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Service Maintenance Company, Inc. Electrical Contractors |
| NEC Article 440 - Air Conditioning and Refrigerating Equipment | 1/2/2004 |
| By Mike Holt for EC&M Magazine Knowledge of Article 440 requirements is increasingly in hot demand. Long-time EC&M reader Joe Territo, president of Territo Electric in Ocoee, FL, knows the importance of Article 440 and its growing role in facilities. A Territo client operates a multi-story building that exists for one purpose: to make and supply chilled water to nearby customers. Chilled water buildings have been popping up in cities across the country. And they have something in common with data centers, clinical labs, temperature-controlled industrial enclosures, and probably your own office-they come under Article 440, because they use electrically driven air-conditioning and refrigeration equipment that has a hermetic refrigerant motor-compressor. What is a hermetic refrigerant motor-compressor? It's a combination consisting of a compressor and motor enclosed in the same housing. These have no external shaft or shaft seals, and the motor operates in the refrigerant. Article 440 does not cover such household appliances as room air-conditioners, household refrigerators and freezers, drinking water coolers, or beverage dispensing machines, because these are appliances and must comply with Article 422. Disconnecting Means Article 440 requires you to locate the disconnecting means within sight of, and readily accessible from, the air-conditioning or refrigerating equipment (Figure 440-2). "Within sight" means visible from, and not more than 50 ft from, each other [Article 100] (Figure 440-4). You can install the disconnecting means on or within the air-conditioning or refrigerating equipment-but not on the access panels (Figure 440-3). This requirement has two exceptions: You can lock the switch open if the equipment is essential to an industrial process and the installation meets certain requirements. Where a plug and receptacle serve as the disconnecting means, they must be accessible. But they need not be readily accessible. Circuit Protection The requirements for protecting the branch-circuit conductors, control apparatus, and circuits supplying hermetic refrigerant motor-compressors against short circuits and ground faults are in [440.22]. The size and type of the short-circuit and ground-fault protection device for air-conditioning and refrigeration equipment are often marked on the equipment nameplate. The manufacturer calculates these ratings per [440.22] and [440.32] (Figure 440-5). If the equipment nameplate specifies "Maximum Fuse Size," use a one-time or dual-element fuse. If the nameplate specifies "HACR Circuit Breaker," use an HACR-rated circuit breaker [110.3(B)]. Short-circuit and ground-fault protection cannot exceed the nameplate ratings. If the equipment does not have a nameplate specifying the size and type of protection device, how do you size those devices? That depends on whether you are sizing for multiple motors or a single motor. Rating for Equipment. Where the equipment incorporates more than one hermetic refrigerant motor-compressor, or a hermetic refrigerant motor-compressor and other motors or other loads, size the equipment short-circuit and ground-fault protection with the "largest load" method. The rating of the branch-circuit short-circuit and ground-fault protective device cannot exceed the largest motor-compressor short-circuit ground-fault protection device plus the sum of the rated-load currents of the other compressors. Size the branch-circuit conductors at 125 percent of the larger motor-compressor current plus the sum of the rated-load currents of the other compressors [440.33]. One Motor-Compressor. The short-circuit and ground-fault protection device for motor-compressor conductors must be capable of carrying the starting current of the motor. Also, the protection device cannot exceed 175 percent of the equipment load current rating. If the protection device sized at 175 percent is not capable of carrying the starting current of the motor-compressor, you can use the next larger protection device if it does not exceed 225 percent of the motor-compressor current rating. Test your knowledge with this question: What size conductor and protection must you have for a 24A motor-compressor on a 240V circuit (Figure 440-6)? 10 AWG, 40A 10 AWG, 60A A or B none of these Answer: (a) 10 AWG, 40A protection. Here's how we arrive at that answer: Step 1: Size the branch-circuit conductor [Table 310.16 and 440.32]. 24A x 1.25 = 30A, 10 AWG, rated 30A at 60°C [110.14(C) and Table 310.16]. Step 2: Size the branch-circuit protection [240.6(A) and 440.22(A)]. 24A x 1.75 = 42A, next size down protection = 40A. If a 40A protection device is not capable of carrying the starting current, you can size the protection device up to 225 percent of the equipment load current rating (24A x 2.25 = 54A, next size down 50A). Conductor Sizing Size these branch-circuit conductors no smaller than the spec on the equipment nameplate. If the equipment does not have a nameplate specifying the branch-circuit conductors, size the conductors per [440.32] (Figure 440-5). One Motor-Compressor. Suppose your equipment is not marked with minimum circuit ampacity. For each single motor compressor, use a branch-circuit conductor ampacity not less than 125 percent of the motor-compressor current. Protect branch-circuit conductors against short circuits and ground faults between 175 percent and 225 percent of the rated-load current. See [440.22(A)]. Several Motor-Compressors. Conductors that supply several motor-compressors must have an ampacity of not less than 125 percent of the highest-rated motor-compressor current of the group plus the sum of the rated-load currents of the other compressors. Protect these conductors against short circuits and ground faults per [440.22(B)(1)]. Room Air-Conditioners The rules in Part VII of Article 440 apply to window or in-wall type room air-conditioner units that incorporate a hermetic refrigerant motor-compressor rated not over 40A, 250V, 1Ø. These units are common in homes, hotels, and mobile shelters such as construction trailers. But, you'll see them in many other applications as well. When calculating branch-circuit requirements, you can consider a room air-conditioner to be a single motor unit only if the installation meets the four conditions listed in [440.62(A)]. You must make three branch-circuit determinations: Conductor and Protection Size. Branch-circuit conductors for a cord-and-plug-connected room air-conditioner rated not over 40A at 250V must have an ampacity of not less than 125 percent of the rated-load currents [440.32]. Maximum Load on Circuit. Where the room air-conditioner is the only load on a circuit, the marked rating of the air-conditioner cannot exceed 80 percent of the rating of the circuit [210.23(A)]. Other Loads on Circuit. The total rating of a cord-and-attachment-plug-connected room air-conditioner cannot exceed 50 percent of the rating of a branch-circuit where lighting outlets, other appliances, or general-use receptacles are also supplied [210.23(B)]. Room Air-Conditioning Disconnecting Means The supply cord for room air-conditioning equipment cannot be longer than 10 ft for 120V units or 6 ft for equipment rated 208 through 240V. An attachment plug and receptacle can serve as the disconnecting means for a room air-conditioner, provided (Figure 440-7): The manual controls on the room air-conditioner are readily accessible and within 6 ft of the floor, or An approved manually operable switch is in a readily accessible location within sight from the room air-conditioner. Unless you have special certification, your work on refrigerant systems does not include breaking refrigerant seals or working with the refrigerant itself. But it does include properly applying Article 440 to protect circuits and ensure reliable operation of the equipment. Whether you are keeping beverages cold at your local convenience store or keeping tissues alive at a medical clinic, your knowledge of Article 440 will allow you to do the job right. Copyright © 2002 Mike Holt Enterprises,Inc. |
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