tap rule

[steve66]

The language in many of the tap rules (240.21(B)(3) to be exact) includes the requirement:

"The secondary conductors terminate in a single circuit breaker or set of fuses that limit the load current to not more than the conductor ampacity that is permitted by 310.15."

I assume this means I can't use 240.4(B) which allows the next higher standard overcurrent device. But, can I use the 90C column of table 310.15? This would allow #1 to be terminated in a 150A breaker. What if the breaker the wire terminates in is only rated 75C?

STeve

 

[tonyi]

Re: tap rule

If the breaker is 75C, then that's the upper limit on how hot you can run the wire. You can't exceed its temp rating regardless of what the tap wire insulations may be capable of.

 

[earlydean]

Re: tap rule

Think of every conductor as an assembly: the CB lug, the conductor, and the termination lug. 110.14(C) tells us that the ampacity of our conductor has an upper limit based upon the temperature rating of the lugs used. If we use 90 degree wire, but 60 degree lugs, then the ampacity of the conductor is no higher than the 60 degree column of Table 310.16. The standard is that lugs for #1 and smaller conductors (or for circuits rated 100 amps or less) are 60 degree by default, Those over 100 amps or conductors larger than #1, are 75 degree by default. No lug is rated 90 degrees by default.