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Author Topic: This will trip a properly wired breaker, right?  (Read 6689 times)

Jeffery Foster

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This will trip a properly wired breaker, right?
« on: September 01, 2014, 05:03:43 pm »

I believe I know the answer to this, but I just want to see it in writing from people smarter than me so that I'll quit second-guessing.

A NEMA 14-50 receptacle wired to a dual-pole 50 amp breaker. A distro connected to this that tried to pull 30 amps per leg simultaneously will indeed trip the OCPD, correct?

Thank you in advance for your knowledge!
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Ray Aberle

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Re: This will trip a properly wired breaker, right?
« Reply #1 on: September 01, 2014, 05:13:10 pm »

I believe I know the answer to this, but I just want to see it in writing from people smarter than me so that I'll quit second-guessing.

A NEMA 14-50 receptacle wired to a dual-pole 50 amp breaker. A distro connected to this that tried to pull 30 amps per leg simultaneously will indeed trip the OCPD, correct?

Thank you in advance for your knowledge!

No. The 50A breaker upstream is protected to 50A on both legs. The distro downstream that is asking for 30A/leg will not trip the 50A/leg breaker. It will have 20A/lef still available.

-Ray
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Jeffery Foster

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Re: This will trip a properly wired breaker, right?
« Reply #2 on: September 01, 2014, 09:30:51 pm »

No. The 50A breaker upstream is protected to 50A on both legs. The distro downstream that is asking for 30A/leg will not trip the 50A/leg breaker. It will have 20A/lef still available.

-Ray

Ray, I've always admired your input to the boards. Thanks for the info. You seem very confident in the correct answer, does some else want to disagree?
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TJ (Tom) Cornish

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Re: This will trip a properly wired breaker, right?
« Reply #3 on: September 01, 2014, 09:52:47 pm »

Ray, I've always admired your input to the boards. Thanks for the info. You seem very confident in the correct answer, does some else want to disagree?
??  A two pole breaker rated at 50 amps can handle a two pole load of 50 amps.  You're suggesting a two pole 30A load.  Why would that trip a 50A breaker?
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Rob Spence

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Re: This will trip a properly wired breaker, right?
« Reply #4 on: September 01, 2014, 09:54:23 pm »

Ray, I've always admired your input to the boards. Thanks for the info. You seem very confident in the correct answer, does some else want to disagree?

Why would 30a trip a 50a breaker?

Do you know something we don't?



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Jamin Lynch

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Re: This will trip a properly wired breaker, right?
« Reply #5 on: September 01, 2014, 10:08:16 pm »

I believe I know the answer to this, but I just want to see it in writing from people smarter than me so that I'll quit second-guessing.

A NEMA 14-50 receptacle wired to a dual-pole 50 amp breaker. A distro connected to this that tried to pull 30 amps per leg simultaneously will indeed trip the OCPD, correct?

Thank you in advance for your knowledge!

Are you maybe thinking that 30amps per leg, 60 amps total, would trip a dual 50amp breaker?
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Ray Aberle

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Re: This will trip a properly wired breaker, right?
« Reply #6 on: September 01, 2014, 10:16:56 pm »

Are you maybe thinking that 30amps per leg, 60 amps total, would trip a dual 50amp breaker?
I suspect that's what he was worried about. (Says the guy who wrote "20A/lef available," whateverthehell a "lef" is. Haha.)

-Ray
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Kelcema Audio
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Jeffery Foster

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Re: This will trip a properly wired breaker, right?
« Reply #7 on: September 01, 2014, 10:29:28 pm »

Are you maybe thinking that 30amps per leg, 60 amps total, would trip a dual 50amp breaker?

Indeed everyone, I was under the incorrect impression that a 50a dual pole breaker would trip if the *total* load presented to it (in my example, 60amps) exceeded its rating. But now I understand that it is rated per 120v leg of that system.  So I could pull 40amps on one branch and 40 on the other branch and never hear the woeful *snap* of the breaker slamming open.

Thanks again for all of the excellent advice here!
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Ray Aberle

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Re: This will trip a properly wired breaker, right?
« Reply #8 on: September 01, 2014, 10:35:19 pm »

Indeed everyone, I was under the incorrect impression that a 50a dual pole breaker would trip if the *total* load presented to it (in my example, 60amps) exceeded its rating. But now I understand that it is rated per 120v leg of that system.  So I could pull 40amps on one branch and 40 on the other branch and never hear the woeful *snap* of the breaker slamming open.

Thanks again for all of the excellent advice here!
That is correct! :) Each pole is on on 'leg' of the service, and it's rated for 50A on each side. If you moved up, to 3 service, then you get a 3-pole breaker such as this-- in this example, you have 50A available on each of three legs (so 150A total). However, if either of the legs exceeds 50A, the whole service trips. And that's the case with your 1/220V service-- if you drew 60A on one leg and 10A on the other, even though the total service is under 100A, since one side exceeded the breaker's capacity, it trips everything.

Hence the reason that balancing your load is very important...

-Ray
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Mike Sokol

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Re: This will trip a properly wired breaker, right?
« Reply #9 on: September 01, 2014, 10:59:22 pm »

Ray, I've always admired your input to the boards. Thanks for the info. You seem very confident in the correct answer, does some else want to disagree?

Ray is 100% correct. There are no electrical connections between the legs of a multi-pole breaker. There's only a mechanical connection in the handle so that if any single pole trips, then it mechanically trips the spring loaded latches and opens up the other pole(s). The reason for requiring all poles to trip is that a 240-volt load with only a single tripped pole would back-feed current through the now non-operating 240-volt appliance through the tripped phase. That could be very dangerous for something like a sub-panel which is fed by a double-pole breaker but then breaks out into both 240-volt and 120-volt appliances.

Also, a 3-phase motor with one leg opened up won't be able to spin up properly and will probably just sit there and overheat. Plus a non spinning motor with voltage on two the legs could be a shock hazard to anyone replacing it. Yeah, they should use a meter, but you know that's not done a lot in places like farm environments.

But back to your original question. Yup, 30+30 amps won't trip a double-pole 50-amp breaker. Nor will 40+40 or even 50+50 amps trip it. However, 60+10 amps will trip the one 50 amp pole, which will then mechanically trip the other pole even though it's only passing 10 amps on that leg. 
« Last Edit: September 02, 2014, 07:45:45 am by Mike Sokol »
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Re: This will trip a properly wired breaker, right?
« Reply #9 on: September 01, 2014, 10:59:22 pm »


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