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I'm trying to find out if THHN 10/2 (No ground) stranded will be ok in place of 10/2 Solid or just 10 ga. There's none solid 10 ga available in my area.

Four 30amp circuits in my panel all run in EMT 3/4. Fill calculator says OK One run of 10ft to an LX-30 Receptacle One run of 50ft to an LX-30 Receptacle Two 50 ft run to two separate Disconnect Boxes for Mini Split System(s)

I'm probably overthinking this but I can't find a definitive answer. I'm familiar with heat issues that can arise from broken strands. Just can't find Solid.

Insights?

Thanks

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    Do you mean L6-30 receptacles?
    – Ecnerwal
    Commented Nov 26, 2022 at 0:45
  • Is this a single 3/4" EMT, or multiple 3/4" EMT conduits?
    – Ecnerwal
    Commented Nov 26, 2022 at 13:23

2 Answers 2

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EMT properly constructed takes care of ground.

Stranded is fine and actually preferable. The only reason to disrecommend it is that it's difficult to put on plain old 120V receptacles - it's not allowed in the backstab and challenging to get it to attach properly to the side screws. However it doesn't sound like any of your attachments will be that (and you can just pigtail if they are - stranded joins solid just fine at a wire nut, I do it all the time.

Thermal derate is a problem

The main problem I see is your thermal derate on those wires. Four circuits in 1 conduit requires a 70% thermal derate (off the 90C thermal rating). This doesn't affect 15A and 20A circuits, but for 30A circuits it's a problem. 10 AWG copper derates to only 28 amps.

Generally 5 or more circuits in the same conduit is Right Out, because the thermal derate is 50% at that point.

Now it's perfectly fine to do that for hardwired loads like mini-splits which are less than 28A after your 125% continuous load derate. (so less than 22.4 amps).

The problem would be the sockets. You can't put a 30A socket on a 28A circuit. NEC 210.21. At the least, you would need to label these outlets for specific appliances, and assure the loads will be under 28 amps (including derate if applicable).

I hope the two L6-30 receptacles aren't for two generators. Making that work is very tricky.

Replace the 2 disconnects with....

You need two disconnects within line-of-sight to the mini-splits. But why take the wall space and expense of two disconnects? You can do that job in a single enclosure by using a subpanel. If you want to be fancy it can be a 2-space subpanel (also known as a disconnect) with a 30/30 quadplex breaker in it.

But here's the value in that: you don't need two 30A circuits anymore. You can have one 60A circuit.

Now you have three circuits in the pipe: two 30A with #10, and one 60A with #6 copper. The thermal derate for three circuits is 80%. So that means

  • #10 THHN derates to 32A (just fine)
  • #6 THHN derates to 60A (just what you need)

This also saves 2 spaces in your main panel. It would be a 240V-only sub - no neutral unless you want to squeeze a #6 neutral into the pipe.

This also throws open the possibility of using #4 or #2 aluminum for some significant cost savings, but that would require larger conduit in the first 10' segment. (or the whole way for #2 aluminum).

Since #2Al is 90A, you could also power the other L6-30 circuit out of that subpanel for additional cost savings.

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    I asked for insights, I couldn't have asked for anything so inspirational and educating, thank you so much for this! Commented Nov 27, 2022 at 14:11
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Properly installed stranded wire is rated the same as properly installed solid wire, and virtually all devices are listed to use either type.

There is no reason to run cable in conduit, and many reasons not to. So just buy spools of wire, or cut to length wire, not 10/2 cable. Stranded wire pulls easier than solid in conduit. Any wire pulls much easier than cable in conduit.

Strands do not break in the middle of properly installed wire, so the main thing you need to be careful of is correctly stripping the ends for your connections, without damaging the wire itself. Cut only partway through the insulation, not all the way through to the point that the wire strands are nicked. And don't bend the wire back and forth 100 times trying to snap it like a coathanger anywhere.

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  • I wound up buying 10ga-stranded THHN and colored tape. Great insights and so appreciate it. Commented Nov 27, 2022 at 14:12

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