MV Feeder Size & Upsizing EGC

[Flapjack]

Working on a project with a 750kVA transformer (13.8kV primary, 4160V secondary) that feeds a lineup with a 200 amp fused main disconnect. The gear feeds a couple of motors with soft starts.

Two separate questions:

1. The transformer secondary FLA is 104 amps. I'm taking the minimum feeder ampacity as being 1.25 x 104 = 130 amps [NEC 215.2(B)(2)]. Typically, I would size the cable to the fuse, but based on NEC 240.101 it looks like this isn't required for MV. And 200 amp fuse is under 250% for Table 450.3(A), so no issues there. The minimum size MV-90 cable per Table 310.60(C)(73) would be 2 AWG. Is this the correct approach for determining the minimum circuit ampacity?

2. For the motor circuit, the motor is rated at 55 amps. Minimum circuit ampacity is 1.15 x 55 = 63 amps [NEC 430.224]. That would be a 6 AWG cable in Table 310.60(C)(73). If a cable manufacturer has a minimum size of 2 AWG, I assume that means the EGC needs to be upsized proportionately per 250.122(B)? The EGC would also be a 2 AWG, correct?

 

[HarrySlother]

1. I'm not sure that 240.101 would apply to a transformer secondary, but someone can correct me.. One of the tap rules might be more appropriate.

2. The power wiring looks right for the motors, but the ground would originally be a #8 I believe from 250.122(D), so the proportional ground could be a #3 instead. Or, if you have shielded cables, I think you may be able to use those shields as the ground (if they have an equivalent combined area of a #3, and if they can handle whatever fault current there is available).

Strange a 5kV cable doesn't come smaller than #2 - what manufacturer is that?

 

[Dennis Alwon]

I agree the equipment grounding conductor is upsized based on what would work. 250.122 based on the overcurrent protective device which Harry stated above would be #8

Take the Cir Mil of the #8 and divide into #2

Then increase the equipment grounding conductor by the same ratio.

 

[Flapjack]

1. I'm not sure that 240.101 would apply to a transformer secondary, but someone can correct me.. One of the tap rules might be more appropriate.

2. The power wiring looks right for the motors, but the ground would originally be a #8 I believe from 250.122(D), so the proportional ground could be a #3 instead. Or, if you have shielded cables, I think you may be able to use those shields as the ground (if they have an equivalent combined area of a #3, and if they can handle whatever fault current there is available).

Strange a 5kV cable doesn't come smaller than #2 - what manufacturer is that?

I was basing the ground size for the motor on a 4R fuse (130 amps).

For the 5kV cable, I was looking at Southwire Spec 46102. Minimum size they show on the datasheet is 2 AWG.