Areas not covered by NEC

[dema]

The NEC requires that calculations be made using their motor amps no matter what the actual motor amps are for most motors. However, they do not cover motors for voltages over 2300V and there are also some other configurations not covered. In order to calculate the required breaker, disconnect and conductor sizing, does one default to the manufacturer's full load current?

The NEC allows for the conductors to be smaller in motor sizing than what would be required for other loads based on the breaker size. Do I have to size the conductors for the breaker sizes for motors that are not covered in the amperage charts in section 430? This would obviously require conduit and wire very much larger than what is required using the full load amps from the charts in lieu of sizing them by the size of the protection.

For motors the breakers can be 800% vs. the normal 125%, and the wires just have to be 125% of FLA.

 

[jim dungar]

For sizing conductors for motors not listed in Table 430.250

200V use 2.75A/HP
230V use 2.4
460V use 1.2
575V use .96
2300V use .24
4000V use .14

These values come from "Electrical Engineering Pocket Handbook" published by Electrical Apparatus Service Association (EASA).

 

[steve066]

I think you could also use the manufacturer's data, if you have it.

800% sizing for a breaker? I think that is only the case if a smaller breaker won't hold the starting load. Anyhow, you do size the wire from the motor load, and then size the breaker from the motor load. So your wire might be somewhat smaller than you would normally put on the same size breaker.

I don't have my book with me, but are there any rules for "Over 600 V motors" at the end of 430? You might want to check there. I am guessing for a motor this size, it will probably have some type of starting that reduces the current drawn. So I can't imagine the breaker would be much larger than the wire size.

 

[haskindm]

The 800% is only for instantaneous trip breakers. These are unusual and usually only found in motor control centers. A "standard" braker is an inverse time breaker. These are limited to 250% and 300-400% if you are allowed to use the exception.

 

[coulter]

The 800% is only for instantaneous trip breakers. These are unusual and usually only found in motor control centers. ...

Yes, instantaneous only, that's true. But I wouldn't call them unusual. They are about all I spec for motors. No, I don't do many single phase - can't think of any in the last ten years.

They have to be part of listed combination starter. They are available as standalone or in an MCC bucket.

carl