Transformer Discconnects

[jbolen]

I have a 3 KVA Single phase transformer. Does it require a disconnect on both sides primary and secondary. Plus maximum over current device.

 

[don_resqcapt19]

See 440.14 for the primary. Note that the primary OCPD can serve as this disconnect. If that is not located at the disconnect, then there must be a sign at the transformer stating the location of that device. There is no specific disconnect requirement for the secondary, however there are transformer secondary overcurrent protection requirements in 450.3, and secondary conductor overcurrent protection requirements in 240.21(C). These secondary protection requirements will almost always result in a secondary OCPD near the transformer.

(EDIT: the code reference should have been 450.14)

 

[Carultch]

I have a 3 KVA Single phase transformer. Does it require a disconnect on both sides primary and secondary. Plus maximum over current device.

If there is a center-tap on the secondary, you'll need OCPD on the secondary per 240.21(C), which likely will be nearby the transformer. If there aren't any center-taps, and both windings only have two wires terminated on them, then your primary side OCPD will protect the secondary. You don't need to count unused center taps, for which you have no intention of connecting a wire.

Certain topologies of transformers qualify for the primary OCPD to protect the secondary conductors "by proxy". In otherwords, a 15A breaker at 480V, will effectively protect a 240V secondary circuit as if it had its own 30A breaker, if the winding arrangement is simple enough to allow it. The idea is that the currents have to line up winding-to-winding, with no possibility of redistributing. Delta-to-delta 3-wire, and single phase 2-wire to 2-wire are what qualify. Anything with a connected center tap or a wye system will not qualify, and therefore needs 240.21(C) protection of the secondary conductors.

Here's an example to show why center taps require OCPD on the secondary:
120/240V split phase secondary
Primary OCPD of 15A
Secondary conductors expecting protection at 30A
Line 1-to-neutral draws 40A overload
Line 2-to-neutral draws nothing
Total VA drawn from the secondary conductors = 4800 VA
Amps at 480V = 10A
10A doesn't trip the 15A primary OCPD. The 40A overload goes "unnoticed" by the primary OCPD.

 

[electrofelon]

See 440.14 for the primary. Note that the primary OCPD can serve as this disconnect. If that is not located at the disconnect, then there must be a sign at the transformer stating the location of that device. There is no specific disconnect requirement for the secondary, however there are transformer secondary overcurrent protection requirements in 450.3, and secondary conductor overcurrent protection requirements in 240.21(C). These secondary protection requirements will almost always result in a secondary OCPD near the transformer.

Don meant 450.14

 

[don_resqcapt19]

Don meant 450.14

Thanks...fat fingers again