I'm running some cables for an electric tankless hot water heater. I haven't decided which one yet. It'll either need 2 x 40A, 2 x 50A, or 3 x 40A, all 240V (there are some that need 60A, but I'll pass on them). I plan on running two sets of 6/2 NM and one 8/2 NM from my breaker panel to some 4x4 metal boxes in the wall where I'll mount the water heater. That way, I can pick the breakers, and use the right two or three runs I need. Once I decide which water heater I'm using, I'll run FMC from the box(es) to the water heater.

I'm wondering what size cable clamp connector (and hence, knockout) I need for each 6/2+G NM cable and for the 8/2+G NM cable (I'm assuming no doubling of cables in a clamp). Also, can I have more than one (maybe even all three) in a single 4x4 box? Or would it be better to use two or three boxes?

Also, what kind of box(es) would I use? I'm only seeing 3/4 and 1" KOs, and I suspect I need something larger...

  • The biggest question I have is "why electric tankless hot water?" Sep 21, 2020 at 2:08
  • 1) More energy efficient. 2) More space efficient. 3) No more burst tanks flooding when we're not there to deal with it. Sep 21, 2020 at 2:37
  • Generally, I find electric tankless to be a poor energy tradeoff (don't make the utility spin up their peaker if they don't have to, modern electric storage water heaters are much better insulated than older units). I do agree with you re: space and burst tanks, but unless gas is an absolute non-starter for you or you're on propane, you're better off going gas if you want tankless. (Going to the gas side also opens you up to boiler+indirect setups, which have some very nice advantages...) Sep 21, 2020 at 3:00
  • I agree with 3phase I have put a new service in for a customer that really wanted a tankless electric. After installing it then point of use small electrics to reduce the fluctuations he was not happy and went back to a tanked water heater he spent +10k with the new service and adding the point of use tankless. I have had gas tankless and it was ok. Another customer was less than happy because his bill jumped because monitored power was based on peak usage so that increased the rate he was paying for all of his power.
    – Ed Beal
    Sep 21, 2020 at 5:44
  • There is currently no gas (natural or propane). I'd prefer not to add it. I don't really want to deal with the venting requirements for gas (tank is not on an exterior wall). Thanks for the feedback. Perhaps I'll add the wiring for the future, and stick with a tank for now. Definitely something to consider. Sep 21, 2020 at 14:49

1 Answer 1


First, if it is at all possible to run conduit between panel and on-demand heater location, that's a great way to do it. That lets you defer the wire size decision until you buy the heater, and simply drop in the individual wires you need. Conduit allows higher ampacity from the same size wire - #8 is good for 40A in NM, but in conduit, 50A (single circuit) or 44A (2-3 circuits). If the conduit is metal and non-flexible, the metal conduit provides the ground - no wires needed. A 1" conduit should be plenty.

Another option, since you need a box anyway and seem focused on a 125A heater, fit a 10x14" box that happens to be a subpanel. Something like a CH8L125SP. Supply that with #2 Cu or #1/0 Al wire or cable at 125A. Then you can feed whatever breakers the heater needs at the time you select it.

If you want to do it as planned, you will need a LARGE box. Each #6 wire needs 5 cubic inches and each #8 needs 3 cubic inches. That is far too big for a 4x4 box. Consider at least a 6x6 box.

A 1" KO should be alright for a #6. Use one KO per cable.

  • I did consider a subpanel but I'm not keen on running rigid conduit through walls and attic. I'm sure an experienced person could do this blindfolded, but as a novice, I was thinking a few runs of NM would be easier. When you say 1" KO, what is the clamp size (or if that's the clamp, what's the real KO)? I ask because a 3/8" clamp goes in a 1/2" KO, so I assume that continues as they get larger. Sep 21, 2020 at 14:53
  • But, let's assume I do want to run conduit to a subpanel. 3@2 XHHW in 1" EMT is almost 37% fill - I'd have at least 2 90 degree elbows (up one wall, down the other). That sounds tight. Or since the WH doesn't appear to need neutral, can I just run 2? Any reason I can't do 1 1/4" PVC Sched 40? That has a fill of < 24% and is much easier to cut pipes to right size. I'd also want to use two pull elbows or LBs for the two turns if possible in order to have the conduit that runs across the attic flat on the joists. Any concerns? Attic run is about 12'. Run in each wall is 3-4' Sep 21, 2020 at 15:40
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    @TomGetzinger you're thinking of Rigid. EMT is easy, I use one of those mini-hacksaws that gives you a handle to hold a hacksaw blade. Pull elbows are great if you can access them. You have to do the pull in segments at that point (which requires access), but it's more novice-achievable than pulling around multiple bends. If you want the access cover on back, left or right, look at LB, LL or LR conduit bodies. Sep 21, 2020 at 18:28
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    Then it sounds like you're on a good track. All access covers must be accessible without needing tools to disassemble any part of the house. Lifting up some insulation shouldn't be a problem. Sep 21, 2020 at 20:27
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    @TomGetzinger You need to double for wires in/out, and you need to count grounds as well (albeit for only 1 wire, so 5 c.i; NEC 2020 revises this to 4 grounds for the price of 1.) Also, you must use the stamped or published cubic inches, you can't just "make stuff up on the fly". The Raco 256 box is 45 c.i. At least pick a Raco 260 which has the KOs you'll need, but even that will be a knuckle skinner. Why not make it easy on yourself? Sep 23, 2020 at 17:44

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