#8 THHN in Conduit for 50 amp circuits

Question

I am installing a new electric point-of-presence hot water heater which requires 3 240v 50 amp circuits (6 hot wires, 1 earth ground). The water heater is roughly 50' from my panel.

I would like to run the wires from my main panel in PVC conduit (the grey sched 40 stuff the big box stores sell) to make a neater installation, rather than attempting to run three 6/2 cables (not to mention there is only 1 connection point inside the water heater for an earth ground, so running three 6/2 cables, each with its own ground wire, would be problematic as to what to do w/ the other two ground wires in the cables.)

I've read through the NEC to attempt to ensure I will be compliant, but I am still unsure as some of the language is confusing...

I read I can run #8 (rather than #6) THWN/THHN for a 50 amp circuit when the THWN/THHN is installed in conduit "as part of a 3-wire set". I am unclear as to what exactly is meant by that... e.g. when they say "3 wire set" do they mean something like 6/3 where you have two hots and a neutral and ground wire, or is 6/2 a "3 wire set"?

Is it permissible under the NEC for me to run all 6 hots plus one earth ground in the same conduit (in which case, from what I see, I can use 1" conduit), or do I have to have a separate conduit run for each "3 wire set"? The conduit fill tables seem to indicate it is permissible, but then I ran into that "3 wire set" language elsewhere which confuzzled me.

I know it is not permissible for me to run NM cable through conduit and do not plan to do so.

Thanks.

Answer 1

TLDR: you would need 3 conduits for #8 @ 50A.

I can't vouch for what "3-wire set" means. I suspect it only made sense in context of that other question (you didn't get it out of NEC).

Cable in Conduit: Yes actually, but it sux

You ARE allowed to run cable in conduit, in two modes. You can either use it as physical armor only (for which sched 40 PVC is inadequate), or you can use it in the conduit wiring method, however that requires excessively large conduit to meet the fill rules. Both require excessively large masochism to pull those stiff, balky things. Not a job I would want!

The three grounds/1 ground point problem can be solved by pigtailing.

A single circuit of 50A is allowed to use #8 THHN, if the terminations at both ends are rated for 75C temperature. You cannot use #8 NM or UF cable at 50A, because it is not allowed to go to 75C temperature. (but weirdly, NM-B uses the 90C column only for the derate I'm about to describe.)

Thermal derate

Let's go through the math. NEC 310.15(B)(3)(a) requires you derate conductors based on how many are in the pipe. In split-phase systems, neutral and ground don't count, so you have 3 circuits and thus 6 wires. 6 wires requires a 20% thermal derate off wire max.

The only thing in the conduit is THHN wire which is allowed to 90C (terminations aren't, but they're not in the conduit). As such, you can use 55A as your basis. The 90C number for #8 is 55A.

That derates to 44A. That is all the capacity you are allowed to use, but you get to round up to the next breaker size. So 45A breaker if offered, otherwise 50A.

Conduit size and aluminum

You'll need 1" conduit for seven #8 Cu wires. If you use 3/4" EMT metal conduit, metal conduit is the ground, and six #8Cu wires will just fit at limits. Aluminum might be your friend

You could use #6 aluminum instead for cost. There's nothing wrong with it at these large sizes. However if the water heater's terminations are not rated for it (CO-ALR) you would need to pigtail the aluminum wires to #8 copper. Price the needed Polaris connectors versus the savings in wire.

You need 1" conduit for six #6Al wires + 1 bare Cu ground.