We are machine builder. Our machines are 460VAC machines. Our electrical enclosures contain multiple motor controllers (35-45). Several motors (3-5) are 30 -50hp motors, and rest of it is from 10hp to 0.5hp. Our enclosure is having main molded case circuit breaker, usially 500 - 700amp. Each power motor circuit is having its own branch circuit protective device sized per motor size.
Question we have is about size of conductors from main circuit breaker to the line side of each branch circuit protective device. According to NEC 430.53(D) these conductors have to be sized not less than 10% of main circuit breaker. If main circuit breaker is 600amp, then conductor to the branch circuit device of 1Hp (4.2A) motor has to be at least 6AWG.
However NFPA 79 2007 paragraph 7.2.8(1) is "reading" that overcurrent protection of conductor at the supply is not required under sertain conditions, and conductors from main circuit breaker to line side of branch circuit protective device of for example 1hp motor (4.2A) can be 14AWG.
We would like to hear opinions of what is right and what other people / companies doing in similar situation.
Thank you
PaulBr.
I look forward to reading the response on this, because of some problems understanding the wording of 430.53.
The Article 100 definition of branch circuit is short and clear:
Branch Circuit. The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
The additional definitions of specific types of branch circuits qualify that but do not change it.
However in 430.52, we suddenly have a new concept of branch circuit wiring that starts at a "branch-circuit short-circuit and ground-
fault protective device" and continues, still as a branch circuit, through one or more "branch protective devices" or motor overcurrent protection devices.
My biggest problem lies in the wording in (D)(3) [2011]:
Conductors from the branch-circuit short-circuit and ground-fault protective device to a listed manual motor controller additionally marked ?Suitable for Tap Conductor Protection in Group Installations,? or to a branch-circuit protective device,....
Is "branch-circuit protective device" being used here as an abbreviation for the full wording, "branch-circuit short-circuit and ground-
fault protective device", even though the full wording is used everywhere else but here? Or is it introducing an additional device type. Maybe short-circuit protection only or ground-fault protection only or overcurrent protection only?
The reason that is an issue is whether the exception in (3) is intended to apply only downstream of a "branch-circuit short-circuit and ground-
fault protective device" (BSGP for short) or is specifically worded to apply to conductors tapping directly off feeder or main service conductors and leading to a BSGP?
The reason this is important is in what the reference point is for "an ampacity not less than one-tenth the rating or setting of the branch-circuit short-circuit and ground-fault protective device. "
If this specifically refers to a BSGP located ahead of the tap wires in question, it makes some sense, and it does not allow the tap to be made to either a feeder or a main conductor.
If this refers to a BSGP located downstream of the tap conductors, it makes absolutely no sense to allow that protection to be 10 times the conductor ampacity.
I think that you are not being given good advice about what is and is not allowed.
The condition which allows motor circuit conductors to have an ampacity lower than that of the branch circuit BSGP, as long as it is higher than the level of the motor overload protector, is conditioned on the fact that the overload protection will take care of a situation where the current is only slightly above the ampacity of the wires, and that such a condition involving the motor could not be sustained indefinitely. While the short circuit protection is still effectively provided by the higher rated BGSP, as long as the conductors are heavy enough to carry the fault current until the fuse or inverse time device trips.
Putting smaller wires directly on a feeder or main conductor would be limited by the tap rules contained elsewhere in the code (primarily 240.21 and later) and not specific to motor circuits. And in that case, I do not think that you are allowed to have just a 10% ampacity rating for taps longer than 10 feet. Also that section specifically refers to the ampacity of feeder breakers or feeder wires, not branch circuit wires.
My interpretation of (D) is that it applies to tap connections made
on the load side of a BSGP, with each tap going to individual motors or groups of motors which are all overcurrent protected at the far end of the tap at a level much lower than the BSGP provides for. It does not allow you to avoid having a BGSP above the tap wires.