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. 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. Red Fox Kits For Sale UkThe companies may have a preferred breaker and wire size, so they may want to redo any work you do anyway.Vinyl Repair Kit Dash Don't forget you'll also need high and low pressure tubing, and control wires between the indoor and the outdoor units.Fiesta Patio Furniture 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. AC Unit Maximum Breaker Re: AC Unit Maximum Breaker Is this new construction? Because it is only 5 amps I would look closely at the wire and insure that it is rated for the higher amperage under an inductive load. It may be wrong IAW the Mfg. but an over current device is there to protect the wire on the branch not the appliance. If the wire to the unit is rated for the breaker size at least you won't have a fire hazzard to the unit. Also a concern is if the breaker is rated for HVAC use. Also consider that the condenser may have internal wiring matched to the Max Fuse/Breaker Rating indicated on the label, and an older oversized breaker may not trip if the compressor motor locks up ... which could complete fry the equipment ...

IMO report it as a defect needing evaluation/repair and call it a day ... every HVAC guy and electrician I speak to say it is alright to go to the next 5 amp size. They want to see that the wire size is consistent with the higher size. None of them will change the breaker when called out to confirm this. At ITA I remember them saying that the wire size can be smaller if matched to the minimum ampacity rating on the dataplate. Being an HVAC tech for a long time that is what would happen. The tech will say no problem. As long as the breaker is correct for the wire size 5 amps isn't a big deal. Start up amps can be in the 60-amp range. I've seen units under this same scenario (25 amp nameplate) that would cause nuisance trips when a 25-amp breaker was installed and when the 30-amp breaker was put in the trips stopped and there were never any problems. It is over fused if a breaker larger than the manufacturer's label calls for is installed. Here is a good explanation I picked up somewhere:

A/C condensers contain a hermetically sealed compressor motor as well as a fan to circulate air across the coils. The rules for protection to motor circuits are different than for circuits with simple resistive loads. When a motor first starts , it draws a much higher amount of current than it does after it is running. The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. The inrush current lasts typically only about 6 electrical cycles, or 1/10th of a second - less time than it would take to damage the wire or its insulation. However, if the overcurrent device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start. Motor circuits get around this problem by dividing the two separate functions of an overcurrent protection device. "Overloads" are currents that can damage a circuit if allowed to continue for a sufficient time, whereas "short circuits" and "ground faults" are high currents that can cause immediate damage.

Large motors and air conditioners separate these overcurrent functions. The breaker or fuse ahead of the air conditioner only protects against short circuits and ground faults. Overload protection is built into the compressor itself. The wire to a condensing unit must be large enough to allow the equipment to start. The device is therefore NOT sized to protect the wire against overloads. It is not uncommon to see a 50 Amp breaker on a #10 wire. The wire is protected against overloading by a separate thermal protection inside the compressor. How do we know what the motor requires for wire size and proper fuse/breaker protection? It's all on the mfg. label attached to the appliance. If the label calls for fuses, there MUST be a fuse in the disconnecting means, not a circuit breaker. If the label calls for a "HACR" type breaker (Heating, Air Conditioning and Refrigeration), then that is the type of breaker that must be used. If the label simply states "maximum sized over current protection" then any form of overcurrent protection may be used.

The HACR designation is not always visible on a circuit breaker once it is installed inside a panel or disconnect. Note on the label taken from a Lennox A/C data plate:FUSE - 25 UL (CSA is a Canadian standard. We use UL in the U. S.)UL - Not allowed in U. S. All this means is the MINIMUM wire size required is #12 (solid copper); the MAXIMUM rating of a fuse (if a fuse is used) must be 25 amps and the MAXIMUM rating of a circuit breaker (if used) must be 25 amps AND the breaker must be a HACR type.However, if the over current device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start. "it draws a much higher amount of current than it does after it is running." This is called an inductive load. "The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. " This is why HVAC breakers must be rated for HVAC use. The wire, breaker (to protect the wire), and type of breaker must be correct and is reflected in the "minimum circuit data".

Michael MerinoMerino's Home Inspection & Education Inc." IF YOUR INSPECTOR IS NOT USING THERMAL IMAGING, YOU'RE NOT GETTING THE WHOLE PICTURE �Jeff Pope JPI Home Inspection Service Santa Clarita Home Inspection Originally Posted by jpope I understand that the circuit is over-fused, but the appliance is connected to that circuit. If for some reason it were to draw more than 30 amps (outside of the inductive load) the breaker will not trip. I am confused by the differing opinions. I have been told by inspectors it is OK, and some say it is not. Some say that you can over-fuse the AC up to 40%. I do not seem to be able to find a definitive answer. I am thinking that if the max breaker is supposed to be 30 amp, then the breaker should be 30 amp not 50 amp, even if the conductor is the proper size. Professional Certified Master Inspector In Edwardsville, Glen Carbon, Godfrey, Maryville, Fairview Heights, O'Fallon and surrounding areas of Illinois Metro-East