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Discussion Starter · #1 ·
Hi everyone; it's been awhile. :) I hope everyone is having a nice New Year!

Got a service call because a main breaker was tripping instead of the branch circuit when a tenant was using a space heater. I overloaded the circuit to confirm and sure enough, the main breaker tripped, but NOT the branch circuit.
The main breaker was warm, but no burn marks/loose connections/burnt smell. I swapped it out. Fine.
I went upstairs and plugged (2) space heaters in the receptacle. After 5-10 mins, the branch circuit wouldn't trip.
So I swap the 20 Amp branch circuit breaker. Same thing; it didn't trip.
I put an amp meter on the line and sure enough, it's reading 24 Amps or so.

Has anyone seen anything like this? Is there a problem with QO breakers that anyone knows of?

It occurred to me that if the voltage was low, maybe the breaker wouldn't trip because the total WATTAGE would be lower even if the amperage was higher. What do you think?

Thanks for any thoughts.
 

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Electricity is Amazing!
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We temped some lights on a 20A GE breaker.

It was pulling just under 25A and held for 12+ hours. It never tripped while we were there; it was hot as well.
 
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It's normal. You're well within the infinite part of the trip-curve. There's no guarantee that the breaker would ever trip until you reach a current level outside that range on the curve.

In your case it looks like somewhere around 26-27A you'd leave that vertical band, but you'd still have a 5 minute delay.
 

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Discussion Starter · #6 ·
Thanks guys.
Tomorrow I'll check voltage just to be sure and ramp up the amperage to 30 to 32A by adding a little toaster. :).

Deep cover: the replacement main didn't trip. The old one had gotten warm enough that the main feeds were getting warm up to about 2" from the CB terminals.
 

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Breakers and fuses have a "Time Current Curve" There is not a single point where they trip. They may hold for hours when slightly above their rating, but will trip in a few milliseconds with a very high fault current. Residential breakers have an AIC of 10,000 amps. This is the Amperes Interrupting Capacity. These breakers can take up to .008 seconds to trip with a direct short. In this small amount of time the fault current will not reach its full value because it is only one half of a cycle (for 60 Hz. systems). A residential transformer can exceed 26,000 amps in a full cycle under a direct short. A #12 copper wire with 75 deg. C insulation can withstand 3800 amps for 1/2 cycle without sustaining any damage. That is why a 20 amp breaker has a 10,000 AIC rating. The manufacturer can supply a TC curve for any breaker they make.
 

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Greg Sparkovich said:
Thanks guys.
Tomorrow I'll check voltage just to be sure and ramp up the amperage to 30 to 32A by adding a little toaster. :).

Deep cover: the replacement main didn't trip. The old one had gotten warm enough that the main feeds were getting warm up to about 2" from the CB terminals.
Sounds like you fixed the problem. Loose connections on the sub feed wore out the main breaker. Don't sweat the rest.
 

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Its late but something doesnt sound right, If a main is tripping (which Im guessing is 100 to 200amps) under a spacer heater load either the main is seriously messed up or the panel is seriously overloaded.

The only thing that might be plausible if the branch breaker is faulty and the space heater is a dead short.

OK edit I see now, the main was replaced. As for the 24 amps the breaker may hold that for hours. You ok, dont loose sleep. BTW, Square D QO is the best money can buy.
 

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Master Of Disaster
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Its late but something doesnt sound right, If a main is tripping (which Im guessing is 100 to 200amps) under a spacer heater load either the main is seriously messed up or the panel is seriously overloaded.

The only thing that might be plausible if the branch breaker is faulty and the space heater is a dead short.

OK edit I see now, the main was replaced. As for the 24 amps the breaker may hold that for hours. You ok, dont loose sleep. BTW, Square D QO is the best money can buy.
Can you back up your statement about qo being the best with some sort of scientific or other than your own opinion proof?
 

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socalelect said:
Can you back up your statement about qo being the best with some sort of scientific or other than your own opinion proof?
I know not of any scientific lab studies, but in the field, the QO, seems to be the best. From the unmistakable "audible click" , to the orange flag, a better terminal screw, a wire landing zone like no other, and better make up when installing into panel... These breakers seem to be of higher quality, and for those reasons and more, QO line cost more at supplier, .....it's just a better product than GE-total crap, more problems than I can count it it's a cheap install, CH- runs a close second, but not as popular down here....in my own personal scientific opinion, square d QO, is the best I can sell a customer
 

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