I recently bought a house with really nice hardwood floors, but the builder didn't put any thought into installing a humidifier system. I'm now looking into my options and steam humidifiers seem to be the highest output solution, albeit overkill. Unfortunately, the only power source I have available in the furnace room is on a 15A breaker shared with two bathrooms, but my tinkering instincts won't accept defeat so quickly.

I'm thinking that I could use a UPS with sufficient output power to support the 7A steam humidifier. The trick is that I would use a current switch to sense when the humidifier is on and subsequently turn off the power source to the UPS (with a relay) and when the humidifier is off the UPS will reconnect and start charging. My theory is that the no load charging current of a UPS is always lower than the maximum load current. Someone correct me if I'm wrong.

The question is: How do I determine a UPS's charging current spec? If this is low enough to put on the 15A circuit then I'm good to go. If it's still too high then I have to accept defeat... unless someone has another clever solution. Given the construction of the building I cannot run a new 20A drop to this room.

Assumptions: Yes, I know, this isn't what a UPS is meant for.. but that isn't a part of the question. Also, I'll probably go through a set of batteries in a year and I'm OK with that.

  • Are you unable to swap the breaker with a higher rated one?
    – lexvegas
    Dec 3, 2012 at 1:48
  • 5
    That would be like putting a bigger fuse in a car. The wires are only rated for so much and to keep things safe the breakers have to stay as is. Otherwise, the wires could get hot, melt, or burn and we don't want that for the holidays! :)
    – Jake
    Dec 3, 2012 at 1:55
  • I guess I should have put a disclaimer if his wiring would allow it. 20A isn't uncommon AFAIK, and might allow him to get by. Otherwise I'd just run little romex. Not too hard.
    – lexvegas
    Dec 3, 2012 at 2:01
  • No worries. I would rather run the wire, but I would have to do some serious demolition to make that happen.
    – Jake
    Dec 3, 2012 at 2:04
  • 1
    "How do I determine a UPS's charging current spec?" - A Kill-a-Watt power meter would give an answer. You need to verify that the humidifier can cope with square or semi-sinusoidal (or whatever) waves if you don't put up the big bucks for a UPS that does output pure sine-wave AC. @lexvegas - 20amp circuits are not common, and have special outlets to indicate that higher current capacity.
    – sawdust
    Dec 3, 2012 at 2:28

2 Answers 2



You definitely want to put the humidifier on its own circuit, or a circuit that is not shared with two powder/bath rooms. You could look for a convenience receptacle in the basement and determine if it's on the same circuit. If not, you can probably come out of that for the humidifier. If the humidifier pulls it's full 7A rating when it starts, you probably want to avoid having it share a lighting circuit, as the lights could dim when it turns on. That can cause premature wear on ballasts and greatly reduce the life of any incandescent lamps you might have in use. Take care to know exactly what else is on any circuit you find in the neighborhood.

NEC requires wiring devices, such as the relays you're talking about to be listed and approved. Assembling something to cut the power to the UPS when the humidifier kicks in using all listed and approved parts does not constitute a listed and approved device. Your approach, while very creative gets a little too far into the "Rube Goldberg" brand of tinkering.

My advice is, have an electrician install a single use receptacle for the humidifier on its own 15A circuit, even if that's slightly overkill. If your panel will not accommodate this, and tandem breakers aren't a possibility, then consider increasing the size of your panel and / or service.

Two bathrooms on a single 15A circuit is problematic enough, it sounds like you need to bite the bullet and get a few additional circuits where they're needed.

  • Definitely a +1 for the last two paragraphs alone. Overkill is good where power supplies are concerned.
    – Rory Alsop
    Dec 3, 2012 at 9:36
  • Excellent technical answer, particularly the NEC standard... I was afraid of that. I will look into my options but I don't think I will be able to install another drop. Thanks!
    – Jake
    Dec 3, 2012 at 18:27
  • Is this a new house? As of 1999 electrical code mandates a separate 20amp circuit for each bathroom!
    – Bryce
    Dec 3, 2012 at 20:19

You may be able to solve this problem with a subpanel. The wiring to the furnace is probably beefy already. Feed the subpanel with the cable that currently goes to the furnace. Add a breaker in the subpanel for the new circuit to the furnace. Add a circuit for the humidifier.

To know for sure if you can do this, we'd need more details:

  • The size of the breaker(s) feeding the furnace

  • The size of the conductors feeding the furnace

  • The load ratings on the furnace

If you're lucky, the electrician may have picked a larger wire than required, perhaps to protect the blower motor.

  • Also, if you do get an electrician to run a new circuit for your humidifier, have him install a small subpanel in the furnace room. That will make it easy to add new circuits in the future.
    – Jay Bazuzi
    Dec 3, 2012 at 15:23

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