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I am planning out my future tankless water heater install. What type of wire will I need to use for a long run from my 40 amp breakers? Heater will be 240v and about 50-55 feet from the panel. I'm doing the ceiling remodel and adding lights and wiring. While I am up there, I will drop in dead wire/conduit for the future tankless install by an electrician to save on install costs. I'm guessing #8 or #6 wire will do, but need to be sure.

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  • Have you considered a gas unit (is that even an option)? It's personal preference of course, but the electric versions draw a lot of current and I've had nothing but bad experience with them.
    – tnknepp
    Nov 2 '20 at 18:09
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    A couple things to consider: 1. can your current panel support this (are the stabs/slots available)? 2. 40A is not a "ton", and it's not "nothing" either. If I were doing this I'd do an ampacity calculation for the house before proceeding. We had one in a house we were renting and when you turned on hot water the lights would flicker. Next thing you know, appliances started dying right and left. The house had a 100A service and the heaters put it over the limit.
    – tnknepp
    Nov 2 '20 at 18:14
  • How many kW of tankless heater are you trying to put in? Nov 3 '20 at 3:09
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Put in big conduit.

The actual wire size for your 40A circuit is 8 AWG - see What Wire Should be Used for a 40A 240V Circuit? for details.

However:

  • You may later upgrade to a bigger heater requiring larger wires.
  • You may later upgrade to a much bigger heater that requires two circuits and twice as many wires.
  • You may later decide to run something else to the same location.

Put in 1/2" conduit and you'll be fine now but have very little room to grow. Put in 3/4" conduit and you'll have some expansion room. Put in 1" conduit and you'll have room for anything from 2 or 3 large circuits to a single circuit for a big subpanel.

At current pricing, 60 ft. (based on "50 - 55 feet from the panel", though title says "75") of 1" Schedule 40 conduit at Home Depot will cost you $13.92 more than 1/2". Plus a little extra on the fittings. $20 now will potentially save redoing everything in the future.

Don't run any wire in that conduit without checking with your electrician. He may be particular about how to do things - or he may say "Any #8 is fine, leave me a few feet on each end to work with."

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Thanks to NEC 240.4(D) disallowing #10 wire, 40A breakers require #8 copper or #6 aluminum wire. (Aluminum may present issues when attaching to the water heater, as not every heater has lugs rated for aluminum.)

If you have 3 circuits in the conduit, #8 Cu will support up to 44A of actual heater load and be breakered as high as 50A. If you have 4 circuits in the conduit, #8 Cu will support 38.5A of actual heater load and be breakered at 40A.

If each circuit had its own conduit, then #8 Cu can support up to 50A of actual heater load and be breakered at 50A.

Note the 125% derate (or 80%) for continuous loads don't apply to tankless water heaters: NEC 422.13. So you can run the wire right up to hard limits (50A, 44A or 38.5A as above).

If you are running cable, such as NM or UF cable, you can only run 40A on #8 copper or #6 aluminum. Advantage: conduit (note the conduit has THHN individual wires in it).

For sizing conduit, remember all circuits can share 1 ground wire, e.g. a 3-circuit heater will require 7 wires (ground + 3 pairs). No need for neutral on most on-demand heaters, and even on the off chance it is required, only on one circuit so only +1 wire. Also if you use metal conduit (that is not flexible) that is the ground path.

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