Single-Phase Transformer OCP Question

[Shackled Designer]

Heya, Folks.

I have an industrial panel fed by 480V, 3-phase, delta without corner ground. A single-phase transformer is connected to one of the branches on this panel's supply circuit. If I read article 240 properly (NEC 2014), then it seems I need to protect each of the primary conductors to the transformer with a fuse, right? In other words, use two fuses, one per primary conductor, instead of just one. This is the conclusion I reach from 240.15 - A, which states:

A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor.

Have I rightly understood? I simply wish to be certain I'm reading the code correctly.

I ask since most writing on this subject seems to address how much overcurrent protection is needed in terms of amp ratings, but not on the number of conductors to be protected.

Thinking a bit more about the issue, a lone fuse would protect the conductor from a phase-to-phase fault in the transformer, but a ground fault in the transformer could present a problem for both primary conductors.

Thanks for feedback. Discussion, critique, etc. is welcome.

Shak

 

[GoldDigger]

You describe the source as delta but not corner grounded.
Do you mean that it is ungrounded? Or 120/240 three phase four wire? Or wye grounded but without the neutral pulled to your location?
Regardless, you do need two fuses.
But if the source is ungrounded a single ground fault will not cause any fault current.

 

[Shackled Designer]

Thanks for your response, GoldDigger.

As a panel builder with multiple customers, I'm generalizing my description of the feed power. As far as I can remember, corner-grounded delta feeds to our panels have been rare. With 240V systems, we often do find the three-phase, 4-wire system in place. Not so much, though, when we use 480V, 3-phase. In that case, we sometimes see 480V from a Wye system, but without a designated neutral conductor.

The specific instance I have in mind with my original post anticipates a 480V-delta system, but it is certainly possible that the end user of the panel might feed 480-Wye + ground, without a conductor designated as neutral. Figuring this is the case somewhere upstream anyway, that's why I made my comment about a ground fault in the transformer. I would hope, of course, that the chances of such a fault are exceedingly remote, but much of the protective measures which we enact are for contingencies that we would expect to be unlikely.

Kind regards,
Shak