Two power sources to one load center


Costa Rica
Hello everyone.

A client is asking me to do something that I think I shouldn´t, but I´m not completely sure if that could actually be a problem.

He has an off grid photovoltaic system and now has also connection to the grid.

The inverter is 120/240V (L-N/L-L), 60 hz, 6000 W and is connecter to a 240 V load center. Phase 1 of the inverter goes to lug one of the load center and phase 2 of the inverter goes to lug 2 of the load center and of course neutral and ground to the respective bars.

Now that he can connect to the grid, he becomes creative and is pretending to connect only one phase of the inverter to lug 1 of the load center and one phase of the grid to lug 2 of the load center. So, he will have 2 different sources to the same load center. For 120v loads maybe won´t be a problem, but what about the 240v loads? what could possibly go wrong here?

One more thing, is ok to share neutral between 2 sources? In case of short circuit will the breaker switch off?

At the beginning I was thinking that is not good to put 2 different sources to one load center, but then I remember that this is normal connection of PV systems when they are grid tied. So, I not sure what to do, or what to answer.


Senior Member
new york
Electrical Engineer / Electrician
I wouldn't allow that. What system would manage the phase synchronization? Even though a single phase system is only 180 degrees out of phase, if you have two separate systems, they could be varied in a harmful way if someone attempted to utilize a 240v load. You couldn't use a main breaker because you wouldn't be able to accurately detect a fault.
What does he want to do with the 2nd leg of each source??

The solar is going to reduce his costs anyway... what benefit does he see bringing in only 120V from the grid?