Motor Overcurrent Protection

[solaeros]

I have a 60 hp motor located 360 feet from the power source (480v 3ph.) I recomended #2 wire for the run But they ordered the starter panel and it came with a 250 amp thermal magnetic breaker. I asked them to try and swap it out for a 125 A. so the wire would be rated for the breaker but, other electricans are telling them that it's fine to run the smaller gage on the larger breaker. I haven't seen anything in 430 that says you have to size your wire for your ocpd, just to size it from you fla. But I have allways sized wire from the ocpd. Any final word on this?

 

[nakulak]

430.52 .

 

[Lcdrwalker]

I'm sure it won't be a final word but... IMHO your coworkers are correct. 430.52 lists ground fault and short circuit protection ratings of to 800% of the fla depending on the type of device used. I normally use inverse time circuit breakers or fuses. These can be rated at up to 250% FLA. Your circuit conductors generally only have to be rated at 125% FLA per 430.22 so there can be a disparity. If you look at the notes to 310-16 there is an exception for motors.

 

[nakulak]

to add to above,
430.22 -> 60 hp, 480 3p 430 table 430.250 flc=77amps


read 430.51

read 430.52, table 430.52 tells you the max ocpd

if your breaker is inverse time, then 250% is the max.
250% of 77 = 192.5

(if breaker is inverse time, then 430.52 C 1 exception c allows you to increase to 400%-> 400% of 77 = 308 amps

(if breaker is instantaneous then see 430.52 C 3 - you can increase ocpd to much greater values)

 

[Jraef]

By way of trying to explain the reasoning behind the NEC (a risky endeavor, I know), the issue is that in a motor circuit as defined in Article 430, there is presumed to be an Over Load Relay or other means of inverse time motor over current protection. THAT is then also the over current protection for the conductors then, not the Circuit Breaker or Fuses.

So technically, the "OCPD" is the entire motor starter as a system. The breaker or fuses become just the SCPD, Short Circuit Protective Device, within that system.

 

[iwire]

To add to Dr Evils post, the 250 amp breaker in this case is only protection for line to line and line to ground short circuits or faults.

The motor starters overload protection prevents any long time over current condition from damaging the conductors just like it protects the motor from damaging over current.