What size Service Entrance Conductors do you need for a 60a fused Service Disconnect when it is being used for an Air Conditioner. FLA of AC is 32a Max fuse 45a? (Tricky)

I'll bite.
8ga copper?

Greg- Look at 230.42(B) and 230.79(D) also consider the terminals. You might change your answer.

I see this as a 230.79(A) "single branch circuit" with a "limited load"
FLA 32a x 1.25 is a 40a load. 316.16 says 8cu is OK.
45a is well within the circuit O/C device limits for a motor load.
I assume overload protection is in the HVAC equipment?

isnt this that trick question?
where #10 is rated for 35 amps??

Unless I losing it- The Service Conductors feeding the fused Service Disconnect need to be sized to meet the rating of the disconnect. Based on 230.42(B) for specific installations. That being the case, a 60a switch would be required to have 60a - 75° terminals could use #6THWN wire on the line side of the switch. On the load side the conductors could be smaller and fused accordingly. 75° terminals THWN #10 would work. (35a)

This is a 45a disconnect if you have 45a fuses in it. I had that out with NFPA in reference to 400a equipment with 250a fuses in it. (there was even an 1999 ROP written)
They said service conductors get sized to the fuse installed, not the maximum fuse that fits.

I still find myself back at 230.79(A).

10ga won't work because you need to use 125% of FLA since HVAC has to be called a continuous load. On a hot day (the top of the design range) a properly sized system should not cycle.

From a practical sense you are just protecting the conductors from the load and that is 32a (plus a 25% cushion) 40a.
Short circuit protection can be at least 175% of FLA in art440.

Greg - I probably could buy into your Service Entrance conductor thing- at least for the time bring, but the #10 is adaquate for the 32a AC. Air Conditioners are not considered continous loads. The wire is rated for continous loading and the 32a. FLA on the AC name plate has already calculated the 125% for largest motor and 100% of the other motor etc.

Edited to say 125% not 1.25%.

[This message has been edited by George Little (edited 08-08-2005).]

OK I agree if the "32a" is the "minimum circuit ampacity" on the nameplate but the terminals in the AC would still have to be labelled 75c to use 10ga.
I was confused when you said "FLA" and not minimum circuit ampacity from the nameplate.


(off topic)
There is no way I would say HVAC is not a continuous load. The manual J calc assumes continuous operation for the max design ambient.
Oversized AC units won't remove enough humidity and you get a clammy house.

The more I read this the more I am still saying #8cu
The disconnect is 45a and needs a 45a conductor per 230.42(B)

I'm sure it is a continous load and the calculations for ACs and motors are figured at 125% for largest motor and 100% for other loads on the same circuit. Some electricians (not you I sure) get continous load and continous duty combined and a motor that is rated for continous duty is built lighter than a motor that has a high duty cycle. A single motor branch circuit conductor is sized at 125% due to the fact that the overload(s) are sized that way. The Ground Fault and Short Circuit protection is really not much of a motor protector.

This was certainly a good question. You always seem to come up with good ones.
Throwing an AC unit in there does add to the FUD since that "min circuit ampacity" is an engineered number that does not translate well into the 230 rules.
(hence our 230.42(B) question)
Refrigeration motors are not exactly like your garden variety motor and that is why they have the label on the cabinet.
Left to our devices electricians and inspectors would usually oversize the circuit.

what about the exception #1 ?
is it possible to invoke?

h2o

Can't follow where you see any exception?? We are using the '02 NEC and looking at 230.42(B) and 230.79.