# Thread: Fault current calculation for autotransformer

1. Junior Member
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## Fault current calculation for autotransformer

Here's a situation that I would like to share. A 240volt to 208volt, single phase auto-transformer.....properly protected by a circuit breaker. I would like to be able to calculate the fault current for it. In my mind, since there is no electrical isolation, I must consider the upstream, two coil transformer to make this calculation.
I think of it this way....if I have a line to line fault on the secondary of the auto-transformer, that effectively shorts out the common coil portion of the auto-transformer....leaving only a small number of windings (what's left) in series with the fault - now being fed by the upstream transformer.

Is my thinking straight on this?

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Originally Posted by basicbill
Here's a situation that I would like to share. A 240volt to 208volt, single phase auto-transformer.....properly protected by a circuit breaker. I would like to be able to calculate the fault current for it. In my mind, since there is no electrical isolation, I must consider the upstream, two coil transformer to make this calculation.
I think of it this way....if I have a line to line fault on the secondary of the auto-transformer, that effectively shorts out the common coil portion of the auto-transformer....leaving only a small number of windings (what's left) in series with the fault - now being fed by the upstream transformer.

Is my thinking straight on this?

Actually to do a fault current calculation even with a isolated transformer you would still have to consider all upstream impedance and or reactors and capacitors that might add to the equation, a buck boost transformer is nothing but an isolated transformer with a primary in series with the secondary in either an additive or subtractive configuration, both add to the impedance of the circuit, although the secondaries are quite large and will add very little.

To do a proper fault current coordination study you need the available fault current at each point in the system from the POCO to the last transformer, now getting this info from the POCO can be a trick sometimes, but luckily I have had a few good utility engineers who would do the math ahead of the service for me, I hear this can be a pain to get from some utilities.

Edited to add: Also running motors can add to available fault current, but thats when I call an engineer to figure that one.
Last edited by hurk27; 11-05-11 at 12:01 AM.

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