Wire Sizing

[gmtt]

A 10 KVA transformer with a 240V, 40A secondary feeding to a 100A breaker panel. Should the wiring size between the X-former and the Panel be based on 100A or 40A ?

 

[augie47]

240 volt 2 wire secondary or 240/120 3 wire ?

 

[gmtt]

3-wire (center tapped secondary)

 

[iMuse97]

wire size has to be based on over-current protection device; therefore, #3 AWG, as I see it.

 

[augie47]

In a case of a 3 wire secondary, 240.4(F) notes that the secondary conductors are not considered to be protected by the primary OCPD, therefore you need to follow 240.21 (C) which gives you a few options as to overcurrent protection but in all cases the OCP would be based on the size of the secondary conductors.
In addition you need to make sure you comply with 450.3.
If your transformer primary protection is 125%, then you would need #3 cu if your secondary protection is 100 amps. If you backfeed the panel thru a lesser rated breaker you could reduce the wire size accordingly.
If your transformer primary protection is greater than 125% (>30 amps), then your secondary would need to be protected at 125% (60 amp)

 

[raider1]

Take a look at 240.21(C).

Chris

 

[charlie b]

And now, the correct answer :grin: to the question that was actually asked : "Neither." You size the wire on the basis of the calculated load. Then you pick an OCPD that will protect that wire.

To be specific, and to answer the question that you thought you were asking, you don't have to use 100 amps, just because the panel rating is 100 amps. The panel rating means nothing. If for example this panel is in a garage that has no loads other than a couple lights, a few receptacles, and a door opener, then you might have a calculated load of less than 20 amps. You could use an 800 amp panel, with a 20 amp main breaker, and use #12 wire to feed the panel.

OK. That example was just off the top of my head, and there might be an issue with tap rules or something. My main point is that you start with the load, and work your way toward the source.

 

[charlie b]

wire size has to be based on over-current protection device. . . .

I see it the other way around.

 

[gmtt]

charlie b
You hit right on the money. But the fact is, the Panle is already there and is equipped with a 100A breaker in it and their load is only about 30A. I have to use this panel.

 

[LarryFine]

. . . the Panle is already there and is equipped with a 100A breaker in it and their load is only about 30A. I have to use this panel.

I that case, the wire selected must be large enough that the 100a breaker provides adequate protection of it.

 

[gmtt]

Thank you.

 

[augie47]

I that case, the wire selected must be large enough that the 100a breaker provides adequate protection of it.

I agree, but still feel it necessary to mention, IF you put a #3 wire and a 100 amp breaker to your existing panel and the transformer is 10 kva you must make sure your primary protection meets the 125% rule in 450.3(B) or you will be in violation.

 

[charlie b]

The Panel is already there and is equipped with a 100A breaker in it and their load is only about 30A. I have to use this panel.

But do you have to keep the 100 amp breaker? Can you replace the panel's main breaker with a lower rated breaker? If so, I would go with a 50 amp breaker, or at highest a 60. That is based on 125% of the tranformer's secondary rating of 41.7 amps, and taking the next lower or next higher standard breaker. Then I would select a conductor rated for that ampacity (i.e., #8 if the breaker is 50 amps, #6 if it is 60 amps).

 

[iMuse97]

You guys got it right, as to all the nuances, and other possibilities, as usual. I just shot off an answer, based on the fact that the OP said he had a 100A breaker.

As someone pointed out, assuming appropriate primary side protection, you can back-feed a 50 or 60A breaker into the panel, which is something I recently did as well; don't use the 100A main breaker; remove it.