Our water heater is a long way from the kitchen, and we waste a lot of water waiting for hot water to the sink. There is a 20a circuit breaker that powers the dishwasher and disposer. The dishwasher will continue to use the hot water from the main water heater. We would like to add a 2 gallon undercounter water heater to our hot water pipe - rated 1500W. Is there anything I can buy that will not allow the water heater to run when the dishwasher is on? The water heater is just for small tasks, but there is not enough power to run both at the same time.

  • Wouldn't it be simpler to split the water line before the water heater, so that the dishwasher never draws from it?
    – isherwood
    Dec 30, 2020 at 16:21
  • Or is your question about power use and not water? You've tagged both topics, so that's not clear.
    – isherwood
    Dec 30, 2020 at 16:24
  • You seem to be looking for the reverse of this product amazon.com/i-Socket-Workshop-Automated-Vacuum-Switch/dp/…
    – MonkeyZeus
    Dec 30, 2020 at 16:32
  • I wonder if you could wire a 2-way or 3-way switch to control which device gets power... Hoping to see the electric gurus chime in :-)
    – MonkeyZeus
    Dec 30, 2020 at 16:39
  • the problem is that the 3 things I want to use on the circuit take more power than the 20a breaker will allow. I want to be able to prevent the water heater from turning on if I am using the dishwasher....automatically....without having to flip any switches.
    – Sue
    Dec 30, 2020 at 16:49

3 Answers 3


You could run the power to the dishwasher and disposal through a current sensor and contacts on the sensor in series with a coil on a relay. The power to the water heater could be taped ahead of the sensor, and connected through "normally closed" contacts on the relay. When current is detected in the sensor the contacts on the sensor would close, energizing the coil on the relay opening the N.C. contacts, breaking power to the water heater.


If you can live with a manual switch, a three-way switch can be wired in the hot lead as a SPDT (single pole double throw). You have to remember to switch it, however.


Have you considered that the dishwasher will likely use more than 2 gallons of water; if it does and you have switched off the water heater, all you will get is the two gallons of hot and limitless cold water.

I took a quick look at under the sink hot water heaters on Amazon and discovered they all seem to require being hard-wired to a Ground Fault Circuit Interrupter (GFCI) circuit. What that means is:

Per the National Electrical Code a Ground Fault Circuit Interrupter (GFCI) Circuit Breaker is required to feed your current outlet.


  • Your existing outlet is already fed from a 20 amp GFCI circuit breaker, then you can hardwire the water heater upstream of your conventional 120V socket.


  • Having two GFCI protection devices wired in series is not beneficial and will probably be problematic. Therefore, if the existing outlet is a GFCI 120V socket you will need to replace it with a conventional 120V socket; $5 for a good one. The required GFCI circuit breaker will provide ground fault protection to all loads which are correctly connected to it.

You might want to consider a larger tank size; 8 gallons are available and seems a better choice and the additional per gallon costs are much lower.

On-Demand tankless heaters would be more efficient, however, they require 240 VAC and higher amperage which probably means running two new 10 AWG and a single 12 AWG ground wire from your load panel. If you choose this option you would instead install a two circuit ganged GFCI circuit breaker instead of a single circuit. A benefit for the time and expense is you would have the benefit of instant and limitless hot water at your sink.

If you own the home and are handy you may be able to do all of this yourself, assuming local ordinances permit that and you file for a permit and licensed electrician inspection. Otherwise I would recommend hiring a licensed electrician to give you an estimate first; think of that $1200 government COVID benefit check coming soon Then if you have a permit and inspection, an electrical fire in the new wiring it will be covered by your home-owners insurance. If you do it yourself without the permit/inspection, then maybe not so much! BTW although I am a degreed electrical engineer, I am not a licensed electrician. They are different skill sets, so please consult an electrician.

Because of the NEC Code requirements of installing a GFCI Circuit Breaker, I would not recommend you wire a switch. You only need to size that new GFCI circuit breaker to accommodate both the water heater and the dishwasher. If you can find the amperage requirements of your dishwasher use that number. If you are sure the heater will not come on, you could subtract 5 amps from the dishwasher's amperage before adding the tanked/tankless hot water heater amperage Add the tank/tankless water heater AND dishwater amps to determine the required minimum amp rating of the GFCI circuit breaker.

  • I see you added a clarification that you are not relying on the localized water heater to service the dishwasher. Most of what I replied is applicable to the water heater and can be edited to remove the dishwasher amperage from the circuit breaker size calculation. You also mention the wire feeding your island comes through the slab. It is very likely it comes out of some sort of conduit which would run back to your load center (fuse box). This might make the whole situation easier. (continued) Dec 30, 2020 at 21:54
  • (comment continued) If you disconnect both ends of the cable or Romex, after securely attaching a heavy nylon string to free end of the cable which you just disconnected from the island's circuit breaker. Pull the cable from the island side allowing the nylon string to feed into the load center; be sure to tie the end of the string off to something so it doesn't get pulled entirely into the conduit When the Romex/Wires exit the conduit inside the island, tie it off for the same reason. Dec 30, 2020 at 21:55
  • (cntd) When you are ready to pull the replacement cable just reverse the process and pull it into the island side of the conduit by pulling the load center end of the string. If you want the pulling back in to be a little easier, just drizzle a small amount of liquid dishwashing detergent onto the cable as it enters the island cabinet. It will lubricate the inside of the conduit as the cable passes by it. I should only be necessary to add the detergent to the first few feet of cable when you start pulling. Keep dry rags handy to wipe the detergent off the cable as it exits the conduit. Dec 30, 2020 at 21:58

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