Paralleling Cable, Compressor Motor Protection and More

By George W. Flach | Jan 15, 2008
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Article 110—Requirements for Electrical Installations; Article 210—Branch Circuits; Article 240—Overcurrent Protection; Article 250—Grounding and Bonding; Article 310—Conductors for General Wiring; Article 334—Non-Metallic Sheathed Cable: Type NM, NMC, and NMS; Article 406—Receptacles, Cord Connectors, and Attachment Plugs (Caps); Article 440—Air-Conditioning and Refrigeration Equipment; Article 680—Swimming Pools, Fountains, and Similar Installations; Article 695—Fire Pumps

Type MC with interlocked armor

Is type MC cable with interlocked metal strip armor permitted to be run in parallel? I ask this question because 250.122(F) of the National Electrical Code (NEC) requires the equipment-grounding conductor be sized based on the ampere rating of the overcurrent device protecting the feeder. I have to install a 1,000-ampere feeder in an existing commercial building and would like to use Type MC cable with interlocked armor because of the large number of bends involved.

The requirement for an equipment-grounding conductor to have an ampacity based on the ampere rating of the overcurrent device reads, “Each parallel equipment grounding conductor shall be sized on the basis of the ampere rating of the overcurrent device protecting the circuit conductors in the raceway or cable in accordance with Table 250.122.”

At least one manufacturer of interlocked armor Type MC cable produces it with an oversize equipment-grounding conductor that is specifically made for paralleling conductors. Here are some examples taken from the manufacturer’s catalog: four 500-kcmil aluminum conductors with a 2/0 equipment grounding conductor or a 250-kcmil equipment-grounding conductor, four 750 kcmil aluminum conductors with a 3/0 equipment-grounding conductor, or a 750-kcmil equipment-grounding conductor. Other combinations also are listed in the catalog. Cable assemblies with the larger size equipment-grounding conductor are suitable for parallel runs. Conductor insulation is Type XHHN-2. Therefore, the 75°C ampacity in Table 310.16 can be used, and three 750-kcmil conductors per phase with a 410 equipment grounding conductor will provide adequate capacity for the 1,000-ampere feeder and satisfy the requirement in 250.122(F)(1).

In the second paragraph [250.122(F)(2)], the size of the equipment-grounding conductor can be reduced, provided that ground-fault protection of equipment is installed by qualified people who will maintain the installation, the ground fault protection is set to trip at not more than the ampacity of a single ungrounded conductor, and the ground-fault protection is listed for the purpose.

Compressed motor overcurrents

An individual, hermetically sealed motor-compressor has a full load current of 32 amperes at 240 volts, single phase. Is a 60-ampere, two-pole circuit breaker permitted for the overcurrent protection of this branch circuit?

Article 440.22(A) allows branch circuit overcurrent protection for this compressor to be 175 percent of full-load current. Doing the multiplication (1.75 × 32) results in 56 amperes. Although not mentioned in this section, the next larger standard-size overcurrent device is permitted. Therefore, a 60-ampere circuit breaker may be used to protect this hermetic motor-compressor from short circuit or ground fault. This increase to the next larger standard-size overcurrent device listed in 240.6 is permitted because the largest size overcurrent device is allowed to be increased to 225 percent of compressor nameplate load current, which is (2.25 × 32) 72 amperes. This increase is permitted where the compressor motor will not start consistently while protected by the 60-ampere circuit breaker. The maximum size overcurrent protection for this motor compressor is 70 amperes. The 225 percent maximum overcurrent protection cannot be exceeded. Therefore, a 70-ampere circuit breaker is the maximum permitted for this installation.

AFCIs for a bedroom switch

Does a wall switch in a bedroom of a residence that controls a porch luminaire (fixture) require arc-fault circuit interrupter (AFCI) protection?

Where a wall switch in a bedroom controls only loads that are not in the bedroom, AFCI protection is not required. Part (B) of 210.12 has this requirement: “All 120-volt, single phase, 15- and 20-ampere branch circuits supplying outlets installed in dwelling unit bedrooms shall be protected by a listed arc-fault circuit interrupter, combination type installed to provide protection of the branch circuit.” The key word in this requirement is “outlet,” which is defined in Article 100 as “A point on the wiring system at which current is taken to supply utilization equipment.” A switch does not qualify as an outlet under this definition and does not need to be protected by an AFCI.

Fire pump service-disconnect location

May the disconnect switch for a fire pump be in the same room as the service disconnect for the building?

The service-disconnecting means for the fire pump can be located in the same room as the building service-disconnecting means, but it must be sufficiently remote from the building service-disconnecting means to prevent inadvertent contemporaneous operation. The words “inadvertent contemporaneous” are used in 695.4(B)(2) to indicate the distance that one switch must be located from the other. Since this distance is a judgment decision, the electrical inspector may not be the authority having jurisdiction for this and other parts of Article 695 where judgment is required.

Before proceeding with the installation, the agency responsible for enforcement of NFPA 20—Standard for the Installation of Stationary Pumps for Fire Protection should be consulted for a decision on the location of the fire pump disconnect switch in relation to the location of the building service-disconnecting means. However, location of the fire pump disconnecting means in the same room with the service disconnect is not prohibited by 695.4(B)(2) in the NEC or in NFPA-20.

Branch-circuit conductors derating

What is the corrected ampacity of four multiwire branch circuits in the same conduit that supply receptacles and are connected to a 120/240-volt, single-phase panelboard? The conductors are 12 AWG copper with Type THWN insulation. Would the ampacity be different if the conductors were connected to a 208Y/120-volt, three-phase system?

The ampacity of 12 AWG copper conductors with Type THWN insulation is 25 amperes, according to Table 310.16. Section 310.15 provides additional details for selecting the corrected ampacity of conductors. Also, 110.14 provides guidance for selecting ampacity based on temperature limitations on terminations. Since the neutral conductors of three-wire, single-phase, 120/240-volt multiwire branch circuits are not considered current-carrying conductors, there are eight current-carrying conductors for derating purposes. According to Table 310.15(B)(2)(a) the derating factor for eight current-carrying conductors in a single raceway is 70 percent. Therefore, the corrected ampacity is (0.70 × 25) 17.5 amperes. Although 240.4(B) allows an increase to the next larger standard size overcurrent device, this permission does not apply to branch circuits that supply receptacles. Therefore, 15-ampere overcurrent devices must be provided for these branch circuits.

Where the supply voltage is 208Y/120 volts and the branch circuits are 120 volts, there are 12 current-carrying conductors, because the grounded branch-circuit conductor is current-carrying. Since there are now 12 current-carrying conductors in the raceway, the derating factor is 0.5, and the corrected ampacity is (0.5 × 25) 12.5. Because 15-ampere branch circuits are the smallest recognized by 210.3, the branch circuits should be changed to three-phase, four-wire. This results in three multiwire branch circuits with neutrals that carry only the unbalanced current from the ungrounded conductors. With nine current-carrying conductors, the corrected ampacity of each ungrounded conductor is 17.5, and 15-ampere overcurrent-protective devices can be used.

8 AWG and larger conductors

Does the requirement for 8 AWG and larger stranded conductors in a raceway apply to a grounding-electrode conductor? Assume the grounding-electrode conductor is 6 AWG installed in a rigid metal conduit with no other conductors.

Yes, the requirement in 310.3 applies to all wires installed in raceways. This section reads, “Stranded Conductors. Where installed in raceways, conductors of size 8 AWG and larger shall be stranded.” An exception reads, “Exception. As permitted or required elsewhere in this Code.” One place where this exception applies is in Article 680. Wet-niche luminaires (fixtures) installed in swimming pools with nonmetallic conduit as permitted by 680.23(B)(2)(b) are required to have an 8 AWG insulated solid or stranded copper bonding jumper in the nonmetallic raceway to bond the luminaire forming shell to the deck box.

A 6 AWG copper grounding-electrode conductor does not have to be installed in a raceway where it is free from physical damage. It is allowed to be run along the surface of the building construction without metal covering or protection where it is securely fastened to the construction. This permission is in 250.62(B). Nothing in Article 250 allows an 8 AWG solid copper grounding-electrode conductor in a raceway.

A dwelling’s increased feeder ampacity

Is a 4 AWG feeder in Type NM cable that supplies the total load in a single-family dwelling allowed to be protected by a 100-ampere, two-pole circuit breaker? Does Table 310.15(B)(6) apply to NM cable?

Permission to use the elevated ampacities of conductors that are used to supply the main power feeder to a dwelling are limited to 75 and 90°C insulations. Although the insulation on the conductors in NM cable is rated at 90°C to comply with 334.112, the ampacity of the conductors is limited to 60°C by 334.80. For these reasons, Type NM cable cannot be assigned a 100-ampere rating. 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 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 inspector for New Orleans and held many other prestigious positions in the electrical industry, including IAEI board of directors and executive committee. He passed away in August 2009.





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