# VFD Wire Size Question



## telsa (May 22, 2015)

If this is a Boeing question -- take it to the in-house EEs.

1) The folks specifying the conductors MAY have a C/B up the line that is holding the current down to NEC specs.

2) INTERMITTENT loading may change the calculus - - a LOT.

It's a RARE machine tool that is not an intermittent load. -- More like an elevator hoist.

&&&&&

Welcome to ET.C... :thumbsup:


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## PlugsAndLights (Jan 19, 2016)

I don't understand where each of the items is (disc/fuses and 000wires),
but if they're all within a UL (or similar) approved assembly, then it's
legal. 

Manufacturers do not have to meet NEC (or OESC or CEC or whatever).
They have their own set of rules. Testing is done in authorized labs to
verify compliance. 

P&L


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## Cow (Jan 16, 2008)

willpoll said:


> Hi all:
> 
> I just received two Yaskawa P1000 configured VFDs, with input rating of 460V, 207A, 3ph. The drives came with 400A disconnects and 500A fuses installed.
> 
> ...


Since it's Yaskawa, the cabinet is most likely UL listed. That means everything preinstalled in the cabinet is outside the scope of the NEC.


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## willpoll (Jun 4, 2016)

Thanks.

Not Boeing...a municipality.

I should have been more clear...The internal wiring from the breaker to the fuses and from the fuses to the drive unit itself is 3/0.

The issue I'm fighting is co-workers insisting that this means we can feed it with 3/0 wire from a 400A breaker in our main distribution. This is replacing a 20 year old, same HP drive.

-WP


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## dawgs (Dec 1, 2007)

You need to size your wire to the full capability of the drive. Don't worry about what pre wired inside the cabinet, that's part of a UL listing. Look at the drive manual. It should tell you your wire size.


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## bill39 (Sep 4, 2009)

dawgs said:


> You need to size your wire to the full capability of the drive. Don't worry about what pre wired inside the cabinet, that's part of a UL listing. Look at the drive manual. It should tell you your wire size.


Do you have a NEC reference for your response? I run into the same issues sometimes and am always unclear what the correct way to do this is.

Thanks


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## PlugsAndLights (Jan 19, 2016)

willpoll said:


> Thanks.
> 
> Not Boeing...a municipality.
> 
> ...


Bottom line is you have to follow NEC. What the manufacturer did 
within the assy has no bearing. 
This principle is common to many types of loads. I just installed some fluorescent fixtures. The wire within the fixture is 18AWG. I need to 
feed the lights with #14 if breaker is 15A or #12 if breaker is 20A. 
I'd check the manufacturers recommendations AND the code, then, if 
the those suggest different sized wire, select the larger of the two. 
P&L


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## dawgs (Dec 1, 2007)

bill39 said:


> Do you have a NEC reference for your response? I run into the same issues sometimes and am always unclear what the correct way to do this is. Thanks


No. I'm away for the weekend and don't have my code book. I believe it's probably in the drive manual.


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## dawgs (Dec 1, 2007)

430.122 maybe? You need to size for the drive.


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## frenchelectrican (Mar 15, 2007)

willpoll said:


> Hi all:
> 
> I just received two Yaskawa P1000 configured VFDs, with input rating of 460V, 207A, 3ph. The drives came with 400A disconnects and 500A fuses installed.
> 
> ...


To clear up the question....

How the heck a 500 amp fuse fit in a 400 amp disconnection switch ????

I would sized the conductors at nameplate plus 25% over


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## Cow (Jan 16, 2008)

dawgs said:


> 430.122 maybe? You need to size for the drive.


That's correct, the NEC requires you to size the wire for 125% of the drive input current rating.

207 amps X 125%=259 amp wire required. That is well beyond a 3/0 copper.


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## willpoll (Jun 4, 2016)

Exactly what I've been saying, 300 kcmil by my calculations. Now to try and figure out how the original installation passed inspection.

Please check my logic:-

For the VFD feeders:-
FLA 207A x 125% = 259A = 300kcmil (250kcmil is only good to 255)
For parallel conductors, this would require 2 x 3/0 or 3 x 1/0.

For the motor conductors:-
150HP - NEC 430.250 = 180A x 125% = 225A which would be 4/0 according 310.15(b)(16)
If I simply halve the amps, 2 sets of 1/0 conductors would suffice.

Since the drive contains a 400A breaker and 500A fuses, is my existing 400A motor branch circuit feeder breaker sized properly or should it be reduced?

And to answer FrenchElectrician, the drive contains a separate 400A breaker and separate 500A fuse blocks, not a fused disconnect.

Thanks again everyone.


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