Hedges
I See Electromagnetic Fields!
- Joined
- Mar 28, 2020
- Messages
- 20,048
You shouldn't do it. It creates a parallel path with the grounding conductor. The only thing that the two sides of an isolation transformer should have in common is the grounding system.
No parallel path because I would not bond center-tap and derived neutral to ground. (That is what would have put ground and neutral wires in parallel.) Neutral of proposed transformer-derived system goes through an extended white wire back to main breaker panel with service disconnect and neutral-ground bond. From there, ground returns to my transformer derived system, where it connects to chassis of any equipment and ground pin of receptacles.
In the absence of fault, no complete circuit to send any current through that extended neutral or ground. If there is a fault from L1/L2 of of transformer secondary to ground, current flows through ground wire to secondary panel, through bond to neutral, back through neutral to center tap of transformer, completing the circuit. OCP trips. This is wired the same and works the same as if no transformer (and no inverter) present.
Of course, this is for 1:1 transformer, and neutral feeder wire large enough for available current. It would be sized according to OCP in main panel feeding the transformer. It had better be large enough for current capability of inverter as well. If this was a system with 20A circuit feeding 50A inverter, the neutral and ground would be undersized.
My system presently looks exactly like this, except no transformer. 4x Sunny Island wired 2p2s can isolate from grid input, and the sub-panel they feed has neutral and ground wired back to main panel which has the bond. (Normally neutral and ground would pass through the inverters, which have two terminals for each, one from grid and one to sub-panel. Mine happens to not pass neutral and ground through, but still just one net for each, bonded at main panel.)
If not planning to backfeed main panel, no need to do this. Just bond center-tap to ground as you say. But if you have interlocked "generator" breaker in main panel so just L1/L2 are switched to feed it from inverter, then I think carrying N and G through as I described, with bond only in main panel, is the way to go. Again, that is how my backfeed works (but with no transformer.)
The secondary neutral is isolated from the primary conductors. Maybe you are thinking of an autotransformer. Which is required to be connected to the primary source neutral. (If one exists)
If I'm allowed to connect center tap of autotransformer to neutral of grid, what is wrong with connecting centertap of isolating transformer?