Double-Pole Circuit breaker questions

Old 09-13-05, 10:56 PM
Robert Hurtado
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Subpanel Wire Size

We're remodeling our house and adding a new panel. We're going to replace the old service panel with a new subpanel, but I was wondering what size wire I need to run if it's going to be a single phase 120/240 panel.

I will be putting a 100amp breaker at the main panel. Those breakers connect the two legs so that when one pops the other one goes down with it correct. Does it take a total of 100 amps to pop the breaker or only 50 amps from one leg?

If it only takes 50 amps from one leg then I can use #6 or #8 cu. wire but if it takes 100 from either leg I would need #3 or #2 right. I'll have to buy the conductors separate and the neutral can be two sizes smaller.

What wire's do I need?
Old 09-13-05, 11:25 PM
Robert Hurtado
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Double-Pole Circuit breaker questions

When does a 100 amp double-pole circuit breaker pop? Do you have to have a 100 amp load on either leg or does it pop when either leg reaches 50 amps.

If you need a total of 100 amps from both legs then each wire should be rated for 100 amps, correct?

I'm wiring a subpanel and I was wondering what size and type of wire to use. Can the neutral be smaller that the hots. Does the ground need to be the same size as the neutral or the hots? I'm guessing it's the hots.

Thanks for your time.
Old 09-14-05, 02:25 AM
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A 100 amp breaker is a 100 amp breaker, it trips at 100 amps.
The two hots and neutral wires must be good for 100 amps.

If you don't have any books on home wiring you should get some. it will save you a lot of time.
Old 09-14-05, 04:30 AM
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Each leg has a breaker rated at 100 amps. You are talking about 100 amps at 240 volts.
Old 09-14-05, 07:07 AM
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A 100-amp double-pole breaker trips when the current though either wire exceeds 100 amps. The wires connected must be rated for 100 amps each.

Adding the current through the two wires is completely meaningless.

In some very special circumstances, the neutral can be smaller than the two hots. But you should definitely make it the same size.

Use #2 copper in a cable assembly, or #3 copper if individual wires and modern equipment (i.e., both terminations rated at 75 degrees). In some circumstances, you can use smaller wire, but we don't have enough information to know if those circumstances apply, so the above sizes should be used. As always, consult with your inspector for the final say.

The equipment grounding conductor (EGC) can be smaller. In your case, #8 copper will do.

If you are using a grounding rod, then the rules for the grounding electrode conductor (GEC) are differnt than the rules for the EGC.

You should also consider voltage drop if the distance is over 150 feet.
Old 09-14-05, 08:17 AM
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"Re-place the old Service-panel with a new sub-panel"-----

If the new panel is equipped with a "Main" circuit-breaker which is the Service Dis-connecting Means on which the Service Entrance Conductors terminate, it is not a "sub-panel".

A "sub-panel" will always have two or more 2-pole circuit-breakers connected "in series" between the Service Entrance Conductors and the individual circuit-breakers in the sub-panel that protect the Branch-Circuits that extend from the sub-panel.The "first" C-B's is obviously the "Main" C-B (100 amps?)in the Service-panel.The 2nd. C-B is the C-B ( 60 amps?) that protects the "Feeder" conductors that extend from the Service panel to the sub-panel which is located at some indefinite distance from the Service-panel.

Good Luck & Learn & Enjoy from the Experience!!!!!!!!!

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