Derating Conductors in a Raceway and More


Article 220 Branch-Circuit, Feeder and Service Calculations

Article 230 Services

Article 240 Overcurrent Protection

Article 310 Conductors for General Wiring

Article 312 Cabinets, Cutout Boxes and Meter Socket Enclosures

Article 404 Switches

Article 410 Luminaires (Lighting Fixtures), Lampholders and Lamps

Article 430 Motors, Motor Circuits and Controllers

Article 440 Air Conditioning and Refrigerating Equipment

Article 695 Fire Pumps

Sizing branch-circuit conductors for air-conditioning units

Q:I have to provide a branch circuit for an outdoor 2-ton condensing unit that has the following electrical data on the nameplate: Branch-Circuit Selection Current 15A; fan motor 1A; compressor 11.7A; maximum overcurrent protection, 25A fuses or HACR circuit breaker; voltage, 230 single-phase. Am I allowed to use 14 AWG copper conductors even though the calculated load current exceeds 15A? Also, are 25A fuses permitted to protect these 14 AWG branch-circuit conductors?

A: Hermetic refrigerant motor-compressors and related electrical equipment are covered by Article 440—Air-Conditioning and Refrigerating Equipment. These rules are somewhat different than those found in Article 430, which deals with Motors, Motor Circuits and Controllers.

Branch-Circuit Selection Current is a term that is only found in Article 440, and is defined as “The value in amperes to be used instead of rated-load current in determining 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 the branch-circuit selection current will always be equal to or greater than the marked rated-load current.”

You are correct in your statement where you indicate that the calculated current according to 430.24 exceeds 15A. This is the calculation: (11.7 x 1.25) + 1 equals 15.6A. But this is an air conditioner containing a hermetic refrigerant motor-compressor. Therefore, Article 440 applies.

Branch-circuit conductor size is determined in accordance with 440.6, which allows the use of ampacity tables 310.16 through 310.19 or by calculation as permitted by 310.15 under engineering supervision. Since the nameplate Branch-Circuit Selection current is 15A, 14 AWG copper conductors are satisfactory if the length of the branch circuit is not excessive.

A horsepower rated 30A, 240V safety switch with 25A fuses or a 25A, two-pole, common-trip HACR type circuit breaker should satisfy the requirements for the disconnecting means mentioned in part II of Article 440.

This size of fuse or circuit breaker provides short-circuit and ground fault protection for the branch-circuit conductors, control apparatus, hermetic refrigerant motor-compressor and fan motor. Overload protection is provided by a separate overload relay or other devices mentioned in 440.52.

Services for fire pump motors and jockey pump motors

Q:Where there are two fire pumps and two jockey pumps for a building and there are five existing service disconnects, am I allowed to have nine service disconnects (two for the fire pumps and two for the jockey pumps)? The disconnecting means for each fire pump is a service-rated fire pump controller. The service disconnect for each jockey pump is on an outside wall. Do the service disconnects have to be grouped at one location?

A:Article 695—Fire Pumps does not cover the installation of jockey pumps. In fact, 695.1(B) says that this article does not cover pressure maintenance (jockey or makeup) pumps; therefore, a separate service for a jockey pump is not required. The general rules in Article 430 apply to motors that drive pressure maintenance pumps, and the branch-circuits for these motors can be supplied from the normal service for the building. If this is done there will be seven service disconnects for the building, which is acceptable.

Generally, the number of services to a building or structure is limited to one, but 230-2(A)(1) allows an additional service for a fire pump. Notice that pressure maintenance (jockey) pumps are not mentioned anywhere in 230.2

You are allowed a maximum of six service disconnects grouped at one location for the normal service to the building by 230.72(A). Part (B) of 230.72 allows additional service disconnects for fire pumps and legally required standby systems. These disconnects are in addition to the six permitted for the normal service to the building. Where a disconnecting means and overcurrent protection is installed ahead of the fire pump controller it must be remote from the service equipment for the building and be marked “Fire Pump Disconnecting Means.” Also, part (E) of 230.2 requires the installation of a permanent plaque or directory at each service that indicates the locations of all other services in the building.

Derating conductors in a raceway

Q: In some home runs that are about 30 feet in length, we have as many as 12 No. 12 AWG Type THHN copper conductors in a single 3/4-inch electrical metallic tubing. Are we permitted to protect these conductors with 20A branch circuit overcurrent devices?

A: Because the number of current carrying conductors is not given and the type of distribution (single-phase or three-phase) system is not known, it will be necessary to make some assumptions. If these are all two-wire circuits (hot and neutral), the number of current carrying conductors in the EMT is 12 and these wires must be derated to 50 percent of their ampacity shown in Table 310.16.

The adjusted ampacity for each conductor is (30 x 0.5) = 15. And according to 240.4, they must be protected by a 15A fuse or circuit breaker. If these conductors supply 3-phase motor loads, the full load current of each motor cannot exceed 12A, but overcurrent protection could be 30A circuit breakers or fuses.

If these conductors are single-phase multiwire (120/240V) branch circuits, there are now eight current-carrying conductors. According to Table 310.15(B)(2)(a), the derating factor is 70 percent. The adjusted ampacity for this arrangement is (30 x 0.7) 21A. Therefore, 20A overcurrent devices are permitted. On a 3-phase, four-wire wye (208/120) volt system where the branch circuits are 3-phase, four-wire and supply 120V circuits that do not have a major portion of nonlinear load, there are nine current-carrying conductors that must be derated to 21A. Overcurrent devices rated 20A are acceptable for this application.

Some of the Code references used in this answer are Table 310.15(B)(2)(a), 310.15(B)(2), 310.15(B)(4), Table 310.16, 430.22, and Table 430.52.

Receptacles, switches and lighting fixtures around bathtubs

Q: Does the NEC allow luminaires (lighting fixtures), switches, and receptacles within the 8-feet and 3-feet dimensions of a bathtub as mentioned in 410.4(D)?

A: The 3 feet of horizontal space around a bathtub and the 8 feet of vertical clearance measured from the rim of the bathtub apply to only those items listed in 410.4(D). They are cord connected small luminaires, hanging luminaires, track lighting, pendants and ceiling paddle fans. Flush- or surface-mounted luminaires may be located closer than 8 feet above the rim of the tub. Switches cannot be installed in wet locations in bathtub spaces, but there is no restriction on the distance that a switch may be located from a bathtub. This information is in 404.4.

Receptacles cannot be installed in a bathtub or shower space; however, they can be installed in the 3 feet of horizontal space that limits certain types of luminaires and paddle fans, but they should not be located in a damp or wet area.

Size of service-entrance conductors for a restaurant

Q:What is the minimum size service-entrance conductors and overcurrent protection for a new all-electric restaurant with an actual calculated load of 828 kVA? The voltage is 208Y/120, 3-phase.

A:Demand factors that are permitted for service-entrance conductors and feeders are shown and explained in 220.36 and Table 220.36. Where the connected load is over 800kVA, the Table allows the service conductors to be sized for a demand load of 410 + (0.5 x 28) 424kVA. Dividing this figure by 208 x 1.73 results in a current of about 1,178A. Four sets of 350 kcmil Type THWN copper conductors in separate raceways or 1,200A bus duct may be used for the service-entrance conductors.

Overload protection for the service-entrance conductors can be 1,200A or higher depending on the air conditioning and motor loads. Overload protection requirements are found in 230.90(A) which requires overload protection to not exceed the ampacity of the conductors, but there are three exceptions that could apply to this installation.

Nonmetallic sheathed cable in a raceway

Q: Short lengths of rigid metal conduit and other raceways have been used to protect Type NM-B cable from physical damage. Does the NEC permit nonmetallic sheathed cable to be installed in a complete raceway system?

A: No, it does not. Although nonmetallic sheathed able is permitted to be protected from physical damage by installation in a short piece of conduit, electrical metallic tubing, schedule 80 PVC, and by other means, there is no permission to pull Type NM cable into a complete raceway system. In fact, the raceway wiring method articles contain this rule: “Cable shall be permitted to be installed where such use is permitted by the respective cable articles.” This sentence appears in Article 342—Intermediate Metal Conduit, Article 344—Rigid Metal Conduit, Article 348—Flexible Metal Conduit and all of the other raceway Articles.

This sentence appears in 312.5(C), “Where cables are used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure.” This sentence is followed by a long exception that allows nonmetallic cable to be pulled into a nonflexible raceway that is not longer than 10 feet and located directly above a surface mounted enclosure; the cables must be fastened within 12 inches of the point where they enter the raceway; a fitting must be provided to protect the cables from abrasion; the raceway must be sealed at the outer end; the cable sheath is continuous and extends into the enclosure at least 1/4 inch; the raceway is secured as required by the applicable article; the allowable fill of the raceway is not exceeded; and derating factors are applied. EC

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720. 


About the Author

George W. Flach

Code Q&A Columnist

George W. Flach was a regular contributing Code editor for Electrical Contractor magazine, serving for more than 40 years. His long-running column, Code Q&A, is one of the most widely read in the magazine's history. He is a former chief electrical...

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