I'm trying to install a new c-wire based thermostat, and I plan to turn off the A/C power before I touch the C and R wires (24v). However, I couldn't locate the power switch.

The breaker box has some switches with suspicious labels: 'SD', 'E.D.O' and 'F.A.U'. Not sure which one belongs to A/C. (BTW: what does those abbreviations mean?)

I also find there is a standalone breaker box with only 1 switch next to the A/C external unit. Is it the right one?

Any cautious needs to be paid attention to when turn off (and turn it back on) this standalone switch?

enter image description here

  • 2
    The external switch is for the external unit. If you're not sure which breaker controls which devices in your house, this is an excellent time to find out. Plug lights into every outlet you can, turn on every device in the house (oven, microwave, dishwasher, disposal, etc), then turn off one breaker and make note of what shut off. Flip that breaker back on and move to the next one. BTW- put the current thermostat blower switch in "ON" not "Auto" to ensure you know when you've found the breaker for that.
    – FreeMan
    Aug 20, 2020 at 15:51
  • Can you post photos of the breaker box please? I have a theory about F.A.U. but would want to see more data to be sure Aug 21, 2020 at 0:03
  • Thanks all. I added a picture of the breaker box. But only the 'Dryer' one has a double breaker...
    – MO1
    Aug 21, 2020 at 16:07
  • @FreeMan, do you mean turning off the external unit won't help me cut the electric goes through the C and R wires?
    – MO1
    Aug 21, 2020 at 16:09
  • @MO1 -- are there two big fat ducts going between the inside of the house and your outdoor unit, or just some pipes and wires? Aug 21, 2020 at 22:49

4 Answers 4


Those abbreviations explained (or FAU is what you're after)

Sometimes, electricians in a hurry wind up leaning on a shorthand they learn from reading blueprints instead of spelling everything out when they label a panel. Fortunately, it's not too hard to crack. We'll start with the most important one for your situation, FAU. That stands for Forced Air Unit, which is another name for a thing with a blower in it that hooks up to ducts, like a furnace does. You may also see AHU used here, which stands for Air Handling Unit; the distinction between an AHU or a FAU and a furnace is that furnaces burn some sort of fuel such as gas or oil, while an AHU or FAU may not have burners and other combustion gear at all, simply using one or more of electricity, hot refrigerant, or hot water to heat the air going through it.

The other abbreviations are not too much harder: GDO is simply short for Garage Door Opener, while SD is most likely the circuit your smoke alarms were put on, the latter standing for Smoke Detector. (The difference between a smoke detector and a smoke alarm, by the way, is that a smoke alarm can make noise about the smoky situation on its own, while a smoke detector can only tell a fire alarm panel to start making loud noises about the problem on the detector's behalf.)


That panel does not appear to have a main breaker in it, so that would mean the Main is somewhere else, probably outside next to the meter. In that box, there is probably another large 2 pole breaker to feed the A/C unit. You are allowed to have up to 6 breakers in that service entrance box, so because this panel looks full, when someone installed the A/C, they put the breaker out with the main.

But also, is your A/C a central air system, where is it combined with the heating system? If so, there must be a separate switch in the closet with the HVAC unit, usually just a toggle switch, sometimes with a fuse holder too, either on the wall or sometimes on the side of the furnace (see picture below). That is required for doing exactly this; it is JUST the control power for the furnace/AC unit so when you turn off that switch, your little 24V transformer for the thermostat circuit is off too. For what you want to do, that's all you need.

enter image description here


what does those abbreviations mean?

Money. That is, money out of your pocket and into the pocket of the next electrician working on this. That was the idea (whether articulated or not) of whoever did this.

People seem to think that such abbreviations make them sound cool and educated. Nah. It's the Essence of Stupid (tm). Nobody who knows their stuff needs to use uncommon abbreviations. GFI or GFCI is about as far as I'd go. We're not living in the days when small fonts were extra and you paid the typesetter by column inch, for crying out loud. Label makers or even plain old paper and pencil and a modicum of penmanship lets you pretty much put an act of Hamlet on those panel doors and have it be readable, so why abbreviate... The mind boggles.

Here's what I've done a decade ago in my own place... electricians that can't do even this are beyond contempt IMHO.

enter image description here


Double or Nothing

(Assuming US/Canada)

A traditional hardwired air conditioning system is almost certainly going to be a 240V system using a double breaker. So look for breakers in your panel that are in pairs - the handles will be together, like:

enter image description here

Most houses will only have a handful of these, typically for:

  • Air Conditioning
  • Electric Hot Water Heater
  • Car Charger
  • Oven
  • Clothes Dryer
  • Feed to another panel (Subpanel)

That should cut down significantly on the number of things you need to try. As noted in a comment, make sure the fan is in "On" instead of "Auto" so that you know when you turn off the system.

First picture doesn't seem to have anything for the air conditioner, unless it is a small (120V 20A) unit with a funny abbreviation. There is almost certainly another panel somewhere.

  • Thank you. I added a picture of the breaker box. Please take a look. But only the 'Dryer' one has a double breaker... Maybe A/C share the same breaker with Dryer?
    – MO1
    Aug 21, 2020 at 16:08
  • A/C can't share with dryer. If it did, every time you ran the dryer in the summer you'd trip the circuit. Are you sure there isn't a second breaker box somewhere? Aug 21, 2020 at 16:12
  • There is one outside the house but it controls the external A/C unit. That one only has a single switch. By turning it off, will it cut the electric goes to C and R wires?
    – MO1
    Aug 21, 2020 at 16:13
  • How about the 'F.A.U.' switch? Although it's not a double breaker.
    – MO1
    Aug 21, 2020 at 16:14
  • 1
    In my house the 24 V a/c output transformer which powers the thermostated controls is located in the air handler unit for the a/c and gas fired furnace. There is a switch on the side of the air handler which turns off the 120 V power to the unit. This is not a dedicated circuit but powers at least some lights and perhaps some receptacles. It is protected by a 15 A breaker and is wired in original 1969-1970 aluminum 12 AWG. The breaker has never tripped in 42 years we have lived in this house. Aug 21, 2020 at 17:34

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