Victron MultiPlus II Generator Fix

Or, “How I rewired my transfer switch without killing myself.”

The Trouble with Features

After installing our Victron MultiPlus II Inverter/Charger, we were very happy with its performance and features. Simply, it slices, it dices.

However, one of the MultiPlus’s features had a downside – we lost half of our generator’s power. So while riding down the road in the summer heat, we now have one air conditioner instead of two. What’s the deal?

Dog Bones

Most RVers probably own a 30A-to-50A “dog bone” that allows connecting a 50A RV to a 30A power supply.

Since 30A power only has a single “hot” line, a dog bone simply connects the single 30A hot line to both 50A power lines (L1 and L2).

With a dog bone everything’s hunky dory, but your RV can only use up to 30A, not 50A.

Enter the Victron MultiPlus.

More Technically Speaking

In order to distribute power to the RV, the Victron handles 50A power (“split phase”) and 30A power (“single phase”) differently.

When the MultiPlus detects 50A split phase AC power, it just passes incoming power on L1 and L2 through to the AC Out (top drawing below).

On the other hand, when the Multiplus senses 30A single phase power, it disconnects the incoming L2 AC power, but connects L1 and L2 AC power internally (see circled areas below). The MultiPlus does this so it can distribute power to the AC Out.

However, if your generator outputs single phase power (like ours), then you’ve just lost half your generator’s power.

Notice the voltage measurements for split phase and single phase systems:

Split Phase:
L1 to Neutral = 120 VAC
L2 to Neutral = 120 VAC
L1 to L2 = 240 VAC

Single Phase:
L1 to Neutral = 120 VAC
L2 to Neutral = 120 VAC
L1 to L2 = 0 VAC

Single Phase RV Generators

Before explaining our generator “fix”, here’s how a single phase generator outputs power:

The takeaway from the above graphic is that the Black (L1) and Red (L2) AC outputs both have the same voltage and the same phase (single phase). In addition, the voltage between L1 and L2 = 0.

Fooling the MultiPlus

To make use of all our RV generator’s power, we rewired our generator-to-transfer switch connection to create a “dog bone”.

To do this, we connected the generator’s L1 and L2 together and also connected the generator’s two neutral wires.

It worked! We can now fire up both our air conditioners without fear of tripping breakers.

Credits

This upgrade would not have been possible if not for the help from several excellent online sources. The single best source was from the Changing Lanes YouTube channel, in particular this video: