PSA: The Mystery Tripping Breaker

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H-M Supporter - Gold Member
H-M Supporter - Gold Member ($25)
Feb 1, 2015
I recently had the main circuit breaker trip on my 100 amp sub panel. None of the individual breakers had tripped so I reset the breaker and we continued preparing for a houseful of party guests that we were expecting in a few hours. The breaker tripped twice more that afternoon and I figured that it was either the perfect storm where the central A/C, the well, and the electric hot water heater were plus a few other appliances chose to come on at the same time or I had a failing breaker.

The next day, I pulled the cover on the panel and put my clamp-on ammeter on the feed wire. The central A/C was running at 20 amps and when the well kicked in the load increased to around 30 amps. The highest current that I saw was 40 amps. Same on the other leg. I left the meter in place to monitor current and turned my attention to other matters with the intention to pick up a new breaker on the next trip into town.

A day later, the breaker tripped again, The current was around 40 amps again and I think I solved the mystery of the tripping breaker. The breaker was quite warm to the touch, as was the feed wire. Then I remembered about aluminum cold flow. Sure enough, the clamping screw were not tight. I tightened them and the breaker temperature is now close ambient and it hasn't tripped again.

In addition to the magnetic portion of the breaker tripping from a dead short condition, there is also thermal breaker. The heat generated by by the load current will trip the breaker with time inversely proportional to the average current draw as shown below. Adding heat to the breaker by virtue of a high resistance connection will shift the curves to the left, causing the breaker to trip when in normal operating range.

I recall some thirty or forty years ago, there was a push to switch to aluminum wire for residential use because of lower cost. It was short lived due to a number of electrical fires caused by cold flow. Aluminum wiring was subsequently disallowed by code, at least in Wisconsin, and most electrical components carry a "not for aluminum wire" warning. However, it still remains used for feed wire.

The sub panel was rewired about 17 years ago. When the wiring was done, the wires were liberally coated with antioxidant paste and tightened securely at the time. Given that the connections were loose in this panel, I will be checking the other three panels on the property where aluminum wire feeds in.


H-M Supporter - Silver Member
H-M Supporter - Silver Member ($10)
Jun 12, 2017
Modern aluminum wire is a much better product, but even then I only use it for main feeds. The current stuff doesn't even need the paste, though it's probably a good idea. The most important thing is to use a torque wrench for the connections. The spec is usually on the side of the breakers or a sticker in the panel. Improperly torqued connections can cause this with copper as well. I actually had a breaker connection arcing in the panel due to a slightly loose connection. It had been going on a while as it had degraded the insulation. I had to pigtail it and replaced the breaker to be sure. I figure it was $10 well spent.

Dave Paine

May 10, 2014
Thanks for posting.

My house was built by the original owner in 1969. The biggest wire is for the electrical stove. This is aluminium.

Good to know about the thermal breaking in addition to amperage. I will have to check the screws on the breaker for the stove.


H-M Supporter - Gold Member
H-M Supporter - Gold Member ($25)
May 15, 2014
in the late 70s.... i worked as an ic tech for a 200+ employee manufacturing plant... once a year during a long holiday weekend we would assist the electricians; opening up breaker panels, lighting panels, supply panels etc. We would tighten each and every compression connection as well as look for overheat issues, testing breakers, megging cables. Even copper connections will loosen over time with temp cycles and vibration .. I would recommend doing it on any home / shop panels at least every few years or so..



Active User
H-M Supporter - Gold Member ($25)
Nov 23, 2013
There are thermal cameras available for phones that can be used to image the panels and connections, among other household uses. Under $200 on Amazon. A little expensive unless you have friends that might want to chip in.
I spent weeks at several factories and hospitals over the years tightening connections that were found by thermal imaging. Even 20 amp breakers would light-up bright white from loose wires.
I remember seeing ads in trade magazines for receptacle cover plate screws that you could install to monitor the temperature of receptacles that were connected to aluminum non-metallic cable. They would turn black if the receptacle was overheating due to a loose connection.


Ned Ludd's bro
H-M Supporter - Gold Member ($25)
Feb 9, 2017
PSA indeed. Wonderful info from everyone.

We went from a house built by college students in the 60's to a prefab house also built in the 60's. It was Dejavu because I had a breaker pop all the time in the first house in the garage/shop. Went through hell trying to find a replacement breaker until I remembered a electrical co we used at the places I worked and called them. They had the oddball breaker. When we moved here it started up again but this time I used the net. Looking back I'll bet I probably had the same panel before. The panel was made by Zinsco and a Google search 'bout scared me to death! The incidents of panel fires with Zinsco was crazy and a failure rate of the breakers of around 80%. Not to mention their breakers are like 10x more expensive than others. A good friend is an electrician and checked with him and he said to go through and check all the connections and start saving up to have the panel replaced. We had decided to go solar and the installers upgraded the panel at no extra cost over the original estimate. Win win all around and no more problems.
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