Sub panel main tripping?

Specific Inspection Topics Electrical Inspections
1

Hello Everyone… I was watching football at my neighbors today when the p ower went out in his basement. I asked him what happened and he showed me the breaker that had tripped twice in the last few days. It was the 100 amp sub breaker in the main panel. He has a 150amp service entering the home and a almost full sub-panel in the basement. If something was overloaded on the sub wouldn’t it have tripped in the sub?? not at the sub disconnect in the main panel?? It had me scratching my head! Faulty breaker? What could be causing this situation???

Regards,
Chris

2

Loose connection at main sub panel disconnect? could be the connection on the breaker itself or at the wire lugs where they connect to breaker???

3

Chris,

Question - Did the sub-panel ( Remote Distribution Center ) have a Sub-Main Breaker or a Main Lug Only panel?

Your question did not state if the "sub" had it's own breaker or not.....so if it did not and their was an overload on the entire panel and possibly causing issues with the feeders to that panel...it may trip the breaker in the Main Distribution Panel......it's possible.

Here is what I would do......

1.) Check the connections to that 100A breaker in the main panel, ensure they are properly tightened and all connections are sound.
2.) View any of those same conductors for overheating…just to be safe.
3.) Does your friend have a Amp-Probe Meter…he could check the amp draw on the feeders to this panel…and see what he had…

Also could simply be a breaker going bad…what is the age of the setup?

It is important to note that knowing exactly if something is overloaded in a panel takes a calculation…however, doing this 20 years all I would have to do it see what is connected in BOTH panels and give you a good guess.

Now…what is important is…does the “sub” have it’s own main breaker for that panel…or is it a MLO panel?

Check all connections…check for discolored connections, terminals or discolored insualtion at the termination points as well.

4

Thanks for all the info Paul…! I was hoping you would see my post… The Sub-does have it’s own main breaker in the main panel and this is the one that is tripping. All other breakers in the Sub are 15-20amp breakers.He doesn’t have a amp probe and the age of the home is 10yrs. I’ll try to take a look at the panel today and let you know what I see.

Thanks,
Chris

5

If you did not notice the lights dimming prior to the 100A breaker tripping then it is just a bad connection or bad breaker.

A warm or hot 100 Amp breaker can be caused by a bad connection or bad breaker also even with just 5-10 amps of load.

6

Chris,

If the MAIN disconnection for this “SUB” panel is in the MAIN panel and no disconnection in the “sub” panel itself other than the individual breakers it is quite possible that alone they do not present a issue that overloads the individual breakers…BUT combined may be overloading the feeders to that panel itself.

Now I am NOT saying this is the case…but it is possible…as well as being a bad breaker as well as a bad connection as all of these issues should be looked at…but understand that many people add “sub” panels without actually doing a load calculation and the only line of defense for the feeders to this panel is the breaker in the main panel…

I would suggest the following: Cheapest methods FIRST

1.) Check the connections in the main panel to the 100A breaker
2.) Check the connections in the “sub” panel to the MLO terminals
3.) Replace the 100A breaker and see what happens
4.) Have a electrician do a load calculation…which I will be happy to do for your friend provided they get me ALL the information on the panel…total loads, wattage and so on run from the “sub” panel.

7

Thanks again Paul…I’ll let you know how I make out!!

Chris