A/C and Refrigeration Equipment, based on the 2020 NEC^®

Figure 01

For EC&M Magazine
By Mike Holt, NEC^® Consultant

Note: This article is based on the 2020 NEC.

Do you know which refrigeration motors have special requirements and which don’t? Or what those requirements are?

Article 440 applies to electrically driven air-conditioning and refrigeration equipment. Each equipment manufacturer has the compressor motor built to its own specifications. The motors are hermetically sealed, so we call them hermetic motors. Cooling and other characteristics differ from those of nonhermetic motors.

The manufacturer typically provides the conductor and protection size, and other information, on the motor nameplate. The math for sizing the overcurrent protection and conductor minimum ampacity has usually been done for you.

Important terms
Three terms are critical to understand:

1. Branch-circuit selection current. The value in amperes to be used (instead of the rated-load current) to determine the ratings of motor branch-circuit conductors, disconnecting means, controllers, and branch-circuit short-circuit and ground-fault protective devices wherever the running overload protective device permits a sustained current greater than the specified percentage of the rated-load current. The value of branch-circuit selection current will always be at least that of the marked rated-load current.
2. Leakage-current detector-interrupter (LCDI). A device provided in a power supply cord or cord set that senses leakage current flowing between (or from) the cord conductors and interrupts the circuit at a predetermined leakage current.
3. Rated-load current. The current of a hermetic refrigerant motor-compressor when it operates at the rated load, rated voltage, and rated frequency of the equipment it serves.
   

Other articles
Not all motors in air-conditioning and refrigeration equipment are hermetic. Those that aren’t must comply with Article 422 for appliances, Article 424 for electric space heating, and Article 430 for motors [440.3(B)].

Equipment such as room air conditioners, household refrigerators and freezers, drinking water coolers, and beverage dispensers are appliances; apply Article 422 [440.3(C)].

Markings
Multimotor and combination-load air-conditioning and refrigeration equipment must have a visible nameplate marked with the manufacturer’s name, rating in volts, number of phases, minimum conductor ampacity, and the maximum rating of the branch-circuit short-circuit and ground-fault protective device [440.4(B)].

If you don’t see this nameplate on new equipment, it’s may be counterfeit and not safe. If your company bought this, your buyer should go back to the supplier for resolution.

If you don’t see this nameplate on old equipment, it may have been mistakenly removed or covered (e.g., with paint). Jot down the serial number and model number, then contact the manufacturer.

Calculations

Multimotor Equipment Example 1
Question: What size conductor and short-circuit and ground-fault protective device does a multimotor air-conditioning compressor require? The nameplate minimum circuit ampacity is 31.40A, the maximum circuit breaker rating is 50A, and all terminals are rated 75°C.
(a) 10 AWG, 50A breaker   (b) 10 AWG, 30A breaker   (c) 8 AWG, 50A breaker   (d) 8 AWG, 20A breaker
Answer: (a) 10 AWG, 50A breaker
Solution:
Conductor: Since the terminals are rated 75°C, we can use 10 AWG rated 35A at 75°C [110.14(C)(1)(a)(3) and Table 310.16].
The circuit breaker protection for air-conditioning compressor equipment must have a rating of no more than the 50A marked on the nameplate [440.4(B)]. Use a maximum 50A breaker per 240.6(A).

Multimotor Equipment Example 2
Question: What size NM cable and short-circuit and ground-fault protection device does a multimotor air-conditioning compressor require? The nameplate minimum circuit ampacity is 24.50A, the maximum circuit breaker rating is 40A, and the terminals rated 75ºC.
(a) 10 AWG, 40A breaker   (b) 12 AWG, 35A breaker   (c) 8 AWG, 40A breaker   (d) 8 AWG, 35A breaker
Answer: (a) 10 AWG, 40A breaker
Solution:
Conductor: The conductor must have an ampacity of at least 24.50A as marked on the air conditioner nameplate [440.4(B)]. 12 AWG is suitable since it is rated 25A at 75°C [110.14(C)(1)(a)(3) and Table 310.16]. However, in this example, the wiring method is Type NM cable. Its allowable ampacity is restricted to the 60°C Column of Table 310.16 [334.80]. Therefore, even though the terminations are rated 75°C, the conductors must be sized to the 60ºC ampacity in Table 310.16. This requires a 10 AWG conductor rated 30A at 60°C.

Overcurrent protection: The circuit breaker must have a rating of no more than the 40A as marked on the air conditioner nameplate [440.4(B)]. Use a maximum 40A breaker per 240.6(A).

Grounding and bonding
If you install air-conditioning or refrigeration equipment on the roof and connect it with a metallic raceway that uses compression-type fittings, you must install an equipment grounding conductor of the wire type [440.9].

A disconnect for air-conditioning or refrigeration equipment must be within sight from, and readily accessible from, the equipment [440.14].Figure 01

“Within sight” means it is visible and not more than 50 ft from the equipment [100].

The disconnect can be mounted on or within the equipment but can’t be on panels designed to allow access to the equipment and can’t be over the equipment nameplate.

Overcurrent protection
The branch-circuit conductors, control equipment, and circuits supplying hermetic refrigerant motor-compressors must be protected against short circuits and ground faults per 440.22.

The short-circuit and ground-fault protective device cannot be more than 175 percent of the motor-compressor current rating. If the protective device sized at 175 percent is incapable of carrying the starting current of the motor-compressor, the next size larger protective device can be used. But in no case can it exceed 225 percent of the motor-compressor current rating [440.22(A)].

The equipment branch circuit short-circuit and ground-fault protective device must be capable of carrying the starting current of the equipment.
Where a hermetic refrigerant motor-compressor is the largest load connected to the circuit, the rating or setting of the branch-circuit short circuit and ground-fault protective device cannot exceed the value specified in 440.22(A) for the largest motor-compressor plus the sum of the rated-load current of the other motor loads [440.22(B)(1)].

Motor-compressors and other motors example 1
Question: What size short-circuit ground-fault protective device is required for a 16.70A motor-compressor with a 1.20A fan?
(a) 30A (b) 40A (c) 50A (d) 90A
Answer: (a) 30A
Maximum Short-Circuit Ground-Fault Protective Device = (16.70A × 175%) + 1.20A = 30.40A, next size down
Maximum Short-Circuit Ground-Fault Protective Device = 30A

Motor-compressors and other motors example 2
Question: Where the short-circuit ground-fault protective device, sized at 175%, doesn’t permit a 16.70A motor-compressor with a 1.20A fan motor-compressor to start, the protection device can be sized to _____.
(a) 35A (b) 40A (c) 50A (d) 90A
Answer: (a) 35A
Maximum Short-Circuit Ground-Fault Protective Device = (16.70A × 225%) + 1.20A
Maximum Short-Circuit Ground-Fault Protective Device = 39A, round down to 35A

Conductor sizing
Branch-circuit conductors to a single motor-compressor for air-conditioning and refrigeration equipment must have an ampacity not less than the greater of [440.32]:
(1) 125 percent of the motor-compressor rated-load current.
(2) 125 percent of the branch-circuit selection current.

Branch-circuit conductors for a single motor-compressor must have short-circuit and ground-fault protection sized between 175 percent and 225 percent of the rated-load current [440.22(A)].

Conductors supplying one or more motor-compressor(s) must have an ampacity at least the sum of [440.33]:
(1) The sum of the rated-load current
(2) The sum of the full-load current rating of all other motors
(3) 25 percent of the highest motor-compressor or motor full load current in the group

Conductor size example
Question: What size conductor and overcurrent protective device are required for an 18A motor-compressor for air-conditioning equipment?
(a) 10 AWG, 30A (b) 10 AWG, 50A (c) a or b (d) 10 AWG, 60A
Answer: (a) 10 AWG, 30A
Solution:
Step 1: Determine the branch-circuit conductor [440.32 and Table 310.16].
Branch-Circuit Conductor = 18A × 125%
Branch-Circuit Conductor = 22.50A
Use 10 AWG rated 30A at 60°C [110.14(C)(1)(a)(2), Table 310.16]
Step 2: Determine the branch-circuit overcurrent protection [240.6(A) and 440.22(A)]:
Branch-Circuit Overcurrent Protective Device = 18A × 175%
Branch-Circuit Overcurrent Protective Device = 31.50A, use next size down: 30A
If the 30A short-circuit and ground-fault protective device is incapable of carrying the starting current, the protective device can be sized up to 225 percent of the equipment load current rating.
Branch-Circuit Overcurrent Protective Device = 18A × 225%
Branch-Circuit Overcurrent Protective Device = 40.50A, use next size down: 40A
A 30A or 40A overcurrent protective device can protect a 12 AWG conductor for an air-conditioning circuit. See 240.4(G) for details.

Keeping it chill
Remember that because hermetic motors are cooled by the refrigerant they pump, they have different heat handling characteristics from non-hermetic motors. They are different enough to justify their own NEC article.

Instead of the Article 430 tables, you’ll use the nameplate information. Instead of full load current (FLC) and full load amperes (FLA), you’ll work with rated load current and branch circuit selection current. When installing refrigeration equipment, make sure you are clear which of your motors drive compressors and which ones don’t.