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#48314 - 02/09/05 07:18 AM Confused about transformers
swampy Offline
Junior Member
Registered: 02/09/05
Posts: 1
Loc: Bancroft, WI U.S.
I 'm studying for my Journeyman's. One question is ; A 10 kva trans. is 240/120v. step down. secondary ocp is 125%. What is max. cb required for primary if eff. is 100%?

Can anyone give me some pointers? Thank you!
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#48315 - 02/09/05 08:20 AM Re: Confused about transformers
cpal Offline
Registered: 05/17/04
Posts: 162
Loc: Cohasset MA
The formula for pri current is kVA/E pri times the phase multiplier. In this case a single-phase multiplier is 1. If it were 3 phase the multiplier would be 1.73.

It would be a good idea to read section 450.3(B) and study the table. You should understand that in general a transformer operating at 600V or less is considered (in general) when the pri OCPD is set at 125% of the pri FLC) The table will go on to indicate that in cases where there is OCP at the secondary of the transformer which is set at 125% of the secondary FLC, the pri OCPD may be increased to 250% of pri FLC.

Your transformer should have a 41A pri and 2.5 X 41 yield 102. (see notes to tables ) I'd call it 100A.
#48316 - 02/09/05 08:33 PM Re: Confused about transformers
Bob Offline
Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
Cpal said "Your transformer should have a 41A pri and 2.5 X 41 yield 102. (see notes to tables ) I'd call it 100A."
That is correct. Note that the table allows you to go up to 250% but not exceed that figure. If the 250% does not yeild a standard
fuse or breaker, you may not round up.

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