Extending shore power circuit to Multiplus II

DIY
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I am in the process of adding a Multiplus II, new batteries etc to a Class C RV so that I can have AC power for a while without needing to run the generator.

I plan to take the existing shore power circuit (the feed), disconnect it from the existing service panel and extend it to the Multiplus. I’ll then take the output from the Multiplus and run it back (the return) alongside the feed, back to the service panel and connect it up.

Clearly I need two phase conductors , one feed and one return, but do I need to run a neutral there and back and an earth wire there and back? aka do I need a full compliment of 6 conductors (3 for feed and 3 for return), or can I reduce the number since the neutral and the earth are essentially “bonded” at the service panel anyway?

fwiw, it’s a 30amp feed and I won’t be using the boost function of the multiplus…
I’ll be using 3/4" flexible non metallic conduit to run the THHN/THWN conductors so even if I run 6 conductors I shouldn’t be running foul of fill limits.

Thanks for letting me know the folly of my thinking!

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Your plan is generally on the right track, but let’s clarify your conductor requirements and bonding concerns for safely wiring the Multiplus II in your RV.

Neutral and Earth Conductors
You cannot omit any conductors, even if the neutral and earth are bonded at the service panel. Here’s why:

Neutral Conductors:

The neutral wire is a current-carrying conductor and forms part of the circuit for return current from connected loads.
A dedicated neutral conductor must run both to and from the Multiplus, as the inverter will switch or interrupt the circuit during certain operations. This ensures safety and proper operation.
Earth (Ground) Conductors:

The ground wire provides a safety path for fault currents.
While the ground and neutral are bonded at the service panel, they serve distinct purposes in the system. You must run a separate earth wire to the Multiplus and back to maintain code compliance and safety.
Conductor Count
Given that you’re wiring a 30A system with a feed and return setup:

Feed: Hot (L), Neutral (N), Ground (G) to the Multiplus.
Return: Hot (L), Neutral (N), Ground (G) back to the service panel.
This results in a total of 6 conductors (3 for the feed and 3 for the return).

Conduit Fill

You’re correct that a 3/4" flexible non-metallic conduit can accommodate six THHN/THWN conductors without exceeding fill limits for 30A. For instance, using #10 AWG wires (appropriate for 30A circuits) is within acceptable limits.

Code Compliance and Safety

Ensure all wiring adheres to the National Electrical Code (NEC) or your local regulations, especially for RVs.
Use the proper breaker size (30A) to protect the conductors.

Verify that the Multiplus inverter’s neutral-ground bonding relay aligns with your wiring plan. The inverter automatically bonds neutral and ground internally when operating in inverter mode and will disconnect when needed for externally bonded input. Check the manual to avoid redundant bonding that can create safety hazards.

Summary
Yes, you need to run all six conductors:

Feed: L, N, G
Return: L, N, G

This ensures proper operation of the Multiplus, maintains compliance with safety codes, and avoids potential hazards. Your plan to use 3/4" conduit is solid and should comfortably accommodate the wiring.

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