0

I want to pull two circuits through 1/2" EMT. Three of the wires would be 12AWG and other three 10AWG. I've found a couple of tables that say I can run 5 10AWG or 9 12AWG. Is my mixed gauge set safe?

I'm having a hard time finding solid 10AWG by the foot. The stores and sites I've found only sell stranded or solid in 500 feet lengths. I only need 35. I understand stranded wire to be larger in diameter than its solid equivalent. Is this a factor in capacity calculations?

The three 12AWG are two loads and a neutral for a NEMA 6-20 receptacle for a 2HP planer (15 amps) and a 3HP shaper (20 amps). The three 10AWG are two loads and a neutral for a 14-30 receptacle for a 5HP bandsaw. The bandsaw doesn't require the neutral. I want to make the receptacle more versatile by adding the ground to it. I'm using the conduit as the ground.

  • For good reason, because nobody wants solid 10AWG THHN. It would be a specialty application certainly not for general pulling. It's super stiff and a nightmare to pull. Believe me, having 1 solid #12 in a bundle of 8 is bad enough. If you are married to solid wire, then I would call that an ambitious pull, and we caution against those here. – Harper Sep 26 '18 at 19:47
2

Solid vs stranded

A lot of people who work 99% in Romex find themselves gravitating toward solid wire because "it's the devil they know", they have no idea the difference, and they fret about terminations.

I work 99% in conduit and stranded is the only way to go. When you have a solid wire in the bundle (I'm cheap, I reuse wire). you know it and it's a fighter. As far as terminations, they're a little more finicky sometimes, but it's not a big deal. If one is out of your comfort zone, pigtail a bit of solid wire (wire nut splices solid-stranded are just fine). You want to be in stranded. Seriously.

#10 is the breakover above which all wire must be stranded. As such, #10 solid is a black swan - nobody who does conduit for a living wants to pull it, and so it is relegated to a specialty item. What's more, every termination meant to take #10 (eg NEMA x-30 receptacles) is designed for stranded wire.

We generally advise people to underfill conduit to keep the effort of pulling easy. That's why we'd tell you to use 2" conduit for three #2’s. Otherwise you could run out of swear words and have to call the guy with a truck full of pulling tools, and he probably won't want to come in for just the pull. Since you are near conduit fill limits, you should expect a challenging pull. If you attempt this with solid wire, you should expect a very, very challenging pull.

You may be tempted to dismantle the conduit and reassemble it around the pulled wire. That is not allowed, and will work particularly badly with solid wire.

Conduit fill

RME describes the gory details and the source material. I just google "conduit fill calculator" and pick one I like. One that lets me state # of wires of each size, and does the lookups for me.

For conduit fill (NOT derate), neutrals and grounds count, as do for instance fiber optic lines or Class II wiring, since they all take space.

Solid vs stranded doesn't matter, the slightly larger size of stranded is a "gimme". There are a lot of gimme's in fill calcs, wait til you get into junction boxes!

You're fine, but you are also essentially full.

Conduit wire derate

This won't affect you because you are full.

Too many wires in one conduit will get too hot (you're a long way from this problem). In residential/split phase, each circuit can only have 2 conductors because like I said, neutrals and grounds don't count. But even if you were doing something odd (lopsided hot/neutral on 3rd circuit? Two 3-phase delta circuits?) it matters not: 4-6 wires have the same derate. 310.15(b)(7).

Look up any wire ampacity chart [called 310.15(b)(16)] and note the top row, which calls out which wires each column applies to. THHN and THWN-2 use the 90C column. If you happen to have an odd duck called THWN (obsoleted by THWN-2), that uses the 75C column. Follow those down to your wire size. That is the point we derate from: example is 40A for #10 copper THHN.

With 4-6 wires we derate 80%. So #10 THHN derates to 32A. #12 THHN derates to 24A. That puts you in the clear with both wire sizes, since you are already capped at 30A and 20A respectively due to other Codes (240.4).

Generally you can have 3 single/split phase circuits in 15, 20 or 30A without even thinking about it, and 4 if they are all 15-20A. Larger than that, you have to break out the sharp pencil, as above.

  • Thanks for the detailed answer. After reading what was posted earlier, I decided to use a second conduit run for the 12AWG. There are four 90 degree bends in the current conduit run plus three jogs. I was worried that I would be able to pull six wires by myself. Besides, its an excuse to buy more tools for cutting conduit and finishing the raw ends. – curt Sep 27 '18 at 0:59
  • Oh, you've got a problem there. You can't have more than 360 degrees of bend between accessible points, and jogs count, so you are over bend budget. Must add conduit bodies or junction boxes at appropriate points. These must remain accessible forever. 1 is mandatory, more help a lot. Given the bends, I concur with either 2 conduits or larger conduit. Also, extra space :) – Harper Sep 27 '18 at 1:06
1

To calculate fill size of different size conductors we have to use Tables 4 and 5 in Chapter 9 of the NEC. Code only allows fill to .122 sq/in and 3 #12 + 3#10 comes to .1032 sq/in if you are using Thhn, Thwn or Thwn-2 type insulation. So from the information you have given us, you are ok to use it. I do caution that just because it says you can I have found some times it's an extremely hard pull. You might want to consider using 3/4" EMT.

Notice, a few days ago there was a large discussion about having to derate conductor due to pulling more than 4 conductors in a conduit. In your case you would have to derate the conductors by at least 80%. This may or may not be able to operate equipment you are connecting up due to overheating and voltage drop. So to a DIYer I would advise you run no more than 4 conductors in any one conduit. Otherwise you may be running into some trouble.

MY recommendation, especially if you are running the conduit new would be to run 2 - 1/2" EMT conduits, one for the #10's and 1 for the #12's.

Sorry I can't help you on finding small cuts of wire. Also you calculate fill on the wire size whether it is stranded or not.

Good luck

PS Just in case your asking how would I find out whether or not I could run all of the conductors in one conduit. We would much need more electrical detail about your project.

  • Ugh, please, just --- the fact that he's running 3 #10s and 3#12 implies two circuits, with the third wire being neutral or ground. Neutral and ground don't count toward conduit derate. So he has only 4 wires for conduit derate purposes. That puts him in the 4-6 conductor range which requires an 80% derate, but again, that is off the 75 or 90C column depending on wire type only, and does not stack with the 240.4 limits. So it will not affect those limits. OP is fine. – Harper Sep 26 '18 at 20:01
  • @Harper You are assuming he is describing a neutral and a ground, but is he? Like I said we need more detail. – Retired Master Electrician Sep 26 '18 at 20:05
  • Not to answer his question, we don't. You want a lot more detail because you are intentionally searching for totally un-asked things to have a handwringing session over -- and that is something new for you and I don't understand why you are doing it. – Harper Sep 26 '18 at 20:14
  • @Harper - In short, the general public ask questions to stack exchange in order to get answers from experienced people. But they are the general public and we do have to assume that they have no experience or training in specialized fields. I believe that if we see something that may become a problem because of the question they are asking, then we have an obligation to point it out and issue a caution. I am not intentionally searching for an un-asked question simply issuing a caution. – Retired Master Electrician Sep 26 '18 at 20:34
  • 1
    Even with derate you can use the 90 degree collum for thhn/thwn so you can safely run more and still have plenty of ampacity. – Ed Beal Sep 26 '18 at 21:39
1

You're fine on fill, but not by all that much

A 1/2" EMT conduit has 78mm2 of usable fill. Your 10AWG THHNs take up 13.61mm2 apiece, and your 12AWG THHNs take up 8.581mm2 apiece (these numbers are from NEC Chapter 9, Table 5, and hold for both solid and stranded wire), so for your circuits, you are taking up 66.573mm2 of that space with your wires, leaving 11.427mm2 for more wires as the EMT supplies the grounding path. (Pro tip: for rough numbers on conduit fill -- use metric units and round wire sizes up to the nearest integer, this makes the fill math quite a bit easier.)

Derate isn't a concern at this point

Conservatively, six wires in two circuits is a pair of multi-wire (120/240V) circuits, which means you have 4 current-carrying conductors (as the neutrals of MWBCs don't count against that number). This puts you into the 80% band on fill derates, which is a nothingburger as you are working from 90°C numbers when derating wire ampacity while the termination temperature limits at 75°C work independently from that.

If those extra wires are equipment grounds though, then don't bother. The EMT is a fine grounding path already, and even if this was the one circumstance where it wasn't (EMT exposed on a flat roof), a single 10AWG EGC could be shared between both circuits without raising Code issues (just like both circuits in a 14/2/2 NM cable share the same ground wire). Furthermore, it's legal to pull a bare ground wire thru conduit, and this saves on fill compared to an insulated ground wire.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.