motor control circuit question

[trinity]

I have a 250 hp 3phase 460 volt motor. It will have time delay fuses for the short circuit and ground fault protection. What size conductor will have to be used for the control circuit if the control circuit is protected by the short circuit and ground fault protection. The wire will leave the enclosure. I came up with 250kcmil. How do I get that on the pushbutton screws?

 

[Bob NH]

I have a 250 hp 3phase 460 volt motor. It will have time delay fuses for the short circuit and ground fault protection. What size conductor will have to be used for the control circuit if the control circuit is protected by the short circuit and ground fault protection. The wire will leave the enclosure. I came up with 250kcmil. How do I get that on the pushbutton screws?

Mr. Instructor: I assume that is a smart a** question for your students. Something along the lines of, "If I am going to drive from Iceland to Greenland in my Volkswagen, how much gas do I need if I get 10 kilometers per liter?"

 

[rcwilson]

Use a control power transformer with secondary protection. #14 awg usually will have the required ampacity and short circuit withstand for the 120VAC control circuit.

Or use a current limitng fuse to protect the 480V control power wiring.

IMHO, Control at 480V is not practical or safe at this HP level.

 

[templdl]

Yes, definitely go with a 120vac control circuit.
Commonly one would take 480v L-L power from the line side terminals of the contactor to (2) KTKR fuses sized for the CPT. Then (1) FNM fuse for the secondary grounding the other side to feed your control circuit.
It is also common to stay with a 480v coil in your contactor controling it through an interposing relay that has a 120v coil. This would allow you to keep the CPT at a reasonable size by not having to provide the VA that is necessary to pull in what I see as a size #6 contactor where the coil may require 1kva to pull in and 100va to seal. But you need to review the specs of the contactor that you choose as some designs maybe less as they actually may use a DC coil feed through a full-wave rectifier and powered with 120v.

 

[petersonra]

Yes, definitely go with a 120vac control circuit.
Commonly one would take 480v L-L power from the line side terminals of the contactor to (2) KTKR fuses sized for the CPT. Then (1) FNM fuse for the secondary grounding the other side to feed your control circuit.
It is also common to stay with a 480v coil in your contactor controling it through an interposing relay that has a 120v coil. This would allow you to keep the CPT at a reasonable size by not having to provide the VA that is necessary to pull in what I see as a size #6 contactor where the coil may require 1kva to pull in and 100va to seal. But you need to review the specs of the contactor that you choose as some designs maybe less as they actually may use a DC coil feed through a full-wave rectifier and powered with 120v.

I don't think I have ever seen it done any other way in this size contactor. This is just the most practical way to handle it.

 

[Jraef]

Mr. Instructor: I assume that is a smart a** question for your students. Something along the lines of, "If I am going to drive from Iceland to Greenland in my Volkswagen, how much gas do I need if I get 10 kilometers per liter?"

I agree. I find it difficult to take that as a serious question.
Kind of like "I'm having dificulty connecting my 4" condiut into my 4" junction box."