I'm having a hard time understanding 310.15(B)(2)(a). Here's the boat I'm in: I hooking up a tankless water heater that calls for 3 40 amp breakers (it's a 24 kw Ecosmart). So I put in a sub panel in the laundry room (forgot what I ran to the sub but it's appropriately sized!). Three 10-foot runs of 6 awg go from the sub to where the heater is mounted. So far I have the heater mounted, and the cable behind the wall opposite the heater is exposed. Each cable has 2 strands of 6 awg and a 10 awg ground. The idea is to exit the wall directly underneath the opening on the heater as to have a short as run as possible. No drilling or cutting has yet occurred. It is not in conduit from the sub to the heater location.

Inspector: The cable must be protected from the point it exits the wall to it enters the heater body. He's non-negotiable on this, doesn't care how, it can be in rain-tite, emc, doesn't care: protect it. In addition, I cannot lose two of the grounds (not conduit them) then pigtail the ground in the heater (it requires 3). Something about not the intent of the cable design.

Tankless manufacturer: Has conveniently provided a 1-inch opening on the bottom of the heater (no knockout, just 1" hole). Per a phone call, their intent is for the cables to exit the wall and go into the heater. They did not design the opening for anything bigger than 1", nor do they feel that NEC should be an impediment to installing their heater. I gather I wasn't the first to call with this question. In answer to my question about enlarging the opening to 1 1/4" or adding a second 1": "The warranty will be voided". There really isn't a lot of room to enlarge or add another hole anyway. Agrees with the inspector about the grounds, also will void warranty. Each ground must connect back to the panel uninterrupted.

Going with 1 1/4" then reducing it just before the heater isn't an option, too big and I'd have to reroute some copper pipe. This is in a tight space.

So I have two dilemmas: If I stuff these 6 "hots" and 3 grounds into a 1" Type-LB Conduit Body am I violating the 3-conductor rule?

Second, let's say I'm not violating the rule, what kind of derating do I need to do here? The cable is rated for 60 c/55 amps, the heater pulls a max of 40 from each breaker, if I take 3 showers at once with the dishwasher going. So if I stuff these in a 1" opening, that's 9 wires (6 @ 6 AWG and 3 @ 10 AWG), which I believe you can only have 6 total before derating.

Here's why I'm obsessing over this: with the 1" LB, this works perfectly! All I need is two nipples (one coming out the top into the heater, one coming out the bottom side into a junction box with metal face). I'm able to easily get the wires through the nipples and the lb with not a lot of wire butter and no nicking or gouging the sheathing.

Or.....do I have to derate based both on over-filling and more than 3 ccc's? I know this is a bit overkill for a total length of about 6 inches inside "conduit" (the 2 nipples and the lb), but I live in an area with a recent several-thousand structure fire and the insurance folks have let it be known that even the slightest code violation (e.g screw holding wire over tightened) will void the policy. This is my response to that, had the whole house rewired, new panel, new meter, arc faults everywhere, &^@&*()!))@@@ tamper-proof outlets..... This is the last step of the electrical upgrade.

Without doing the math (which I can't figure out anyway), 310.15(B)(2)(a) seems to suggest that I can safely pull this off, and the cable, rated at 55 amps, will do fine at 40 amps when 6 of them (and 3 ground) are shoved into this 1". However I'd sleep better if I can confirm it with the math (as well as explain it to the inspector).


Tank location. A 1 inch lb fits perfectly underneath the opening at the bottom right of the tank. A 1 1/4" will NOT fit behind the pipes, which are closer to wall then appear in photo. Cannot do anything to left of tank, a sink with a wall mount sprayer is going there.

enter image description here

Wall directly behind tank. The tank opening is approx where the red piece of tape on the tank brace stud is.The 8 awg cables are stuffed behind the stud to the left just to keep them out of the way for now, I have about 6 feet to work with, plenty extra! It was 3 20-ish foot "remnants" that the BORG was selling cheep cheep. enter image description here

What I want to do. In test (dry) fitting, this fits perfectly.

  • With a 24 kW tankless water heater don't expect to be able to run three showers and a dishwasher at the same time . . . unless you have very warm feed water and have low flow shower heads. If you do use ultra low flow shower heads you could have trouble when only one shower is being taken. The flow might be so low that the heater shuts off. What can happen is that you turn the hot flow high at the start and then add add more and more cold. The pressure from the cold in the shower mixing chamber impedes the flow of hot and reduces it so low that the water heater turns off. This can happen. – Jim Stewart Apr 13 at 21:02
  • I think 24 kW is equivalent to 82,000 BTU/h. Most houses with three showers would have a 180,000 BTU/h gas fired tankless water heater. However, I do think that a group of cooperative people could get acceptable use out of the 24 kW heater you have – Jim Stewart Apr 13 at 21:18
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    It's a 3 BR/2BA cabin. The chance of 3 showers at once + dishwasher, as long as I live in it, are nil. I am very electricity conservative (I file formal complaints when my bill exceeds 60$...hence the new meter. I get hourly usage and text alerts when I go over 1.50$ per day of usage). With that in mind, I do have to install at rated wire capacity. Next owner could very well run all hot water fixtures at once, for hours. I DO have warm water coming in, 57 winter/62 summer. I picked 24 k based on available amps I could use based on other average electrical load. – tpcolson Apr 13 at 21:46
  • how difficult would it be to run 1" EMT conduit from subpanel to heater? plastic conduit is also fine. – Harper Apr 13 at 21:51
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    @tpcolson -- why the tankless, by the way, for that matter? It seems like an awful lot of grunt for not much gain (although if it's used only seasonally, that does weigh somewhat in favor of tankless due to the issues a seasonally used tank can develop) – ThreePhaseEel Apr 14 at 0:51

You're barking up the wrong tree. 310.15(b)(2)(a) isn't the issue, you are a ways from hitting that. The issue is the other kind of conduit fill.

With conduit fill, you are limited to 40% of the conduit cross-section with 3 or more wires.

The company gave you a 1" knockout and they are correct. You need to bring in six #6 wires and 1 bare ground wire up to #6. That will only give you about 33% conduit fill on a 1" conduit with THHN wires.

There is an additional rule that oval cross-section cables are treated same as a round wire of the maximum dimension. #6 Romex is .68" in the long dimension so it is a wire of .68" diameter. That is 0.363 sq.in. in surface area, triple that is 1.09 si. Divide by 40% you need a pipe with 2.7 si. A 1.5" pipe isn't close, so you need a 2" pipe.

Needless to say, trying to stick three cables in a 1" knockout is a lost cause. It will never be legal. It will never be anywhere near legal. It will always get redflagged by the inspector. That is what he is trying to tell you, and you are misconstruing this to be a 310.15(b)(2)(a) issue. It's not.

Again, this is easily done with conduit. I happen to like EMT conduit because it's a good balance of "easy to work with" and "allows the conduit to be the ground, so no ground wires needed".

How to solve it

Given that running conduit back to the panel is not a possibility, what remains is to run 1" conduit to an intermediate point or point(s), and splice from your cable to THHN wires at that location. Each cable will require 20 cubic inches of splice space. In addition each box will require 7.5 cubic inches to allow for cable clamps and grounds. So if you can get one big box with 67.5 cubic inches, that will do. Or one with 27.5 ci and another with 47.5 ci.

One 6x6x2" box should suffice, or a 4-11/16" square box that is either deep with a domed cover, or has a box extension.

Ironically, you are at a disadvantage getting a big box at a big-box store - a proper electrical supply house will have better selection and be cheaper.

And here's a relief: you don't need to buy 2 colors of THHN wire. You are not paralleling, so you do not need to distinguish red from black - black and black are perfectly appropriate colors for a 240V circuit. In fact, when dealing with multiple circuits, it is far more important to distinguish circuits from each other.* So grab a 5-pack of colored electrical tape and call it golden. The 3 circuits can be black-black (unmarked), blue-blue and yellow-yellow.

Seriously, you don't want to accidentally attach one of the 3 heating coils to Black1 and Red2. It will seem to work, but will cause weird problems and would trip a GFCI if the supply breaker was GFCI.

Stuff that doesn't matter

As far as 310.15(b)(2)(a) first that's a non-issue in conduits shorter than 24", but even so, every circuit fed from a split-phase panel counts as 2 conductors. Because it is concerned with heat, and grounds don't matter, and neutrals between 2 hots do a magic thing that cancels out. So you have 3 circuits, therefore 6 conductors. 6 conductors gets an 80% derate off the highest temp rating allowed for that wire, and NM gets an exception allowing it to pull from the 90C column even though it's 60C wire. So at #6 you are reading 310.15(b)(16) and getting 75A, derating x 80% = 60A. Which is a big nothingburger, because #6 wire is already limited to a 60A breaker (actually 55A but that size isn't made, so you get to round up.) Now if you had 8 conductors, that's a 70% derate and #6 gets knocked down to 52.5 amps.)

Also in the "doesn't matter" category, their three 40A breakers actually call for #8 wire, but this changes nothing. Three #8 cables still can't fit in a 1" knockout/conduit, and three #8 wires fit with ease. So using #6 wire has no impact.

* I have one installation where the guy ran 2 1" conduits from a panel to a machine room. In one of the conduits he put four 120V circuits. The other got four #10 wires for 4 circuits - red-black, red-black, red-black and red-black. And you can't tell them apart because his cheap labels fell off. Ugh. I plan to rearrange it to black-black and red-red in conduit 1, ditto in conduit 2.

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    @tpcolson -- the wire in the cable isn't marked as THHN, so it can't be used as such. You need to have a splicing-point here; if you can't find a suitably sized junction box (I'd look at 5" square boxes in addition to 4 11/16" and 6" squares), a 6x6x4 NEMA 1 flush-mount enclosure can be used instead, considering that all it's housing is a pile of splices. – ThreePhaseEel Apr 14 at 14:21
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    yes, junction box cover needs to be accessible without needing tools. i like tasteful cabinet doors or built in bookcases. I also like tasteful cabinet doors etc. on the backside of the tub spigot. – Harper Apr 14 at 18:18
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    @tpcolson keep in mind if you do this in EMT, IMT or Rigid, the conduit is the ground and the 3 #10 grounds simply need to be landed on 3 ground screws attached to the metal junction box. So you only have 6 wires in the conduit. Even better for box fill! And again #8 is allowed if the circuit is breakered 40A. 310.15(b)(2)(a) is still not a problem #8 wire is 55A@90C and with 3 circuits that derates to 44A. 310.15b2a really doesn't become a bear until 5 circuits (50% derate). – Harper Apr 14 at 20:01
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    Talk is talk. Does the labeling and instructions require the continuous ground back to the panel? Because the UL listing is based on that, and (therefore) that is what you must comply with (NEC 110.3b). – Harper Apr 14 at 20:23
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    @tpcolson -- you will need a fully UL listed distribution block for what you are doing, as per NEC 314.28(E)(1). Unfortunately, the block you linked is not UL listed, just recognized (or Rather Useless to you if you want a mnemonic device for remembering which is which). Examples of listed blocks include this one from Mersen, and it's somewhat cheaper but more than adequate for the job little brother. – ThreePhaseEel Apr 15 at 23:49

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