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I'm installing a whole house tankless water heater and I'm a little lost as to wiring this power hungry unit and yes I'm upgrading to 200a service(by electrician). I've done some wiring before but not with double pole breaker. My water heater recommend Min of 2 X 50A breakers and 2 X 8/2awg. I'm planning on using a 6/2awg instead. So my question is with 6/2 (black, white and ground), the ground hookup to ground bar, white to neutral bar and black to breaker? Do I leave the other hole empty? So just one hot wire to the breaker?

Thank you.

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    It sounds like the heater does not use neutral, but uses white as another hot, if cable has a white. Might not be code to use white as hot at that size. What does heater installation instructions say about how to connect wires? Wait till the experts come.
    – crip659
    Jul 30 at 15:11
  • How about posting a picture of the water heater instructions diagram for the electrical hookup.
    – JACK
    Jul 30 at 20:23
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First of all, keep in mind that you have two double-breakers, not one. So that's two 8/2 or 6/2 cables, not one. You can't simply use a single 6/2 (even though it has higher capacity than 8/2) instead of two 8/2. Not sure if you intended that or if it just ambiguity in the description.

Back to the breaker installation. In the US/Canada, you can generally use wires or cables. A cable has multiple wires assembled with an outer covering that keeps everything together and provides some limited physical protection. Typical cables (at any AWG) are /2 = Black + White + ground or /3 = Black + Red + White + ground. While normally white = neutral, since cables have white (you can't normally get a black/red/ground cable without white), code allows for use of white as hot if no neutral is needed. Normal breaker installations when used with cables (not counting ground - that always goes to the ground bar) are:

  • 120V = Black hot, White neutral
  • 120V/240V (e.g., typical dryer or oven) = Black hot, Red hot, White neutral
  • 240V (e.g., typical water heater) = Black hot, White hot

For a water heater (in most cases) you ignore neutral. It isn't needed by the water heater.

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    If the water heater documentation states to use 50 A breakers for each of two heating elements, then the mfgr is indicating that the expected current to each element will be no more than 80 % of this or 40 A (per heating element). So if #8 wire is rated for A 50 A breaker, then you could use that. Of course #6 would have less line loss, but check to see if the 50 A breakers are listed for #6 because #6 might not fit in the breaker. Jul 30 at 17:46
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    Almost certainly a 24 kW tankless WH will be unacceptable in the winter in WI. If you are going electric, you are going to need a tank. Jul 30 at 18:33
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    Why do you want a tankless? There is lot of misinformation out about the benefits of a tankless WH. We got a NG fired tankless WH because we have a 1-story 2000 sq ft house on a slab and when our 40-gal natural gas fired tank failed catastrophically it flooded the house. This happened twice. Jul 30 at 18:44
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    Yikes! Just checked some random site about sizing water heaters. For 4 GPM = 2 showers at the same time (or a shower and washing dishes or whatever), in my area (determines incoming temperature) 160 Amps at 240 Volts That's more than a lot of entire houses. Jul 30 at 19:44
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    Electrician ended up connecting the WH for me. All I did was routing the wire prior to the service upgrade. I'm happy to report that my tankless WH is working out nicely. It takes about the same amount of time for hot water to reach the point of use like the conventional tank WH. Water doesn't get hot when small streaming water is use, that's something I have to get use to. Otherwise, all good. I don't have to worry about not having hot water anymore when it's my turn to shower. Hope I'll still get plenty hot water in winter when the inlet water is much colder. Thanks.
    – Jordan
    Aug 13 at 18:48
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You must re-mark the white wire

You need both hots and ground, but you don't need a neutral. So what you need for that job is red-black-bare cable.

Unfortunately, that cable is not made.

So what you actually need is red-white-bare cable, and some red tape. You will re-mark the white wire red, using red electrical tape.

You must re-mark the white, but to tell you the truth, it doesn't need to be red. It can be any legal color for a hot wire, which is any color except white, gray or green. Black and black is perfectly fine. Feel free to use black tape instead.


Now, on a 2-pole breaker, you have 2 terminals for hot wires. And by golly, you have 2 wires that are hot colors. You can figure it out :)

We should see other metals

One more thing. If you are about to spend hundreds of dollars on long runs of wire, stop. You should price an alternative: heavy aluminum feeder wire at a tiny fraction of the price.

First, you can definitely run large aluminum (1 AWG or larger, 100A) to a subpanel near the water heater. In the subpanel, place your two 50A breakers and run #6 copper the short distance from the subpanel to the heater. You could also upsize the aluminum wire, to bring more amps to the subpanel - either for shorter wire runs for additional circuits, or for a larger water heater.

Or second, if the water heater's terminals are rated for 75C thermal, and they are also rated for aluminum wire, then you can simply replace the 6/2 copper with 6/2 aluminum wire, which is good for 50A.

That seems illogical, how can the same size aluminum wire do the same ampacity? It's a fluke. Because of the NM or UF copper wire is an inferior insulation type limited to 60C thermal, so 55A. Aluminum wires in the US are made with better insulation good to 75C thermal, so 50A.

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  • red-white-bare cable Really? Wouldn't it likely be the same as in 10/2, 12/2, 14/2 - i.e., black-white-bare cable? Aug 1 at 1:41

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