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I am getting ready to hang drywall in my basement but before I do that, I need to run wire from the main panel upstairs to the future location of the central AC furnace through the basement ceiling joists. I do not know yet who will be installing my AC but I will probably outsource it. At this point I just need to know what the electrical requirements for AC are so that I can bring the cable and hang the drywall. The house has three bedrooms and two living rooms, 1800 sq ft, two floors.

What amperage/AWG am I looking at? The wire distance from the panel to the furnace is only about 20' +/- and then I plan to sit the condenser on the roof (it is a low pitched mod bit roof so I can do this, many of my neighbors with similar homes do) directly above the furnace, two floors above (so another 16-20' total). It also may be worth mentioning that I plan to use this for just AC and not heat -- I have already a radiator system in place and I love radiant heat.

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What would the length of this wire be? It's important to know for the calculation. –  longneck Nov 11 '13 at 16:29
    
thanks, @longneck, i updated the OP –  amphibient Nov 11 '13 at 16:31
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2 Answers

It's going to completely depend on the unit that's being installed, but commonly it'll be 30-60 amp with 10-4 AWG wire. You're probably looking at a 3.5 ton unit, so you'll be in the 30-40A (10-8 AWG) range. It might be worth it to get a few quotes from local HVAC companies, and see what they want to install. The companies may have a preferred breaker and wire size, so they may want to redo any work you do anyway.

Don't forget you'll also need high and low pressure tubing, and control wires between the indoor and the outdoor units.

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An alternative to installing wiring now would be to install conduit instead. –  longneck Nov 11 '13 at 16:41
    
no, @longneck, it needs to go through ceiling joist holes –  amphibient Nov 11 '13 at 16:52
    
do the condenser and air handler run on the same or separate circuits? –  amphibient Nov 11 '13 at 19:20
    
@amphibient Usually the furnace is on a 120V circuit, and the condensing unit is on a separate 240V circuit. –  Tester101 Nov 11 '13 at 22:12
    
but do you run just a single cable (10-4?) hooked into two circuits from the panel to the air handler inside and then another cable from the air handler to the condenser? –  amphibient Nov 11 '13 at 22:15
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You should either sort out what the AC is going to be, so you can get specific power supply and cabling needs sorted out now, or line up your joist holes nicely and follow the excellent suggestion from @longneck to run conduit, leaving the wiring for later. Use flexible conduit, if needed to get it in place. Run at least two - there may well be low voltage control circuits that need to be run separated from the power supply. Alternatively, leave a section of the drywall open for now, allowing for both cabling and refrigeration tubing to be run before you close it up.

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