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I've looked at multiple AC disconnect boxes, though haven't ordered any yet.
On every one I've seen so far, they have a connection for both phases of the 220v, and also a connection that appears to be for Ground.
Where do I connect Neutral? I'm bringing 10-3wG into the quick-disconnect, and haven't yet purchased the air conditioner itself. Do air conditioners even use Neutral?
Do I just tie the two Neutral together with a wirenut, akin to a regular lightswitch?
Yes, you just tie the neutrals together. There are terminals for ground to allow the box to be grounded. The terminals for the hots, of course, are to allow the disconnect.
Just be careful that the wire it is large enough for the wires involved. Also, if the wires are aluminum, you’ll need a special connector, as well as anti-ox compound on all of the connectors.
The air conditioners I've seen either use hot and neutral if they are 120 volt, or two hots with no neutral if they are 240 volt. Ground always passes through the disconnect, either via a splice or straight through if it's THHN or similar. The two poles in the disconnect thus are either used for the two hots or for hot and neutral.
How do I wire Neutral on a AC disconnect box?
Read the instructions that come with the box.
I've looked at multiple AC disconnect boxes, though haven't ordered any yet.
Don't, yet.
On every one I've seen so far, they have a connection for both phases of the 220v, and also a connection that appears to be for Ground.
That's typical.
Where do I connect Neutral?
You probably don't. Cap it off.
What did you attach it to in the electric panel???
I'm bringing 10-3wG into the quick-disconnect
All outdoor A/Cs I'm familiar with simply take 2 hots and a ground; 10/2 would have sufficed.
and haven't yet purchased the air conditioner itself. Do air conditioners even use Neutral?
Bingo, pick out your A/C and then do your electrical per the unit's instructions.
Do I just tie the two Neutral together with a wirenut, akin to a regular lightswitch?
The A/C won't have one so just cap off the extra wire inside the disconnect box.
I think you're getting confused with electric dryers and stoves which usually need 120v for the electronics; the heating elements are 240v.
Picture added per comments:
A 240v appliance with 120v electronics will require a plug like this and the appliance internals will be wired to take the power source they require.
The compressor/condenser unit for a traditional "split system" air conditioner in USA pretty much universally runs on 240 volts. It has no need for a neutral. However...
Code requires a 120 volt outlet within 25 feet of the air conditioning compressor to facilitate service of the air conditioning system (such as recovering refrigerant or drawing a vacuum on the system). The outlet must not be downstream of the disconnect for the compressor (ie it must remain live when the disconnect is pulled). Do you already have such an outlet?
If not, look at disconnect solutions that can also accommodate this service outlet. There are a variety of ways of doing it, but in every case, having the neutral in that 10/3 cable is what makes it possible. Some ideas:
Do note that not all AC compressors can be served with 10 gauge wire on a maximum 30 amp circuit. Some compressors, around 3 tons and up, may require a 40 amp circuit and corresponding (heavier) wire gauge. I haven't encountered any yet, but I have to expect there might be a few with even higher current requirement.
