I am running wire for a 240 V, 20 A split-system AC over a hundred feet and the voltage drops below the allowed NEC 5% voltage drop.
So I have to step down the a wire a gauge and run a #10 wire which will allow me to send the correct voltage. Usually, when running a 20 amp wire, you don't need to run a neutral because the load is balanced.
My question is that as I'm running a 30 A wire and only using 20 A, do I need to run a neutral to send the unused 10 A back the supply?
My question is now that I'm running a 30 amp wire and only using 20 amps, do I need to run a neutral to send the unused 10 amps back the supply?
There is no 'unused 10 A'.
However (and this is physics, not building codes) if because you now have 30 A phase wires and you put 30 A breakers on them, and there is any possibility of you connecting an unbalanced 120 V 30 A load at the end, then this will use the neutral wire at 30 A. Your breakers should not be larger than the smallest wire in the circuit.
If there is no neutral there, and you can only use a balanced 240 V supply, then there is no problem. Size the breakers for less or equal to the phase wire rating.
There is no "unused" 10 amps. You use the #10 wire because it's big enough to handle the 20 amps without overheating and in your case, without dropping the voltage.
If you are running cable (flex, armored, whatever) you should run a /3 one with a neutral that you will not use but will be there in case needed in future. In case, for example, you replace the A/C later with one that requires a neutral.
If you are running conduit, don't run a neutral, just make sure you install the conduit properly so that you can add one later.
A 30A circuit does not push 30 amps. It pushes 230 volts, and the appliance draws however much it needs. As such, no need to return any extra amps... they just won't be taken.
The circuit breaker is a gatekeeper that prevents anything from drawing significantly more than 30 amps (or whatever).
A 20 amp mini-split that calls for #12 wire... that can't be right.
Your mini-split has a specification or a nameplate amp rating. You really need to go look at that. I suspect it will be more like 12-16 amps, given that they are asking for a 20A circuit.
We take that number (say: 15.2A) and we derate that by 125%, because that's the law. 15.2 x 125% = 19A. That decides the circuit size we must run.
For 19A, the next larger circuit size is 20A, so we must run a 20A circuit. OK.
With those numbers nailed down, we can compute.
First and foremost, for amps, we use 15.2 amps because that is our actual load. Voltage drop is based on our Actual Load, not some number on a breaker trip handle. If your circuit were actually running 20A, you'd be in trouble because of the 125% thing.
Suppose you are buying silver. Someone comes in with 15.2 ounces. But their container says "20 ounce max". Do you pay them for 15.2 ounces or 20 ounces? LOL Amazing how money clears things right up.
Second make sure you entered 240V in the voltage, and not 120V. Not 220 either. US power is 240V.
If it asks for parallels, say 1. Always.
For percentage drop, enter 5.2% because the calculator is dumb, and won't tell you about "close enough" numbers like 5.1%.
Now, when you calculate with proper values, I am 100% sure that it will recommend the standard #12 wire, unless your distance exceeds 180 feet.
Let's be clear about this: Code absolutely requires a disconnect switch near the mini-split. The cheapest disconnect switch is a sort of subpanel. It can be a bigger subpanel that might support other stuff too. As long as it has a breaker/switch for the mini-split, and is within sight distance and within 15 feet of the unit.
120V circuits are affected twice as much by voltage drop as 240V circuits, so big wire to a subpanel is also a great way to power 120V circuits in that local area. It sure beats a long >100’ run of #12 or #14 copper clear back to the panel, both in performance and cost.
All that argues for upgrading the mini-split disconnect switch to a proper subpanel capable of supporting other circuits as well. Breaker spaces are cheap, aluminum wire is cheap.
The neat thing about subpanels and most disconnects, is their terminals are rated for aluminum wire. Your main panel's breakers are rated for that too, which means the long run can use inexpensive aluminum wire instead of pricey copper. This is perfectly safe; the industry learned a lot about aluminum. You simply need to follow the rules: use larger wire sizes, only put it on lugs made for aluminum, torque the nuts to spec, use the goop, and use the new AA-8000 alloy.
You could use aluminum wire as small as #10 for a 20A feeder. However, the small sizes did have problems and that still spooks some people, so I prefer to be up in a bit larger size, as larger wires have always been reliable. And the stuff is so inexpensive it doesn't hurt to oversize. For instance #6 Al is the smallest size I'd use, just because it works better with lugs.
For the price of the #10/3copper you were about to use, you can use fat #2 aluminum, which is 90 amps.
As others have said, there is no "unused" power to return. Think about it on a simple 15 amp 120v circuit: You plug a 100 watt lamp into it that uses just under an amp (old style incandescent) that leaves 14 amps "unused". So just saying, you have no problem/issues providing a larger wire/cable. But be sure to provide over-current protection for the unit as specified by the manufacturer. Just because you CAN supply 30 amps, doesn't mean you should. And just because you are running 10ga to reduce voltage drop doesn't mean you have to run a 30 amp breaker, you could stick with the 20 amp breaker. Lots of choices, I know.
Regarding your A/C unit, it's good to run larger wire to reduce voltage drop. Assuming it's operating normally, your A/C unit will not draw more than it needs regardless of the wire size or breaker.
If I were doing this, I'd pull 10/3 with ground, "just in case" I wanted to add a 120v outlet or lighting circuit. Also, in most places an outlet within a certain number of feet (check local codes), for service personnel to use. A small sub-panel would provide local disconnect as well as being able to connect an outlet.
There is never a need to "send back the current in excess": things are different.
Even more if you talk about cables size: the difference between cables of different size is that the thicker cables have smaller resistance, so they heat less, so they dissipate less energy and the voltage drop is reduced.
It comes to my mind some other person asking "my xxxx runs at 12 volts, but the original power supplier is broken. It was 12V 3A. I've found a replacement, but it is 12V 5A. Can that more current harm?". Well, the reply is no: the xxxx device cares about the voltage and "at least" the current it needs. If the power supply can give more current, that is not a problem: it can, but it will not be asked to supply more than those 3A the xxxx asks.
Your situation is the same: your HVAC will consume no more than 20A and, even if the supply and/or the cables can carry more, those 20A will remain 20.
Only if you need a split feed, otherwise no. You of course need a ground for your run to be up to code.
That said, I’d run the neutral anyway, for convenience later. For example, if this is a run to a pool equipment pad or a spa it’s handy to have 120V for lighting, controller, etc.
My question is now that I'm running a 30 amp wire and only using 20 amps, do I need to run a neutral to send the unused 10 amps back the supply?
There is no need to run the neutral.
Only the rated current of the AC Unit will be drawn from the source.
There is no question of the source feeding more and taking back the excess.
Increasing your wire size is the the proper solution. The posts mentioning the neutral (which will serve no purpose for your AC) being handy are valid, but it's an added cost now with limited chance of future use. Even though #10 can be used for a 30A circuit, you would still use a 20A breaker(actually need to check the label on the ac for "Max fuse or HACR Breaker size")(US). This rating from the manufacturer should be used. There is no issue as to unbalanced or unused load.
An addendum point - you're already running two wires. It is very little more work to run the third wire at this time, but running it later will be a lot more work.
You're balancing the cost of one length of wire that will lie unused, vs the labour and effort required to add it later, or to buy some kind of 240V>120V stepdown transformer later.
Make Future-you grateful to Present-you by planning ahead and run the neutral wire too.
