Autotransformer output capacity

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brianv asked Jun 13, '21

Autotransformer output capacity

I am confused as to what the actual output of the autotransformer 32 and 100 amp is. I see that the neutral caps out at 32 amps on both. So does that mean that the total output of either autotransformers is 3840VA in split phase 120/240? It doesnt make sense because if you are connecting let's say two 120v 5000VA Quattros to the AT to create balanced split phase current, you basically lose a bunch of output capacity. Or is the 100 amp version able to output 100 amps total between the two 120 lines?

MultiPlus Quattro Inverter Charger wiring Autotransformer split phase

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Answer 1 · 2021-06-13T05:28:43Z

Matthias Lange - DE answered · Jun 13, 2021 at 05:28 AM

I never installed a autotransformer because here in Europe(Germany) we don't have split-phase systems.

But if I read the datasheet and schematics correctly it should be like this:

1. The 240V input of the autotransformer is passed trough via a relay and that relay is rated for 32A or 100A depending on the model of autotransformer. In that case the transformer is not used.

2. Only for 120V loads the transformer is used and in that case you are limited to a neutral current of 32A (for 30min; 28A continuous at 40°C/100°F) in total because that is the limit of the transformer winding.

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Answer 2 · 2021-06-13T05:50:18Z

wkirby answered · Jun 13, 2021 at 05:50 AM

It is important to pay attention to the neutral current and balance your 120V loads as well as possible to minimise the neutral current.

Let's say you have two 120V 20A heaters for example. If you connect both heaters to the same 120V leg then the Autotransformer would be overloaded because the neutral current would be 40A.
However, if you put one heater on each 120V leg then the neutral current would be approximately 0A yet the line current for each phase is still 20A.

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