Overcurrent protection for 5kVA control transformer to North American requirements 3

[electrickiwi]

I'm not at all familiar with UL/NEC requirements for switchboards, so please help with recommendations for protection of a single-phase 5 kVA control transformer. It's a 277/120 V unit, tapping off 1 phase of a 480 V system and supplying non-motor loads of about 25 A continuous plus around 30 A non-continuous. I'd prefer to use circuit breakers rather than fuses. Please advise what the primary and secondary protection should be (and any references to requirements would be useful, too).
Many thanks, DW

 

[waross]

It is more common to use two phases and a 480:120V transformer. There is nothing wrong with using 277 Volts except that there will have to be a 4 wire feeder rather than a three wire feeder. If there is a 4 wire feeder for some other reason than just to supply the control transformer no problem, but there may be an economic hit if a long, large cable has to be upgraded to 4 wire just for a small control transformer. Some MCCs are fed with three wire cable and not just the cable to the control panel but the feeder to the MCC also must be upgraded to 4 wire.
The second issue that may or may not be applicable is the use of line to neutral loads on some systems. North American Codes require the neutral to be solidly grounded when line to neutral loads are served. A 277:120V transformer will not be usable on an impedance grounded system.
Primary protection:
26-256 Overcurrent protection for dry-type transformer circuits rated 750 V or less (see
Appendix B)
....each ungrounded conductor of the transformer feeder or branch circuit
supplying the transformer shall be provided with overcurrent protection rated or set at not more than
125% of the rated primary current of the transformer, and this primary overcurrent device shall be
considered as protecting secondary conductors rated at 125% or more of the rated secondary current.
(2) Notwithstanding Subrule (1), a transformer having an overcurrent device on the secondary side set at not
more than 125% of the rated secondary current of the transformer shall not be required to have an
individual overcurrent device on the primary side, provided that the primary feeder overcurrent device is
set at not more than 300% of the rated primary current of the transformer.
(3) Where a value not exceeding 125% of the rated primary current of the transformer as specified in
Subrule (1) does not correspond to the standard rating of the overcurrent device, the next higher standard
rating shall be permitted.
26-258 Conductor size for transformers
(1) The conductors supplying transformers shall have an ampacity rating
(a) not less than 125% of the rated primary current of the transformer for a single transformer; or
(b) not less than the sum of the rated primary currents of all transformers plus 25% of the rated primary
current of the largest transformer for a group of transformers operated in parallel or on a common
feeder.
(2) The secondary conductors connected to transformers shall have an ampacity rating
(a) not less than 125% of the rated secondary current of the transformer for a single transformer; or
(b) not less than 125% of the sum of the rated secondary currents of all the transformers operated in
parallel.
(3) Notwithstanding Subrules (1) and (2), primary and secondary conductors shall be permitted to have an
ampacity rating not less than that required by the demand load, provided that they are protected in
accordance with Rules 14-100 and 14-104.
Appendix B Notes on Rules:
Rule 26-256
Selection of overcurrent devices with too low a rating for the primary of a dry-type transformer can result in
unintended operation when the transformer is being energized (such as might occur after a power outage). To
avoid such operation, the overcurrent device should be able to carry
(a) 12 times the transformer rated primary full load current for 0.1 second; and
(b) 25 times the transformer rated primary full load current for 0.01 second.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electrickiwi]

Thanks Bill, very helpful!
In this case the main feed will already be 3-ph + N, and neutral is solidly earthed, so it sounds like that's ok. However, nothing has been purchased yet, so are there any other advantages/disadvantages in picking 277/120 over 480/120? If the latter is more common, is their pricing/availability better? Would the switchgear typically all be ~600V rated, or would the protective devices be different for the two transformer options (aside from being able to go smaller for lower pri current)?
D

 

[waross]

Those are Canadian rules. USA NEC is generally well harmonized but perhaps one of our US friend will comment on the corresponding NEC rules.
US standard voltage is 480 Volts. Canadian standard voltage is 600 Volts but, that said, many Canadian plants have chosen to stay with 480 Volts to take advantage of the much greater availability of 480 Volt gear just to the south.
It is almost universal for MCCs to use line to line voltage to feed control transformers in the cells. There are lots of 277:120 Volt transformers, but a much greater selection of 480:120 Volt transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter