250.24(C) clarification

Aenergies

Hey all. I’m interested in better understanding 250.24(C) in regards to projects with inverters that don’t require a grounded (neutral) wire and have a line side connection. Should:

a)     No neutral be required anywhere in the wiring

b)    A neutral be required from the main service panel to the fused service disconnect only (and bonded there)

c)     A neutral be required from the main service panel to the fused service disconnect (where it’s bonded) and then continue on to any other AC disconnects in the system (additional unfused disconnects, combo panels, etc) even though it's not bonded in those locations.

Thanks for any thoughts.

BB.

Normally, I would go with "B"... You need to ground reference the wiring so that if there is a short from L1 (or L2 if 240 VAC), it will trip a protective breaker/fuse.

If you have a truly "floating" AC output (option "A")... You would need a dual circuit breaker as you would on a 120/240 Spit phase system. That way a short on either L1 or L2 would trip a breaker if there was a short circuit. This is assuming you have a higher current rated AC inverter output going to lower rated branch circuits (i.e., a 30 Amp AC inverter, and several 15 amp branch circuits). If you only had breakers on "L1" (single pole breakers), there are short circuits that could over current the 15 amp wiring on the L2 / non-breaker protected circuit.

If you have a ground reference "neutral", then you would only need a single pole circut breaker to protect against a Hot to Ground short... The ground referenced "neutral" is never above zero volts, so a short from Neutral to Ground will not pass much current.

I am guessing, but this sounds like a 240 VAC or higher system voltage? Will the inverter(s) be 120/240 VAC split phase (i.e., there is a center tapped "white wire" from the inverter/transformer output) with single phase 240 VAC (or higher) load...

For systems that had floating outputs (transformer isolated), they would have a circuit dedicated to make sure that the outputs were floating, and not have one shorted to ground somewhere--Such as on a (probably) older ship. The would have two lamps. One L1 to gnd and a second lamp L2 to gnd. If the output was floating, then both lamps would be equal brightness and relatively dim. If there was a short from Lx to gnd, one lamp would be full brightness, and the other would be dark (ground fault). (It has been decades since I even read about ship systems--So I am sure there is new stuff out there that is much better).

In the end, need to know a bit more about the power source (120/240 North American split phase inverter or 230 VAC only European inverter) to figure out if three wire (L1/L2/N + ground) or 2 wire (L1/N + ground).

If you have a "Neutral" i.e., 120/240 VAC split phase--There can be some arguments about a pure 240 VAC single phase load, and the sizing of the neutral wiring--I.e,. size as a ground wire for short circuit current if shorted (smaller AWG), or size per the nominal rated current of the source (larger AWG neutral--Even though no Neutral loads):

https://forums.mikeholt.com/threads/art-250-24-c.26293/

Anyway--More of an "engineering" discussion vs "code" requirements at this point.

-Bill