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I am considering buying a MultiPlus II 2x120V for my Grand Design 337RLS Fifth Wheeler, which has 50amp electrical.
The MultiPlus II manual indicates AC-out-1 can supply up to 75A using PowerShare. Is there a configuration setting on the MultiPlus to limit the AC-out-1 to 50A per leg during a PowerShare scenario? My panel is limited to a 50A main.
The 50-amp per leg limit is correct. I think the "PowerAssist" you're referring to is just for short periods of use drawing from the battery.
There is no way to throttle back power assist. It adds available inverter power to incoming shore power. So if you have 50 amps of shore power you get 25 additional amps on leg 1 from the inverter for a total of 75 amps. L2 is not connected to the inverter when 120/240 mains power is present so it's a max of 50 amps out.
That said, you don't need to use that additional 25 amps. Your distribution panel is probably fitted with dual 50 amp mains breakers which will prevent drawing more than 50 amps per leg (at least continuously).
While you can use power assist with the full 50 amps of shore power, that is not the only scenario where it is useful. For example, when shore power is limited, e.g., when you are running the RV on a 120 volt 20 amp receptacle. You'd set the input current limit to 20 amps then if the loads draw more than that, the inverter will kick in and supply up to 25 amps of additional power. Of course this comes from the batteries so you don't get this assist indefinitely.
The distribution panel's 50 amp breakers won't trip unless you draw more than 50 amps from one of the legs. I would guess that under normal conditions you aren't anywhere close to 50 amps on either leg. If you were over 50 amps, you would be tripping the distribution panel's main breaker or the breaker in the pedestal now (before you install the Multi). Installing the Multi won't change that. If you draw more than 50 amps per leg, the distribution panel's breakers WILL trip just like they would now. But if you stay under 50 amps per leg, all is well even though the Multi COULD supply an additional 25 amps to L1 if there wasn't a 50 amp breaker in the distribution panel. But don't remove it because it protects the wiring as well.
