Boost Transformer Breaker 1

[DRWeig]

Hi Group,

I'm confused on this one for some reason. See attached diagram, I'm boosting 208V to 240V to feed a 31 FLA motor. The size I calculate for the boost transformer is 2 kVA, so its full load input current is 8.3A. But the motor needs a 50A breaker. According to NEC 450.4, though, I can't have over a 10A breaker or fuse in the line.

Do I need to get a 10 kVA boost transformer instead? Or is it OK as-drawn with 50A protection and 2 kVA boost transformer?

Best to you,

Goober Dave

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[waross]

Hi Dave;
The transformer current ratings for 2KVA will be;
208 Volt winding; 2000VA / 208V = 9.6 Amps
32 Volt winding; 2000VA / 32V = 62.5 Amps.
The input current to the transformer at full load will be 9.6 Amps + 62.5 Amps = 71.1 Amps.
No problem with a 50 Amp breaker.
For an auto-transformer boost application, the input current is the sum of the primary winding current (shunt winding) and the secondary winding current.

Such over-current device shall be rated or set at not more than 125% of the rated full-load input current of the auto=transformer.

Note that for auto-transformers the term "full-load input current" is used rather than rated primary current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DRWeig]

Thanks Bill!

Best to you,

Goober Dave

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