My electrician is installing a tankless water heater he told me to get 4/2 AWG with ground for 240v. I went to Home Depot and they have no idea what I’m talking about. Can someone tell me what I need to say to explain what I need?

  • 3
    Given that you need a real electrical supplier, your electrician should get the required cable, or run conduit and get the required wires, IMHO. FYI, the closest I could find at homely (...you made me look) was a full reel of 4/3 cable (one more wire than you need) at slightly less than $3000 - "not sold in stores."
    – Ecnerwal
    Jan 18, 2021 at 0:44
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    For oddballs like this, I'd just drop a 1/2" or 3/4" ENT ("smurf tube") in and pull individual THHNs through it. Probably easier and cheaper than 4/2 W/G NM-B :P Jan 18, 2021 at 1:00
  • Maybe you ought to link the tankless water heater that you are installing, so we can figure out for ourselves why the electrician is saying that. Jan 18, 2021 at 5:47
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    I'm curious why the electrician is expecting you to go get the cable/wire and isn't supplying it himself. Usually they're more than happy to supply it, as they're confident that they'll get the right thing the first time, and usually, they buy in bulk at a discount and mark up the price as they pass the cost along. I'd ask him to provide it himself...
    – FreeMan
    Jan 18, 2021 at 13:08
  • You cannot - your electrician did not say whether they wanted Al or Cu cable. At this gauge, it's not OK to simply assume copper. The water heater manual will indicate the required ampacity and that will determine what cable you should buy. Your electrician should not be asking you to buy it for them - that's just sketchy. I wouldn't trust them. This is a red flag.
    – J...
    Jan 18, 2021 at 19:43

2 Answers 2


A cable with two insulated 4 AWG wires and an uninsulated grounding wire in a common sheath (which is what makes them a cable.)

You might need a real electrical supplier, rather than Homely's Despot to find that. It's a bit out of their usual range of product.


This is better done with conduit

As it turns out, 4/2 NM-B cable isn't even made; you could use a 4/3 NM-B cable, but that's upwards of $4/ft. You are much better off wiring this using a 1" ENT ("smurf tube") with a couple of 4AWG THHNs for the hots and an 8AWG THHN ground. Even in copper, this costs less than $3/ft; if you went with 2AWG aluminum instead for the hots, you could get the costs down to under $2/ft. Just remember that with conduit, you'll need to insert boxes or bodies as-needed so that you don't have more than 360° of bending between pull points.

An extra bonus is that if you decide you want something else there instead, it's now trivial to pull a neutral over and drop a subpanel in. Beats having to rip out and replace an inappropriate cable, eh?

  • Absolutely with the 1" smurf. That way when you find out the tankless is inadequate and you need a bigger one, you can just huck more/bigger wires into the existing pipe. Jan 18, 2021 at 5:49

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