How to Wire an A/C Unit

This isn't an especially difficult project but it will involve applying for an electrical wiring permit from your the building department, working with the NEC (National Electrical Code,) and using some basic applied math. The most difficult part of this project will be routing the cable from the service panel to the safety-disconnect switch. This may involve fishing the cable through finished walls and floors/ceilings.

Planning the Installation

1. Locate the nameplate on the A/C unit and note the specifications for the "Continuous Duty Full Load Current" and/or the "Branch Circuit Selection Current."

2. Calculate the minimum required Ampacity (Ampacity refers to the maximum current that a conductor of a given size can carry safely) for the branch circuit conductors by multiplying the "Continuous Duty FLC (Full Load Current)" or the "Branch Circuit SC (Selection Current,)" whichever is greater by 1.25, per NEC Article 440.32.

3. Compare the computed value with the values given in NEC Table 310.16, column one. If the computed value falls between two listed values, select the wire size with the next higher rating.

4. Calculate the minimum rating for the safety disconnect switch by multiplying the "Continuous Duty FLC" or the "Branch Circuit SC," whichever is greater by 1.15 per NEC Article 440.12(A)(1.)

5. If this minimum rating falls between two standard ratings, select the next higher rated switch.

6. Calculate the minimum circuit breaker rating by multiplying the larger of the two current ratings by 1.25 per NEC Article 210.20(a).

7. Compare this value with the values given in NEC Article 240.6. If the computed value falls between two standard ratings, select the higher rated circuit breaker.

8. Apply for your wiring permit. Don't start the wiring until the permit arrives.

Wiring the A/C Unit

1. Mount the safety disconnect switch next to the A/C unit.

2. Route the type UF cable between the service panel and the safety-disconnect switch.

3. Install a cable connector in one of the safety switch's bottom knockouts.

4. Remove 12 inches of the cable's outer jacket and secure the cable in the connector by tightening the clamping screws.

5. Route the red and black circuit conductors to the top of the switch, strip off ¾ inches of insulation, and attach them to the main switch lugs.

6. Attach the white circuit neutral conductor and the green/bare-grounding conductors to the switches neutral and grounding bars, respectively.

7. Install liquid tight connectors in the switch and the A/C terminal box and make up the flex.

8. Install liquid tight connectors in the switch and the A/C terminal box and make up the flex.

9. Secure the flex to the switch and attach the red and black conductors to the bottom lugs on the switch. Attach the neutral conductor and the system-grounding conductor as you did above in step 8.

10. Attach the flex to the terminal box and make up those connections by attaching the conductors to the appropriately marked terminals.

11. Remove the cover to the service panel and install the double-pole breaker.

12. Install a cable connector and the cable in the service panel.

13. Connect the red and black circuit conductors to the circuit breaker terminals.

14. Connect the neutral and ground conductors to the service panel's neutral and grounding bars. Only one wire per screw or your installation will fail inspection.

15. Call the local government building department for inspection.