# 可以將12 AWG THHN / THWN電線放置在埋入式導管中嗎？

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

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.

## 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.

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.