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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?

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    Call it neutral. Do not call it common. Neutral must be kept separate on each and every circuit. If you have 2 circuits in the same junction box, you must have 2 groups of neutral - they must not all go together! Commented Dec 21, 2022 at 20:33
  • You might have a panel where neutrals and grounds go onto the same bar. That is a weird exception because neutral and ground are supposed to be bonded in that panel. Normally, neutral and ground must be kept separate from each other (and both brought separately). You never combine them (except as I say, inside the panel with the main breaker). Commented Dec 21, 2022 at 20:37
  • @Harper-ReinstateMonica I'm pretty sure I was just confused, and the bar is just for ground, with neutral either being capped off or twisted and capped to the AC neutral if present.
    – Jamin Grey
    Commented Dec 22, 2022 at 22:34

5 Answers 5

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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.

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  • The 10-3wG is copper. Won't have a clue about the AC side of things until I purchase one about a year from now.
    – Jamin Grey
    Commented Dec 21, 2022 at 16:03
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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.

enter image description here

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  • Thanks! I have already ran 10-3wG, and wired up the common to the circuit breaker box's common bar like normal. I can just cap it off if the AC doesn't use Common. Do electric dryers and stoves get their 120v from their 240v connection, or do I have to add a seperate 120v outlet behind the stove?
    – Jamin Grey
    Commented Dec 21, 2022 at 16:08
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    @JaminGrey That's fine, just cap it off inside the disconnect box. Too much wire is better than not enough =). I edited my answer to explain how 240v appliances get 120v to the electronics. Do note that some appliances have 240v electronics so that's why you always need to read the appliance installation manual to find out exactly what you need.
    – MonkeyZeus
    Commented Dec 21, 2022 at 16:18
  • @JaminGrey most appliances that use both 240 and 120V get the 120 from the 240V supply by using one of the two hot legs combined with neutral. However appliances do exist that require a separate 120V supply from a separate 15A or 20A breaker. As with the A/C, it helps to know what you're installing. If you want to prepare a kitchen or laundry room to be highly adaptable to arbitrary future appliances, ask that question!
    – jay613
    Commented Dec 22, 2022 at 11:52
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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.

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  • Ground generally does not just pass straight through - if the box is metal (most disconnects are) it must attach to the box to ground it. Neutral can pass through.
    – nobody
    Commented Sep 8, 2023 at 3:21
  • I meant that it is not switched. Yes, the box should be grounded.
    – KMJ
    Commented Sep 8, 2023 at 4:41
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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:

  • A disconnect box that also has a GFCI receptacle and a 20 amp breaker all in one housing
  • A small subpanel, in which you install a circuit breaker to serve as the disconnect for the compressor and a second circuit breaker to protect the GFCI service outlet

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.

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Dont confuse yourself. 110/120 has neutral spliced together. 220/240 No neutral If you have a neutral it can be capped off. However The AC disconnect is considered service equipment. Just like your main load center. What that means is your house is basically the electric company and the AC disconnect now becomes your service panel. Based on that per the national electric code. The answer is yes. You could have an incoming neutral wire and connect it to the same ground lug bar with in the AC disconnect box. That is, providing that the AC disconnect is fusible. If the AC box has either old, solid fuses or a breaker, that means that it is fusible. If it has the little handle that you pull out then it is not. that little handle does not automatically trip. It must be pulled out!

Now, as we roll into the 2020 national electric code, and the need for ground, fault interruption. You will be able to connect the neutral to any ground fault, interruption device.

So if you have a 120 system, you just basically splice the neutrals together.

If you have a 240 V system, then the neutral coming in can just be capped off.

If you have a 240 V system and a AC disconnect that is fusible. It is more advisable to connect that neutral wire and bond it to the ground lug along with the ground wire.

In the event of a ground fault on the AC unit. The excess current can now travel along the neutral wire from the AC disconnect box back to the load center. Which is a secondary measure of safety.

Wilhelm

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    "It is more advisable to connect that neutral wire and bond it to the ground lug along with the ground wire." Is flat WRONG! Neutral and ground are ONLY to be grounded at the MAIN panel and no place else.
    – FreeMan
    Commented Sep 8, 2023 at 12:35

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