Can I run my Rheem T-Rex tankless water heater on same 60amp breaker as my 240v dryer? Can I put in a junction box at my dryer area and run power to the tankless water heater?

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    Most dryers are the 30A not 60A. You need a subpanel. Jan 23, 2022 at 18:27
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    And two single 30 amp breakers handle tied does not equal 60 Amps
    – JACK
    Jan 23, 2022 at 18:38
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    It would be a code violation to have a standard dryer on a 60a breaker. What are the requirements for the T-Rex as listed in the mfg instructions? A sub off that 3 wire may be possible if black white red, you can add a ground. Note the neutral is normally required for a dryer so moving wires around and adding a ground to feed a sub is possible, more info is needed.
    – Ed Beal
    Jan 23, 2022 at 19:09
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    I'd have concerns about the performance of a tankless heater that is only 60 amps. That's barely bigger than a British electric shower. The #1 mistake people make with tankless heaters is scrimping on heater amps, or "compromising" because they can't power a large enough heater. This usually ends in project failure and blaming tankless tech (wrongly). OTOH 60A would be ideal for point-of-use e.g. in a bathroom. Instant shower - no pipe wait! Jan 23, 2022 at 19:43
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    @Harper-ReinstateMonica And the #2 mistake is getting a really big tankless heater without considering their overall electric service capacity. Jan 23, 2022 at 20:44

3 Answers 3


A typical US residential clothes dryer uses a 30A 240V circuit. The wiring is almost always sized appropriately (10 AWG) and not suitable for any larger load. There seems to be a bit of a debate about whether you could use 2 30A receptacles on a single 30A circuit, but while that might allow a 30A dryer + 30A kiln or 30A dryer + 30A water heater, it won't allow a 30A dryer + 60A water heater or a 60A water heater by itself.

Looking at it in reverse, if you had a 60A circuit with appropriately sized wires, you could not put a 30A receptacle on the same circuit. So you really need to have each circuit sized correctly to the load and to the wires. Most of the time that is quite straightforward, though there are some exceptions at (15A and 20A receptacles allowed on 20A circuits, certain types of motors allowed to have larger breakers based on startup current, etc.)

So you need more power and bigger wires and a way to connect 2 or more circuits to get their portion of the power. That's a subpanel.

The recommendation right now, based on wire pricing, is to use large aluminum (e.g., 2 AWG for 90A) to feed from the main panel to the subpanel, and then install whatever you need in the subpanel. A 90A subpanel can handle a water heater + 30A dryer, and can normally have some smaller circuits in the same panel as well, even though the total based on breaker handles will be more than 90A.

Related is a load calculation. Before adding a new large load such as a 60A water heater, you need to make sure your existing main electric service is up to the task, in the utility feed, main breaker and panel specs. If your panel is rated for 200A and has a 100A main breaker, you can't just swap it for a 200A breaker without (most likely) upgrading the wires from meter to panel and checking with the utility to make sure they can supply that level of power. Similarly, even if the utility says 200A is fine, if your panel has a 150A breaker and you already have lots of other large electric loads, you may find that adding the 60A water heater will push you over the 150A, leading to main breaker trips (not fun). You can upload a picture of your panel and answer a bunch of questions here and get a reasonable recommendation, or get a qualified electrician to review everything on-site.


In order to avoid overload or high installation costs, a special device could be used, which is a sort of XOR power switch. There are many different names for this item, f.e. shower priority switch/contactor, load shedding switch, Lastabwurfrelais/Lastabfallrelais/Lastabwurfschalter (german).

The english wikipedia page on this subject seems to be still missing, here is a translation: https://de-m-wikipedia-org.translate.goog/wiki/Lastabwurfrelais?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en-US&_x_tr_pto=wapp

A tankless heater for showers can/should be combined with a counterflow heat exchanger, which saves money and energy and/or improves the comfort. For DIY- exchangers, see the youtube videos of "Rob the plumber". Local codes need to be considered, f.e. in some locations, between fresh and drain water a double wall construction is mandatory.


As @xeeka alludes to, this can be done, but needs to be done right. You'd be well advised to get a professional electrician and a permit. Set it up like so

  main panel ---60A breaker---switch------------------- Tankless
                                  \-------30A breaker-- Dryer

Your single 60A breaker needs to feed a switch. At any one time all the power must to either the tankless or the 30A breaker for dryer, never both. The switch must be rated for hot disconnect, in case a full 60A is being pulled by the tankless unit at the time the switch is actuated.

Let me be clear: this is a DIY activity only at a high level. You're well advised to get a professional to make this setup.

A device I sometimes install is a tank booster. That device boosts the outgoing temperature of a tank heater to 120F. It runs off the same 30A breaker as the electric water heater, but it's greedy. When it's running it temporarily turns off power to the tank water heater.

This device is used when the main tank is too small for the need, deliberately kept at a low standby temperature for energy savings, or when recovery is slow as in with solar or heat pump systems. That lukewarm water is boosted to full strength instantly.

  • @FreeMan take a look at the diagram. The 30A dryer is on a 30A breaker, as diagramed above.
    – Bryce
    Jan 25, 2022 at 2:17

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