Cable Bus termination to switchboard busing, NEC 110.14 vs 370.

[JacobsDave]

We need some help. Our city inspector is concerned about the cable bus terminations as they relate to NEC 2017 section 110.14(C). It states that the terminations on the equipment must be based on table 310.15(B)(16) unless specifically noted otherwise.

NEC section 370 relates to cable bus and allows table 310.15(B)(17) to be used for cable sizing. This is the free air rated table which allows us to get more amps out of a given cable size due to the cooling effect that happens in the air. The cable and lugs that are provided by MPHusky are rated for 75 degree C, and the terminations (bus bars) of the 4000A equipment are rated at 90 degree C.

This appears to me to be a contradiction in the code and is a point of contention between engineers and AHJ inspectors.

The inspector is saying that in order to meet the code, we can’t terminate the cable bus conductors on the equipment unless it is rated per table 310.15(B)(17). Even though the temperature ratings are being adhered to. (But 110.14 requires equipment to be tested with 310.15(B)(16) as the default.

Mike discusses conductor temperature ratings in this video and at the 13:50 mark he talks about the difference between terminating at equipment and a "connector". I believe my situation described above is the later of the two and there should be no problem terminating cables and lugs that are sized for 75 degrees to a bus that is rated for 90 degrees. Thoughts?

 

[don_resqcapt19]

Most cable bus terminations that I have done or seen are terminated to bus bar and not to equipment. The 2020 added Informational Notes in 370.20 to refer to 110.14(C) and to 110.40. The panel statement for this change said:

Users of Article 370.80 may not be aware that the requirements in Sections 110.14(C) and 110.40 apply when the conductors terminate on equipment with a termination temperature limitation. Correct application of the termination temperature rating is a critical safety consideration for electrical systems.

Typically bus bar does not nave such a temperature limitation.

However if you are directly connecting to equipment such as a breaker that does have termination temperature limits, you would have to compy with those limits.