parallel feed to DC motor

Question

parallel feed to DC motor

resqcapt19 (Electrical)

(OP)

18 Dec 01 12:49

I'm going to be installing a large DC motor that will require that the armature leads be installed in parallel from the drive to the motor. Can I install the 3 positive leads in one conduit and the 3 negative leads in the second conduit? The conduits will be rigid steel. I know that you can't do this on AC systems because of the inductive reactance. I don't think that is a problem on a DC system. If I can install the positive in one and the negative in the second conduit, I can save 2 conductors. If I must place the positive and negatives in the same conduit, the circuit will require 2 positive and 2 negative in each conduit because of the derating required for four current carrying conductors in a raceway.
Don(resqcapt19)

gordonl (Electrical) · Answer 1 · 2001-12-18 15:11:40

I say yes, you want to make sure all parallel conductors are the same length and preferably free of splices.

djs (Electrical) · Answer 2 · 2001-12-19 12:34:30

The derating for current carrying conductors applies wether the same polarity cables are run in the same conduit or not. Therefore the desicion to run all the positive leads in one conduit and all the negative leads in a second will not effect the conductor sizing or quantity.

resqcapt19 (Electrical) · Answer 3 · 2001-12-19 13:22:50

When conduits are run in parallel, they must be matched. I can't put two sets in one conduit and one set in the second conduit. Also when I put 2 sets in one conduit the derating requires larger conductors. If I can place all of the + in the first conduit and the - in the second conduit, only 3 conductors will be required in each raceway and derating will not be required.
Don(resqcapt19)

busbar (Electrical) · Answer 4 · 2001-12-19 17:02:25

In the ’99 NEC, It seems like article 300-20 [Induced Currents] is specific to alternating-current circuits, but articles 300-3 and 310-4 *are not.* Specifically, as I understand it, 300-3(b)(1) [Paralleled Installations] does permit the arrangement you describe in any instance (whether DC or AC).

busbar (Electrical) · Answer 5 · 2001-12-19 17:09:52

Correction: 300-3(b)(1) [Paralleled Installations] does NOT permit the arrangement you describe.

resqcapt19 (Electrical) · Answer 6 · 2001-12-19 19:09:09

Busbar,
Thanks. I miised that point in 300-3(b)(1). However, I still have the question if the circuit that I have proposed will cause any problems. The code is not an issue in this installation.
Don

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parallel feed to DC motor