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.
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.