Main 100A Breaker keeps tripping


Old 08-27-05, 11:45 AM
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Question Main 100A Breaker keeps tripping

Got a stumper here. I have 100A service, and the main service breaker is a 100A. My A/C breaker is a 50A (correct rating for my A/C unit). Never had any problems until the recent Jul-Aug heat wave (A/C unit is new - installed in last 4 mos.) here in So Cal. During the mid-morning to late afternoon, with nothing else in the house on (at all!) except the A/C, the 100A main trips. Not the A/C breaker. The main trips. Now, the paperwork (sticker) on the side of our new Rheem unit says it requires a minimum 45A, and a maximum 60A breaker, but it never trips? Just the main. And to worsen matters, the times it seems to trip repeatedly, is when the hot sun is beating down on our service panel. Now - I don't know if the breakers we have are thermal or magnetic, or if the breakers are affected by that type of heat, but the 50A for the A/C, is located right next to the 100A main in the service box, and boy do they get hot! I put a thermometer in there just to see, and it was averaging 140-150F degrees just prior to tripping. If I turn my thermostat up so the A/C doesn't come on, or stay on (to about 80 degree F), then everything seems ok. If I try and run the dryer (different breaker) or the dishwaher (again - different breaker) at the same time as the A/C, in mid day - trips within a few minutes (not right away), and seems to be heat related (in the panel). Ran all these appliances (and then some!) in the winter/spring wthout issue. We just bought the house in December (the house is eight years old), so I can't give much more on the electrical history than that. Any ideas? I thought maybe to add a subpanel in the garage (right behind the outside panel), out of the sun, and break out the 50A service for the A/C, and locate it in the subpanel, but to be honest, since the main is tripping and not the 50A, I wasn't sure that was the right way to go. Do these breakers have a service life that is affecting it's ability to handle the heat? And if so, what's the best way to go when replacing the 100A main? The only thing between that breaker and the main power wires coming into the service panel (that I can see) is the meter. Thanks for any advice.
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Old 08-27-05, 02:08 PM
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nothing else in the house on (at all!) except the A/C
Is your refrigerator unplugged? Is everything unplugged? There are a lot of loads in a house that turn themselves on without you having to do so. This may or may not be contributing to your problem. I'd try turning off all the breakers except for your A/C and see if it still trips the main. That way you'd know that nothing else is really on.

the times it seems to trip repeatedly, is when the hot sun is beating down on our service panel
Modern breakers are both thermal and magnetic. The thermal mechanism handles overloads, and the magnetic mechanism handles shorts. The thermal forces on a breaker are the sum of the heat generated by the electrical current, plus the heat generated by ambient factors. The sun may very well be causing the breaker to trip. It is not uncommon to erect some sort of screen to shade the panel. Try it.

It is also possible (likely even) that nobody did a demand load calculation when your A/C was added. Your service may indeed be overloaded. Many A/C companies, afraid of losing your business, don't even check for this.
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