# Autotrasformer for US split phase and 15 kva Quattro

I would like to connect 2 Quattro 15 kva in parallel for an output of 24 kW. I would also like to connect that output to the 100 amp Autotransformer to obtain a 120-N-120 split phase (USA). I have seen the circuit diagram describing a similar connection with 1 Quattro, but I have not been able to confirm if you can do with two inverters in parallel. Thanks!

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Sure, you can do it with two. Keep in mind, though, that a single AT can only sink 32A of neutral current (difference between L1 and L2).

With that large of a system, you may need two ATs or one larger AT.

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@ben Thanks Ben! When you say 32 amps on the neutral, are you referring to the 100 amp autotransformer or the 32 amp autotransformer?

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Both models from Victron, the 32 amp and 100 amp, are defined by their passthrough amperage, not their ability to balance the difference in amperage per phase (leg).

The 32 amp model can passthrough 32amp, and can balance up to 32 amps BETWEEN the 2 x 120v outputs (28 amp continuous imbalance).

The 100 amp model can passthrough 100 amp, and can balance up to 32 amps BETWEEN the 2 x 120v outputs (28 amp continuous imbalance).

For example:

With the 32amp model, you could (not advisably... but) have Leg 1 (120v phase1) running 32 amps (28 amp continuous) and Leg 2 (120v phase2) having no load/nothing connected.

-> Leg 1 could be 120v 28 amp, Leg 2 could be 120v 0 amp - and the autotransformer can handle that imbalance. (not the best setup, but it would be within the spec sheet of the product)

With the 100 amp model, you could (not advisably... but) have Leg 1 (120v phase1) running up to 100 amps and Leg 2 (120v phase2) running almost 100 amps - BUT you can still only have a DIFFERENCE in output between Leg 1 and Leg 2 of 32 amps (28 amp continuous).

-> Leg 1 could be 120v 100amp, Leg 2 could be anything from 68amp to 100amp. (32 amp difference). Or vice versa. So long as there is never a difference of greater than 32 amps between the output legs.

In essence:

Buy the 32 amp model if you are never going to use more than 32 amps, and never going to have all the power COMPLETELY on 1 phase output. (overheat, since max continuous is 28 amps)

Buy the 100 amp model if you need the 100 amp capacity AND can ensure that your split-phase system is somewhat balanced to the point where no more than 32 amp DIFFERENCE in load is experienced by either output leg(phase).

In the real world, most off-grid or inverter run systems would not even have an autotransformer, and design their electrical system so that loads are fairly evenly balanced.

The need for this product comes in when there is a chance of a large load that runs only on ONE leg of the output - for reasons outside of this discussion, large differences in loads on a split-phase system can be very problematic.

My personal experience (before Victron) was the purchase of a 7 kva Schneider inverter that wanted to charge the batteries ONLY from phase 1 from our generator. So whenever the inverter would start charging, the generator had to supply a very imbalanced large current which - a few months later - led to generator's demise. (The Schneider manual and tech we had install it did not know that it charged from only 1 leg......)

Damage from unbalanced loads can also effect the equipment on the load side. As voltage drops and current fluctuations are damaging.

Jason

CanWest Renewable

Jason Bolduc

Jason Bolduc, thanks for the thorough explanation.

With that in mind, should I go with two inverters at 120 V to make the two legs of the split phase or should I go with one 240 V inverter connected to an autotransformer? I feel the 32 amp balancing limitation would make a one inverter setup pretty limited. Is there another autotransformer that can balance more that 32 amps for a setup like this?

Best,

Gustavo

Hello again @ben. I just checked the specs, and now I see that both the 32 amp and the 100 amp have the 32 amp limitation on the neutral. The follow up question would be: is there any issues in connecting two autotransformers in parallel?

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There’s no theoretical reason you couldn’t parallel them. You need to verify that they balance well; connection resistances need to be similar and there might be slight tolerance differences. Verify current with a meter, etc.

Or, simply source a bigger AT.

ben ♦

Thanks @ben! I looked for larger ATs, but I am having a hard time finding any. I may not be searching for the right keywords. Do you know who makes larger autotransformers for this type of application?