Lightly Loaded Transformers?

[TwoBlocked]

Had a discussion with a coworker. I agree with him but want to see what you folks think. Let's say a 30kva, 3-phase, 480 delta / 208 wye transformer will only be loaded to less than 20kva, 50 amps on the secondary legs. On the secondary feeder you run #6 wires and 70 amp fuses. For the primary, you are kinda short on 60 amp cubicles so you use a 30 amp cubicle with 30 amp fuses. We do not see a code violation. What do you fine folk say?

 

[Tulsa Electrician]

Had a discussion with a coworker. I agree with him but want to see what you folks think. Let's say a 30kva, 3-phase, 480 delta / 208 wye transformer will only be loaded to less than 20kva, 50 amps on the secondary legs. On the secondary feeder you run #6 wires and 70 amp fuses. For the primary, you are kinda short on 60 amp cubicles so you use a 30 amp cubicle with 30 amp fuses. We do not see a code violation. What do you fine folk say?

Larger wire or smaller fuses.
Assuming your using CU conductors.
The wire on the secondary has to be sized based the OCPD it feeds. You need a 70 amp conductor at 75 c. This be a #4 cu or a #3 AL.
Be sure to size your SSBJ correctly.

 

[don_resqcapt19]

Had a discussion with a coworker. I agree with him but want to see what you folks think. Let's say a 30kva, 3-phase, 480 delta / 208 wye transformer will only be loaded to less than 20kva, 50 amps on the secondary legs. On the secondary feeder you run #6 wires and 70 amp fuses. For the primary, you are kinda short on 60 amp cubicles so you use a 30 amp cubicle with 30 amp fuses. We do not see a code violation. What do you fine folk say?

No code violation on the primary, but a 30 amp OCPD for may trip on transformer inrush. The secondary load does not change this, and it could be an issue.
For the secondary, the rule in 240.4(B) does not apply, so the transformer secondary conductors must have an ampacity that is equal to or greater than the rating of the OCPD at the load end of those conductors.

 

[TwoBlocked]

No code violation on the primary, but a 30 amp OCPD for may trip on transformer inrush. The secondary load does not change this, and it could be an issue.
For the secondary, the rule in 240.4(B) does not apply, so the transformer secondary conductors must have an ampacity that is equal to or greater than the rating of the OCPD at the load end of those conductors.

Please, why would it not apply? Actually, I was considering the secondaries as feeders, article 215.

 

[infinity]

Please, why would it not apply? Actually, I was considering the secondaries as feeders, article 215.

The #6 conductors cannot terminate on a 70 amp OCPD since their ampacity is only 65 amps.

240.21(C) Transformer Secondary Conductors.
A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). Section 240.4(B) shall not be permitted for transformer secondary conductors.

 

[TwoBlocked]

Thanks! (Just when I think I got this stuff figured out...)

 

[infinity]

Thanks! (Just when I think I got this stuff figured out...)