conduit and wire size question

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tacomafc

tacomafc

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Long Island New York
I need to size wires and conduits for a project. I'm adding a VFD for a 100 hp 3phase 480v motor. I'm using the same motor start but I'm going to add the vfd next to it and just splice the load side of the VFD in the starter. My question is wire sizing.
250awg x .3970x6=2.382
6awg x .0507x1=.0507
Total = 2.4327 cu in

Emt = 3"

250 awg = 290 amps x 70%=208A

Would this be correct?

So you won't be confused I'm not mixing line and load. The fuses are in the starter. I'm just adding a VFD. Six wires plus a ground.
 
Without checking your math, Table C1 shows (6) 250s in a 2-1/2" so 3" is probably more than adequate even if you add an equipment ground (and the conduit may even be less than 24")
Without the nameplate info on the drive it's not possible to tell but your 250 looks like overkill. 430.122 only requires 125& of the drive input and even if the motor was across the line start a 2/0 would be sufficient.
 
based on 430.122, your conductor ampacity would need to be 167.5.
If your interconnecting conduit contains 6 current carrying conductors and is over 24" long, you would need a minimum of a 3/0.
A 2-1/2" emt would be very adequate.

Does you drive instruction manual have any input on needed short-circuit/ground fault protection ?
 
Ok so i think i could do this:

134a x 25% 33.5

33.5 + 134a = 167 amps

3/0 = 225 a (90 degree) x 70% = 157.5 amps no good

i will need a 4/0 thhn wire

or i could use 2 conduits so i would only derate by only 80% not 70%
 
tacomafc said:
Ok so i think i could do this:

134a x 25% 33.5

33.5 + 134a = 167 amps

3/0 = 225 a (90 degree) x 70% = 157.5 amps no good

i will need a 4/0 thhn wire

or i could use 2 conduits so i would only derate by only 80% not 70%
Click to expand...
I don't follow your 70%. I would think you would only have (6) curreent carrying for a 80% adjustment (if the conduit is 24") 225 x .8 = 180
 
Oops 310.15 b.2.a does not apply to the grounding conductor

3/0 = 225 amps (90 degree) x 80% (4-6 conductors)= 180 amps :)

2.5 inch emt with six # 3/0 and a # 6 awg ground (250.122)
 
Cant find any info on short circuit or ground fault info. The fuses in the motor control center are 200 amps.

I wonder what would cost less 2 conduits with smaller wire like 2/0 in a smaller conduit or one larger conduit with more larger conductors???
 
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