Installing tankless water heater - 18kW, requires 2 x 40A circuits.

I have a lot of 8ga Black TW wire AND 3/4” conduit.

Is TW still legal ( I have some THHN, but want to use up the TW where I can)?

Can I run both circuits in one conduit?

Can I use a single ground for both circuits?

Is painting the ends or color coding with tape adequate for identification?

What issues am I overlooking?


TW is still legal and good for 40 Amps. You can run 5 #8 AWG in a 3/4" conduit so 4 hots and a smaller ground (see your instructions for additional wiring instructions) will fit in your conduit. Yes, you can use tape on the ends for identification purposes but your ground has to be a green wire.

  • Instructions assume 2 Romex runs with separate grounds...
    – Scott K
    Sep 19 at 20:27
  • Can I run a bare 8ga (stripped TW) for ground?
    – Scott K
    Sep 19 at 20:29
  • @ScottK -- yes, you can use a bare grounding wire in conduit Sep 19 at 21:36

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