My understanding is all buried wire in conduit must be THWN-2.

Per Southwire's spec sheet here the end of the first paragraph reads:

All Sizes Rated Both THHN and either THWN (sizes 14, 12, and 10 AWG) or THWN-2 (sizes 8 AWG and larger).

It is baffling me that there doesn't seem to be THWN-2 in 12 AWG.

What do folks do for 20A circuits buried in conduit?

Background, I'm doing all this in conjunction with running 4-8's for a separate 35A 240V circuit to a mini-split. And figured I should fix the previous homeowners error of running 12/2 romex in that buried 3/4" PVC conduit.

Upsizing a 20A circuit to 8 THHN just to get a THWN-2 rating doesn't make sense to me, and I'd run into fill issues. I would then need to upsize the conduit (not out of the question, but seems an extreme path to take).

Running 12 AWG UF-B in the conduit seems to be the other code compliant. While permitted that seems extreme as well and I'm not sure how to calculate fill using 12 UF-B + 4 8 THHN. Again, likely having to upsize the conduit.

I assumed I could just run 2-12 THHN/THWN (20A circuit A) and add the 4-8 THHN/THWN-2 (35A circuit B), and using one of the 8s as a common ground on both circuits. 6 wires total, fill would be 33%.

I'm wondering if there is some footnote in the NEC that says wire under 8 AWG actually only need to be rated THWN in buried conduits. Or if someone actually makes 12 AWG THWN-2, just not SouthWire?

  • Circuits up to 60A only require a #10 ground. Do not run UF in conduit, besides being a hard pull and hard to legally support when entering a cabinet, the code requires calculating conduit fill of a cable assembly as a single round conductor based on the largest dimension of the cable. Normally that requires bigger conduit than would seem obvious. Jul 12, 2020 at 16:00
  • 1
    @NoSparksPlease Because UF is very, very flat and wide instead of round like it could be. Making it a worst case scenario. A 6/3UF requires a 2” conduit. (You can easily use the Harper shortcut: 1 wire in conduit needs conduit ID to be 138% of wire OD). Jul 12, 2020 at 16:05
  • @Harper-ReinstateMonica And directly to this question 53% fill of 3/4" sch 40 PVC is only 0.269 in². Encore 12-2 uf has a large dimension of 0.415" making it's area 0.54 in², 53% fill requiring 1.25" pvc. Jul 12, 2020 at 16:20
  • @NoSparksPlease I think you made a math error there. I try to simplify it by using “circular inches” (millions of circular mil if you prefer), so 0.415” diameter squared becomes 0.172 circular inches, or 172 kcmil. Handle conduit diameters the same way, and it takes pi out of the equation. Jul 12, 2020 at 17:43

3 Answers 3


THWN is rated for wet locations

As Ecnerwal discusses.

The issue is thermal

The difference is at the top of Table 310.15(B)(16).

  • THWN is allowed 75 degree C running temperature.
  • THWN-2 is allowed 90 degrees C.
  • THHN also is allowed 90 C

Southwire is making a disclaimer: they are saying they don’t guarantee #14-10 will be THWN-2. What’s on their mind is a different table, NEC 240.4(D), which places artificial limits on #14, #12 and #10 wires, to limit them to 15A, 20A and 30A regardless of tables or thermal calcs.

Their logic is “heck, you can’t run these wires at thermal limits anyway, so why do they need to be -2?”

However this still can bite you if you have multiple circuits in the pipe. You must still do thermal derate calcs based on 310.15(B)(3)(a). Those derate calcs come off the highest temp allowed for the insulation. If the wire is THWN and not THWN-2, you’re forced to calculate off the 75C number instead of the 90C number. That won’t make a difference with up to 3 circuits in single/split-phase territory (2 in 3-phase territory) but it’ll be problem getting to 4 circuits (or 3 in 3ph).

Cable in conduit is about swept area

Anytime you put a non-round cable in a conduit, you must treat it like helicopter blades: it occupies space like a round wire of the large dimension. Because it twists.

This means an oval cable, especially a very wide oval cable like UF, sweeps an awful lot of area in the conduit - to the point that UF requires outlandishly large pipe. One 6/3 UF requires a 2” pipe, no kidding, do the calc yourself! Here’s Harper’s shortcut: A single cable needs 138% of its wide width as the interior dimension of the conduit, but this gets much worse if there are 2+ wires present.

NM or NM-B is already wrong in outdoor conduit because of the “wet rating” problem of course, but UF isn’t an automatic upgrade because UF takes more space.

You do need to remove the NM-B, but not just over the wet rating; it’s because the circular area of the NM-B, plus the #8 wires, has most likely overstuffed the conduit.

  • Thank you for the through and referenced answer. I understood the rated temp difference, but I thought (not sure why now) all outdoor buried in conduit wire had to be 90C, and hence THWN-2.
    – jb510
    Jul 12, 2020 at 16:20
  • @jb510 Yup, that only applies if you’re bundling enough wires that you need the 90C for derating purposes, i.e. using my “no more than four” rule of thumb. Jul 12, 2020 at 17:34

Not sure where you got the idea that you need THWN-2, only.

All you need is the W.

TWN, THWN, RHW, XHW, XHHW, etc. are all perfectly fine, because they have the W that means they are waterproof, that means they can go in outdoor conduit. Most THHN is also THWN and MTW (multiply rated.)

If you are in a situation where the wire temperature may be extreme you might need to get picky for insulation temperature rating; but to be in exterior conduit (buried or not, they are all wet locations), all it needs is the W.


For most applications you would use wire in household installations, you don't need the THWN-2. The 90C rating is pointless for anything other than when you need to de-rate wire for having a lot of conductors in a single conduit. Most residential products have terminals rated for 60 or 75C anyway, so you still end up sizing the wire for that anyway, meaning the "-2" is pointless.

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