Tripping 100 amp Breaker

[edselectric]

I have a 100 amp subpanel @ about 150 ft away from the main breaker. The main trips rather than the breakers in the subpanel. The subpanel feeds a number of outdoor lighting circuits as well as pool equipment. Wires origianly run in metal conduit. (Around 1960 to '65 installation occured) I know this is a problem, I am changing out wire as I find them. Moisture in conduit also a problem. I realize the best way to correct this is to run new PVC pipe, but not an option. My question is: Why does the main trip rather than the subpanel circuits? What would be the proper course of action to troubleshoot this problem? Any help would be very appericated.

 

[charlie]

Re: Tripping 100 amp Breaker

What is the total load on the panelboard?

 

[petersonra]

Re: Tripping 100 amp Breaker

The type of conduit used will not affect whether a breaker trips or not.

I can come up with several things that might cause the main to trip rather than a branch CB>

1. The load on each branch is within the limits of that breaker but the total load exceeds the rating of the main. Measure the current at the main and see what it is.

2. The main CB is defective. A replacement ought to cost no more than $20.

3. You have an intermittant ground fault on the wires coming from the main out to the subpanel. You could test for this with a megger.

 

[charlie tuna]

Re: Tripping 100 amp Breaker

if the total normal load isn't close to 80 per cent of the main breaker's rating buy or rent a data recorder and monitor the current at the main breaker --- or just keep guessing.

 

[don_resqcapt19]

Re: Tripping 100 amp Breaker

If the faults are short circuits, then there is no way of knowing which breaker, branch or main, will trip. The trip curves for short circuit conditions are overlapping on most molded case breakers 225 amps or less.
Don

 

[petersonra]

Re: Tripping 100 amp Breaker

Originally posted by don_resqcapt19:
If the faults are short circuits, then there is no way of knowing which breaker, branch or main, will trip. The trip curves for short circuit conditions are overlapping on most molded case breakers 225 amps or less.
Don

I wonder if that is true with 150 feet of wire going out to the subpanel?

 

[charlie b]

Re: Tripping 100 amp Breaker

Originally posted by don_resqcapt19: If the faults are short circuits, then there is no way of knowing which breaker, branch or main, will trip.

Quite true. It?s like that ?Toss Up Round? on the ?Wheel of Fortune? show. They show a blank puzzle, and start filling in the letters one by one. This goes on until someone recognizes the answer and hits their buzzer. But it is not a matter of who first recognizes the answer (i.e., which breaker first recognizes the fault), but is rather a matter who is the first player to push the button (i.e., the first breaker to actuate its trip mechanism).

 

[Ed MacLaren]

Re: Tripping 100 amp Breaker

Relatively simple co-ordination is one of the main advantages of fuses over circuit breakers.

Ed

 

[stud696981]

Re: Tripping 100 amp Breaker

One more possibility is to check your connections on the main breaker. I have seen before where a loose connection heats up and causes the breaker to trip. This is even more common when AL wire is used.

 

[rbalex]

Re: Tripping 100 amp Breaker

Molded Case Circuit Breaker performance is becoming enough of an issue that IEEE is in the process of developing its own Standard in addition to those of NEMA and UL?s safety standards. They have done similarly for motors and cable flame tests. The issue is reliability as well as safety.

Both Charlies (59 & 125) noted that you need to look at total actual load.

Don, Charlie (29), and Ed noted the issue coordination. (faults)

Stud696981also noted the terminations.

All of these are legitimate things to review.

If the issue is really faults, I?m surprised that none of the branch circuits breakers tripped at least occasionally. However, if they are low-grade faults you have two problems. One is the coordination problem. Even at 150?, both the main and branch circuit see the same fault-currents and, as pointed out, either one or both could trip. Second, if the fault were low enough that the thermal unit of the main also attempted to interrupt, a little damage could have been done to it (the thermal unit.) This makes it more prone to respond in subsequent low-grade faults because the internal mechanical restraint tab on the thermal unit gets worn a bit under this interrupting condition. The condition then escalates until only the main will respond.

 

[pierre]

Re: Tripping 100 amp Breaker

Great responses to this situation!!
Strictly based on the post and my personal experience, I will present my 2cent

Based on the fact he said the installation is almost 40 years old, I am going to make an ASSumption. That circuits have been added and the load may have increased to the point that the main opens.
But all of the others ideas are very good and should be investigated.

Pierre

 

[edselectric]

Re: Tripping 100 amp Breaker

Thanks for all of the great advice. I will investigate further and report what I find. Thanks again, you guys great!