I'm installing the wiring for a mini-split system which requires 8 ga (40 amp) wire. There is an existing conduit for a dedicated circuit (12ga) for an over the stove microwave. It's mounted on an exterior wall. The run to the mini-split outdoor unit will follow the same route. Maybe I'm being picky, but having a bunch of exterior mounted pipes/conduits looks tacky, so I'd like to combine the microwave circuit and the mini-split circuit into one conduit. My question how to determine fill capacity when using 2 different gauge wires. It will be 3 12 ga and 3 8 ga. I'm guessing 1" would suffice, but I"m not sure.

  • First what type of wire are you talking individual conductors like THHN or UF multi conductor cables? (Standard Romex or NMB can not be used outside in conduit). There are tables in the NEC starting in chapter 9 and conduit fill tables in annex C that can help but we need to know the type of conduit and type of wire. With more than 2 conductors or cables the cross sectional area max is 40% NEC chapter 9 table # 1. The next thing we need is your mini split 120/240 or straight 240 (this affects the number of wires) we add the cross sectional area for the different sizes to get the area then siz
    – Ed Beal
    Jun 9, 2022 at 23:50
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    @Ruskes I think you mean the circumference = Pi x R2 = Pi x D.
    – JACK
    Jun 10, 2022 at 0:38
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    What type of conduit are you trying to use for this run? I suspect 1" will be rather on the generous side Jun 10, 2022 at 0:42
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    @EdBeal It will be THHN/THWN, for all wires This is just a follow up comment to complete the post. Thanks again for your answer. Jun 10, 2022 at 1:48
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    The one advantage to pvc is it quick to put together , I looked at some thhn the other day and want to say it had like 5 listings I think it was thhn , thwn, thwn-2 , MTW and something else soon other than high temp fixture wire THHN will be an everything wire well it’s close now.
    – Ed Beal
    Jun 10, 2022 at 1:53

2 Answers 2


Let’s be lazy and say you have 6ea # 8 or smaller thhn as thhn is normally duel rated as thwn / thwn-2 it can be used outside in conduit. The fill tables will allow 6 conductors in 3/4 EMT table C.1,, 6 on rigid metal C.8 or going to 1” schedule 80 pvc 7 conductors so the type and size of conduit makes a difference, if the outside unit was protected by elevation and we sharpened the pencil 3/4 sch 40 may work table C.10 at 5# 6 so if 3# 8 and 3#12 it would be close.

The cost difference between 3/4 and 1” is significantly higher for 1” especially for conduit bodies that are normally used when exiting the structure. So it all depends on the type of conduit

As 3/4 EMT would be the smallest outside at mini split ground level it would look the best and EMT holds paint better than PVC so 3/4 EMT would be my choice.

  • Thanks Ed and all for the rapid response. UV to Ed! I went ahead with 1" just to be safe and know it will make for an easier pull. Not only that, I had a number of various 1" fittings left over from a previous project. And another, "not only that", I went to HD and they were completely out of both 1/2" and 3/4" pvc and the price difference wasn't all that bad between 3/4 and 1. $4 extra per stick for 1" and I only needed 7 sticks, so it becomes a $28 decision. So being on the safe side, I'll go with 1" . Prices for electrical stuff is just plain nuts right now. Jun 10, 2022 at 1:44
  • @GeorgeAnderson Oh yeah... prices through the roof.
    – JACK
    Jun 10, 2022 at 1:49
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    @JACK yeah. All the wire at Home Depot and Lowes is locked up. They won't even give it to you when they unlock the cages. They bring it up to a certain cashier and say you have to go thru check stand x to get your wire. I bought 500' of 8 gauge copper THHN today and the employee walked with me to the check out station. a 2.5" 10' length of PVC conduit is now $71, probably about $10 a couple of years ago. Just nuts. Jun 10, 2022 at 1:55
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    @GeorgeAnderson hope you had an armed guard going out to your car.
    – JACK
    Jun 10, 2022 at 2:00
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    I ordered a couple spools of #6 a few weeks ago and when they were delivering it they called me , I went out to the loading dock and there was an armored truck I did not see the platt small van on the other side the guy from platt saw my face and really thought it was funny, copper has gone nuts in the last year.
    – Ed Beal
    Jun 10, 2022 at 2:00

Physical space

If you have a multi-conductor cable with a sheath, you must take the widest width of the cable and treat it as a single wire of the wide dimension.

Multiple circuits can share a single ground wire. The ground wire must be large enough for the largest circuit. #10 ground will suffice for 25-60A circuits. However, you can only "fork" the ground with a splice, and a splice requires you treat the junction box as a junction box and follow the "box fill rules" (which I won't cover here).

With the above settled, we can talk about physical conduit fill, or how many wires you can have in the pipe. You don't need to crunch numbers - there are online "conduit fill calculators" which will do that for you, such as Southwire's.

Gory details? OK, you consider the inside diameter of the conduit to determine cross-section in area. Then the diameter of the wires (or wide dimension of cables) to figure its cross-section. The wire cross section is summed up and compared to the conduit.

  • 53% for 1 wire
  • 31% for 2 wires
  • 40% for 3+ wires

Thermal derate.

Wire ampacity limit isn't just about copper/aluminum cross-section. It's also about skin surface area - the wire must be able to shed the heat made. That is why 1000 kcmil aluminum is barely more than double the ampacity of 250 kcmil aluminum - while it has 4x the cross-section it has less than 2x the skin for radiating heat.

With conduit, you now have a bunch of wires making heat in a confined space. So wires must be thermally derated with many wires in same pipe.

TLDR: with 3 or fewer circuits, you don't have much to worry about.

Long version: For any given wire or cable, you look up the highest thermal rating the insulation permits, except with NM cable you use 90C (even though it is 60C wire) per 334.80. You look this up on Table 310.15(B)(16).

For instance #8 copper and #6 aluminum THWN-2 is allowed 90C which is 55A. However in practice you can only get 50A out of it because of 75C thermal limitations on terminals.

#12 copper NM-B is allowed 90C (for this calculation only) which is 30A. However NEC 240.4(D) limits it to 20A.

Now the derates.

In 'normal' 120/240V split-phase power, all circuits count as 2 conductors under the thermal rules. I'll talk under that assumption.

If you have 2-3 circuits (3-6 wires that count) you must derate that number to 80% of ampacity. Now #8Cu/#6Al is limited to 44A, and #12Cu is limited to 24A. That pinches on the formerly 50A #8Cu/#6Al, but doesn't bother the #12 wire at all since it's already limited to 20A.

If you have 4 circuits (7-9 wires which count) you must derate to 70% of ampacity. Now #8Cu/#6AL is pinched worse down to 38.5A, and #12 copper is limited to 21A which is still no problem.

If you have 5-10 circuits (10-20 wires which count) you must derate to 50% of ampacity. This is punishing for all wire sizes. #12 is now 15A wire, #10Cu is now 20A, And #8Cu/#6AL is now 27.5A wire (can't even run a dryer). As such, "more than four" circuits in a conduit is really best avoided.

10-20 circuits (21-40 wires which count) is a brutal derate to 35% - forget it. You're using #6AL for a 20A circuit at this point. This is the fatal flaw with novice ideas of using one giant conduit pipe the full length of a house.

Your case

For your 40A circuit, assuming 2 circuits, #8 copper or #6 aluminum can handle 44A. Note that you need a disconnect at the mini-split, and the disconnect will be rated for aluminum wire even if the mini-split isn't.

You need a 120V outlet within 25' of the mini-split outdoor unit for the serviceman's benefit. If you don't already have plans for that, change the disconnect to a 4+ space subpanel and stick a 120V breaker in there for the 120V receptacle.

For your 20A circuit, as we crunched in the above example, you are fine with #12.

Ground can be combined if you have an appropriate place to fork the splice, but let's assume we don't. Also, grounds can be bare, and if so, they are much smaller diameter. But you must enter this manually into the fill calculator as they don't have it pre-programmed.

  • 3 x #12 wires
  • 1 x #10 Cu or #8 AL wire
  • 2 x #8 Cu or #6 AL wire.

Plug those into the conduit fill calculator and it will spit out the pipe size needed. No DIYer ever ever got fired for buying extra-large conduit! 3/4" conduit will do this without doubt. You could squeeze it into 1/2" conduit with #8 and bare grounds.

Oh, one other thing. If you go with EMT metal conduit the ground wires can go away. Now you have 2x#12 and 2x#8, fullstop. Now you can get it done in 1/2" conduit (31% fill for #8 and 39% fill for #6).

  • WOW, thanks for the thorough answer, Harper. I thought I had most of it covered, but missed the 120v outlet for service to the outdoor unit. Thanks for that. I'll probably just add another hot to the 120 v circuit and turn it into a MWBC, of course properly connected to opposite poles with handle tied breakers for the service outlet. Jun 10, 2022 at 4:35

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