Looking into ways to connect a tankless water heater that requires 3 40A double pole breakers and has 113A max draw 125 feet away from my 200Amp main service panel.

Running 3 separate 8 AWG wires (probably need 6 AWG for 125 feet anyway) is prohibitively expensive (about $1500 for copper).

I was thinking about connecting a subpanel near the tankless and feed it with 1/0 1/0 1/0 2 aluminum.

I would use a 125A (CH2125 breaker) in the main panel to the 1/0 1/0 1/0 2 aluminum and over to the new 125A panel.

Is there any flaws in this design, or other recommendations? Also do I need another 125A main breaker in the subpanel (my understanding is no since it has the 125A breaker at the main)?

2 Answers 2


Just say no.

I know it isn't the question you asked, but in my opinion this is an absurd use of resources. You are taking more than half the capacity of your home electric service and sending it to the other side of the house (125' - another building?) in order to produce (based on looking up some tankless water heaters based on 113A) ~ 5 GPM of hot water output. Actually, I see 3 possible scenarios:

  • Most of the time you don't need much, but once in a while (2 showers plus kitchen sink all going full blast at one time) you need that much output.
  • You rarely need that much output but want to have it in case you need it.
  • You actually need huge amounts of hot water at a rate of 5 GPM for large amounts of time.

In the first 2 cases, an appropriately sized tank heater, which typically uses a single 30A or 40A circuit, will do just fine. Yes, there is significant recovery time, but if the stored water can get you through the peaks then you're fine. Maybe 50 gallons isn't enough, so go for 80 gallons. You will save a lot on wire, you cut down your peak demand, which is becoming a real concern for electric utilities (and that peak demand might someday cost you quite a bit extra $ every month) and it just makes a lot more sense.

The main exception is the last case. If you truly have a need for that much water - literally hundreds of gallons per hour on a regular basis, then tankless may actually make sense. Short of a commercial laundry or similar operation, I'm not sure why you would actually need that much hot water. And if you really did, I would seriously consider natural gas, if available in your area, as being more cost-effective at those usage levels. The raw energy cost of natural gas vs. electric varies by location and current (pun intended) rates, but overall they are comparable and natural gas doesn't come with a peak demand surcharge.

Based on comments, there is already natural gas in the building. If you're going to go for tankless, I would definitely go for gas rather than electric.

Also based on comments, OP is considering a tank + tankless setup. Be careful. That doesn't always work the way you want it to work. Not dangerous, but not necessarily as effective as you might think.

One more concern is your total electric capacity. 200A is enough (more than enough) for a typical house, even with all electric appliances, until you add either electric tankless water heating or Level 2 electric vehicle charging. You may be just fine. But consider that 200A - 113A leaves you with just 87A for everything else. The correct thing to do is a load calculation to figure out how much capacity you actually have available. You may find that 113A is OK. You may find that you can squeeze out 80A and add a reasonable-but-not-quite-as-big tankless heater, or you may find that you can only spare 50A and that you're better off adding another tank or going with gas.

Rethink this. Running essentially a full service entrance cable from one end of the house to the other for hot water just doesn't seem to make sense.

  • 1
    We currently have a 75 gal, 75,100 BTUH gas hot water heater, but run out of hot water a lot. Between a 2 person jacuzzi tub and 8-10 people in the house needing showers, and laundry, and dishes its hard to schedule things so that we don't run out. I actually want to connect the tankless AFTER the 75gal tank hot water heater. It sucks that the house was designed with the power coming in one end and the water supply (and current 75 gal water heater) on the other end.
    – Garfield
    Jan 5, 2022 at 1:11
  • 1
    @Garfield -- what sort of First Hour Recovery rating does the existing 75gal heater have? Jan 5, 2022 at 3:10
  • 2
    Electric tankless is the worst of all options. If you are depleting at 75 gal water heater often, there is NO WAY an electric tankless could keep up. And the way electric prices are going, it will kill you. If you really have that much demand for hot water, either replace your existing WH with a larger one, or get another conventional WH and place it in series with your existing one. Electric on demand water heaters are the worst. Jan 5, 2022 at 5:46

Use alumi - - -

You could try a sub - - -

You know what, I don't think you need my help lol!

You have a good plan. You sized the heater correctly, and are being sensible about power delivery. I wish the subpanel was a big bigger because I'm sure those aren't the only loads in that area, but realistically 125A is the largest breaker that is reasonably available or affordable.

You only need a breaker in the main panel, unless it's an outbuilding in which case you need a main disconnect, but then you really will need to have all loads in this panel. Can't have 2 feeders to an outbuilding.

As far as the wisdom of slamming 113A of power anytime you want hot water, hey, using it is what electricity is for. If the PoCo is alright with it, it beats wasting energy keeping a tank of water hot all the time.

  • 1
    PoCo may be OK with it. But if they get permission to start charging $ 10 per kW for peak monthly usage, this could suddenly cost a couple thousand $ per year for what should otherwise be a slight savings over tanked in terms of pure electric usage. (Except starting point here is tanked gas, as it turns out.) Jan 5, 2022 at 1:22
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    Meaning tankless gas probably makes a lot more sense than this idea...unless space allows for two tanks, both gas...
    – Ecnerwal
    Jan 5, 2022 at 1:26
  • Gas tankless would require upgrading our service and was about 2-3X the cost in the end.
    – Garfield
    Jan 5, 2022 at 1:33
  • Maybe. But as you have found out with the cost of wire + subpanel etc. the cost here may not be as much less as you thought originally. And based on all the climate-change-mitigation related regulations in various places, some of which are totally logical and some...not so much, residential demand charges are starting to show up. The other thing to consider is whether your 200A supply is actually even large enough - I'll add to my answer. Jan 5, 2022 at 1:54
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    Harp: I usually agree with most of your posts, but relatively modern water heaters are so well insulated that there is very very little loss by "keeping a tank full of hot water all the time" . I've been without power for days and still have some hot water for domestic use, provided we don't take showers or baths. Modern water heaters are extremely well insulated. The other benefit is they don't draw all that much power at once...about 25 amps, which puts much less strain on the power grid, which is going to become a more and more important consideration. Jan 5, 2022 at 5:56

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