240.21(C) vs Art 430

[augie47]

240.21 vs 430 question…. assume one has a 100 HP 480v motor supplied by an in-house step-up transformer.
430.52 will allows us a 300 amp breaker. 430.22 will allow us a 2/0 conductor. 240.21(C) rules call for the transformer secondary conductors to be sized to the OCP device plus no “next size up. Does this mean our secondary conductor supplying our 300 amp breaker must be a 350 Cu (310 amp) conductor ?

 

[iwire]

Darn good question and I am curious what the answer will be.

 

[Carultch]

240.21 vs 430 question…. assume one has a 100 HP 480v motor supplied by an in-house step-up transformer.
430.52 will allows us a 300 amp breaker. 430.22 will allow us a 2/0 conductor. 240.21(C) rules call for the transformer secondary conductors to be sized to the OCP device plus no “next size up. Does this mean our secondary conductor supplying our 300 amp breaker must be a 350 Cu (310 amp) conductor ?

In the event that a secondary OCPD is required, you need at least as much wire ampacity as the secondary OCPD rating.

In the event that no secondary OCPD is required due to certain topoligies, and the primary OCPD protects the secondary conductors by proxy, then what you do is convert the primary OCPD by the voltage ratio to determine the corresponding secondary amps, and make sure that secondary conductors at least have that much ampacity.

I assume you have a 120/208 Volt primary, where the 300A breaker is located. When you convert this to a corresponding secondary current, you get 300*208/480 = 130A. This requires #1 Cu wire. I'm not familiar with motors, so other factors may come in to play.

 

[kwired]

I kind of think it is pointless to require larger conductors on the transformer secondary if the 100 HP motor is the only load served, but also can see how one may read into it being required for them to be larger. As said if no secondary protection is required (like a delta-delta transformer then there is definitely not a problem.

I am likely using a pump panel or combination starter in many cases for this install - just put in one with fuses and you don't have as many questions. 2/0 conductor and 175 amp fuses works even if you disregard that you are supplying a motor. Worst case up the conductor to 3/0 and use a 200 amp fuse.

Also a good chance for performance reasons you will be using reduced voltage starting method and can get by with lower short circuit/ground fault protection than the mentioned 300 amps anyway.

 

[wwhitney]

A related question is how would a transformer be sized for this motor load? If the transformer is sized based on 125% of the motor FLC, and the transformer is delta-delta with primary only protection at 125% of the transformer rating, then that would be akin to protecting the motor branch circuit conductors with an OCPD that is 156% of motor FLC. Would there be a risk of the primary transformer protection opening during motor starting?

Cheers, Wayne

 

[Dennis Alwon]

A related question is how would a transformer be sized for this motor load? If the transformer is sized based on 125% of the motor FLC, and the transformer is delta-delta with primary only protection at 125% of the transformer rating, then that would be akin to protecting the motor branch circuit conductors with an OCPD that is 156% of motor FLC. Would there be a risk of the primary transformer protection opening during motor starting?

Cheers, Wayne

That was my thought. I am wondering whether the transformer has to sized so the overcurrent protective device is as large as the req. ocp for the motor.

 

[don_resqcapt19]

I don't think that there is any provision that permits one Chapter 1-4 rule to modify another Chapter 1-4 rule. I think you are stuck with the full sized conductors.