# Overcurrent protection and wire size for small trannies

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#### lakee911

##### Senior Member
I had a shop drawing come in that was as follows:

480VAC single phase power feed (from 2P 20A CB, wiresize #12) to a panel. In that panel, the main CB is 7A. From that they are feeding only a 480VAC x 120VAC 2KVA transformer. There is no secondary protection and they wire to power distribution blocks and feed various loads with their own seperate CBs.

According to the NEC (if you make the assumption that industrial control panels are covered by the NEC), the incoming CB can be sized to a maximum of 167% of the tranny rated current. There is no round-up rule here. 2000VA/480V*167% = 6.958A. Is this close enough or is 7A too big?

Given that 2000VA/120V = 16.667A, what size wire should be used after the transformer? I would think that #12 is too small because 16.667 is greater than 80% of 20A. Would 10A be correct because 16.667A * 167% * 125% = 34.792A?

Thanks,
Jason

Bump! Anyone?

#### pfalcon

##### Senior Member
For industrial control panels in the US the NFPA79 is a preferred standard. Even so, transformer protection is still under the NEC even for them.

7A would be the correct size. Your calculation only shows less than 3 significant digits therefore the correct answer is 7A not 6.958A

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#### lakee911

##### Senior Member
Thanks for the response.

What sized wire should be used after the transformer?

Thx

#### pfalcon

##### Senior Member
Which table are you using to determine ampacity?

#### steve66

##### Senior Member
#12 is normally good for 25 amps per the table in 310. (Although it is normally limited to use on a circuit protected at 20 amps.)

If you look at 240.21(C)(1), if you multiply the 25A capactity of #12 by 120V/480V, you get only 6.25 amps. This is less than the 7A fuse on the primary, which is not allowed by (C)(1).

So if we want to split hairs, I would say they should either have a 6 amp fuse on the primary, or they should have #10 wire on the secondary.

Steve

#### pfalcon

##### Senior Member
Bussman usually recommends 125% protection even when 167% is allowed. Higher fusing is only recommended when the lower rated fuse won't hold due to equipment characteristics. Of course, that could suggest design failure.

We never fuse the primary only in our industrial environment. We also individually fuse all solenoids and similar devices for service reasons.

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