Neutral for Supply side connection feeding to a transformer?

[SunFish]

I'm using a stepdown transformer to take an inverter output from 480 wye and stepping down to 208 delta on the utility side and then to a fused disconnect before making a supply side connection. Since the transformer doesn't have a neutral connection point on the 208 delta (primary side) we're not running a neutral between the transformer and the fused disconnect.

However I'm unclear if I need a neutral run from the fused disconnect to the supply side connection. I've run one on past projects just to be safe, and also to bond neutral to ground at this fused disconnect since we're connecting upstream from where the service N/G bond is made. But is this even necessary? Seems silly to run a neutral only to the fused disco, and then not run a neutral to the transformer.

 

[ggunn]

I'm using a stepdown transformer to take an inverter output from 480 wye and stepping down to 208 delta on the utility side and then to a fused disconnect before making a supply side connection. Since the transformer doesn't have a neutral connection point on the 208 delta (primary side) we're not running a neutral between the transformer and the fused disconnect.

However I'm unclear if I need a neutral run from the fused disconnect to the supply side connection. I've run one on past projects just to be safe, and also to bond neutral to ground at this fused disconnect since we're connecting upstream from where the service N/G bond is made. But is this even necessary? Seems silly to run a neutral only to the fused disco, and then not run a neutral to the transformer.

I have designed a bunch of systems like this and I don't run a neutral anywhere on the utility side of the transformer..

 

[wwhitney]

A neutral is required where it is needed to ensure a fault clearing path should an ungrounded conductor fault to the case or EGC, right?

So under the 2020 NEC, IIRC 250.25 requires that the service neutral be extended from the supply side connection to the fused disconnect, with an N-G bond in the fused disconnect. Under the 2017 NEC and earlier, you could plausibly do the weird thing of instead grabbing an EGC from the other service disconnect and running it to the fused disconnect downstream.

Cheers, Wayne

 

[jaggedben]

The transformer is pretty much irrelevant to the question. You are performing a supply side connection to a circuit that doesn't require a grounded conductor. As Wayne said, the 2020 NEC clarifies that you must bring the grounded conducto from the point of the supply side connection to the disco and bond it just like if it were a regular service disco. Prior to the 2020, it was unclear if you should bring the white wire or a green wire to the disco and which should be bonded at the disco. But physics doesn't care what color the wire insulation is as long as it provides a grouns fault current path.