My small office has a 20A breaker and we have to run a space heater in the winter or else we freeze to death. Whenever someone turns on the coffee maker, the office circuit breaker trips and everyone's computers are down for 10 minutes while someone goes and resets the breaker. We've tried implementing a policy of shutting off the space heater before starting the coffee maker, but it's too easy for people to forget.

I plugged both devices into a power strip rated at 15 amps, and I assumed since it had a reset switch, it would actually trip at 15A. Much to my surprise, the office breaker still tripped. I also tried a different 15A power strip with the same results.

I then plugged the power strip into a Kill-A-Watt meter and it measured over 22A with the coffee maker and space heater both running, before I unplugged everything. My best guess is that both the office breaker and the power strip have time-delay breakers, but the office breaker is on a shorter delay. Another theory is that the power strips are manufactured to very loose tolerances and will only trip if the current far exceeds 15A.

So my questions are:

  1. Why isn't the 15A power strip tripping? Should it even have a time delay?
  2. If we just need to buy a better power strip, is there an easy (and safe) way to test at what current a power strip will trip, aside from plugging a bunch of stuff in and potentially tripping the office breaker instead? (Edit: I think I found the answer to this one.)
  3. If all else fails, what can I install between the wall and the appliances which will trip the space heater and coffee maker before the office breaker trips?
  • 4
    Are you sure the power strip has overload protection?
    – Tester101
    Commented Dec 15, 2011 at 21:11
  • Good question...how do I tell for certain? I had assumed since the switch was labeled Reset/Off, that implied it had overload protection.
    – rob
    Commented Dec 15, 2011 at 21:20
  • 2
    If it's not tripping at 22 Amps, I'm going to say it doesn't. The packaging my say 15Amp, but that only means it's rated to be connected to a 120v 15Amp circuit. If it has an internal breaker, it should clearly state that on the packaging.
    – Tester101
    Commented Dec 15, 2011 at 21:31
  • 2
    The rating may be for how much the power strip handles before it melts itself. The trip on many power strips is surge protection for when you get a voltage spike (e.g. near by lighting storm). Sounds like you need a better power strip with overload protection like Tester mentions.
    – BMitch
    Commented Dec 15, 2011 at 21:47
  • Thanks for the suggestion...I looked up the model of one of the power strips and it does claim to have a 15A circuit breaker. cyberpowersystems.com/products/surge-protectors/home-surge/… Hmm...I think they're lying.
    – rob
    Commented Dec 15, 2011 at 21:52

4 Answers 4


Put your computers on a UPS, even a small one. Just protecting yourself from short power outages will save you a bunch of downtime. Even if you resolve your overload issue, this is still worth it.

Most computer users today can get by with a laptop that's under $500, giving you built-in battery backup & portability in a compact, low-power package. Plug it in to your keyboard, mouse, and monitor the same as your desktop today, so your work experience doesn't change.

The power strip you linked to (http://www.cyberpowersystems.com/products/surge-protectors/home-surge/6050S.html?selectedTabId=specifications&imageI=#tab-box) doesn't appear to have a breaker. I think that whoever wrote that was just confused about what it means to be rated for 15A.

If your coffee maker and space heater are in the same location, you could plug them into mutually exclusive switched outlets. You'll need:

  • A steel square box, and an appropriate face plate

  • A regular duplex receptacle. 15A or 20A can work

  • a 3-way switch

  • cable

  • plug (15A or 20A, to match the recep)

  • fittings

Snap off the tab on the hot side of the recep, then run short leads from those 2 screws to the 3-way switch. This will let the switch choose one socket or the other. Plug in the heater on one and the coffee maker on the other. Now it's easy to make sure only one is in use at a time.

(Later I will add some pictures and other details. If anyone has pointers to the correct fittings and cable, please comment.)

  • 9
    While I was writing this, I got interrupted because my wife turned on the electric tea kettle while the electric space heater was on, and it flipped the 20A breaker. Heh.
    – Jay Bazuzi
    Commented Dec 16, 2011 at 0:07
  • +1 for the UPS suggestion; it's simple. However, I believe that a "three way" switch is DPST, while switching between outlets requires SPDT.
    – kdgregory
    Commented Dec 16, 2011 at 0:09
  • After checking Wikipedia, it appears that 3-way switches are indeed SPDT. I'll delete my answer.
    – kdgregory
    Commented Dec 16, 2011 at 0:11
  • Thanks for the detailed info...I had been trying to think of something controlled by a relay but I think this would also do the trick. Now that you mention it, I think I misread the specs of my power strip. So when they list "Circuit Breaker: 15 Amp" under the "Input" section, are they actually just restating that the power strip is only rated for an input voltage of 15 amps, not that it internally has a 15A circuit breaker?
    – rob
    Commented Dec 16, 2011 at 19:34
  • 1
    Also, I guess I forgot to mention, we do have UPSes on all the computers but on a couple occasions it took more than half an hour to find someone with a key to the breaker room, and the UPSes automatically hibernated the computers because their batteries were almost depleted. I like the 3-way-switch idea because it saves us the hassle of resetting the breaker and because it will be much more economical than buying higher-capacity UPSes.
    – rob
    Commented Dec 16, 2011 at 19:40

Let me try and explain the breakers using this hypothetical scenario.

Let's say you have a 20A circuit which has 3 computers on it, totaling 12A.

Let's say you have a power strip with a coffee pot and a heater, totaling 10A.

Now plug in the power strip to the 20A circuit. Your 20A circuit now is overloaded with 22A, while your power strip is not overloaded as it is rated for 15A. Your 20A wall breaker should trip but the power strip breaker will not.

Breakers are thermal devices. If you're only slightly over the breaker rating, it could take several minutes before it warms up enough to trip.

It sounds to me like you need a power strip with an A-B Selector. I can't find one on the net, but you could toss something together with a SPDT (three-way) power switch and a receptacle.

  • Good call on not everything on the circuit being plugged into the power strip.
    – Tester101
    Commented Dec 16, 2011 at 11:49
  • To clarify, the power strip was plugged into a Kill-A-Watt meter which read more than 22A with the coffee maker and space heater running. Usually the building breaker trips within seconds of both devices being turned on, but if the power strip's 15A breaker really does take longer than the 20A breaker to trip, that seems a little scary to me. Thanks for the 3-way switch suggestion.
    – rob
    Commented Dec 16, 2011 at 19:13
  • 1
    Because the computers and all the other office load is also there. Say you have 8A of computers etc. You run a 10A coffeemaker, you are at 18A, alright. You run a 12A heater, and you are at 20A and you are at breaker limit with the thing slowly warming You run both, you are at 8+10+12=30A, and you are on the 150% line of the thermal trip curve, except the breaker is already preheated, so it trips much sooner. The wire in the walls is also preheated, which is why it's good it tripped! Resetting it immediately is the WRONG thing to do, as the wire in the walls haven't cooled. Commented Nov 5, 2018 at 23:48

Proper solution: run another circuit to this location for the space heater and/or coffee maker.

Lazy but error prone solution: get a power strip with overload protection included.

If all else fails, there's the low-tech solution: cover all the outlets except one, and tell everyone to unplug one device to plug the other in.

  • We're checking with the building manager how much it would cost to add a circuit, but I'm guessing it will be at least a few hundred bucks. I thought our power strip did have overload protection, but now I'm thinking I misread the manufacturer's spec sheet.
    – rob
    Commented Dec 16, 2011 at 19:42

The "high"-tech solution: get a single-pole, double-throw switch where the two switched hot wires are connected to two different outlets. Plug the coffee maker into one outlet and the space heater into the other. Only one outlet will work at a time courtesy of the switch.

Fancier yet: attach a normally open relay to the circuit so that when the coffee pot is turned on, the heater is turned off.

Fanciest: a low-voltage amperage-sensing control circuit that controls the heater based upon the coffee maker's usage. That will allow your coffee maker to to keep running if it has something fancy like a clock -- or maybe your relay won't be that sensitive and will work directly.

  • 1
    In other words, a 2 way light switch. You can even connect the two hots from this switch to the top and bottom of the same outlet and break off the tab that joins them in the middle. Probably out of scope for the original question, but it's the DIY solution that I was just about to add to my own answer. +1
    – BMitch
    Commented Dec 16, 2011 at 17:29
  • I really like the relay and control circuit ideas, but might just go with the switch that seems to be the popular suggestion here, for the sake of simplicity. Is the control circuit something I can just pick up at Lowe's or wire together myself for a few bucks more than the switch or relay solutions? Our space heater has electronic controls and won't turn back on automatically after being disconnected, so it would be nice to just limit the current it gets when the coffee maker starts heating up.
    – rob
    Commented Dec 16, 2011 at 19:49
  • You can't just "limit current." Especially if it needs electronics to stay on. Some advanced heaters will have separate always-on and switched power inputs, but that's not your case.
    – Zdenek
    Commented Dec 28, 2018 at 20:30

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