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Installing an A/C unit can be daunting if you do not understand how the nameplate values can assist you in selecting the proper wire size, overcurrent protection, and disconnect rating. In this article, we will figure out each piece required for the installation using the nameplate values. Next month we will look at the calculations involved in determining how the manufacturer came up with their nameplate ampacity and fuse sizes.

1 Manufacturer’s number: ‘13’ is the SEER rating (Season Energy Efficiency Ratio), which is the average ratio of how much cooling a system will put out for each unit of energy it consumes over a season. Theoretically, the higher the SEER rating, the more efficient your A/C unit is. ‘X’ refers to the type of coolant used. ‘036’ cooresponds to the Ton rating of the unit, 3.5 Ton in this case, and ‘230’ is the voltage rating.

2 Coolant: R-410A is one of the acceptable non-ozone-depleting alternatives to HCFC-22. HCFC-22 is a hydrochlorofluorocarbon which is known to deplete ozone. It is scheduled to be phased out of production by January 1, 2020.

3 MIN: Minimum voltage rating of the equipment. As voltage decreases, the current will increase roughly the same proportion, creating more heat than the motor windings were designed to carry. The startup torque can also be lowered, making it harder for the motor to start.

4 MAX: Maximum voltage rating of the equipment. If voltage gets too high, it can stress components of the A/C unit, such as the capacitors used for starting and running the compressor leading to premature failure. This can also cause the motor’s magnetic field to saturate causing motor current to increase, increasing heat.

5 RLA: Rated Load Amps of the compressor. Section 440.12 covers sizing of the disconnect and states that the ampere rating of the disconnect shall be 115% of the rated load current (RLA) or branch-circuit selection current, whichever is greater. The branch-circuit selection current is equivalent to the minimum circuit ampacity value given on this nameplate. In this case we would take 115% of 18.6A requiring a minimum of a 30A disconnect (standard size).

6 LRA: Locked Rotor Amps of the compressor. This is how many amps the motor would pull if it was not spinning which is also the same as the inrush current. This value can be used to help size the required rated horsepower of the disconnecting means. Using Table 430.251(A), 77A at 230V would require a disconnect rated 3 HP.

7 8 FLA/HP: Fan motor values. These values have already been taken into consideration by the manufacturer to determine the nameplate values given in items 9 and 10, so we do not need to consider these at this time.

9 Minimum Circuit Ampacity: Use this number to size the conductor used. Since there is no termination temperature rating on this equipment, we will use 110.14(C)(1) and consider worst case scenario, the termination temperature rating is 60°C. So no matter which type of wiring we are using, NM cable to THWN individual conductors in conduit, we need to select a wire from Table 310.15(B)(16), 60°C column, rated a minimum of 18.6A. This would require a 12 AWG copper conductor. I chose NM cable for the interior
portion of the circuit, transitioning to THWN since the conductors will be considered in a wet location accoring to 225.22 and 300.9. Note that we did not have to worry about the small conductor rules of 240.4(D) because conductors covered by Article 440 are excluded from that according to 240.4(G).

10 Maximum Fuse or Circuit Breaker Size: This one is simple, we can install no larger than a 30A fuse or a 30A HACR (Heating, Air-conditioning, Refrigeration) rated circuit breaker. Technically we could install smaller but it is safe to go with the nameplate value.

Article by Laura Benedict


A free information orientation is held the third Tuesday of every month at 4:00pm. Pre-registration is not required. If you have attended an orientation in the past, we suggested you attend the orientation again, as information and policies may have changed. You may also view the information orientation by clicking on the link below. 

NECA-IBEW Electrical Training Center is now selling the 2017 NEC Code Books in both Soft Bound and Loose Leaf versions. 
Prices are as follows:
2017 NEC Code Book - Soft Bound: $75.00
2017 NEC Code Book - Loose Leaf: $85.00
2017 NEC Code Book - Spiral Bound: $80.00
2017 NEC Handbook : $141.00

These prices are specially discounted for our IBEW contributing members. Come by and pick up your new 2017 NEC Code Book today!