NEC Section for isolating neutral from ground

250.32 (b)(1)for buildings or structures detached from the structure or building with the service equipment.

250.24(a)(5) for load side of service equipment.

Note prior to 1996 code cycle you were allowed to bond a 3 wire feeder to a detached structure or building. There are many thousands of those existing today, especially in farm and rural settings. Not to mention the 3 wire dryer and ranges out there.

What about for attached subpanels? Is there any code stipulation? Thanks.

Do you mean in the same building as the service?

Exactly, Jim. Sorry.

They have always required a 4 wire feeder and isolated neutrals and grounds as long as I know.

The exception was to a detached structure as was already mentioned. This was allowed only if there were no metallic paths between the structures like a water line, phone or cable etc.

Which code article describes the NEED for the isolated ground?

250.24 (a)(5)

load-side grounding connections. A grounded conductor
shall not be connected to normally non–currentcarrying
metal parts of equipment, to equipment grounding
conductor(s), or be reconnected to ground on the load side
of the service disconnecting means except as otherwise permitted
in this article.

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