I am installing a Multiplus 3000 in a RV. The #1 output of multi will supply two circuits. One 15A & one20A. I am installing a small breaker panel between the multi and these two circuits. The breaker panel has only one neutral / ground bar where both neutral and grounds are combined. My question is do I combine the neutral and ground wire together?
You must not have two neutral - ground bonds in the system.
Either the N-E relay in the MultiPlus shoul create the N-E bord ot the bond is created in the panel. Using one busbar in the panel for both neutral and ground has the same effect as combining neutral and grounds in the panel.
The N-E relay can be disabled, but that's only half the story. We don't know what the grounding arrangements are upstream of the MultiPlus when the backfeed relays are closed.
The best advice is to consult your electrician about this and to make sure that the wiring will be safe and your circuit protection devices will operate correctly.
You should add a separate neutral busbar in your panel downstream of the Multi. Many low cost panels only include one but most will accept a neutral bar purchased separately.
