3

If I'm reading NEC correctly, my mere 1000-watt mini splits will require whopping 6 AWG wire, for the legs from the attic junction boxes through the exterior conduit to the panel.

MCA (Minimum Circuit Ampacity) per condenser nameplate: 20A

Wire Ampacity (Table 310.16) for THWN/THWN-2:

  12 AWG:  25 / 30A
  10 AWG:  35 / 40A
   8 AWG:  50 / 55A

Three 115V units, two 230V units = 12 conductors, 15 wires. Table 310.15(C)(1) says wire capacity is reduced by 50% for 10-20 conductors in raceway. So after adjustment for number of conductors:

  12 AWG:  12.5 / 15A
  10 AWG:  17.5 / 20A
   8 AWG:  25 / 27.5A

Only about 2' of the wire will be in the a attic, but I don't see any exception in NEC for that, my reading is that if any portion of the wire is in the hot environment, I have to reduce the wire ampacity. So as per Table 310.15(B)(1)(1) at 134°F (Texas attic), reduce THHN by 0.58 factor, THWN-2 by 0.71 factor.

  12 AWG:  7.250 / 10.65A
  10 AWG:  10.15 / 14.2A
   8 AWG:  14.50 / 19.53A

If that's true even 8 AWG doesn't get me up to 20A wire ampacity, unless I'm missing something. Am I missing something?

Of course, I guess I could bury the wires in insulation, to reduce the ambient temperature they're exposed to, but I favor making junction boxes visible and not buried if possible.

14
  • 2
    I think your count of 12 conductors is wrong: 240V units do not need neutral and 120V units also have only 2 conductors each, so 10 conductors. There's an opportunity to use MWBC for powering two of the 120V units though, which can save you 1 conductor, so total of 9 current-carrying plus 1 ground. Even so, you might consider adding a subpanel nearer to the outdoor units to reduce the conductor count for part of the journey. Another possibility to reduce conductor count is higher-amp circuits feeding into fused disconnects so that one source circuit powers eg 2 or 3 disconnects.
    – Greg Hill
    Commented May 24 at 21:24
  • 2
    Why in heck are you running a bunch of wires in one raceway rather than multiple separate raceways where the derate (from 90C assuming 90C wire) doesn't impact your ampacity? That's an expensive, foolish choice.
    – Ecnerwal
    Commented May 24 at 21:29
  • 2
    Yes, de-rating can get pretty onerous if you try to put lots of circuits in one conduit. Use two conduits or run one feeder to a sub-panel.
    – nobody
    Commented May 24 at 22:14
  • 3
    Even if 240v circuits need neutral you still don't count them since they just carry the imbalance. You also don't count neutral of Multi-Wire Branch Circuit (two 120v of opposing legs sharing a neutral). See 310.15(E)(1) up.codes/viewer/colorado/nfpa-70-2023/chapter/3/… Commented May 24 at 22:55
  • 1
    "bury the wires in insulation, to reduce the ambient temperature" Since wires generate heat, I wouldn't expect that to work. Also, if the attic is 120 F, over time the heat will seep through the insulation and the insulation will then hold it in overnight when the temperature drops to 80 F. But if the wires are under load, they might go to 140 or 160 without ambient heat, and the insulation will hold that in and keep it from dispersing. Insulation will hurt you far more than it will help you here. If that's unconvincing, perhaps ask a separate question about insulation.
    – mdfst13
    Commented May 25 at 13:20

2 Answers 2

8

The solution is to not put too many wires in one conduit or raceway. With one exception - there is an exception for up to 24", commonly used when joining two panels but may also apply if you have a large conduit from a big indoor junction box (with wires splitting off in various directions) to a panel.

Three 115V units, two 230V units = 12 conductors, 15 wires.

Each 115V (normally 120V but whatever) is 2 wires (hot and neutral) and each 230V (normally 240V but whatever, can be anywhere from 208V to 250V) is 2 wires (two hots, no neutral). Grounds don't count. Neutrals are not normally needed for 240V circuits except for clothes dryers and ovens and a few other things. But HVAC normally is either 120V or 240V.

So that means 10 current carrying conductors, not 12 or 15.

Next: Wire type. As I understand it, most common wire has multiple ratings and is rated for 90 C. If not, get wire that is rated for 90 C. That gets you a baseline of 25A for 14 AWG and 30A for 12 AWG.

If you simply use two conduits/raceways instead of one then your multiple conductor derate drops to 80%. Which gives you 20A for 14 AWG and 24A for 12 AWG. Which is still above the actual size you can normally use for a circuit.

Then if you need to derate for attic temperature (I'm not sure that's the case, but let's assume that it is) that is a factor of 0.71, so 14.2A for 14 AWG and 17A for 12 AWG. Which means 12 AWG will work for 15A circuits just fine - enough for up to 1,440W at 120V and 2,880W at 240V.

How big are your units and do they require 20A circuits? If they require 20A circuits then you may need to bump to 10 AWG (40A * 0.8 * 0.71 = 22.7A) but if they only require 15A circuits then 12 AWG with no more than 3 circuit/6 conductors in each conduit and you are all set.

3
  • @manassehkat-Moving Thank you, this is helpful. About this: "12 AWG will work for 15A circuits just fine", the limiting factor is that the nameplate of the equipment specifies that the Minimum Current Ampacity (of the wire) is 20A. The nameplate also specifies the maxmimum breaker, in this case 30A. But again, as you know, I have to make sure the wire can carry the 20A, after accounting for multiple wires in conduit, and temperature. Commented May 24 at 22:49
  • 1
    When I wrote the answer, I didn't know about the nameplate 20A/30A rating. Which I suspect is NOT for "mere 1000-watt mini splits". If it is, please provide the make/model as 1,000 W mini-split should be fine on 15A even at 120V. Commented May 26 at 1:45
  • 1. "I didn't know about the nameplate 20A/30A rating." Well, the 20A MCA is in the 2nd line of my post. 2. I used to run the most popular site on the web re: saving electricity, so I'm pretty versed in energy use. 3. 3 of the units are from diff. "manufacturers" (Cooper&Hunter, Pioneer, Blueridge) but they're actually all made by Midea. The EER2/SEER2/HSPF2 specs are same, only diff. is the C&H is 6kBTU vs 9k for others. They look identical. The B. spec sheet says 900W input (220-1200W in paren). From the COP we can derive 7000 BTU ÷ 3412 BTU/kW ÷ 2.16 COP (17°F) = 950W. Commented May 26 at 5:54
4

Use an MWBC to reduce the circuit count by one.

For thermal adjustment factors in 310.15(C)(1), and 120/240V split-phase power (NOT 120/208V "2 of 3 phase" power), all circuits count as 2 wires. That is, grounds don't count, and neutrals don't count in a 4-wire hot-hot-neutral configuration in split-phase.

Also, 240V HVAC equipment doesn't use neutral unless it does, so that saves you a couple bucks on THHN.

So we can now simplify the adjustments in 310.15(C)(1): 1 circuit = 100%. 2-3 circuits = 80%. 4 circuits = 70%. 5-10 circuits = 50% (very bad news). From this, I simplify the rule further to "No More Than Four" - but usually I don't have Texas attics to deal with.

So. There is a type of circuit called a Multi-Wire Branch Circuit or MWBC or shared-neutral circuit. This has the usual black-white-bare triplet of wires, except a red wire is added on the opposite phase and shares that same neutral. This works because neutral only carries differential current in that case. And we're going to have a conversation about wire colors in a minute.

So you connect two of your 120V mini-splits to one MWBC.

Now you have four circuits, and by the rule of two that's 8 wires, which use the 7-9 wire adjustment in 310.15(C)(1). That adjustment at 30% isn't so bad.

Now let's talk about wire colors. The idea that the two hots should be red and black is a myth, sourced from the fact that /3 cables are colored that way. In 120V/240V power, there is no benefit from distinguishing the two poles from each other - no load cares if they are swapped. However, in multiple circuits, you must distinguish the circuits from each other - this is a Code requirement. You should use THHN wire colors accordingly, because having to tape or paint wires is no fun and the marks fall off. BTDT.

So the easiest way to do this is make both hot wires the same color. For instance in the above configuration, you might go

  • black-white for the single 120V circuit
  • red-red-gray for the MWBC - gray is a valid neutral color and distinguishes it from the other one.
  • Blue-blue for 240V circuit 1
  • Orange-orange for 240V circuit 2

Or, consolidate circuits.

If your mini-split labeling/instructions don't require a dedicated circuit per unit, you may be able to put 2 or more on 1 circuit. Another way to get circuit count down.

You need disconnects anyway. How about use a subpanel?

I don't know if they'll be near each other, but if they will, you need a disconnect for them. A subpanel within the required distance certainly qualifies as a disconnect. Now you can run one feeder, and even aluminum feeder for significant cost savings. And then use the subpanel as both power distribution and disconnect.

1
  • Another advantage of a subpanel (which might be relevant as OP mentioned being an energy efficiency enthusiast) is that just a single pair of current transformers around the feeder is enough to monitor all your HVAC energy use. (Yes, I know you can sometimes stuff multiple conductors into a single CT, but having a single one is so much easier.)
    – TooTea
    Commented May 27 at 9:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.