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. electrical wiring hvac furnace

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. how to fix a carrier ac unitYou're probably looking at a 3.5 ton unit, so you'll be in the 30-40A (10-8 AWG) range. best standing air conditioner unitsIt might be worth it to get a few quotes from local HVAC companies, and see what they want to install. what size ac unit for 1500 square foot houseThe 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. 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.Browse other questions tagged electrical wiring hvac furnace or ask your own question. The heating and air guys that we work around want the circuit conductors sized according to the maximum circuit capacity given on the unit. The units give a minimum and maximum circuit ampacity. I was reading in article 440 and my interpretation is that you size your circuit conductors according to the minimum circuit ampacity, and your overcurrent protection, whether being a fuse or a circuit breaker at the panel, would be the maximum circuit ampacity. For example, a unit that has a maximum circuit ampacity of 40amps and a minimum of 27amps would have a number 10 copper conductor with a 40amp breaker.

Is this correct or what is the correct way? It sounds like the HVAC guys may be confused about the code rules. Many folks who haven’t taken a close look at Articles 430 and 440 of the NEC usually fall back on the basics of Article 240 – match the wire ampacity to the circuit breaker or fuse size. If you use #10 wire, then you can't have larger than a 30A breaker or fuse. If you use #8 wire, you can't have greater than a 40A breaker or fuse, etc. However, for motor and compressor loads the NEC rules have a little more substance than that basic view. Here are the basic items you find on HVAC units: MCA - minimum circuit ampacity. This is the calculation of what the minimum size wire is for the particular unit. The MCA is actually calculated out of UL 1995 - the standard for HVAC equipment. It generally follows our 125% of the largest motor plus the other loads rules. This value is taken as is on the nameplate (in other words, you don't multiply it by 125%, etc.). So if the MCA on a unit is 30A, you can use a #10AWG copper wire for the unit.

MOP - maximum overcurrent protection. This is the maximum size fuse or breaker permitted to protect the unit. Again, the manufacturer has already done the calculations to arrive at this value. If it says 50A maximum overcurrent device, then you can use a 50A breaker or fuse. So, if I had a unit that said MCA = 30A, and MOP = 40A, then I could use a #10AWG copper wire and protect the unit with a 40A circuit breaker. This is usually where, if folks are not familiar with how motor loads are handled in the NEC, you will hear “You can't put a 40A breaker on a #10 wire”. However, you can do this for motor and HVAC loads under Articles 430 and 440. The overload protection is provided by the thermal protection elements of circuit and not by the circuit breaker or fuse. This may be separate motor overloads, or it may be built directly into the motor or HVAC compressor. The 40A breaker is providing short circuit protection for the circuit. The answers provided are the opinions of the authors and are not formal interpretations of the National Electric Code.

Formal interpretations may only be issued as part of the NFPA standards development process. See Section 6 of the NFPA Regulations Governing Committee Projects.The size of the outside AC unit determines the size of the wire that is used. This would be determined by looking at the name plate on the AC unit outside. The larger the tons, the larger the wire size and breaker. Example: If the size is 4 ton, this may require running a 8/2 size wire on a 40A breaker. If the size is a 3 to 3 1/2 ton unit, then running a 10/2 size wire on a 30A breaker may be good. The AC unit requires a disconnect near it. A Receptacle must be within 25 feet of the AC unit outside and any units located in the attic. One light must be a located near the air handler located in an attic. The switch for the lighting outlet shall be at usual point of entry. The air handler located in the attic must have a disconnect means to turn power off to the air handler located near it. How many amps does my unit draw?

What size wire should I use?Here are some basic tips to help you determine what you need. Wire is sold based on several factors. The most important part that will be covered here is the gauge, sometimes refered to as AWG (American Wire Gauge). Wire guage is measured from high to low with higer numbers meaning a smaller wire size. The chart below will tell you approxiamtly what wire size is needed for the amount of power that will be running over the line. Under certain circumstances a large wire size may be needed even if your amp requirment does not seem to justify it. Go to the next larger wire size if your run is more than 100 feet, inside a conduit, or ganged with other wires where the heat dissipation may be inhibited. As with all electrical work, consult a professional with any questions about special cirucmstances. After the gauge number, there will be a number signifying the amount of service wires in the cable. The picture above shows 2 service wires plus a ground.

If this wire was 14 gauge, it would be labeled "14-2" or "14/2". (The ground wire does not count.) In the example above, if this wire was 12 gauge it would be labeled "12-3" or "12/3". This wire has 3 service wires plus a ground. The example above is 14 gauge 4 conductor stranded wire which is used for most mini split air conditioners. This is known as the control wire. This is the minimum gauge that should be used between the indoor and outdoor units. This is not the correct wire to connect your outdoor unit to your circuit box. For the main unit, refer to the chart above. When updating an appliance or air conditioner, it is important to determine the correct current draw. If your circuit breaker is 20 amps you cannot install a product that requires 30 amps. It is not as simple as just changing the circuit breaker. The wiring to the appliance also needs to be changed. Although, when the opposite occurs, the solution is easier. If you are going from an appliance that required 30 amps and your new appliance requires only 20 amps, the wire does not need to be changed.