8s with a 35. NM cable would have to be #8

What makes this not work for OP is the fact he is supplying it with NM cable, which must have ampacity determined with 60C ampacity values. Otherwise most other wiring methods used today can use 75C ampacity values. Had he used AC/MC cable or THHN/THWN then 10 AWG would have enough ampacity.

Since it is marked with a minimum circuit ampacity it doesn't matter if it is motor load only, you must supply it with the marked minimum circuit ampacity. Having such MCA marked however means that is your final ampacity and you do not need to add 25% for continuous loads or anything like that as it is already factored into the MCA.