I had an electrician install a dedicated 20 amp GFCI duplex outlet and 20 amp circuit breaker (both customer supplied) two years ago. The outlet is directly below the panel box. Just this month, I mistakenly operated a 12 amp vacuum cleaner, a 12.5 amp space heater (on max heat), a 1/2 amp fan (on max speed, an air purifier, and three LED lamps, at the same time, out of that outlet for a few minutes. That’s 26 amps, or 130% of the rated current.

According to Waytek’s Table of “trip time versus percentage of rated current”, it should’ve tripped somewhere between 100 seconds (one min 40 secs) and 1000 seconds (16 mins 40 secs). That’s too big a range for me. And the chart was too hard to interpret for me. In any case, it did not trip. How long should it have taken before tripping?

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    What make/model is the breaker in question? Dec 25, 2019 at 15:29
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    If you really want to know the actual draw of your variety of appliances, get a clamp ammeter (aka clamp meter). Like others, I doubt that your draw is as high as the math might suggest. Dec 25, 2019 at 15:59
  • ThreePhaseEel...It is a Square D HOMC30UC. It was installed in 2015 by another electrician.
    – user62948
    Dec 26, 2019 at 2:01
  • AD...It since occurred to me to use my “KILLAWATT” plug-in electrical meter to measure the amperage of my devices, and then add them up. You’re right, the amperage ratings on the appliance spec plates are overrated.
    – user62948
    Dec 26, 2019 at 4:19

5 Answers 5


Infinite time.

Your 12 amp vacuum cleaner is probably labeled according to the maximum current it can draw (to be sure, look at its electrical specifications rather than the big marketing label some manufacturers put on the front to make the vacuum sound powerful). It is unlikely that it was actually drawing 12 amps at that particular time. If it was drawing say 8 amps, maybe the total load was closer to 22-23 amps.

If the total load was about 10-15% over the circuit breaker rating, it is expected that it would not trip. Circuit breakers are rated for the current they will definitely allow for an unlimited period, and there is some tolerance on top of that depending on the ambient temperature (the hotter the breaker is, the lower the tolerance above labeled current will be). We can assume the ambient temperature was less than 20C since you were using the space heater, and this gives roughly a 5% "bonus" to the current before the breaker trips.

If for some reason you want to make sure 22 amps cannot be drawn on that circuit, you should probably install a 15 amp circuit breaker instead. But since this is recent work done by an electrician and the outlet is near the breaker, it's unlikely a cause for concern. But do make sure the power strip plugged into the outlet is rated for 20 amps as well (or that you are using two separate strips with their own 15 amp breakers).

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    Space heaters even on max will cycle on and off, mine does anyway.
    – JACK
    Dec 25, 2019 at 14:39
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    For the vacuum specifically, often they use DC motors, which draw the most current when not moving. 12 amps may be the draw at stall speed.
    – MooseBoys
    Dec 25, 2019 at 17:34
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    Most vacuum cleaners also use more amps if the garbage compartment is getting more full
    – Ferrybig
    Dec 25, 2019 at 21:59
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    "the hotter the breaker is, the lower the tolerance above labeled current will be" Interesting. Citation? (Not doubting, just interested in specifics like rate of fall-off.)
    – bishop
    Dec 26, 2019 at 1:48
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    @bishop one of the trip mechanisms in a modern breaker is a bimetalic strip that heats up as current passes though it, this causes it to bend. Once it bends enough it will hit a release trigger that will trip the breaker. The other mechanism is a solenoid that will hit the same trigger when a high current passes through. Dec 28, 2019 at 3:09

I find it interesting that you looked up the breaker rating on Waytek’s Table but did not use an amp meter to verify your true amperage usage on the circuit.

Many appliance makers advertise the amperage or horsepower of a device because they believe it attracts buyers. Yet we have constantly found that their advertising doesn't hold up.

Try to simulate the same conditions on that circuit with an amprobe (amp meter) attached. It would be better if it is the type that records minimums, maximums, and average. Then we have an accurate measurement of what is truly happening on the circuit and we can then compare it to the trip charts.

Good Luck and Happy Holidays

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    Although it's probably not an issue here, it's possible that the total current consumed by different types of reactive and inductive loads may be less than the sum of the individual currents because reactive loads push power back into the mains during part of each cycle; if other loads consume power at those times, some of their power may be supplied by the reactive loads. In an extreme case, if one had an almost purely capacitive load in parallel with a purely inductive one, the total current could be less than the individual currents.
    – supercat
    Dec 25, 2019 at 16:48
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    Retired Master Electrician…It has since occurred to me to measure the amps of my appliances using my “KILLAWATT” plug-in electrical meter. Part of what must be factored in is that my air purifier uses 1/2 amp less when not in “turbo mode” (which it wasn’t). Those amp ratings on appliances assume the device is run at max power. I plugged the appliances into the KILLAWATT, and then added them up. The total amps was only about 21 amps. It appears that the rated currents listed on the device spec plates (not the ad hype) are all STILL overrated. And I did not run them 16 mins. Mystery solved!
    – user62948
    Dec 26, 2019 at 3:53

15 amperes or 20 amperes on a typical circuit is not a magic critical number. The wires (14 AWG or 12 AWG) can actually carry far more current. The concern is heat. The electrical code is based on an understanding of how much heat, for how long, actually poses a problem. While you should not push the limits, an occasional overage is built in which is why the breakers are designed with these trip curves. A short circuit is dangerous and results in an immediate trip. And a ground fault can be extremely dangerous, so GFCIs are designed to trip super fast.

All that being said, there have been some poorly designed /fraudulently approved circuit breakers that don't trip when they should. If you have Federal Pacific or Zinsco, google for more details.

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    Manassehkatz-Reinstate Monica...My circuit breaker box was a Square D that I had installed in 2015. I gave the exact model number to another poster who asked. The circuit breaker in question was also a Square D. I believe that your explanation, plus my having found out that the amperage wasn’t that high in the first place after I measured them with my KILLAWATT plug-in electrical meter (as I explained to RME) best help to explain the mystery.
    – user62948
    Dec 26, 2019 at 4:03
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    M-RM...Once again I accidentally hit “send”...(This is part two)…I’m aware re: FPE and Zinsco panel boxes. In fact, my present panel box replaced an FPE box in 2015. The problem is, almost all of the apartments in my three-story building still have their original FPE boxes that were installed in 1977. I told my nearby neighbors about the danger. But apathy rules! Is it possible to edit or delete these comment posts.
    – user62948
    Dec 26, 2019 at 4:10
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    @MJCallinall For some unknown-to-me reason, you can only edit your Comments for 5 minutes. But you can always Delete your comments. FYI, sometimes comments will magically disappear - that happens when a Moderator decides to either move a batch to Chat (which most people don't use) or deletes them as either obsolete (e.g., incorporated into Question or Answers in some way) or as not focused on the Q&A (e.g., as this comment once you have read it). Dec 26, 2019 at 4:13
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    @MJCallinall Aha! Another one of the mysterious design decisions of the SE system. If you mouseover your own comments, a red "Delete" should show next to the timestamp ("11 mins ago", etc.). Click on "Delete". Dec 26, 2019 at 4:25

Some good points already:

But significant numbers of breakers in recent production are failing to meet the standard trip on 130% of rated current. The Federal stablok and their chinese knockoffs are the worst.


enter image description here

  • ITE and Murray, yikes! Both are now part of Siemens. Dec 26, 2019 at 20:28
  • @Harper-ReinstateMonica ITE doesn't look so bad, but Murray looks like the center is way above the max. must trip! Dec 26, 2019 at 20:37

If I am correct, Waytek supplies automotive breakers.

Mains breakers are quite different. They have a magnetic (instant) trip for extreme overloads. And a thermal trip for lesser overloads. The thermal trip mode is to protect the wires from melting.

The thermal trip must trip (eventually) at 135% of rating, in order to get a UL listing.

Also, thermal trip is influenced by panel temperature.

However I suspect your actual load was somewhat shy of 26A. You are stating nameplate ratings, actual performance may be less.

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