1000-watt mini split needs 6 AWG?

Question

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.

Answer 1

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.

Answer 2

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.