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Secondary conductor size for transformers

Posted By: mr_electrician

Secondary conductor size for transformers - 12/05/07 01:41 PM

Hello, can someone tell me the rule for sizing secondary conductors of a dry type transformer. I know it is 125% minimum, but what rule is it when you do not have secondary protection and you are sizing your panel board based on
14-606(2)
Thanks
Posted By: Rick Kelly

Re: Secondary conductor size for transformers - 12/05/07 02:03 PM

14-100(d) comes to mind...
Posted By: jay8

Re: Secondary conductor size for transformers - 12/05/07 08:07 PM

I think you have it covered there, 26-256 for the 125%, but that is maximum, not minimum.
Posted By: bill woods

Re: Secondary conductor size for transformers - 12/05/07 08:53 PM

Doesn't 26-256 cover overcurrent protection? Aren't we talking about conductor size here? Shouldn't we be looking at 26-258? If you install overcurrent devices in the primaries, and these devices limit the current on the secondaries to an amperage less than the KVA rating of the transformer allows don't you size the secondary conductors to this value?
Posted By: Rick Kelly

Re: Secondary conductor size for transformers - 12/05/07 10:13 PM

I take back my earlier mention of 14-100(d) for the following reason.

14-100(d) can not apply as its subrule (ii) requires that the circuit teminate at a single overcurrent device, which in this case we do not have.

So it looks like...

As per rule 26-258(2a) the minimum ampacity of the secondary conductors must be sized at 125% of the transformer's rated secondary current.

Good catch Bill, your comment made me rethink this.
Posted By: jay8

Re: Secondary conductor size for transformers - 12/06/07 04:08 PM

Yes, oops, I read that as overcurrent when the post mentioned not using secondary protection, so that would be 125% minimum on conductor size.
But Bill, the wording on 26-256 refers to rated current of the transformer, so you wouldnt limit current on the secondary conductors to less than the KVA rating of the transformer unless you wanted to do so, since 125% is the maximum, you could go less if you chose to.
Posted By: mr_electrician

Re: Secondary conductor size for transformers - 12/07/07 12:21 PM

Thanks guys for taking the time on this subject. I was talking with the local inspector and he said it like this....125% of full load current on the secondary when there is no protection on the secondary (what rule is that?). So even when your choking transformers you would size your secondary conductors based on 14-100(f)(ii) and multiply by 125%
Example: A 45KVA can put out 115 amps when the primary is fused at 40Amps. Now if I were to choke the transformer and fuse the primary at 30Amps, then the secondary conductors would be sized for 108A (30 X 2.88 X 1.25 = 108)
Clear as mud!
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