Subpanel breaker tripping but not individual breakers

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Old 08-17-15, 02:53 PM
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Subpanel breaker tripping but not individual breakers

Hello,

We have a 90A double pole breaker in our main box that serves a subpanel. It has tripped a few times and one side of the breaker gets hot. None of the breakers on the actual subpanel have tripped. I was able to isolate the specific circuit (15A) in the subpanel because it was a GFCI circuit that has tripped before when we've plugged in some extra appliances in our garage. I overloaded it as a test and sure enough the 90A breaker got hot but the individual breaker did not trip.

We've pulled all the extra stuff off this circuit and all is well for now but I'd like to understand the issue if it's a simple fix.

Thanks - Jeffzoomer
 
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Old 08-17-15, 03:01 PM
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How far is the sub-panel from the main panel? What gauge wire is between the two panels? What is the total amperage, in the garage?
 
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Old 08-17-15, 04:06 PM
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Best to run a new circuit to the garage and bring it up 20 amps. If the garage is detached I'd suggest a multiwire circuit.

As to what breaker trips first it just depends on which one gets hot enough to trip first. Breakers get "weaker" over time. You might need a new 90 amp breaker but I'd suggest doing a load calculation first.
 

Last edited by ray2047; 08-18-15 at 09:02 AM. Reason: Remove incorrect code information.
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Old 08-17-15, 04:44 PM
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If either of the legs of the 90 amp breaker is significantly over 90 amps it will trip even if the branch breakers do not. The 90 is seein all the power used by the panel.
 
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Old 08-17-15, 05:00 PM
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Subpanel is about 50 feet from the main panel. It was installed as part of a house addition about 15 year ago. The wiring from the main to the sub is Triple E Alloy Type SE 600 V about an inch thick. There are 5 x 2-15A, 1 x 30A double pole and 1 x 25A double pole on the subpanel. We've only ever had a trip on the subpanel with the one circuit that seems to cause a problem with the 90A on the main. Right now there are only wall outlets on the problem circuit. The only thing plugged in is a 20 yr old refrigerator that we put in the garage after getting new appliances. If I plug in a normal window fan to exhaust garage air, I can feel the 90A breaker warm up.

Thanks
 
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Old 08-17-15, 06:08 PM
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The wiring from the main to the sub is Triple E Alloy Type SE 600 V about an inch thick.
Only describes the cable jacket not the wires.
If I plug in a normal window fan to exhaust garage air, I can feel the 90A breaker warm up.
An Amprobe reading would be really helpful.
 
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Old 08-18-15, 08:46 AM
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I believe immediately I'd check the connection at the 90 amp breaker lugs and also check the tension of the breaker on the bus stabs and the condition of the stabs themselves. Like Ray said, an Amprobe reading would be helpful too.
 
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Old 08-18-15, 08:50 AM
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I agree with Joe, Check the terminal screws of the 90 amp breaker, sounds like a loose connection. Also, since you have aluminum wire, check to see they installed anti-oxidation paste on the wires.

IIRC - It is not required to be a 20 amp circuit in a garage, although it is a good idea.
 
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