Why are NEC Cable Ratings Are Rarely followed

[7carisfast]

Why is it when we, as an engineering firm, follow the NEC for industrial motor applications, Electrical contractors always fight conductor sizing. For example, when sizing a motor, and taking into account temperature deratings and cable fill deratings, a 100A conductor is sometimes only rated for 40 Amps. Why is this always questioned by the contractors but never enforced by local inspectors? I mainly see them complain (and rightly so) when we need to connect the feeder wires (say parallel 500MCM) to a motor which only has single 500MCM terminations. In a lot of cases, the motor terminal boxes are entirely undersized. Just curious.

 

[dpc]

Maybe if you give us a specific example, we could offer a better response.

Derating from 100 A to 40 A is pretty signficant. Most designs would not be done in such a way as to require this level of de-rating, so that could be why you get questions.

It's common for motor leads to be much smaller than the field wiring coming in.

BTW, my experience is that electricians will complain about pretty much EVERYTHING.

 

[jraef]

Amen to that!

Realize that for most contractors, their job is to finish the project within the price they estimated. If they made an error in judging cable sizing when they bid it, they will fight tooth and nail to not have to eat the mistake. Inspectors only care if the code has been violated, they are not concerned about value engineering or best practices. You as the engineer on the other hand, have a much more vested interest in the job being done right; it's your name all over it.

That said, I agree with dpc that a 60% de-rate seems a bit excessive on the surface. I'd love to see the details on one like that. Maybe you are perhaps over-engineering a bit? Hopefully you are aware that some of the NEC rules for conductor sizing in article 250 are superseded by article 430 when it comes to motor circuits.

 

[dpc]

Also, when you have to de-rate, it is generally OK to start from the 90 deg C ampacity instead of the 75 deg C ampacity.