I am running an electrical line for an a/c unit 220 it calls for 20 amp breaker about 15' underground and the rest overhead in conduit 80' total. My question is what size wire should I use? Thankyou in advance

  • Overhead in conduit? Do you mean running along a wall? – ThreePhaseEel Jun 26 '18 at 2:23
  • No where I'm picking up electric from is out from building so from there underground 15' then up a pole on carport and overhead to a wall on the room with the new a/c unit, all this cause can't get in attic (too low) – Ronald Fisher Jun 26 '18 at 2:40
  • Is the underground section in conduit as well? – ThreePhaseEel Jun 26 '18 at 3:02

Why we cannot accurately answer this question

In order for someone to give you an accurate answer to a question such as yours, we need to know at least three things and maybe a forth.

Length or the run - you have

Run load amperage - you don't have

Type of conductor (copper or aluminum) - we are assuming copper

Ambient temperature - we can get back to this one.

Just because the AC you are installing calls for a 20A breaker doesn't mean it needs the entire 20A. 20A breaker is the overcurrent protection device that trips if the circuit is in danger of damage by a short, compressor lock, or some else that goes wrong. It does not indicate the operating amperage of the unit.

Somewhere on the unit there is a nameplate and it should have the RLA (run load amperage) FLA (full load amperage) or just an amperage or maybe even a HP rating.

So what if I can't find the nameplate rating?

Then rather than just taking a WAG (wild ass guess). We take a SWAG (scientific wild ass guess) and run a calc based on the breaker size, but all guarantees go out the window. enter image description here

This chart is part of a cookbook I gave to the field so they could do a quick reference in the field without have to run a calc.

So you can see that if we have AC running at a 20A breaker and at a 3% voltage drop we can run a feeder 166' with a number #10 copper conductor. Remember the fourth thing you needed? This chart was developed using 75 degrees C. Since you are running conduit in an exposed environment we would need to know the climate you live in, your compass exposure and how you intend to mount your conduit (distance from wall).

So can we run #12? - Don't know not enough information.

Can we run #10? - Probably safe and since it's only 1/2 the allowable distance, SWAG best option.

Can we run #8? - Super safe depending on how much you are willing to spend.

Would you like to get a tighter answer? We will need to get more information.

Good luck


#10 wire, not for the reasons you'd think

Most whole-home A/C units are engineered on the expectation that the wiring to the pad will be #10. Wiring #10 means you won't have to do this job again.

You must follow the installation instructions, but you are allowed to upsize wire even if the instructions don't say you can.

This will also moot the engineering study that others want to do.

With voltage drop calcs... think

Most voltage drop calculations are done slapdash, on two assumptions:

  • that the circuit will be running at full braker ampacity
  • that any voltage drop over 3% is fatal

Neither of these is true. Near as I can figure the 3% thing was made up by wire and cable suppliers to sell you thicker wire. NEC never makes mention of it. Certainly machines can handle more drop, right off the bat people talk about "110/220V" when the power has actually been 120/240V since WWII, well gosh, that's 8.3% voltage drop right there and nobody says boo about that. As far as running full ampacity, that doesn't work - the breaker will trip. You're required to derate circuit capacity 20% for continuous loads.

So if we're only allowed 16A on a 20A circuit, why are we plugging 20A into the voltage drop calcs? In fact even that is wrong. You should be plugging in expected current flow of actual loads. Mindful of the load's tolerance for voltage drops.

Take the guy running power 600' to a post light at the end of his drive. Conventional calc is #2 copper or 2/0 aluminum "coz of the distance". No. His LED post light draws 0.1 amps, and that figure should be used in the voltage drop calcs. And the tolerable percentage should not be 3%, but should be based on the fact that multi-voltage LEDs can work anywhere in the 100-264V range. Actually run the numbers and he could easily put a post light every 15' and feed it all with a #14 run... without even bothering to kick the voltage to 240V, which is also an option.


What you need is a wire size calculator. In the US that would be a NEC compliant one. Here is one http://www.paigewire.com/pumpWireCalc.aspx

There are several things to set and you must consider that your load has a motor (compressor for AC). Still if calls for 20amp breaker then I suppose you can assume that covers it but your might determine the current draw instead of depending on that (see the calculator). To be safe I would probably go up one wire gauge. So from that calculator I get 12awg for 20amps and copper. I would use 10awg. Because of the 15' of underground looks like you can't use nm/b Can Romex (NM-B) be run through conduit?. A 10awg should fit in most 20amp breakers. So if you get a 90 degree wet/dry wire (THHN and the like) you should be fine.

just a disclaimer I am not an electrician. Maybe one will comment to my post to confirm what I say.

BTW an app for the phone I really like is mobile electrician. I prefer to use its NEC wire size calculator as it has a power factor for motors, wire type and you can enter power instead of amperage

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