I'm trying to troubleshoot an issue that's been happening with one of our circuits in our '87 house (home office). At least twice now, the 15 A circuit breaker has been tripped on our 200 A breaker. Here are the details and steps I've taken to troubleshoot:


  • Load sources in home office: Work desk with laptop, external monitor, overhead light fixture, 500W electric space heater on desk, laptop charger (Note: I believe the bathroom next door is also part of this circuit, and it has 1 GFCI outlet and vanity lights overhead).
  • My wife has said that her laptop/desk space heater all turn off first, followed by the overhead lights a short while later (not all at once). This usually happens after the space heater has been on for about an hour.


  • Replaced the outlets in the room (needed anyways, a lot of them couldn't hold prongs in anymore)
  • Plugged in a kill a watt to determine total system wattage draw. With the whole desk going, as well as the space heater, total known system load is around 600W.

From what I understand, because this is a 15 A circuit, we should be able to draw around 1800W of power before anything trips. My reading of 600W is well below this limit, yet it seems that after a short while the circuit still heats up enough to trip the breaker.

Any idea what could be happening? Is this an indication of a faulty fuse in the circuit breaker? Is there a phantom load somewhere that is pushing us past the 1800W "limit" (1200W though??)

Thanks in advance!

Edit: for clarity, the 15 A circuit breaker is tripping, not the entire 200A circuit breaker. The 15A circuit breaker has a TEST button in it as well.

Edit 2: pictures of the circuit breaker - number 14 is the one in question.

Breakers involved

Panel label

Edit 3: upon testing the affected circuit breaker (number 14) it immediately tripped, which seems to indicate a fault to ground issue.

  • 1
    Confirm it is tripping the 200 amp breaker or just the 15 amp breaker in the 200 amp panel. Is the 15amp breaker simple type or does it have a test button(GFCI?AFCI)? Edit this into your question.
    – crip659
    Commented Jan 18, 2022 at 19:42
  • Sorry, yes, it is tripping the 15A breaker specifically (in the 200A panel). I don't think the affected 15 A breaker has any GFCI test buttons on it.
    – Shultzy
    Commented Jan 18, 2022 at 20:08
  • 2
    Is there a "test" button on the breaker? For what is considered a "continuous load" like a space heater, code requires the load to be derated by 20% which means 1,440 watts. Is there anything else on that circuit? If you are careful and OK with working in a live electrical panel, can you put an "amp clamp" over the wire on the breaker that's tripping? Be very careful around a live panel and if you're not comfortable with getting in there, DON'T DO IT, call a pro. If you do, you'll know the current draw. Amp clamp style DMMs are about $40-$60. ....continued in next comment..... Commented Jan 18, 2022 at 20:12
  • 1
    Any chance of a photo of the breaker panel?, breaker access door open, deadfront still bolted on is fine. Certain questions arise about its health and age. I see you asked another question 3 months ago so I gather you have a registered account; that will make it easier to log in from another device e.g. camera phone. Commented Jan 18, 2022 at 21:25
  • 1
    A Square D "QO" panel will support "Time Saver Diagnostics". Can you give it a whirl and tell us what it indicates? You must be able to reliably distinguish 2 seconds from 5 seconds, so have a "second counting" device in sight. Commented Jan 19, 2022 at 3:14

2 Answers 2


As George says the load should be de rated from the max of 15 amps but the breaker should not trip for over current until it reaches 15 amps.

What concerns me the lap top and heater go off at a different time than the lights this sounds more like an arc fault issue.

since the laptop and heater go out followed by the lights I would be checking for a loose connection where the desk & heater plug in at that receptacle.

From memory the QO’s with a white button were combination or cafci breakers. This would support a loose connection as it arcs and your laptop dies the breaker senses it and kills the power.


Since this seems to be an intermittent problem, if you're comfortable working in the main panel, swap the wire for your home office to another 15 Amp breaker in the panel and see if the problem continues on that breaker or stays on the original one. Breakers, like everything else, fail. They are energized 24/7 for many years so yes, they can fail. I've replaced dozens over the last 50 years just based on this simple test.

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