Ecmweb 7584 Nec Code Quandaries July 2015 Pr
Ecmweb 7584 Nec Code Quandaries July 2015 Pr
Ecmweb 7584 Nec Code Quandaries July 2015 Pr
Ecmweb 7584 Nec Code Quandaries July 2015 Pr
Ecmweb 7584 Nec Code Quandaries July 2015 Pr

Stumped by the Code? NEC Requirements for Sizing Service and Feeder Conductors

July 15, 2015
Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2014 NEC.

Q. What are the rules for sizing service and feeder conductors to a single dwelling unit?

A. For one-family dwellings and individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single phase, 120/240V system can be sized using the requirements set forth in Sec. 310.15(B)(7)(1) through (4) (Fig. 1).

Fig. 1. Service and feeder conductors for a single dwelling unit — supplied by 120/240V, 1-phase — can be sized using 310.15(B)(7)(1) through (4).

1)  Service conductors supplying the entire load of a one-family dwelling or an individual dwelling unit in a two-family or multifamily dwelling can have an ampacity of 83% of the service rating. This rule doesn’t apply to service conductors for two-family or multifamily dwelling buildings (Fig. 2).

Fig.2. Service conductors supplying the entire load of a one-family dwelling or an individual dwelling unit in a two-family or multifamily dwelling can have an ampacity of 83% of the service rating.

For example, what size service conductors are required if the calculated load for a dwelling unit equals 195A, and the service disconnect is rated 200A?

A 200A rated circuit breaker multiplied by 83% = 166A. As per Table 310.15(B)(16), a 2/0 AWG conductor would work since it’s rated 175A at 75°C.

2)  For a feeder rated 100A through 400A, feeder conductors supplying a one-family dwelling, or an individual dwelling unit in a two-family or multifamily dwelling can have an ampacity of 83% of the feeder rating, but only if the feeder supplies the entire load of the dwelling. This rule can’t be used to size feeder conductors where a feeder doesn’t carry the entire load of the dwelling unit, except as permitted in Sec. 310.15(B)(7)(3).

Here’s a sample problem to review: What size feeder conductors are required if the calculated load for a dwelling unit equals 195A, the service disconnect is rated 200A, and the feeder conductors carry the entire load of the dwelling unit?

Once again, a 200A rated circuit breaker multiplied by 83% = 166A, and as per Table 310.15(B)(16), a 2/0 AWG conductor would work since it’s rated 175A at 75°C.

WARNING: Section 310.15(B)(7) doesn’t apply to 3-wire service or feeder conductors connected to a 3-phase, 120/208V system because the neutral conductor in these systems always carries neutral current, even when the load on the phases is balanced [310.15(B)(5)(b)]. For more information on this topic, see Sec. 220.61(C)(1).

3)  Feeders for an individual dwelling unit are never required to be larger than the conductors in Sec. 310.15(B)(7)(1) or (2).

4)  Neutral conductors are sized using the rules of Sec. 220.61 and Sec. 230.42 for services and Sec. 220.61 and Sec. 215.2 for feeders.

CAUTION: Because the service neutral conductor is required to serve as the effective ground-fault current path, it must be sized so it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accomplished by sizing the neutral conductor in accordance with Table 250.102(C), based on the area of the largest ungrounded service conductor [250.24(C)(1)].

Here’s a sample problem to review.

What size neutral conductor is required if the calculated load for a dwelling unit equals 195A, the maximum unbalanced load is 100A, and the service disconnect is rated 200A with 2/0 AWG conductors?

As per Table 310.15(B)(16), a 3 AWG conductor is a good choice here since it’s rated 100A at 75°C. Section 310.15(B)(7)(3) doesn’t allow for the 83% deduction for neutral conductors. In addition, Sec. 250.24(C) requires the neutral conductor to be sized no smaller than 4 AWG based on 2/0 AWG service conductors in accordance with Table 250.102(C).

Q. What are the disconnecting means requirements for a building supplied by a feeder?

A. A disconnect is required for all conductors that enter a building [225.31] (Fig. 3). The disconnecting means for a building must be installed at a readily accessible location either outside or inside nearest the point of entrance of the conductors [225.32]. Supply conductors are considered outside of a building or other structure where they’re encased or installed under not less than 2 in. of concrete or brick [230.6].

Fig. 3. The disconnecting means for a building must be installed at a readily accessible location either outside or inside nearest the point of entrance of the conductors.

Exception No. 1: If documented safe switching procedures are established and maintained, the building disconnecting means can be located elsewhere on the premises, if monitored by qualified persons.

Exception No. 3: A disconnecting means isn’t required within sight of poles that support luminaires.

According to Art. 100, “within sight” means that it’s visible and not more than 50 ft from one to the other.

Exception No. 4: The disconnecting means for a sign must be controlled by an externally operable switch or circuit breaker that opens all ungrounded conductors to the sign. The sign disconnecting means must be within sight of the sign, or the disconnecting means must be capable of being locked in the open position [600.6(A)].

The building disconnecting means can consist of no more than six switches or six circuit breakers in a single enclosure, or separate enclosures for each supply grouped in one location as permitted by Sec. 225.30 [225.33(A)].

The building disconnecting means must be grouped in one location, and they must be marked to indicate the loads they serve [110.22 and 225.34(A)].

To minimize the possibility of accidental interruption of the critical power systems, the disconnecting means for a fire pump or for standby power must be located remotely away from the normal power disconnect [225.34(B)].

In a multiple-occupancy building, each occupant must have access to the disconnecting means for their occupancy [225.35].

Exception: The occupant’s disconnecting means can be accessible to only building management, if electrical maintenance under continuous supervision is provided by the building management.

The building disconnecting means can be comprised of a circuit breaker, molded case switch, general-use switch, snap switch, or other approved means. If an existing building uses the neutral conductor for the bonding of metal parts [250.32(B) Ex], the disconnect must be listed as suitable for use as service equipment [225.36]. “Suitable for use as service equipment” means, among other things, that the service disconnecting means is supplied with a main bonding jumper so a neutral-to-case connection can be made, as required in Sec. 250.24(C) and Sec. 250.142(A).        

Holt is the owner of Mike Holt Enterprises, Inc. in Leesburg, Fla. He can be reached at www.mikeholt.com.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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