Stumped by the Code?

All questions and answers are based on the 2017 NEC.

Q. What uses are not permitted by the NEC for liquidtight flexible nonmetallic conduit?

A. Liquidtight flexible nonmetallic conduit cannot be used if:

(1) subject to physical damage [Sec. 356.12].

(2) the ambient temperature and/or conductor temperature is in excess of its listing.

(3) longer than 6 ft, except if approved by the authority having jurisdiction (AHJ) as essential for a required degree of flexibility.

(4) in any hazardous location, except as permitted by Sec. 501.10(B), Sec. 502.10(A) and (B), and Sec. 504.20.

Liquidtight flexible nonmetallic conduit is more or less a flexible version of PVC and is actually made from a more pliable type of PVC.

Q. Which alternating current systems are required to be grounded by the NEC?

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

(1) A 277V or 480V system.

(2) An ungrounded system.

The following 50V to 1,000V systems must be grounded (connected to the earth) [Sec. 250.20(B)]:

(1) Single-phase systems where the neutral conductor is used as a circuit conductor.

(2) 3-phase, wye-connected systems where the neutral conductor is used as a circuit conductor.

(3) 3-phase, high-leg delta-connected systems where the neutral conductor is used as a circuit conductor.

Q. The neutral of a grounded electric utility service must be installed and terminated at what location in accordance with the NEC?

A. A service neutral conductor must be run from the electric utility power supply with the ungrounded conductors and terminate to the service disconnect neutral terminal [Sec. 250.24(C)]. A main bonding jumper [Sec. 250.24(B)] must be installed between the service neutral terminal and the service disconnect enclosure [Sec. 250.28].

The service neutral conductor provides the effective ground-fault current path to the power supply to ensure that dangerous voltage from a ground fault will be quickly removed by opening the overcurrent protection device [Sec. 250.4(A)(3) and Sec. 250.4(A)(5)].

Even if the service equipment is only supplying loads with no neutral conductor, such as a 3-phase motor, a service neutral conductor between the electric utility and service is part of the effective ground-fault current path needed in case there is a ground fault condition (Figure).

DANGER: Dangerous voltage from a ground fault won’t be removed from metal parts, metal piping, and structural steel if the service disconnect enclosure isn’t connected to the service neutral conductor. This is because the contact resistance of a grounding electrode to the earth is so great that insufficient fault current returns to the power supply if the earth is the only fault current return path to open the circuit overcurrent protection device.

If the neutral conductor is opened, dangerous voltage will be present on metal parts under normal conditions, providing the potential for electric shock. If the earth’s ground resistance is 25 ohms and the load’s resistance is 25 ohms, then the voltage drop across each of these resistors will be half of the voltage source. Because the neutral is connected to the service disconnect, all metal parts will be elevated to 60V above the earth’s voltage for a 120/240V system.

These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.