可以將12 AWG THHN / THWN電線放置在埋入式導管中嗎?


4

我的理解是導管中的所有埋線必須為THWN-2。

根據Southwire的規格表here,第一段結尾為:

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

令我感到困惑的是,似乎沒有12 AWG的THWN-2。

人們對於埋在導管中的20A電路做什麼?

背景,我將所有這些與4-8的運行結合起來,以便將一個單獨的35A 240V電路連接到一個微型分路器。並認為我應該修復以前的房主在該埋入的3/4" PVC導管中運行12/2 romex的錯誤。

將20A電路增大到8 THHN以獲得THWN-2的額定值對我來說是沒有意義的,我會遇到填充問題。然後,我需要加大導管的尺寸(並非不可能,但似乎是一條極端的道路)。

在導管中運行12 AWG UF-B似乎是另一種符合規範的方法。雖然允許,但看起來也很極端,我不確定如何使用12 UF-B + 4 8 THHN計算填充量。同樣,可能必須加大導管的尺寸。

我假設我可以只運行2-12 THHN / THWN(20A電路A)並添加4-8 THHN / THWN-2(35A電路B),並使用8s之一作為兩個電路的公共接地。總共6根電線,填充率為33%。

我想知道NEC中是否有一些腳註,指出實際上8 AWG以下的電線只需要在埋入式電纜中的額定THWN即可。或者,如果有人實際生產12 AWG THWN-2,而不是SouthWire?

8

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.


8

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.


2

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.