VFD Input Conductor Size and 430.122

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textmi

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EC
I have been reading previous threads and can't find a definite answer to this question.

I have a 75 hp VFD with an input rating of 96A. Per NEC 430.122, I should size the line side conductor to 125%*96A=120A. This can be done with #1 conductors and #6 ground (will handle 125A).

However, the breaker from the panel serving this VFD is a 150A and the VFD also comes with a 250A thermal magnetic breaker. Which of the following is correct?

1) I can use line-side conductors per NEC 430.122 (handle up to 125A) although the line-side breaker ratings are all higher amperage than what my conductor can handle.

2) I will need to upsize my conductors to handle the OCPD (150A) or replace the OCPD at the feeding panel to be 125A so that my conductors are protected.
 
The conductors are fine. This is like any other motor circuit. The OCPD is just there for short circuit protection. Overload protection is provided by the VFD itself.

The EGC had to be sized to the OCPD rating though.
 
petersonra said:
The conductors are fine. This is like any other motor circuit. The OCPD is just there for short circuit protection. Overload protection is provided by the VFD itself.

The EGC had to be sized to the OCPD rating though.
Click to expand...
In this case, I would need to size the ground for 150 A so #1 phase conductors and #1 ground would be sufficient?
 
petersonra said:
The conductors are fine. This is like any other motor circuit. The OCPD is just there for short circuit protection. Overload protection is provided by the VFD itself.

The EGC had to be sized to the OCPD rating though.
Click to expand...
On the LINE side of the VFD would the conductors and OCP not have to comply with the basic Art240 rules ?
(ie: conductor size and OCP match)
 
augie47 said:
On the LINE side of the VFD would the conductors and OCP not have to comply with the basic Art240 rules ?
(ie: conductor size and OCP match)
Click to expand...
That comes down to 240.4(G). It lists Article 430 Parts 2-7 as permitted exceptions to the basic rule of 240.4.

VFDs are under Article 430 Part 10, but 430.120 says Parts 1-9 apply unless modified by Part 10. And 430.130(A)(1) on SC/GF protection for "single motor circuits containing power conversion equipment" refers us back to 430.52(C), which is in Part 4.

So I guess the argument is that since the maximum allowable valued is determined from the text of Part 4, that means it is covered by 240.4(G).

Cheers, Wayne
 
textmi said:
In this case, I would need to size the ground for 150 A so #1 phase conductors and #1 ground would be sufficient?
Click to expand...
Never mind, #6 EGC would be sufficient per table 250.122 for up to 200A so in this case it would be sufficient for the 150A OCPD ahead of the VFD.
 
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