MP-II 48/8000/110-100 With AT-100 For Split-Phase Output - Excessive Neutral Current

Q&A and troubleshooting
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1

I am in the USA. I configured the MP-II for 240VAC, 60 HZ. It is connected to a Victron AT-100 autotransformer. The Critical Loads sub-panel does not have the neutral bonded to the ground. The Main Service Panel does have the neutral bonded to the ground.

The transformer output is wired split-phase to a breaker panel with only 120VAC loads connected. I disabled the Internal Ground Relay in the MP-II settings. The External Ground Relay output is wired to the AT-100 Earth Relay input.

I am using a Klein CL-800 clamp on current meter for all voltage & current measurements.

When the inverter is powered via battery, the AT-100 split-phase output is perfect. I get ~120VAC on each hot leg. With 2.4 amps of current on one hot leg, and 1.6 amps of current on the other hot leg, I was measuring roughly 1 amp of current on the neutral leg.

When I power on the mains to the MP-II, the current on the hot legs of the AT-100 remains the same. However, the neutral current immediately jumps to 15 amps.

There are no error messages or fault conditions on the MP-II. The Cerbo / GX-Touch 50 report no error conditions.

Why is there so much current on the neutral leg of the AT-100 when the MP-II is powered by the mains?

House System AC Only
House System AC Only1563×1036 193 KB

2

What are the input current and voltages to the Multiplus?

3

If the AT neutral and Grid Service neutral are connected (even indirectly through ground bonds), the AT will effectively be in parallel with the utility transformer. The AT is probably trying to balance loads located in your main service panel. This might explain your observed neutral current.

If you believe that the two neutrals should be isolated from each other and that this cannot be happening, check the current on your ground wire going from the critical load panel back to the main service panel.