Motors used for refrigeration equipment (including air conditioning systems) are hermetic. When we say “hermetic,” what do we mean? It’s a motor that’s sealed from the outside. In the case of refrigerant motor compressors, the motor and the compressor it drives are enclosed in the same housing and operate in a refrigerant [Art. 100]. This has implications for ampacity and other design and installation decisions.
In most cases, the manufacturer has worked out the details and identified the minimum conductor ampacity, maximum overcurrent protective device rating, and other information (such as running-load amperes) on the nameplate. Article 440, which applies to hermetic refrigerant motor compressors (such as those used for pool heat pumps and air-conditioning equipment) takes this fact into consideration.
A hermetic refrigerator motor compressor must have a nameplate that indicates (at a minimum) the manufacturer’s name, trademark, or symbol and shows the phase, voltage, and frequency [Sec. 440.4(A)]. The manufacturer must mark on the motor-compressor nameplate and/or the nameplate of the equipment in which it is used the rated-load current. Additional marking requirements are given in Sec. 440.4(A).
For multi-motor air-conditioning equipment, the requirements are similar. But it must also have the minimum supply circuit conductor ampacity, the maximum rating of the branch-circuit short-circuit and ground-fault protective device, and short-circuit rating of the motor controllers or industrial control panel. Further, the conductor ampacity must be calculated by using Part IV and counting all the motors and other loads that will be used at the same time [Sec. 440.4(B)], as shown in Fig. 1.
Another requirement for multi-motor air-conditioning equipment is the rating of the ground-fault protective device cannot exceed the value calculated using Part III. Finally, multi-motor or combination-load equipment for use on two or more circuits must be marked with all of this information for each circuit.
Hermetic refrigerant motor compressors typically have an integral thermal device that provides overload protection [Sec. 440.51]. Branch-circuit short-circuit and ground-fault protection is provided with a circuit breaker or fuse, which must be installed by the electrician in accordance with the manufacturer’s nameplate marking.
Question: What size conductor and short-circuit and ground-fault protective device is required for a multi-motor air-conditioning compressor? The nameplate minimum circuit ampacity is 31.40A and the maximum circuit breaker rating is 50A, where the equipment is rated for a 75°C conductor.
a) 10 AWG, 50A breaker
b) 10 AWG, 30A breaker
c) 8 AWG, 50A breaker
d) 8 AWG, 20A breaker
Solution:
Conductor: Since the terminals are rated 75°C, we can use 10 AWG rated 35A at 75°C [Sec. 110.14(C)(1)(a)(3) and Table 310.16].
The circuit breaker protection for air-conditioning compressor equipment must have an ampere rating of not more than the 50A marked on the nameplate [Sec. 440.4(B)]. Use a maximum 50A breaker in accordance with Sec. 240.6(A).
Answer: (a) 10 AWG, 50A breaker
Tubs and showers
ir-conditioning equipment is not permitted within a zone measured 3 ft horizontally and 8 ft vertically from the top of a bathtub rim or shower stall threshold [Sec. 440.8]. This requirement would seem unnecessary. After all, who puts an air conditioner in their bathroom? But notice the language here. The issue is the location of air-conditioning equipment, not whether there is an air conditioning vent.
You could violate this in a single-family home or a duplex by installing air conditioning equipment in the attic above a shower stall. If you have 8-ft ceilings, the rim of any bathtub will create a “no install zone” in the attic.
For multi-family homes or hotels with multiple floors, air conditioning equipment is often located on the roof. You can see where this rule then comes into play for the occupancies on the top floor.
Equipment grounding conductor
Outdoor portions of metal raceways on a roof using unthreaded fittings must contain an equipment grounding conductor (EGC) of the wire type [Sec. 440.9].
Note that you do not make an EGC by connecting a wire to the building steel or a ground rod. The EGC doesn’t actually ground (connect to the earth), it bonds. It’s used to reduce dangerous differences of potential between equipment. Ultimately, the EGC system does connect to ground. The earth has a far higher impedance than any of the approved EGCs listed in Sec. 250.118. This high impedance is why the earth cannot bring objects to the same electrical potential.
The outdoor portions of rooftop metal raceways with compression fittings are exposed to a higher likelihood of physical damage and are often stepped on and broken from roof activities such as snow removal or roof repair/replacement. The installation of an EGC of the wire type within outdoor portions of metal raceways ensures an effective ground-fault current path.
Disconnecting means
If the air-conditioning disconnecting means is readily accessible to unqualified persons, the disconnect enclosure or hinged door that exposes energized parts when opened must require a tool to open or be capable of being locked [Sec. 440.11].
A disconnect for air-conditioning equipment must be within sight and readily accessible from the air-conditioning equipment. It must also meet the required working space requirements of Sec. 110.26(A) [Sec. 440.14], as shown in Fig. 2.
“Within Sight” means it is visible and not more than 50 ft from the location of the equipment [Art. 100]. “Readily Accessible” means capable of being reached quickly for operation, renewal, or inspection without requiring the use of tools (other than keys). It also means people don’t need to climb over or under obstructions, remove obstacles, resort to using portable ladders, etc. [Art. 100].
The disconnect can be mounted on or within the equipment, but it cannot be on panels designed to allow access to internal wiring or where it obscures the equipment nameplate [Sec. 440.14].
Overcurrent protection
For an individual motor compressor, the branch-circuit short-circuit and ground-fault protective device must be able to carry the starting current of the motor. The rating or setting must not exceed 175% of the rated load current [Sec. 440.22(A)].
Exception No. 1: If the values for branch-circuit short-circuit and ground-fault protection per Sec. 440.22(A) do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices, or available settings of adjustable circuit breakers, you can use a higher size, rating, or available setting that does not exceed the next higher standard ampere rating.
Exception No. 2: If the values for branch-circuit short-circuit and ground-fault protection per Sec. 440.22(A) or the rating modified by Exception No. 1 is not sufficient for the starting current of the motor, you can increase the rating or setting up to 225% of the motor rated-load current or branch-circuit selection current (whichever is greater).
The equipment branch-circuit short-circuit and ground-fault protective device must be able to carry the starting current of the equipment. Where the equipment incorporates more than one hermetic refrigerant motor-compressor or a hermetic refrigerant motor-compressor and other motors, the equipment branch-circuit short-circuit and ground-fault protection must comply with
Sec. 440.22(B)(1).
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 refrigerant motor-compressor plus the sum of the nameplate current ratings of the other motor loads [Sec. 440.22(B)(1)].
Question: What size branch-circuit short-circuit and ground-fault protective device is required for a 17.60A refrigerant motor compressor with a 1.20A fan?
a) 35A
b) 40A
c) 45A
d) 50A
Solution:
Branch-Circuit Short-Circuit and Ground-Fault Protective Device = (17.60A × 175%) + 1.20A
Branch-Circuit Short-Circuit and Ground-Fault Protective Device = 30.8A + 1.20A
Branch-Circuit Short-Circuit and Ground-Fault Protective Device = 32A, use the next size up [Sec. 440.22(A) Exception No. 1]
Branch-Circuit Short-Circuit and Ground-Fault Protective Device = 35A
Answer: (a) 35A
Conductor ampacity
Conductors supplying hermetic refrigerant motor compressors with other motors must have an ampacity of at least the sum of the following [Sec. 440.33] (Fig. 3):
- The hermetic refrigerant motor-compressor nameplate current rating.
- The motor(s) nameplate current rating.
- 25% of the hermetic refrigerant motor-compressor current rating.
Question: What size conductor is required for a 16.70A refrigerant motor compressor with a 1.20A fan, where the conductors are required to be sized at 60°C in accordance with Sec. 110.14(C)(1)(a)(2)?
a) 12 AWG
b) 10 AWG
c) 8 AWG
d) 6 AWG
Solution:
Conductor Size = (16.70A × 125%) + 1.20A
Conductor Size = 20.88A + 1.20A
Conductor Size = 22.08A
Use 10 AWG rated 30A at 60°C [Table 310.16].
Answer: (b) 10 AWG
Avoiding rework
The nameplate plays an important role in applying Art. 440. So, always review the nameplate data before performing any calculations. You may find the calculations are already done for you; in that case, install them per the nameplate requirements. If not, perform them per Art. 440 — not Art. 430. Take a photo of each nameplate and add it to the asset information in the CMMS (maintenance) or project documentation (construction).
