Derating 6 current-carrying conductors?

Question

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).

Edit:

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.

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.

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

Answer 1

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.

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* 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.