3 way switch to toggle between 2 devices


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Old 04-29-20, 08:34 AM
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3 way switch to toggle between 2 devices

Full disclosure - I am NOT an electrician and don't claim to be. Have done a lot of small, straightforward electrical jobs like replacing switches and adding outlets to an existing circuit, but this one is a bit different. I want to install a 1400 watt under-sink hot water heater (HWH). It uses a simple 3 pronged plug and I can easily install an outlet box under the sink and tap into one of two existing 120v circuits for the power. Both are in a gangbox above the counter, so it was pretty easy to pull 12-2 Romex straight down the stud to the opening under the sink. I have a 20 amp circuit that feeds multiple outlets including the ones used by the fridge, toaster oven, and coffee maker. My concern there is that the HWH at full draw will likely trip the breaker when toaster oven is running or the fridge kicks in. I can deal with the toaster oven turning off, but I don't want to lose a fridge full of food if this trips in the middle of the night or while we're not home.

The other is the dedicated 15 amp circuit for the garbage disposal, which draws 8 amps, so it can't be on at the same time the HWH kicks in (8+12 > 15). However, you're never needing hot water while you're running the disposal, it's an either-or thing. (I'll also install a separate switch on the outlet for the HWH so it can just be shut off manually.) What I'm wondering is can I use a 15 amp capacity 3 way switch to effectively have the HWH on when the disposal is off, and when you turn the disposal on, the HWH is powered off? (each appliance will be grounded directly to the existing ground.) I'm pretty sure that this would actually work from a technical perspective - the HWH and disposal can share the neutral and the hot will be controlled by the switch. The question is, would it be to code, is it even a good idea?
 

Last edited by EricH2020; 04-29-20 at 11:06 AM.
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Old 04-29-20, 09:29 AM
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1. You can't under code add the heater to the counter 20 circuit.
2. The three way will as you propose.
 
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Old 04-29-20, 10:23 AM
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I agree that using a 3-way switch would be code-compliant, but I don't think I'd go that direction. It would be a pretty unusual installation.

If you have access below, I would definitely consider running a new 15A circuit from the panel to the water heater. Add an under-cabinet receptacle and you're all set. No need for an extra switch because unplugging the heater can serve as a disconnect.
 
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Old 04-29-20, 10:42 AM
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The three way switch would work to select power to the disposer or to the heater.
However..... you'd still need another switch (single pole) for the disposer.
 
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Old 04-29-20, 11:04 AM
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Thanks for the responses. It's not easy to pull wire from the existing panel because the basement is finished, and the panel is on the other side of the basement (as is the water heater which is why I'm putting a small one under the sink.)

One person said the 3 way switch wouldn't work without a separate switch for the disposal. But I believe that if both devices share the same neutral, then switching the hot from one to the other will complete the circuit and things should just work. Either the disposal will run, or the heater, which is what I need.

I suppose another option is a double throw switch, but it's basically doing the same thing here, just in a different way, and the ones I have seen have a middle Off position, which I don't really want here as people could accidentally turn off the heater after using the disposal.

Thanks for the help.
 
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Old 04-29-20, 11:07 AM
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One person said the 3 way switch wouldn't work without a separate switch for the disposal.
Yes.... I just said that and it's true. Nothing to do with the neutral.

The three way switch is on/on.
If it's in the disposer position.... what keeps the disposer from running ?


 
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Old 05-04-20, 06:48 AM
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Nothing, but that's the idea: To recap, I want to disable the heater only when the disposal is on to keep the breaker from tripping, but otherwise the heater is on full time. So, with the 3 way switch, if you flip the switch up, the disposal is on, and the heater is off. Flip the switch down, and disposal turns off and the heater is on, which should be the norm. (I checked the continuity on the switch poles and wired according to my needs). Note that I do have a way to disconnect the heater if needed - it's plugged into a dedicated outlet (that the switch controls) under the sink.

I appreciate all the replies. I wrapped up the installation over the weekend and my kids are already tired of hearing me say "Look, hot water as soon as you turn on the tap!"

 
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Old 05-04-20, 04:21 PM
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with the 3 way switch, if you flip the switch up, the disposal is on, and the heater is off.
Not to split hairs..... but you did not say you had a batch feed type disposer.

The bulk of disposers have a wall switch. If you had a standard disposer and the wall switch was in the disposer position..... it would runs continuous.
 
 

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