Wire sizing

[petersonra]

Similar to the EGC sizing question.

I have a VFD with 232A FLC input. The VFD manufacturer says I can go as high as a 700A CB to feed it.

For whatever reason I select a 700A CB.

Am I trapped into sizing the input conductors so they are protected by a 700A CB?

(2) 1/0 wires per phase [125% of 232 is just under 300A - the ampacity of (2) 1/0 wires] is a whole lot nicer then (2) 500 MCM wires per phase.

 

[texie]

Similar to the EGC sizing question.

I have a VFD with 232A FLC input. The VFD manufacturer says I can go as high as a 700A CB to feed it.

For whatever reason I select a 700A CB.

Am I trapped into sizing the input conductors so they are protected by a 700A CB?

(2) 1/0 wires per phase [125% of 232 is just under 300A - the ampacity of (2) 1/0 wires] is a whole lot nicer then (2) 500 MCM wires per phase.

You are good to go with the 232 amp X1.25 for conductor ampacity. See 430.122(A).

 

[petersonra]

You are good to go with the 232 amp X1.25 for conductor ampacity. See 430.122(A).

that only tells me what minimum size conductors to use. I don't disagree with that. but does not tell me anything about the rating of the CB.

If I put a 300A CB in the feed, I don't have an issue.

If I go with a 700A CB, what protects the wiring between the CB and the VFD if I have wires that are only good for 300A?

 

[texie]

that only tells me what minimum size conductors to use. I don't disagree with that. but does not tell me anything about the rating of the CB.

If I put a 300A CB in the feed, I don't have an issue.

If I go with a 700A CB, what protects the wiring between the CB and the VFD if I have wires that are only good for 300A?

Part X, Art 430.120 says to use Part I through Part IX unless modified by Part X. Size the OCP based on the tables. I think the manufacturer is saying you can apply the VFD up to a motor that would require a 700 amp OCPD.

 

[petersonra]

Part X, Art 430.120 says to use Part I through Part IX unless modified by Part X. Size the OCP based on the tables. I think the manufacturer is saying you can apply the VFD up to a motor that would require a 700 amp OCPD.

based on what tables?

So I can take a 700A CB feed it thru a couple of 1/0 wires 500 feet out to a VFD located somewhere in my plant? What protects the wiring for that 500 feet?

 

[texie]

based on what tables?

So I can take a 700A CB feed it thru a couple of 1/0 wires 500 feet out to a VFD located somewhere in my plant? What protects the wiring for that 500 feet?

You did not indicate the HP and voltage, so let's take a hypothetical. The drive could be used for a 460 volt, 150 HP motor. Table 430.250 tells us 180 amps (180 X 1.25=225 whitch falls within the range of your drive).
Table 430.52 says if you use an inverse time breaker, you can have a 450 amp breaker. If you use an instantaneous breaker you could have 800 amps, but this exceeds the drive spec. You would need 180 X 1.25 of conductor ampacity for this motor with a 450 amp inverse time breaker. I'm sure you know that the OCPD does not have to match the conductor ampacity for a motor load.

 

[don_resqcapt19]

... If you use an instantaneous breaker you could have 800 amps, but this exceeds the drive spec. ...

I don't think you can use an instantaneous breaker with a drive. The only code permitted application of that type of breaker is when used as a part of a listed combination starter.

 

[texie]

I don't think you can use an instantaneous breaker with a drive. The only code permitted application of that type of breaker is when used as a part of a listed combination starter.

Yes, I agree. It was a poor choice to use as an illustration. I was trying to illustrate that 2, 1/0 are fine no matter what OCP he uses.