Conductor Sizing Question
Conductor Sizing Question
(OP)
Hello,
I'm having trouble figuring out what exactly is the legal way to size conductors in one of my typical motor circuits. My typical circuit includes an MCP (motor circuit protector) and overload protection inside of the MCC and a fusible disconnect switch near the motor. Logically, I would size the conductors based on the FLA of the motor (1.25x), because the overload protection would not permit the circuit to reach a higher ampacity. But is this the legal way based on the NEC? If not, how should I go about this?
Here is an example: MCP = 30A, DISC = 30A, FUSE = 25A, MOTOR = 10HP (14 FLA), OL set to trip at just above 14FLA.
Thanks!
I'm having trouble figuring out what exactly is the legal way to size conductors in one of my typical motor circuits. My typical circuit includes an MCP (motor circuit protector) and overload protection inside of the MCC and a fusible disconnect switch near the motor. Logically, I would size the conductors based on the FLA of the motor (1.25x), because the overload protection would not permit the circuit to reach a higher ampacity. But is this the legal way based on the NEC? If not, how should I go about this?
Here is an example: MCP = 30A, DISC = 30A, FUSE = 25A, MOTOR = 10HP (14 FLA), OL set to trip at just above 14FLA.
Thanks!





RE: Conductor Sizing Question
This is not the same procedure used for general purpose feeders.
RE: Conductor Sizing Question
The motor branch circuit breaker or fuses only provide short circuit protection. However, there is overload protection in form of the motor overload protection. protection. Do as what dpc said.
RE: Conductor Sizing Question
Question: Have the Rules been changed? I remember the NEC requiring most motor conductors to be sized based on the horsepower with the FLA being determined from the tables rather than nameplate rating. This ensures that the wiring for a given horsepower motor is adequate for all motors of that horsepower, should a high efficiency motor be replaced with a lower efficiency motor.
Thanks
respectfully
RE: Conductor Sizing Question
Basically, yes - you are supposed to use the FLA data in the NEC Tables. It hasn't always been that way, but it is now. It's Article 430.6.
For motors less than 1200 rpm, and where the nameplate FLA is higher than the table, you have to use the nameplate value.
RE: Conductor Sizing Question
Respectfully
RE: Conductor Sizing Question
Anyway, NEC permits you to size motor conductors (and a few other specific types of feeders and branch-circuit conductors) differently from most other branch circuits in 240.4(G).
RE: Conductor Sizing Question
RE: Conductor Sizing Question
In particular, 240.4.
The NEC Handbook (NFPA) is also a good investment - although you should wait for the 2008 version at this point.
RE: Conductor Sizing Question
Article NEC 430.
RE: Conductor Sizing Question
RE: Conductor Sizing Question
It better!!:)
RE: Conductor Sizing Question
Since you have a fused disconnect switch next to the motor, The feeder from MCC to Fused disconnect switch falls under branch circuit (not a motor circuit/feeder) and therefore, the correct wire size for your example is #10AWG. Now, if you use a non-fused disconnect switch, then the whole circuit will be considered a motor feeder/circuit and NEC allows using #14 AWG wire on it (based on 125% of FLA). Hope this helps.
RE: Conductor Sizing Question
I disagree. I don't see how adding a fuse to a circuit would result in a requirement for a larger conductor size.
I follow your logic & reasoning, I just don't agree with that interpretation. There may well be plenty of AHJ's who do agree with you, though.
RE: Conductor Sizing Question
While not the easiest to use and you can't cut and paste or print, you can view the NEC online here.
http:
The NFPA has made all of there codes and standards available in this manner.
Don
RE: Conductor Sizing Question
John
RE: Conductor Sizing Question
RE: Conductor Sizing Question
The 3 hour concept is for general purpose feeder loads and has nothing to do with sizing motor feeders.