Home Improvement Asked on February 26, 2021
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 4×4 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 4×4 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…
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.
Answered by Harper - Reinstate Monica on February 26, 2021
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