450.4 OCPD

[PowerdT]

I need some help clarifying Article 450.4 regarding overcurrent protection for the primary.

If I have 1KVA boost transformer 208v primary to 230v secondary and the load is 230V, 14A, 1phase. What size ocpd is acceptable?

Based on 450.4,
1000/(24) * 230v = 9.58kva
9580/230v —> 42A*1.25 gives us about 60A.

However Article 450.4 say, “overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer”

Does this mean, ocpd can be rated 20A on primary if my load is only 14A?

 

[JoeStillman]

Yes, 20A is ok. When the Code says "rated full-load input current" they are not referring to the non-autotransformer rating (1,000VA/240V=4.2A). Your 1kVA 240:24V two-winding transformer, when connected as a boosting autotransformer, will be loaded to way less than 1kVA. Only the low-voltage winding carries the full current of the load. So the kVA load on the transformer in your example will be 14A x 24V = 0.336kVA.

The calculation for autotransformer-rated output current is 1000VA/24V = 41.7A. It's autotransformer-rated kVA is 41.7A x 240V = 10kVA and it's input current is 10,000VA / 208V = 48A. So you could us a 48 x 1.25 or 60A breaker, bur the code says "not more than" so 20A is fine.

You could have done the job with a smaller autotransformer.

 

[JoeStillman]

Corrected secondary volts...

 

[PowerdT]

Yes, 20A is ok. When the Code says "rated full-load input current" they are not referring to the non-autotransformer rating (1,000VA/240V=4.2A). Your 1kVA 240:24V two-winding transformer, when connected as a boosting autotransformer, will be loaded to way less than 1kVA. Only the low-voltage winding carries the full current of the load. So the kVA load on the transformer in your example will be 14A x 24V = 0.336kVA.

The calculation for autotransformer-rated output current is 1000VA/24V = 41.7A. It's autotransformer-rated kVA is 41.7A x 240V = 10kVA and it's input current is 10,000VA / 208V = 48A. So you could us a 48 x 1.25 or 60A breaker, bur the code says "not more than" so 20A is fine.

You could have done the job with a smaller autotransformer.

Thank you. Now I am 450.4 expert.