By Mike Holt for EC&M Magazine

Try your best to answer these questions without looking at the answers. You are welcome to use these questions as a quiz. Feel free to print and email to your peers.

Q1. How is the grounding electrode conductor sized to a concrete encased electrode?

A1. If the grounding electrode conductor is connected to one or more concrete-encased electrodes, as permitted in 250.52(A)(3), that portion of the grounding electrode conductor that’s the sole connection to the concrete-encased electrode(s) isn’t required to be larger than 4 AWG copper [250.66(B)].

Q2. What is the NEC requirement for GFCI protection of swimming pool pump motors?

A2. For this question, it is important to understand the NEC definition of an outlet. An outlet is a point on the wiring system where current is taken to supply utilization equipment [Article 100]. The outlet can be a point where a receptacle is installed for a cord-and-plug type connection of the pool pump motor. This outlet can also be the point where a pool pump motor is hard wired to the branch circuit.GFCI protection is required for outlets supplying pool pump motors connected to single-phase, 120V through 240V branch circuits, whether by receptacle or by direct connection [680.21(C)]. The GFCI protection can be provided by a GFCI receptacle or a GFCI breaker protecting the branch circuit supplying the pool pump motor.

Q3. Are cord-and-plug connections allowed for swimming pool pump motors?

A3. Cords are permitted if the length doesn’t exceed 3 ft and it contains a copper equipment grounding conductor, sized in accordance with 250.122, based on the rating of the overcurrent device, but not smaller than 12 AWG [680.21(A)(5)].

Q4. When I parallel conductors, can each phase be in a different raceway?

A4. To minimize induction heating of ferrous metal raceways and ferrous metal enclosures for alternating-current circuits, and to maintain an effective ground-fault current path, all conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray. See 250.102(E), 300.3(B), 300.5(I), and 392.8(D).

Author’s Comment: When alternating current (ac) flows through a conductor, a pulsating or varying magnetic field is created around the conductor. This magnetic field is constantly expanding and contracting with the amplitude of the ac current. In the United States, the frequency is 60 cycles per second (Hz). Since ac reverses polarity 120 times per second, the magnetic field that surrounds the conductor also reverses its direction 120 times per second. This expanding and collapsing magnetic field induces eddy currents in the ferrous metal parts that surround the conductors, causing the metal parts to heat up from hysteresis heating.

Magnetic materials naturally resist the rapidly changing magnetic fields. The resulting friction produces its own additional heat—hysteresis heating—in addition to eddy current heating. A metal which offers high resistance is said to have high magnetic “permeability.” Permeability can vary on a scale of 100 to 500 for magnetic materials; nonmagnetic materials have a permeability of one.

Simply put, the molecules of steel and iron align to the polarity of the magnetic field and when the magnetic field reverses, the molecules reverse their polarity as well. This back-and-forth alignment of the molecules heats up the metal, and the more the current flows, the greater the heat rises in the ferrous metal parts.

When conductors of the same circuit are grouped together, the magnetic fields of the different conductors tend to cancel each other out, resulting in a reduced magnetic field around the conductors. The lower magnetic field reduces induced currents in the ferrous metal raceways or enclosures, which reduces the hysteresis heating of the surrounding metal enclosure.

 

WARNING: There’s been much discussion in the press on the effects of electromagnetic fields on humans. According to the Institute of Electrical and Electronics Engineers (IEEE), there’s insufficient information at this time to define an unsafe electromagnetic field level.

 

When single conductors are installed in nonmetallic raceways as permitted in 300.5(I) Ex 2, the inductive heating of the metal enclosure must be minimized by the use of aluminum locknuts and by cutting a slot between the individual holes through which the conductors pass [300.20(B)]. Figure 300-20B0 01

 

Author’s Comment: Aluminum conduit, locknuts, and enclosures carry eddy currents, but because aluminum is nonferrous, it doesn’t heat up [300.20(B) Note].

All underground conductors of the same circuit, including the equipment grounding conductor, must be inside the same raceway, or in close proximity to each other in the same trench [300.5(I)]. See 300.3(B).

Ex 1: Conductors can be installed in parallel in raceways, multiconductor cables, or direct-buried single-conductor cables. Each raceway or multiconductor cable must contain all conductors of the same circuit including the equipment grounding conductor. Each direct-buried single-conductor cable must be located in close proximity in the trench to the other single conductor cables in the same parallel set of conductors, including equipment grounding conductors.

Ex 2: Parallel circuit conductors installed in accordance with 310.10(H) of the same phase or neutral can be installed in underground PVC conduits, if inductive heating at raceway terminations is reduced by the use of aluminum locknuts and cutting a slot between the individual holes through which the conductors pass as required by 300.20(B).

 

Author’s Comment: Installing ungrounded and neutral conductors in different PVC conduits makes it easier to terminate larger parallel sets of conductors, but it’ll result in higher levels of electromagnetic fields (EMF).

Q5. When multiple type NM cables pass through a bored hole in the wood framing that will be sealed with foam, must the ampacity of the conductors be adjusted?

A5. Section 334.80 requires that if multiple Type NM cables pass through the same wood framing opening that’s to be sealed with thermal insulation, caulking, or sealing foam, the allowable ampacity of each conductor must be adjusted in accordance with Table 310.15(B)(3)(a).

Author’s Comment: This requirement has no effect on conductor sizing if you bundle no more than nine current-carrying 14 or 12 AWG conductors together. For example, three 14/2 cables and one 14/3 cable (eight current-carrying 14 THHN conductors) are bundled together in a dry location, the ampacity for each conductor (25A at 90°C, Table 310.15(B)(16)) is adjusted by a 70 percent adjustment factor [Table 310.15(B)(3)(a)].

• Adjusted Conductor Ampacity = 25A x 0.70
• Adjusted Conductor Ampacity = 17.50A

14 AWG is limited to a 15A overcurrent protection device [240.4(D)]. The 14 AWG conductors with an ampacity of 17.50A after adjustment is okay for the 15A overcurrent protection

Q6. When walls are insulated by filling them with spray-applied polyurethane foam insulation, or other types of insulation, must the ampacity of Type NM cable in that wall be adjusted?

A6. Where more than two NM cables are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor must be adjusted in accordance with Table 310.15(B)(3)(a) [334.80].