What size wire in conduit before feeding 20A nm circuit?

Question

I want to run five 20A 120V circuits and one 20A 240v circuit from my panel (13 circuit carrying conductors, 6 grounds), through a 25' long conduit in my crawlspace, to a junction box where I then splice those in to romex.

My understanding is that the romex will need be #12. What I'm wondering is what size do the conductors in the conduit need to be?

Is the following thinking correct:

1. Using Romex limits the circuit to 60degrees, so using THHN in conduit at 60 degrees means the conductors must be #10
2. Using 13 circuit carrying conductors in the conduit means I must derate by 50%, which for THHN would be to use #10 (though this is for 90degrees)

Can I say both of these restrictions are met by using #10's in the conduit? Or do I need to apply 2 on top of 1 and then size to #8's?

Does the size of my conduit matter here beyond fill restrictions (which I just punch in to a calculator online)?

(or is there something else I'm missing entirely)

Answer 1

(or is there something else I'm missing entirely)

You only need one ground wire per conduit (or metallic conduit and it is the ground.) That is sized for the largest ampacity circuit in the conduit, and serves all of them. In your junction box you join all the ground wires to the box if it's metallic, and to each other.

2 parallel but separate conduits lets you (effectively, because the applicable 90°C derate does not go below the circuit ampacity) avoid the conduit derate and use 12AWG THHN/THWN2. That is almost always less costly than upsizing the wires. If it happens to be the case that it makes sense to have them go to separate junction boxes at the far end rather than the same big one, that's also typically a benefit, as @JACK mentions in a comment.

Also, you've miscounted your Current Carrying Conductors. The 240V circuit is 2 CCC, even with a neutral, as the current in the neutral is only the imbalance between the hots, so it only counts as 2 CCC for derate, so you have a total of 12 CCC, not 13.

This math is one reason why MWBCs can be useful for avoiding derate. 2 120V circuits on 4 wires are 4 CCC - 1 240V MWBC on 3 wires is 2 CCC and supplies the same 2 120V circuts. It is more difficult/expensive to get AFCI/GFCI protection for, though, and impossible to get both at the same time for some panel makes/models. But in any case a 240V circuit with (or without) neutral is only 2 CCC.

Put 6 CCC in each conduit and you only have 80% derate, which is 24A on 12 AWG copper at 90°C, so it's above 20 and you're good.

NM/B (Romex) is limited to 60°C, but that only applies to the NM/B, not to other wiring methods (such as THWN2 in conduit) on the same circuit.

If you choose to put them all in one conduit, you'll need 10AWG, derated 50% (10-20 CCC) to 20A. Not really applicable here, but 7-9 CCC is 70% derate which comes out 21A and would also allow use of 12AWG.