# Sizing autotransformer primary and secondary conductors



## Doug584 (Jan 24, 2013)

I am a little confused about my install. I have a piece of equipment that is 230v 3ph 25a. I am using (2) Sola HD transformers (HS19F750B) to boost the voltage from 208v to 230v. When using Article 450.4A...

Primary 
12400VA/ (208x1.73)= 34.4A
34.4x125%=42.88 (45A breaker)
#8 THHN

Then using 450.3B table I see that currents of 9 amps or more are not required to have secondary protection. So how do I size the secondary conductors? If I go off the equipment, the conductors won't be properly covered by the 45A breaker. Also, the equipment won't have adequate protection.

Is there a different calculation that I should be using since I am boosting the voltage?


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## Black Dog (Oct 16, 2011)

Doug584 said:


> I am a little confused about my install. I have a piece of equipment that is 230v 3ph 25a. I am using (2) Sola HD transformers (HS19F750B) to boost the voltage from 208v to 230v. When using Article 450.4A...
> 
> Primary
> 12400VA/ (208x1.73)= 34.4A
> ...


".......

You should have a disconnect switch on the secondary's fused for the wire size.


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