Bonded Generator, Transformer, Bonded Service Panel

Question

This forum has extensively fielded questions on the requirements for interconnecting the home electrical system with a portable generator with an intact neutral-ground bond. From what I can recall, the options seem to be to sever the generator’s neutral-ground bond or to utilize a neutral-switching transfer switch.

As I was daydreaming about purchasing an F-150 Lightning and cord connecting the onboard 240V/30A power socket to a hypothetical generator inlet at my service panel, I came across the fact that the F-150 Lightning has GFCI protection on all its receptacles. This would not play nice with the neutral-ground bond in the service panel since it would allow normal operating (non-fault) current to return down both neutral and ground paths back to source, tripping the Lightning’s GFCI.

Now whether using a Lightning or a bonded neutral-ground generator to power the house, I am wondering: does code permit one to wire a 240-to-120/240 isolation transformer between the generator inlet and the service panel, thereby serving the panel with a newly regenerated floating neutral?

The potential benefits being agnosticism to these details of the supply source and the ability to use a simple sliding plate interlock across the supply breakers in the service panel.

Perhaps one issue is, unless properly sized, the transformer can’t necessarily flow the current necessary to magnetically trip the source’s breaker in a secondary-side bolted fault to ground scenario?

Answer 1

Yes, that is fine. There is a commodity style of transformer called 240x480V-120/240V. They are widely used in industry and can be seen on Craigslist or similar marketplaces from time to time at sane prices. (local pickup is essential because of the weight).

This type of transformer has a 240x480V primary, meaning two matched 240V primaries which can be wired series or parallel. And two matched 120V secondaries, which could be paralleled for 120V, but are usually set in series with a center tap for 120/240V.

Since your application is generators (of one kind or another), you are dealing with an inherently current-limited source. Put it another way, a 30A breaker will need 180-315 amps to instant-trip. That is never going to happen from a 9 kW generator with a 15 horsepower ICE. The ICE simply is not capable of creating that much power because it can't move enough air through the ICE. Likewise the silicon inverter in the EV would vaporize if it attempted to deliver such current, and so its onboard protective circuits will not allow this.