Main breaker throwing very odd. Help Troubleshooting.

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whackit

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Los Angeles, CA
Hi Folks,

Both me and my electrician (Im a GC) are scratching our heads on this one. I thought I would give a try here before I have to start the expensive stuff...

Here's the situation.

325 Amp service panel (CA style main panel with just two main breakers in it) feeds 2 Sub panels in seperate buildings. Main breakers Rated at 100 amps and 200 amps at service entrance panel. These then feed (wire correctly sized, actually oversized to allow for expansion on 100 amp leg) a 125 amp sub panel in garage building which has a 100amp main breaker, and a 200 amp sub panel in main house. Everything is wired correctly on the panel level, separate grounds and neutrals at subs, no connection between buildings etc. Professionally wired by licensed electrician I have used for the last 10 years.

Here's the rub, we had a circuit in the garage (100 amp side) that has a short, not sure if it is to ground or neutral yet. When the 20amp breaker throws... It kicks the main 100 amp breaker at the service entrance. It does this every time. We cant figure out why a 20 amp short is throwing a 100amp main. It will do this if everything else on that panel is disconnected. Just to test we overloaded another circuit to see if it was specific to the shorted one and it also kicked the main. Everything seems fine between the sub panel and the main... It's only like a 35' run.

Any Ideas?
 
Amp wires on main and let me know what it trips out at. Could be faulty main

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Sounds like a bolted short. First you need to find what is shorted before resetting the 20A breaker. Sounds like you have tripped it more than once. After finding the short and testing with a meter then the 20A breaker, I would replace the 20A breaker since you reset it into a bolted fault.

As for the main tripping, the trip curves for each breaker will vary and sometimes it just a race as to which one will trip first. That being said, it does sound like the main breaker might have a problem since it happened with a different circuit.
 
whackit said:
We cant figure out why a 20 amp short is throwing a 100amp main.
Click to expand...

How do you know it is a 20A short?

Unless you have taken some extra-ordinary steps it is extremely likely that these two breakers are not selectively coordinated. I would not be surprised to find out that a fault of 400-500A would cause both devices to open.
 
When the fault current is in the instantaneous trip range differences in the details of the time curve can lead to seemingly paradoxical tripping of a large breaker before the smaller breaker.
If the fault is in the inverse time region the results should at least be predictable based on the published curves *except* for the fact that each set of curves is really a tolerance band subject to variations from one unit to other nominally identical units.

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Could be a faulty main but it could also be that your shorts are pulling 100A. Any metal-metal-contact short could easily do so. If so it's not that much of a mystery, the breakers are doing what they're supposed to do. You can't necessarily count on the higher breaker not tripping on a bolted short, especially if they happen not to be the same brand. I've seen breakers trip in series more times than I'd care to admit.

Ideas: 1) Fix the short. 2) Replace the 100A main (not that expensive) to rule out a bad breaker.
 
whackit said:
Hi Folks,

Both me and my electrician (Im a GC) are scratching our heads on this one. I thought I would give a try here before I have to start the expensive stuff...

Here's the situation.

325 Amp service panel (CA style main panel with just two main breakers in it) feeds 2 Sub panels in seperate buildings. Main breakers Rated at 100 amps and 200 amps at service entrance panel. These then feed (wire correctly sized, actually oversized to allow for expansion on 100 amp leg) a 125 amp sub panel in garage building which has a 100amp main breaker, and a 200 amp sub panel in main house. Everything is wired correctly on the panel level, separate grounds and neutrals at subs, no connection between buildings etc. Professionally wired by licensed electrician I have used for the last 10 years.

Here's the rub, we had a circuit in the garage (100 amp side) that has a short, not sure if it is to ground or neutral yet. When the 20amp breaker throws... It kicks the main 100 amp breaker at the service entrance. It does this every time. We cant figure out why a 20 amp short is throwing a 100amp main. It will do this if everything else on that panel is disconnected. Just to test we overloaded another circuit to see if it was specific to the shorted one and it also kicked the main. Everything seems fine between the sub panel and the main... It's only like a 35' run.

Any Ideas?
Click to expand...
Coordination between these two to make sure the upstream higher rated device doesn't trip on a short circuit/ground fault is not easy to do with the lower grade devices like you find in residential and light commercial. The fault current you are experiencing is high enough it is above the instantaneous trip setting of both breakers. This really is not all that uncommon.
 
I really don't want to replace the main breaker. It's a Milbank 22k that go for $400. We will trouble shoot the short. The thing I found odd was that it tripped on an overload as well... We installed a temporary 20A breaker (this is a new renovation) and loaded it up so it tripped and it tripped the main simultaneously with the circuit. This is on a thermal overload... not a bolted short.
 
whackit said:
I really don't want to replace the main breaker. It's a Milbank 22k that go for $400. We will trouble shoot the short. The thing I found odd was that it tripped on an overload as well... We installed a temporary 20A breaker (this is a new renovation) and loaded it up so it tripped and it tripped the main simultaneously with the circuit. This is on a thermal overload... not a bolted short.
Click to expand...
That seems to me to be truly unusual. If you gradually increase current as an overload) till something trips and the 200A goes first something is wrong. Did you measure the current (and time) at the moment of trip? If it is less than 200A then something is wrong with the 200A breaker without question.
If it is more than 200A, then something is almost certainly wrong with the 20A breaker. If you hold the current below 160A so that the main should not trip, then the 20 had better trip within a couple of seconds, max.

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Read what GoldDigger said again. What you are experiencing is not at all unusual. When you have a “bolted” fault, meaning almost no resistance in the short circuit, whether to ground or line to line, all bets are off as to which breaker trips first. You CAN coordinate the trip curves of the breakers and someone SHOULD have done that if you want to avoid this exact circumstance.

But the truth is, the consequences of having an uncoordinated system in a residential installation are not that dire, it’s mostly just an inconvenience, so it’s often ignored. In a commercial or industrial installation, a Main tripping on a short in a small circuit however could be financially disastrous.
 
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whackit said:
I really don't want to replace the main breaker. It's a Milbank 22k that go for $400. We will trouble shoot the short. The thing I found odd was that it tripped on an overload as well... We installed a temporary 20A breaker (this is a new renovation) and loaded it up so it tripped and it tripped the main simultaneously with the circuit. This is on a thermal overload... not a bolted short.
Click to expand...
I also agree that isn't likely to happen. If the 200 amp main tripped on a 20A+ overload then it should never hold when you have a circuit requiring 30/40/50 amp breaker and run whatever item that circuit supplies, or if all you have is 20 amp circuits at very least you would never be able to run enough at a time to exceed 20 amps.
 
GROUND FAULT TRIP?

GROUND FAULT TRIP?

Kwired mentioned it in post #7. I wonder if the main is tripping on a ground fault instead of a thermal or instantaneous thip when the 20A breaker is closed.
I cannot tell you how many times we hear stories of an EC changing a ballast (277v lighting) in a high-rise, hot, and the 20A branch panel breaker trips as well as the 4000A main 20 stories down in the basement. The instantaneous trip level on a 20A breaker can be well in the hundreds of amps and exceed the ground fault pickup of the main breaker which has a setting GF pickup range of only 100A to 1200A. High-rise designs are often cheap and dirty with no secondary level of GF protection as compared to a well designed (and expensive) found in hospitals, data centers, telcom CO's, etc. Often you see Pringle switches equipped with GF schemes for the main disconnect instead of circuit breakers because of cost.
I am describing 480V, 3-phase, 4-wire systems so I may be way off base when it comes to solving the OP's problem but the OP needs to look at the main (or send us a pic of the 100A main, panel cover off) to determine if it has GF protection on the trip unit. If not then if adjustable, what is the instantaneous dial setting (min-int-max, 1x-10x, etc) set at?
We can speculate here forever,........ we need to get more information.
 
ATSman said:
Kwired mentioned it in post #7. I wonder if the main is tripping on a ground fault instead of a thermal or instantaneous thip when the 20A breaker is closed.
I cannot tell you how many times we hear stories of an EC changing a ballast (277v lighting) in a high-rise, hot, and the 20A branch panel breaker trips as well as the 4000A main 20 stories down in the basement. The instantaneous trip level on a 20A breaker can be well in the hundreds of amps and exceed the ground fault pickup of the main breaker which has a setting GF pickup range of only 100A to 1200A. High-rise designs are often cheap and dirty with no secondary level of GF protection as compared to a well designed (and expensive) found in hospitals, data centers, telcom CO's, etc. Often you see Pringle switches equipped with GF schemes for the main disconnect instead of circuit breakers because of cost.
I am describing 480V, 3-phase, 4-wire systems so I may be way off base when it comes to solving the OP's problem but the OP needs to look at the main (or send us a pic of the 100A main, panel cover off) to determine if it has GF protection on the trip unit. If not then if adjustable, what is the instantaneous dial setting (min-int-max, 1x-10x, etc) set at?
We can speculate here forever,........ we need to get more information.
Click to expand...
His breaker in question is a main breaker at service equipment for dwelling application - not to likely to have ground fault protection incorporated into it.
 
Ask your supplier if you could swap it out.

Or, could you purchase the whole disconnect? Take the problem breaker out, switch with the new one. Then take it back,
 
Buck Parrish said:
Or, could you purchase the whole disconnect? Take the problem breaker out, switch with the new one. Then take it back,
Click to expand...
You are one of those that takes defective things back and doesn't tell them anything is wrong with it so that someone else gets stuck with it?:huh:
 
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