A/C and Refrigeration Equipment

Article 440 applies to electrically driven air-conditioning (A/C) 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.

Markings

Multimotor and combination-load A/C 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 [Sec. 440.4(B)] (Fig. 1).

Fig. 1. This type of nameplate should be present on all new equipment.

If you don’t see this nameplate on new equipment, then it 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, and then contact the manufacturer.

Calculations

Multimotor Equipment – Example 1Question: What size conductor and short-circuit and ground-fault protective device does a multimotor A/C compressor require? The nameplate minimum circuit ampacity is 31.40A, the maximum circuit breaker rating is 50A, and all terminals are rated 75°C (Fig. 2).

Answer: Since the terminals are rated 75°C, we can use 10 AWG rated 35A at 75°C [reference Sec. 110.14(C)(1)(a)(3) and Table 310.16].The circuit breaker protection for A/C compressor equipment must have a rating of no more than the 50A marked on the nameplate [Sec. 440.4(B)]. Therefore, use a maximum 50A breaker per Sec. 240.6(A).Multimotor Equipment – Example 2Question: What size NM cable and short-circuit and ground-fault protection device does a multimotor A/C compressor require? The nameplate minimum circuit ampacity is 24.50A, the maximum circuit breaker rating is 40A, and the terminals are rated 75°C.Answer: The conductor must have an ampacity of at least 24.50A as marked on the air conditioner nameplate [Sec. 440.4(B)]. 12 AWG is suitable since it is rated 25A at 75°C [Reference Sec. 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 [Sec. 334.80]. Therefore, even though the terminations are rated 75°C, the conductors must be sized to the 60°C ampacity in Table 310.16. This requires a 10 AWG conductor rated 30A at 60°C.The circuit breaker must have a rating of no more than the 40A as marked on the air conditioner nameplate [Sec. 440.4(B)]. Use a maximum 40A breaker per Sec. 240.6(A).

Grounding and bonding

If you install A/C or refrigeration equipment on the roof and connect it with a metallic raceway that uses compression-type fittings, then you must install an equipment grounding conductor (EGC) of the wire type [Sec. 440.9].A disconnect for A/C or refrigeration equipment must be within sight from, and readily accessible from, the equipment [Sec. 440.14] (Fig. 3).

Fig. 3. The disconnect can be mounted on or within the equipment but can’t be on panels designed to allow access to the equipment.

“Within sight” means it is visible and not more than 50 ft from the equipment [Art. 100].The disconnect can be mounted on or within the equipment, but it 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 Sec. 440.22.
The short-circuit and ground-fault protective device cannot be more than 175% of the motor-compressor current rating. If the protective device sized at 175% is incapable of carrying the starting current of the motor compressor, then the next size larger protective device can be used, but in no case can it exceed 225% of the motor-compressor current rating [Sec. 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 Sec. 440.22(A) for the largest motor-compressor plus the sum of the rated-load current of the other motor loads [Sec. 440.22(B)(1)].
Motor-Compressor – Example 1
Question: What size short-circuit ground-fault protective device is required for a 16.70A motor-compressor with a 1.20A fan?
Answer: Maximum Short-Circuit Ground-Fault Protective Device = (16.70A × 175%) + 1.20A = 30.40A
Choose the next size down protective device, which is 30A.
Motor-Compressor – 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 _____.
Answer: Maximum Short‑Circuit Ground‑Fault Protective Device = (16.70A × 225%) + 1.20A = 39A.
Choose the next size down protective device, which is 35A.

Conductor sizing

Branch-circuit conductors to a single motor-compressor for A/C and refrigeration equipment must have an ampacity not less than the greater of [Sec. 440.32]:
(1) 125% of the motor-compressor rated-load current.
(2) 125% 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% and 225% of the rated-load current [Sec. 440.22(A)].
Conductors supplying one or more motor-compressor(s) must have an ampacity at least [Sec. 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% of the highest motor-compressor or motor full load current in the group.
Conductor Sizing Example
What size conductor and overcurrent protective device are required for an 18A motor-compressor for A/C equipment?
Step 1: Determine the branch-circuit conductor [Sec. 440.32 and Table 310.16].
Branch-circuit conductor = 18A × 125% = 22.50A
Use 10 AWG rated 30A at 60°C [Sec. 110.14(C)(1)(a)(2) and Table 310.16]
Step 2: Determine the branch-circuit overcurrent protection [Sec. 240.6(A) and Sec. 440.22(A)].
Branch-Circuit Overcurrent Protective Device = 18A × 175% = 31.50A
Choose the next size down protective device, which is 30A.
If the 30A short-circuit and ground-fault protective device is incapable of carrying the starting current, then the protective device can be sized up to 225% of the equipment load current rating.
Branch-Circuit Overcurrent Protective Device = 18A × 225% = 40.50A
Choose the next size down protective device, which is 40A.
A 30A or 40A overcurrent protective device can protect a 12 AWG conductor for an A/C circuit. See Sec. 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 nonhermetic motors. They are different enough to justify their own Article in the National Electrical Code (NEC).
Instead of the Art. 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 on which of your motors drive compressors and which ones don’t.

These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.