I'm installing a mini-split and the outdoor condenser requires a 15A circuit. I have installed a sub-panel about 10 ft away on the same outdoor wall. I'm planning to use the 15A breaker in the panel as a disconnect for the condenser, since it is line of sight. The condenser will have a 1/2" NM flexible conduit whip. I want to install a 20A service receptacle (required per 210.63) between the panel and the condenser.

I will have 2 breakers, one double-pole 15A breaker for the condenser and one 20A single-pole for the service receptacle.

Can I use one 1/2" EMT conduit from the panel to an outdoor single gang box with a 20A GFCI, and then transition to the NM whip? One circuit with a hot+neutral would feed the receptacle and two hots would feed the condenser. The two hots for the condenser would run un-interrupted from the panel through the outdoor single gang box to the condenser. See schematic below. Schematic showing conduit from panel to condenser

I have some left-over pieces of THHN that I would like to use. I assume there is no issue running a black and blue #10 to the condenser, and a #12 white and black to the receptacle. I would use a green #12 for ground. I calculated the conduit fill at 26% for these wires in 1/2" EMT.

Any issues with this setup? My AHJ right now accepts both 2017 and 2020 NEC, I just have to tell them which one I'm following. I'm planning to use 2017 since I don't want to have to use a GFCI/AFCI breaker for the condenser. Can the 20A GFCI receptacle be on a regular breaker per the 2017 NEC?


  • 1
    I think you mean LFNC, not NM, since NM would not be compatible with running wires through it, nor be a legal wiring method outside.
    – Ecnerwal
    Nov 8, 2022 at 16:50
  • 2
    That's Liquidtight Flexible Nonmetallic Conduit but NM as shorthand for "nonmetallic" is already taken for an interior-only cable type many people call Romex (a brand name).
    – Ecnerwal
    Nov 8, 2022 at 17:24
  • Yes, I meant LFNC. I used NM because of how Southwire brands it: "Ultratite® Type NM, Liquidtight Flexible Nonmetallic Conduit". I will refer to it as LFNC Nov 8, 2022 at 18:06

3 Answers 3


There's no such thing as a "service receptacle"

What it means, quite directly, is this:

Most houses have general-use receptacles around the outside of the house, for holiday lights, barbecues, bouncy castles, hedge trimmers and all sorts of things. Make sure one of them is within 25' of the heat pump so the maintainer can plug in their vacuum pump, which has a 25' cord.

That's it. That's the whole of the rule.

If there's already such an outlet within 25' as the cord flies, you already comply. Finished, move on.

Otherwise, expand your network of general-purpose outdoor receptacles so there's one nearby. Putting it right next to the heat pump is pointless and counter-productive. Dedicating the circuit is worse. Why? GFCIs don't last outdoors, and heat pumps don't need vacuum-pump service very often. A GFCI on a dedicated circuit will be stone dead next time it is needed. What good is that?

You want that outlet to be on a circuit you use all the time, with GFCI protection ideally indoors but at least where a GFCI problem will be soon detected. As such, you are better off extending an existing outdoor outlet circuit, or if you don't have one, extend this one to be as generally useful as possible.


I would use a green #12 for ground. I calculated the conduit fill at 26% for these wires in 1/2" EMT.

EMT provides the ground, and there is no need for a ground wire. However if you do fit a ground wire, you must follow the rules. One is that you must land at every junction box - so you must splice to the box and not ignore the box and splice only to onward wires.

To save conduit fill, use a bare copper ground.

is no issue running a black and blue #10 to the condenser, I would use a green #12 for ground

Negative, Ghost Rider. Your 15A circuit requires #14 hots and #14 ground. You are enlarging to #10 hots but only #12 ground.

250.122(B) Increased in Size. Where hots are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type grounds, where installed, shall be increased in size proportionately according to the circular mil area of the hots.

So #10 hots = #10 ground.

Hey, you're the one who wanted a ground wire in EMT :)

Other than that, the plan looks fine.

However, with the "service receptacle circuit" possibly removed, you might only need a simple disconnect instead of a subpanel. Remember too that subpanels have a working space requirement that must be flat and kept 100% clear 100% of the time - 30" wide (not centered), 36" deep and 6'6" tall. Nothing can ever be stored or let to grow there.

I personally am very against putting subpanels in places where people will tend to be inclined to put things. That forces someone to be the "working space nazi" and nobody wants to do that.

Can I use one 1/2" EMT conduit from the panel to an outdoor single gang box with a 20A GFCI, and then transition to the NM whip?

With two circuits in the EMT? Sure. With 15-20A circuits in conduit, there's a simple rule (derived from a stack of rules that KMJ overshared lol): Up to four 15-20A circuits in any size conduit. With 1/2" conduit that's not even a concern, since 5 circuits won't fit lol.

Also, there is no useful use in distinguishing the colors of the 2 hot wires. In fact, it helps identification in a crowded conduit if both hots are the same color. So if you have enough blue, use blue-blue.

  • Hey, I usually feel better when I show my work. :)
    – KMJ
    Nov 9, 2022 at 7:25
  • Harper, thanks for your usual thorough comments. I was under the impression that the ground could always be one size smaller then the hots, but that is clearly not true. I will not use my leftover #10 wire. I have a bunch of #12, and could run: #12 blue and white to receptacle, two #12 blacks to condenser, with a #12 ground for everything. I do have 30" wide clearance in front of panel. Don't need disconnect since I have panel nearby, and don't want to run 10 ft of horizontal LFNC because it looks bad. I figured I transition from EMT to LFNC in the service outlet. Nov 9, 2022 at 15:02

Only trick, and it's not much of one, would be box fill (having an adequate sized box.) two #10 passing through is 2.5 cubic inches each, 2 #12 terminating in box is 2.25 c.i. each, the #12 ground is 2.25 ci and the device is 2x 2.25 c.i. - 16.25 c.i. minimum for the box, and that's easy.

There's no need for two different hot colors on the 240 circuit. If you have enough blue to use two blues, it would offer less room for confusion than black hot on the 120 and one black hot on the 240 circuit.

The EMT does not require an additional ground wire (it is a listed ground, if done correctly) but you can have one in there if you like.

Your #10 wire is vast overkill for the 15A circuit, and you might consider swapping it for some large amount of #14, or cash, rather than going for overkill, but there's nothing wrong with using it if you really want to.

  • Don't you have to count the device (receptacle) as well? I came up with 19.75 rather than 16.25 when I did the math. Either is easy, so maybe it doesn't matter.
    – KMJ
    Nov 8, 2022 at 17:08
  • 1
    and the device is 2x 2.25 c.i. already in there. Not sure how you came up 3.5 c.i. higher, TBH. There are no internal clamps in this type of box, and it's a peculiar number anyway since it's not a multiple of either of the wire sizes involved. Admittedly, a near-full to spec box is often far more unpleasant to work in than one vastly oversized.
    – Ecnerwal
    Nov 8, 2022 at 17:14
  • Not sure either as I didn't show my work, I will correct my number to match yours.
    – KMJ
    Nov 8, 2022 at 17:17

Your plan seems sound. Running multiple circuits in a single conduit is common and accepted by code. Here's the factors I am aware of to keep in mind.

  1. Derating due to conductor count. Table 310.15(C)(1) requires you to derate to 80% of capacity since you will have four current carrying conductors in the same conduit.
  2. Derating due to temperature. I suspect your conduit is going to exceed 86F so you need to check Table 310.15(B)(1) for derating. You take the lower of the two factors from this and 310.15(C)(1). In this case that probably means you are working with 80% as your rated max unless you're in Arizona. Once you have both of those figures, you can consult Table 310.16 to determine the necessary conductor size. The good news in this case is that your 12GA THHN is actually rated for 30A on that table, so even with the 80% factor your wire is still properly sized. Your 10GA THHN is overkill at 40A rated on 310.16, but you already knew that.
  3. Box fill. There's going to be a lot of conductors in that box. Just make sure you're not overfilling it. I show 16.25 cu in required based on your description.
  4. Conduit fill. Not an issue here, this is a reasonable number of conductors for 1/2 EMT, giving you 27% fill.
  5. Color coding. I would use black, white, and green as usual for the 120V circuit and the shared ground, then another color for the 240V circuit. Two reds, two blues, two violets, whatever you've got or can get. It will make things obvious for the future. Yes the 10ga/12ga difference makes it obvious but I'm all about making it so obvious even an idiot can figure it out.
  6. Grounding - your whip is not conductive so you need a ground for at least that portion of the run. Personally I would still run a conductor the whole way even though EMT can be a ground, for the same reason that 440.9 is in the code. Even though this isn't a rooftop install, it's subject to some of the same physical abuse across time.
  • Good point about the box fill. I had bought a 18.3 cu in box, but will see if I can find the deep one that is 22.5 cu in, just to be safe. The GFCI outlet will take up some space as well Nov 8, 2022 at 18:16
  • Yes, I have enough blue, so I could just run 2 blue's to the condenser. I do want to make things obvious for the future Nov 8, 2022 at 18:17

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