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Stumped by the Code?

Mar 23, 2011 4:18 PM, By Mike Holt, NEC Consultant

Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2011 NEC.

Q. What are the field marking requirements for available fault current at electrical service equipment?

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)], as shown in the Figure. 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)].

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

Q. Are flexible metal conduit and liquidtight flexible metal conduit allowed to serve as equipment grounding conductors per the NEC?

A. In order for flexible metal conduit or liquidtight flexible metal conduit to serve as an equipment grounding conductor, the following conditions must be met [250.118]:

Listed flexible metal conduit (FMC):

1. The raceway terminates in listed fittings.
2. The circuit conductors are protected by an overcurrent device rated 20A or less.
3. The combined length of the flexible conduit in the same ground-fault current path doesn’t exceed 6 ft.
4. If flexibility is required to minimize the transmission of vibration from equipment or to provide flexibility for equipment that requires movement after installation, an equipment grounding conductor of the wire type must be installed with the circuit conductors in accordance with 250.102(E), and it must be sized according to 250.122, based on the rating of the circuit overcurrent device.

Listed liquidtight flexible metal conduit (LFMC) where:

1. The raceway terminates in listed fittings.
2. For 3/8 in. through ½ in., the circuit conductors are protected by an overcurrent device rated 20A or less.
3. For ¾ in. through 1¼ in., the circuit conductors are protected by an overcurrent device rated 60A or less.
4. The combined length of the flexible conduit in the same ground-fault current path doesn’t exceed 6 ft.
5. If flexibility is required to minimize the transmission of vibration from equipment or to provide flexibility for equipment that requires movement after installation, an equipment grounding conductor of the wire type must be installed with the circuit conductors in accordance with 250.102(E), and it must be sized in accordance with 250.122, based on the rating of the circuit overcurrent device.

Q. Are there any AFCI requirements for circuit modifications or additions to existing circuits?

A. Yes, according to 210.12(B), where the branch circuit wiring in a dwelling unit is modified, replaced, or extended in any of the areas specified in 210.12(A), the branch circuit must be protected by:

1. A listed combination AFCI located at the origin of the branch circuit; or
2. A listed outlet branch circuit AFCI located at the first receptacle outlet of the existing branch circuit.

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