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Question DetailsAsked on 5/5/2017

Cost to run dedicated 220V circuit with 30amp breaker and 8/3 copper wire to new AC on outside wall. A 120 ft run

There is space in panel for the breaker.
90 feet of circuit will run through attic with
30 ft total conduit on outside walls to AC and breaker Box.

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1 Answer


Make sure the breaker and circuit wiring is rated for 125% of the maximum sustained run load on the unit (not just the compressor, the total unit draw including fan and TXV valve and such) - might bump you above 30A if you used only the full-load amperage, which might significantly change the picture for your case.

Also - you say there is space in the panel - many panels cannot use 240V or high-amperage loads in other than the top 2-4 slots in the panel, and of course check that the panel and the service wiring/main breakers have the capacity for the extra load. Just because there is physical room in the box for added breakers does not mean it has the load capacity for it, especially for a large sustained load like an A/C. One needs to run a panel load calculation, which figures in the actual likely load on the panel under normal conditions. And of course, common household use of electric water heater or furnace, electric clothes dryer, electric range, etc should be based not only on the formula but also on your household normal usage. I have seen houses where the Residential Service Load Calculation said it was OK but the actual loads kept tripping out the main breaker - a very dangerous situation, not to mention really ticking off the cook when the oven loses power in the middle of baking. All-electric houses and ones where there is a lot of dryer use (say baby/infant or a lot of kids in house) or a lot of long-duration range/oven use may need a bump-up in overall panel/main breaker/service capacity when you add in an A/C. Ditto if there is high-amperage garage use like electric car charging or high-amperage shop uses like wqelder, air compressor, large saws or planers, etc.

An electrician should certainly verify the wire type and size needed (likely THHN or THWN-2 rated because of the outside conduit run and the potentially high-temp attic run). Depending on your local code requirements, or if not running the outside run in metal conduit, this may well have to be a 4 wire conductor - 2 lives, neutral, and ground in the cable. Or the A/C may have to be bonded externally to the electrical sytem ground rods, which adds up to the same thing - so personally I would run 4 conductor wire like THHN/THWN-2 /3 WG, bringing the ground back to the panel for best protection. This also gives some protection in the attic in case the wire gets damaged by someone inspecting or working up there, which is all too common when the wiring is spanning through or over the top of joists and is covered with insulation - I have seen/heard of several house fires from attic wiring runs being stepped on and damaged to the point of catching fire.

In some code areas, especially lightning-prone areas, you may wantto or have to (by code) put in an independent ground at the A/C as well - likely another $100 cost or so in driveable ground conditions.

So - cost - probably about 2-3 hours work at $100-200/hour for labor in most areas, so that is about $200-600. Materials - 125' cable at about $1.50-2.00/LF, 30' conduit (say 3/4" - 1/2 is awful tight for that wire) @ $2/LF including connectors/pull corner box at house wall. Connection box and miscellaneous $50. 2-pole breaker $10-400 depending on make/model of your box - that is the biggest variable. Possible separate ground rod and wire $100.

So - I would estimate from $500-1450 depending on local labor costs, attic accessibility and working conditions, whether breakers/circuits have to be moved around in the breaker panel to put in another 240V circuit, whether an independent ground is installed, and the cost of the breaker itself (which is normally under $50 but with some older panels can be up to $400 or occasionally even more for brands where compatible ones are no longer made). I have seen cases where a new modern sub-panel had to be put in because the old panel breakers were no longer available or were incredibly expensive to find and buy.

This presumes that the breaker panel is not one of the ones that is banned from expansion use (and many electricians will not even work on them except to replace them), and that the panel and service and main breaker have the capacity for the new A/C load.

You definitely need to have an electrician work out the exact needs and system capacity for this upgrade.

On the attic wiring - I would definitely make sure the wiring is buried in the insulation if legal in your area (in some areas has to be marked on top of insulation so it does not get stepped on) because in many areas your attic can get into the 150-170 degree range, which will put added load on the circuit and compressor by increasing the voltage loss along the way. I would not be at all surprised if the electrician recommended (or demands) #6 copper wire, or alternatively outside-run conduit, for this case. Pushing the limit on wire size (which you are - my wire size table says 30A in #8 copper wire is limited to 120' run, right where you are WITHOUT the higher-amperage starting load, which can sometimes reach 3 times the normal full-load run amperage) will result in slower compressor start, meaning more motor winding heating and shorter life. With high-amperage motors, especially ones starting under load like many or most A/C compressors do, you can commonly really extend their life and the life of starting condensors by cutting the line voltage drop to a bare minimum - which going with a larger wire size does.

Actually, to limit voltage drop to 3%, your run limit on #8 wire at 30A load is 100' - so I am almost certain your electrician will insist on #6 copper wire, or #4 if you go to aluminum.

My practice is to generally use one wire size larger than the table requires for all 240V and other high-amperage sustained loads at least if the load is anywhere near the limit for that wire size and run nconditions - also for convenience outlet runs with a lot of outlets like in kitchens. Just pays off in better and safer performance, and provides some protection against the minor damages at pretty minimal added wire costs. So I would definitely use #6 copper in this case, which will add maybe $50-70 to the total job cost. (This assumes 30A is the correct circuit size, needs confirmation as discussed above).

Answered 3 years ago by LCD

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