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Underground distribution design

Underground distribution design

Underground distribution design

(OP)
Scenario:
New customer needs single phase 120/240v.
Closest source is an existing primary radial fed 3 phase (120/208v) loop type pad mount transformer.

I have been taught that it is a big "no-no" to tap single phase primary from the loop side of a three phase transformer. A three phase enclosure should be used and then tap the single phase primary from the enclosure. No one can explain why, just that it is an "unwritten" rule.

Can anyone tell me why not?

RE: Underground distribution design

If I understand your question: "Why is it not OK to loop one phase through a single phase padmount on the way to feeding a three phase unit?"

Electrically, its OK (assuming everything is rated appropriately for the load). But operationally its a problem. There is no easy way* to remove the single phase transformer for maintenance without dropping a phase to the three phase unit. So the preferred method is to tap off a junction box to feed a single phase padmount or lateral.

*It can be done, but it requires an alternate three phase primary feed. Many utilities don't like to switch to alternate feed for only one phase (resulting in the three phase transformer's supply to come in from different directions) so the number of operations to switch over all three phases is greater.

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