45kVA step-up transformer secondary OCPD

[kchap]

Showing my ignorance in regard to power circuits here... A customer had me build a control panel for some equipment that includes five motors with a total load of about 100 Amps, with the largest motor load being a 20 HP. The supply power is 208V. After the fact, the customer realized that the 20 HP motor is rated for only 480V, and the others are rated 240/480V. So he got with his electrician and came up with a solution that involves splitting the 208V downstream from the feeder circuit OCPD. Two motors will remain on 208, and then they will install a 45kVA transformer to step up to 480V for the other three motors. Should the secondary side of the transformer be treated as a feeder circuit, requiring a main breaker in addition to three breakers for the motors? If yes, I assume it should be sized according to UL 508A, section 32.3. Would that be correct?

 

[petersonra]

You will need to have both primary and secondary transformer protection, primary and secondary transformer conductor protection, and protection for the three branch circuits.

There is a good chance you can protection the transformer primary and its conductors with one device,same with the transformer secondary and its conductors.

Personally, this being a field modification, I would not be looking at UL32.3 but whatever the NEC requires, which in this case I think is the same thing.

 

[d0nut]

I would look into buying the motor at the correct voltage. It will probably be less expensive than the fix you are trying to create.

 

[kchap]

I would look into buying the motor at the correct voltage. It will probably be less expensive than the fix you are trying to create.

I'm guessing the availability of an alternative motor is an issue. Like I said, the path forward was decided upon by my customer and his electrician.

You will need to have both primary and secondary transformer protection, primary and secondary transformer conductor protection, and protection for the three branch circuits.

There is a good chance you can protection the transformer primary and its conductors with one device,same with the transformer secondary and its conductors.

Personally, this being a field modification, I would not be looking at UL32.3 but whatever the NEC requires, which in this case I think is the same thing.

I have selected a breaker for the primary. That part is easy. I just wasn't sure about how to size the secondary. The calculated available current on the secondary is 54 Amps, so the secondary breaker cannot exceed 67.5 Amps (125% of secondary available Amps) according to NEC table 450.3(B). Since the next standard size up is 70 Amps, that is what I am going with. So I answered my own question by getting off my butt and looking in the NEC.

What caused me doubt is that the 20 HP motor pulls 27 Amps (per UL 508A table 50.1...actual nameplate FLA is probably lower), so its breaker can be sized up to 67.5 Amps - the same as on the transformer secondary. That just felt weird to me.

 

[ron]

Is it also weird that it is a 240V motor on a 208V source? It is already at the top end of its service factor. The standard SF for open drip-proof (ODP) motors is 1.15.
We typically use 200V motors for 208V applications.

 

[petersonra]

What caused me doubt is that the 20 HP motor pulls 27 Amps (per UL 508A table 50.1...actual nameplate FLA is probably lower), so its breaker can be sized up to 67.5 Amps - the same as on the transformer secondary. That just felt weird to me.

Then you could, if you wanted to, use the transformer secondary breaker to feed the pump rather than having two 70 A breakers in series.

Keep in mind the breaker is only providing short circuit protection for the motor conductors. Overload protection is being provided by whatever overload protection you have.

Incidentally, you could probably use what is called a motor protection circuit breaker in combination with an appropriate IEC contactor to run the motors. The MPCB provides both short circuit, and overload protection. The only thing about them I do not like is at 480 V they can mostly only be used on wye systems, and not delta. However, they are generally quite cost effective and the combination is usually rated for 65 kA SCCR, although there are a few that are lower so you have to check.