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Can I have one 240V 30A circuit with 3x8AWG (clothes dryer) and two 20A circuits (washer and bathroom circuit, 120V with neutral each) together in one 3/4” flex SMC?

I believe so but I haven’t found a definite answer. In terms of fill it should be definitely ok (3x8AWG+4x12AWG is 28.39% fill for 3/4” flex FMC.

In terms of de-rating, if all conductors were 8AWG I could have up to 6 of them while maintaining 40A ampacity. However, I have only 3 8AWG and 4 12AWG. If I had only 12AWG, I could have 9 of them. I don’t know how to do it mixed but given that only 3 are 40A I feel it should be ok.

Background: My dryer has a dedicated 240V 30A circuit over 3x8AWG within said 3/4” flex FMC.

The washer and the bathroom are currently on a random 15A circuit with plugs and stuff in other rooms.

Per code, washer should be dedicated circuit as should be the bathroom (latter should be 20A). Since all of them are close it would be perfect if I could pull two dedicated 20A circuits and reuse the existing dryer conduit.

EDIT: Dryer circuit is 30A, not 40A.

divB
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    Can you confirm that the circuit/s have a ground or are using the conduit as ground? The way the 120v circuits are set up, it is possible the dryer is using old NEMA 10 type(hot,hot,neutral) and that really should be upgraded to NEMA 14. – crip659 Jun 21 '23 at 11:20
  • @crip659 Yes I know it's grandfathered but I'd like to make it "proper" if there's a fairly straight forward to do it. I don't get too much breaker trips but that's maybe I am careful not using that circuit too much while laundry is on. There is so much other stuff on it and I don't feel good about it. – divB Jun 21 '23 at 17:23
  • @crip659 The current 15A bathroom+laundry+others circuit (squeezed into overstuffed conduit currently) uses EMT as ground. Dryer has 3 wires (2xhot, 1xneutral) and also uses conduit (flex FMC) as ground. As it seems, this is already problematic. I was pretty sure it is a NEMA14 plug though but I will cross-check – divB Jun 21 '23 at 17:25
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    Some useful info in https://diy.stackexchange.com/questions/174380/flexible-metal-conduit-length-limits-with-grounding As you hint at, using the metal shell of flex as ground has issues and is only permitted up to 6' even if you use approved fittings on each end. IIRC even with that you are limited to circuits of 20A or less, so not fitting your setup. – Armand Jun 21 '23 at 17:44
  • @crip659 ok it says “NEMA 10-50R”. Seems I should upgrade this? – divB Jul 11 '23 at 02:04

1 Answers1

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It fits, and it fits (at 34.63%) if you add the 10 AWG copper ground you need, because flex does not get the pass on needing an explicit ground wire ground that rigid metal conduit does.

Unless perhaps your dryer is a 240V-only hot-hot-ground? 40A is unusually large for a dryer circuit, so it might be otherwise unusual and not have any 120V components (commercial unit?) - or it may be oversized for your current dryer and you should leave the wires but install a smaller breaker suitable to what your dryer actually needs...

In terms of ampacity derating, you only have 6 current-carrying conductors, because even if the third 8 AWG is a current carrying neutral, it's only carrying the imbalance current of the two 8 AWG hots, and is therefore not counted as a current carrying conductor. So those are good for 44A (80% of 55A) on the thermal derate, thus 40A in practice. The total current of all three can't be any more than the current of just two of them fully loaded. Unless you're really out in the weeds and have 3-phase...

Ecnerwal
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  • I checked and dryer is 30A, not 40A. I updated my question. Good point with the ground. I was sure that flex could FMC also be used as a ground. I will cross check the plug (NEMA10 vs 14) and whether the flex conduit was used as a ground illegally and will get back. – divB Jun 21 '23 at 17:27
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    FMC *might* be permitted as ground if it's less than 6(?) feet long, but I take that as an indication that it's not really a *good* one and assign it a wire regardless. I don't worry about that with EMT, by contrast. If you're into "nothing more than code minimum" and it's short, you can go there, if you choose. – Ecnerwal Jun 21 '23 at 18:49