Stumped by the Code? Type NM Cable, Sizing Branch Circuit Conductors & More

Stumped by the Code? Type NM Cable, Sizing Branch Circuit Conductors & More

Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2011 NEC.


Q. What must be done to protect Type NM cable where it is run through or parallel to framing members?

A. Type NM cable installed through or parallel to framing members or furring strips must be protected against physical damage from penetration by screws or nails by 1¼ in. of separation or by a suitable metal plate [300.4(A) and (D) and 334.17], as shown in Fig. 1. If Type NM cable passes through holes in metal studs, a listed bushing or listed grommet is required [300.4(B)(1)] to be in place before the cable is installed.


Q. What is the Code rule for sizing branch circuit conductors?

A. Conductors must be sized no less than 125% of the continuous loads, plus 100% of the noncontinuous loads, based on the terminal temperature rating ampacities [110.14(C)(1)] as listed in Table 310.15(B)(16), before any ampacity adjustment [210.19(A)(1)]. An Exception to this rule states: If the assembly and the overcurrent device are both listed for operation at 100% of its rating, the conductors can be sized at 100% of the continuous load.

It’s important to note that equipment suitable for 100% continuous loading is rarely available in ratings under 400A. See the definition of “Continuous Load” in Art. 100. In addition, see 210.20 for the sizing requirements for the branch circuit overcurrent device for continuous and noncontinuous loads.


Q. What is the Code rule for marking of service equipment with available fault current?

A. Service equipment in other than dwelling units must be legibly field-marked with the maximum available fault current, including the date the fault current calculation was performed, and be of sufficient durability to withstand the environment involved [110.24(A)]. When modifications to the electrical installation affect the maximum available fault current at the service, the maximum available fault current must be recalculated to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) must be adjusted to reflect the new level of maximum available fault current [110.24(B)].

Ex: Field markings aren’t required for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.


Q. What are the working space requirements for equipment?
A. For the purpose of safe operation and maintenance of equipment, access and working space must be provided about all electrical equipment [110.26]. Equipment that may need examination, adjustment, servicing, or maintenance while energized must have working space provided in accordance with (1), (2), and (3) [110.26(A)]. The phrase “while energized” is the root of many debates. As always, check with the AHJ to see what equipment he or she believes needs a clear working space.

Depth Requirements

The depth of the working space, which is measured from the enclosure front, must not be less than the distances contained in Table 110.26(A)(1). Working space isn’t required for the back or sides of assemblies where all connections and all renewable or adjustable parts are accessible from the front. If special permission is granted in accordance with 90.4, working space for equipment that operates at not more than 30VAC or 60VDC can be less than the distance in Table 110.26(A)(1). See the definition of “Special Permission” in Art. 100. If electrical equipment is being replaced in an existing building, Condition 2 working space is permitted between dead-front switchboards, panelboards, or motor control centers located across the aisle from each other where conditions of maintenance and supervision ensure that written procedures have been adopted to prohibit equipment on both sides of the aisle from being open at the same time, and only authorized, qualified persons will service the installation.

Note: The working space requirements of 110.26 don’t apply to equipment included in Chapter 8 — Communications Circuits [90.3].

Width Requirements

The width of the working space must be a minimum of 30 in., but in no case less than the width of the equipment. The width of the working space can be measured from left-to-right, from right-to-left, or simply centered on the equipment, and the working space can overlap the working space for other electrical equipment.

Height Requirements

The height of the working space (headroom) in front of equipment must not be less than 6½ ft, measured from the grade, floor, platform, or the equipment height — whichever is greater. Equipment such as raceways, cables, wireways, cabinets, panels, and so on, can be located above or below electrical equipment but must not extend more than 6 in. into the equipment’s working space.

Ex 1: The minimum headroom requirement doesn’t apply to service equipment or panelboards rated 200A or less located in an existing dwelling unit. (See the definition of “Dwelling Unit” in Art. 100.)

Ex 2: Meters are permitted to extend beyond the other equipment.
In all cases, the working space must be of sufficient width, depth, and height to permit all equipment doors to open 90°.


Q. What does the NEC recommend with regard to voltage drop when sizing conductors?

A. To provide reasonable efficiency of operation of electrical equipment, the Code recommends branch circuit conductors be sized to prevent a voltage drop not to exceed 3%. In addition, the maximum total voltage drop on both feeders and branch circuits shouldn’t exceed 5% [210.19(A)(1) Note 4 and 215.2(A) Note 2], as shown in Fig. 2.

Many believe the NEC requires conductor voltage drop, as per Note 4, to be applied when sizing conductors. Though this is often a good practice, it’s not a Code requirement because “Notes” are only advisory statements [90.5(C)]. The NEC doesn’t consider voltage drop to be a safety issue, except for sensitive electronic equipment [647.4(D)] and fire pumps [695.7].

Per ANSI/ASHRAE/IESNA Standard 90.1-2007, Energy Standard for Buildings Except Low Rise Residential Buildings, 8.4.1: Feeders need to be sized for a maximum voltage drop of 2% and branch circuits sized for a maximum of 3%, calculated at design load. This is part of the federal energy code and is adopted by many states.


Q. When installing a new electrical service in an existing building, does the Code require the concrete be chipped up to access the rebar to establish a concrete-encased electrode?

A. Not according to the exception to 250.50. First off, the rule in 250.50 states that any grounding electrode described in 250.52(A)(1) through (A)(8) that is present at a building/structure must be bonded together to form the grounding electrode system.

  • Underground metal water pipe [250.52(A)(1)]
  • Metal frame of the building/structure [250.52(A)(2)]
  • Concrete-encased electrode [250.52(A)(3)]
  • Ground ring [250.52(A)(4)]
  • Ground rod [250.52(A)(5)]
  • Other listed electrodes [250.52(A)(6)]
  • Grounding plate [250.52(A)(7)]
  • Metal underground systems, piping systems, or underground tanks [250.52(A)(8)].

Ex: Concrete-encased electrodes are not required for existing buildings or structures where the conductive steel reinforcing bars are not accessible without chipping up the concrete.

Note: When a concrete-encased electrode is used at a building/structure that does not contain an underground metal water pipe electrode, then no
additional electrode is required.

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