Main panel breaker operation

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mmm_max

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Since a main breaker furnishes current to two phases of the panel, will it trip if all the current is on one leg. For example a 100 amp residential breaker with drawing 150 amps on one side only?
 
I concur, and the time to trip will be determined by the manufacturer's inverse time/ current trip curves for the breaker. Higher the current on either phase, the faster the trip time. For a short circuit, instantaneous trip times typically less than 1 sec.
 
mmm_max said:
Since a main breaker furnishes current to two phases of the panel, will it trip if all the current is on one leg. For example a 100 amp residential breaker with drawing 150 amps on one side only?
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Yes. A two pole breaker (main or otherwise) is two single pole breakers with a common trip mechanism that disconnects both of them if either one trips.
 
Yes. Same w 3 pole breakers. A breakers purpose is to protect the wiring and devices downstream. If there is any type of short or overload in any of the phases, it MUST trip to protect!

That being said, I once asked this silly question way back: "If you have a 60 amp 3 pole breaker, is it 60 amps per phase or a total of 60 amps from all the phases....lol. " What a numb nuts.

I also once asked the question....."what's a substation"? Ahhhh...to be young again.
 
powerpete69 said:
That being said, I once asked this silly question way back: "If you have a 60 amp 3 pole breaker, is it 60 amps per phase or a total of 60 amps from all the phases....lol. " What a numb nuts.
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That's a common misconception among folks who haven't dealt with power distribution. I can remember having to grasp the way it works, myself, even after I got my EE degree.
 
If I was not concerned about inspection or the NEC at all; and assuming a 100 amp main breaker panel and assuming that the main will trip if 100 amps is exceeded, could I not size the wire feed for the two mains for 50 amps if the two circuits are balanced? I would then size the return wiring for the full 100 amps. Anyway that is what I would do to minimize the cost if copper was the price of gold. Of if it was going to space where every pound costs $2000.00.
 
Trip ratings for breakers are not total, but rather per pole. A 100A breaker is made to carry 100A on leg A, or 100A on leg B, or 100A on both simultaneously.

This means that you need 100A rated conductors for this 100A panel.

(I am intentionally ignoring the nuance of 80% rating, trip curves, or neutral sizing, and just focusing on the fact that a 100A 2 pole breaker does _not_ mean 50A per leg.)


Jon
 
mmm_max said:
If I was not concerned about inspection or the NEC at all; and assuming a 100 amp main breaker panel and assuming that the main will trip if 100 amps is exceeded, could I not size the wire feed for the two mains for 50 amps if the two circuits are balanced?
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No.
 
mmm_max said:
If I was not concerned about inspection or the NEC at all; and assuming a 100 amp main breaker panel and assuming that the main will trip if 100 amps is exceeded, could I not size the wire feed for the two mains for 50 amps if the two circuits are balanced? ......
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This is being discussed and answered: Thread 'argument with electrician' https://forums.mikeholt.com/threads/argument-with-electrician.2569893/.
NO, your wire being undersized for amperage of protection device, will become a sort of fuse and burn out at most any place along the wire when current exceeds 50A depending on duration of overcurrent and other factors, but well before the 100Amp device might trip, and with catastrophic resulting damage, and potential personal injury. Your electrician will have sized for you an appropriate, safe, compliant wiring, if you are not satisfied with the electrician's answer get another electrician. There are way to many variables that play into sizing and adequate protection and safe installation than can be addressed via a forum like this when asked by a non electrician unfamiliar with the basics of electricity.
 
Thanks Jon. Your explanation clarified the situation well. So when I buy a 100 amp main distribution panel it is actually capable of supporting 200 amps in total including both legs. Got it!
 
mmm_max said:
Thanks Jon. Your explanation clarified the situation well. So when I buy a 100 amp main distribution panel it is actually capable of supporting 200 amps in total including both legs. Got it!
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Another way to say it is, it is capable of supporting 24,000 watts whether it is 120V or 240V or a combination of both. With that said this appears to be a DIY thread so we will close it based no the forum rules.

This site is designed for:

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* This NEC� Forum is for those in the electrical and related industries. Questions of a "How-To" nature by persons not involved in the electrical industry will be removed without notice.
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powerpete69 said:
…
That being said, I once asked this silly question way back: "If you have a 60 amp 3 pole breaker, is it 60 amps per phase or a total of 60 amps from all the phases....lol. " What a numb nuts.
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We still get that in here once in a while from the DIY crowd that we remove from view because they are not in the trades. In unmoderated forums like Reddit, it’s a very common question still.
 
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