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Stumped by the Code? October 2011

Oct 21, 2011 12:10 PM, Mike Holt, NEC Consultant

Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2011 NEC.

Q. When is the neutral considered to be a current-carrying conductor for the purposes of ampacity adjustment?

A. There are a couple of cases when the neutral is considered to be current carrying. According to 310.15(B)(5), these include:

• The neutral conductor of a 3-wire circuit from a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) [310.15(B)(5)(b)]. Note: When a 3-wire circuit is supplied from a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system, the neutral conductor carries approximately the same current as the ungrounded conductors (click here to see Figure).
• The neutral conductor of a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) if more than 50% of the neutral load consists of nonlinear loads [310.15(B)(5)(c)]. Note: Nonlinear loads supplied by a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system can produce unwanted and potentially hazardous odd triplen harmonic currents (3rd, 9th, 15th, and so on) that can add on the neutral conductor. To prevent fire or equipment damage from excessive harmonic neutral current, the designer should consider increasing the size of the neutral conductor or installing a separate neutral for each phase. See the Informational Notes in 210.4(A), 220.61, and 450.3 for more information.

The neutral conductor of a 3-wire, single-phase, 120/240V system, or 4-wire, 3-phase, 120/208V or 277/480V wye-connected system isn’t considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) [310.15(B)(5)(a)].

Q. What field labeling is required to identify circuits served by electrical equipment such as panelboards and motor control centers?

A. All circuits and circuit modifications must be legibly identified as to their clear, evident, and specific purpose. Spare positions that contain unused overcurrent devices must also be identified. Identification must include sufficient detail to allow each circuit to be distinguished from all others, and the identification must be on a circuit directory located on the face or inside of the door of the panelboard. See 110.22 [408.4(A)]. Circuit identification must not be based on transient conditions of occupancy, such as Steven’s or Brittney’s bedroom.

All switchboards and panelboards supplied by a feeder in other than one- or two-family dwellings must be marked as to the device or equipment where the power supply originates [408.4(B)].

Q. When are systems required to be grounded?

A. Systems operating below 50V aren’t required to be grounded or bonded in accordance with 250.30, unless the transformer’s primary supply is from [250.20(A)]:

• A 277V or 480V system.
• An ungrounded system.

For systems more than 50V, the following systems must be grounded (connected to the earth) [250.20(B)]:

• Single-phase systems where the neutral conductor is used as a circuit conductor.
• 3-phase, wye-connected systems where the neutral conductor is used as a circuit conductor.
• 3-phase, high-leg delta-connected systems where the neutral conductor is used as a circuit conductor.             

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