HVAC equipment wire sizing

[KHall]

According to NEC 440, HVAC equipment wire and OCPD can be sized according to the manufacturer's nameplate ratings. This often results in a wire size per the minimum circuit ampacity that does not correspond to the maximum protective device allowed by the nameplate. (For example, often a 5 ton split-system condenser can be served per the nameplate with a max of50A breaker and a min of #8 wire) I have used this calculation since first allowed by the NEC and have experienced no problems, complaints, or "call backs". Recently, engineers on several jobs have required that the exception allowed in 440 not be used and that the wire be sized according to ampacity tables in 310. Obviously, I am going to do that the engineer specs! But they could give me no justification for disallowing the use of the art. 440 exception other than 'he was not comfortable with that method.' (voltage drop was not a factor ... the equipment was within 20' of the service panels) Is there any evidence or expereince to indicate that using the code allowed sizing is the source of real problems? I know the code is not a design standard, rather a safety minimum. But whatever wire size you select, someone could always say the next size up would be safer!

 

[don_resqcapt19]

There are no issues using the minimum wire size and the maximum OCPD size.
The branch circuit OCPD only provides the short circuit and ground fault protection. The overload protection is provided by the HVAC unit.

Don

 

[infinity]

Those engineers need to learn that the manufacturer has done all the work in calculating the conductor size. Requiring conductors sized for the OCPD is a waste of somebody's money. I would think that even a neophyte engineer would have learned about ground fault and short circuit protection in engineering 101.

 

[peter d]

As a side note - sizing circuit conductors for HVAC equipment is one of the most misunderstood sections of the NEC. It's amazing how much misinformation is out there on this subject. This is an area that need to be better taught in the classroom.

 

[nakulak]

As long as someone else is footing the bill, who cares ? If they aren't, however, and its your dime . . .

 

[iwire]

Issues like voltage drop may have influenced the design. Or perhaps the EE knows that the HVAC load may be increased at some point.

But whatever the reason for the larger wire size if you bid it per plan you need to install it per plan.

 

[petersonra]

As long as someone else is footing the bill, who cares ? If they aren't, however, and its your dime . . .

I concur. As long as someone else is paying for something not required by the code, who cares why they want it.

 

[LarryFine]

I would introduce the engineer to the customer (or whomever will foot the bill for the difference.)