Wire Size for MV Switchgear Control Circuits

[Phlapjak]

MV Switchgear vendors will install 14 AWG wire for circuit breaker close/trip circuits that are fused at 30A. If you wire in an aux contact from the field into the close/trip circuit, do you have to use 10 AWG wire for the field aux contact so that it meets NEC requirements for 30A fuse?

 

[che12345]

Phlapjak (Electrical)(OP)11 Dec 23 19:22
" ....MV Switchgear vendors will install 14 AWG wire for circuit breaker close/trip circuits that are fused at 30A. If you wire in an aux contact from the field into the close/trip circuit, do you have to use 10 AWG wire for the field aux contact so that it meets NEC requirements for 30A fuse?"
My personal opinion, for your consideration.
1. Copper conductor 14 AWG with insulation rated say 60 deg C would have an ampacity of 20A. It would usually be fine for circuit breaker close/trip circuits load, voltage-drop within say 20m and with sufficient mechanical strength. Therefore, NOT necessary to increase it to copper 10 AWG.
2. With copper 14 SWG, the over-current protective device shall be NOT exceeding 15A. See Art 240-3 (d).
3. Proposal: Replace the 30A fuse with an over-current protective device NOT exceeding 15A. Unless special cases. See Art 384-32 . Exception No.1.
Che Kuan Yau (Singapore)

 

[wcaseyharman]

It’s my understanding that a rated/listed assembly does not have to follow the NEC, hence why you find strange wiring in manufactured electrical equipment. I believe if you add any new wiring that new wiring would require you to follow the NEC and yes you’d have to use #10.
Unless you are a utility, if it’s a utility it does not have to follow the NEC.
However, I’ve re-fused trip and close coil circuits per the NEC, but generally check the trip coil and close coil ratings to make sure the coil current is nowhere near the fuse rating. Have not had any issues so far.

As an aside, I’ve re-wired switchgear so the close coil power comes off the trip coil circuit (with a smaller fuse) so if the trip coil fuse blows you can’t close the breaker.

 

[dpc]

From an engineering perspective, it's not a problem. The fuses are sized to handle to high momentary current associated with close and trip circuits. I'm too lazy to research it in the NEC, but in over 40 years of doing design, we never had an issue with inspectors or plan check with running #12 external to the switchgear for these control circuits, regardless of the fuse size. It's not a branch circuit that is carrying power, but rather a control circuit.

 

[che12345]

Phlapjak (Electrical)(OP)11 Dec 23 19:22
You had already received numerous learned advice. I would like to add the following for your consideration.
1. In general, copper AWG 14 is fine. The current rating would be fine for (momentary only) closing/opening coil. Attention: in case use of low control voltage or with long distance, the voltage at the instance of coil operation, Shall > 0.85 rated operation voltage of the coil.
2. Irrespective whether the board is UL listed or in compliance with NEC, the 30A fuse is far too high for the copper AWG 14 conductor. The conductor is NOT protected irrespective whether it is energized momentarily, during operation.
3. Suggestion: Strongly recommend a) replacing the 30A fuse with a < 15A breaker attached with an auxiliary signaling contact, or a 2-pole breaker using 1-pole for open/trip signaling.
b) an un-noticed blown fuse on the trip coil circuit may result to very serious consequence, unless it is continuously monitored by a trip supervision relay.
Che Kuan Yau (Singapore)

 

[wcaseyharman]

A lot of the older gear from GE and other OEMs used 35A fuses for years and years, all wired with #12.

I’m assuming it was left over from older circuit breakers required larger trip coils. Most of the vacuum breaker trip coils we have are in the hundreds of ohms or more, so a 20A fuse is more than sufficient.

Casey