NEC Questions and Answers – Based on the 2011 NEC

June 2012

By Mike Holt for EC&M Magazine

Here’s the follow up to yesterday’s newsletter.This includes all of the answers to the questions sent, so you can see how you did.

Q1. What are the NEC rules for securing and supporting raceways?

A1.Raceways, cable assemblies, boxes, cabinets, and fittings must be securely fastened in place. The ceiling-support wires or ceiling grid must not be used to support raceways and cables (power, signaling, or communications). However, independent support wires that are secured at both ends and provide secure support are permitted [300.11(A)].

Author’s Comment: Outlet boxes [314.23(D)] and luminaires can be secured to the suspended-ceiling grid if securely fastened to the ceiling-framing members by mechanical means such as bolts, screws, or rivets, or by the use of clips or other securing means identified for use with the type of ceiling-framing member(s) used [410.36(B)].

Electrical wiring within the cavity of a fire-rated floor-ceiling or roof-ceiling assembly can be supported by independent support wires attached to the ceiling assembly. The independent support wires must be distinguishable from the suspended-ceiling support wires by color, tagging, or other effective means [300.11(A)(1)].

Wiring in a nonfire-rated floor-ceiling or roof-ceiling assembly can be supported by independent support wires attached to the ceiling assembly. The independent support wires must be distinguishable from the suspended-ceiling support wires by color, tagging, or other effective means [300.11(A)(2)].

Raceways must not be used as a means of support for other raceways, cables, or nonelectrical equipment, except [300.11(B)]:

• If the raceway or means of support is identified for the purpose [300.11(B)(1)].
• Class 2 and 3 cable can be supported by the raceway that supplies power to the equipment controlled by the Class 2 or 3 circuit [300.11(B)(2)].
• Raceways are permitted as a means of support for threaded boxes and conduit bodies in accordance with 314.23(E) and (F), or to support luminaires in accordance with 410.36(E) [300.11(B)(3)].

Q2. Can you explain how to size 90°C conductors terminating on 75°C rated equipment?

A2. Conductors are to be sized using their ampacity from the insulation temperature rating column of Table 310.15(B)(16) that corresponds to the lowest temperature rating of any terminal, device, or conductor of the circuit [110.14(C)].

Unless the equipment is listed and marked otherwise, conductor sizing for equipment terminations must be based on Table 310.15(B)(16) in accordance with (a) or (b):

(a) Equipment Rated 100A or Less [110.14(C)(1)(a)].

• Conductors must be sized using the 60°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(1)].
• Conductors terminating on terminals rated 75°C are sized in accordance with the ampacities listed in the 75°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(3)].

(b) Equipment Rated Over 100A.

• Conductors must be sized using the 75°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(b)(1)].
• Separate Connector Provisions. Conductors can be sized to the 90°C column of Table 310.15(B)(16) if the conductors and pressure connectors are rated at least 90°C [110.14(C)(1)(b)(2)].

Q3. What are the installation requirements for equipment disconnecting means?

A3. Switches and circuit breakers used as switches must be capable of being operated from a readily accessible location. They must also be installed so the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, isn’t more than 6 ft 7 in. above the floor or working platform [240.24(A) and 404.8(A)].

Author’s Comment: The disconnecting means for a mobile home must be installed so the bottom of the enclosure isn’t less than 2 ft above the finished grade or working platform [550.32(F)].

Ex 1: On busways, fusible switches and circuit breakers can be located at the same level as the busway where suitable means is provided to operate the handle of the device from the floor.

Ex 2: Switches and circuit breakers used as switches can be mounted above 6 ft 7 in. if they’re next to the equipment they supply, and are accessible by portable means [240.24(A)(4)].

Q4. What are the grounding requirements for a building supplied by a feeder?

A4. Each building/structure’s disconnect must be connected to an electrode of a type identified in 250.52 [250.32(A)].

Author’s Comments:

• The grounding of the building/structure disconnecting means to the earth is intended to help in limiting induced voltages on the metal parts from nearby lightning strikes [250.4(A)(1)].
• The Code prohibits the use of the earth to serve as an effective ground-fault current path [250.4(A)(5) and 250.4(B)(4)].

Ex: A grounding electrode isn’t required where the building/structure is served with a 2-wire, 3-wire, or 4-wire multiwire branch circuit.

Q5. How do I size the EGC for a branch circuit or feeder if the conductors are increased in size for any reason?

A5. If ungrounded conductors are increased in size from the minimum size, equipment grounding conductors must be proportionately increased in size according to the circular mil area of the ungrounded conductors [250.122(B)].

Author’s Comment: Ungrounded conductors are sometimes increased in size to accommodate conductor voltage drop, harmonic current heating, short-circuit rating, or simply for future capacity.

Question: If the ungrounded conductors for a 40A circuit are increased in size from 8 AWG to 6 AWG, the circuit equipment grounding conductor must be increased in size from 10 AWG to _____.

(a) 10 AWG (b) 8 AWG (c) 6 AWG (d) 4 AWG

Answer: (b) 8 AWG

The circular mil area of 6 AWG is 59 percent more than 8 AWG (26,240 Cmil/16,510 Cmil) [Chapter 9, Table 8].

According to Table 250.122, the circuit equipment grounding conductor for a 40A overcurrent device will be 10 AWG (10,380 Cmil), but the circuit equipment grounding conductor for this circuit must be increased in size by a multiplier of 1.59.

Conductor Size = 10,380 Cmil x 1.59
Conductor Size = 16,504 Cmil
Conductor Size = 8 AWG, Chapter 9, Table 8

Q6. Is a 15A or 20A, 125V rated maintenance receptacle required for an outdoor evaporative cooler at a dwelling unit?

A6. Not according to the exception to 210.63. The basic rules is that a 15A or 20A, 125V receptacle outlet must be installed at an accessible location for the servicing of heating, air-conditioning, and refrigeration equipment. The receptacle must be located within 25 ft of, and on the same level as, the heating, air-conditioning, and refrigeration equipment [210.63].

Ex: A receptacle outlet isn’t required at one- and two-family dwellings for the service of evaporative coolers.

Author’s Comments:

• A receptacle outlet isn’t required for ventilation equipment, because it’s not heating, air-conditioning, or refrigeration equipment.
• The HACR receptacle must be GFCI protected if located outdoors [210.8(A)(3) and 210.8(B)(5)] or in the crawl space or unfinished basement of a dwelling unit [210.8(A)(4) and 210.8(A)(5)].
• The outdoor 15A or 20A, 125V receptacle outlet required for dwelling units [210.52(E)(1)] can be used to satisfy this requirement.

Ex: A receptacle outlet isn’t required at one- and two-family dwellings for the service of evaporative coolers.

Q7. What is the Code rule regarding mixing different voltage systems such as 120V and 480V in the same raceway or enclosure?

A7. Power conductors of alternating-current and direct-current systems rated 600V or less can occupy the same raceway, cable, or enclosure if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(1)].

Author’s Comments:

• Control, signal, and communications wiring must be separated from power and lighting circuits so the higher-voltage conductors don’t accidentally energize the control, signal, or communications wiring:
• – CATV Coaxial Cable, 820.133(A)
• – Class 1, Class 2, and Class 3 Control Circuits, 725.48 and 725.136(A)
• – Communications Circuits, 800.133(A)(1)(c)
• – Fire Alarm Circuits, 760.136(A)
• – Instrumentation Tray Cable, 727.5
• – Sound Circuits, 640.9(C)
• Class circuit conductors can be installed with associated power conductors [725.48(B)(1)] if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(1)].
• A Class 2 circuit that’s been reclassified as a Class 1 circuit [725.130(A) Ex 2] can be installed with associated power conductors [725.48(B)(1)] if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(1)].

300.3(C)(1) Note 2: PV system conductors, both direct current and alternating current, are permitted to be installed in the same raceways, outlet and junction boxes, or similar fittings with each other, but they must be kept entirely independent of all other non-PV system wiring [690.4(B)].

Q8. What is the disconnect Code rule regarding transformers?

A8. A disconnect is required to disconnect all transformer ungrounded primary conductors. The disconnect must be located within sight of the transformer, unless the location of the disconnect is field-marked on the transformer and the disconnect is lockable [450.14].

Author’s Comment: Within sight means that it’s visible and not more than 50 ft from one to the other [Article 100].

Q9. What are the overcurrent protection requirements for transformers under 600V?

A9. The primary winding of a transformer of 600V and less must be protected against overcurrent in accordance with the percentages listed in Table 450.3(B) and all applicable notes [450.3(B)].

Table 450.3(B)

Note 1. If 125 percent of the primary current doesn’t correspond to a standard rating of a fuse or nonadjustable circuit breaker, the next higher rating is permitted [240.6(A)].

Question: What’s the primary overcurrent device rating and conductor size required for a 45 kVA, three-phase, 480V transformer that’s fully loaded? The terminals are rated 75°C.

(a) 8 AWG, 40A (b) 6 AWG, 50A (c) 6 AWG, 60A (d) 4 AWG, 70A

Answer: (d) 4 AWG, 70A

Step 1: Determine the primary current: I = VA/(E x 1.732) I = 45,000 VA/(480V x 1.732) I = 54A

Step 2: Determine the primary overcurrent device rating [240.6(A)]: 54A x 1.25 = 68A, next size up 70A, Table 450.3(B), Note 1

Step 3: The primary conductor must be sized to carry 54A continuously (54A x 1.25 = 68A) [215.2(A)(1)] and be protected by a 70A overcurrent device [240.4(B)]. A 4 AWG conductor rated 85A at 75°C meets all of the requirements [110.14(C)(1) and 310.15(B)(16)].

Q10. What are the requirements when running conductors through one panel cabinet to feed another panel?

A10. Cabinets, cutout boxes, and meter socket enclosures can be used for conductors as feeding through, spliced, or tapping off to other enclosures, switches, or overcurrent devices where all of the following conditions are met [312.8]:

• The total area of the conductors at any cross section doesn’t exceed 40 percent of the cross-sectional area of the space.
• The total area of conductors, splices, and taps installed at any cross section doesn’t exceed 75 percent of the cross-sectional area of that space.
• A warning label on the enclosure identifies the disconnecting means for feed-through conductors.

Q11. Is AFCI protection required in nondwelling occupancies?

A11. No. Here is the rule for AFCI protection; and it that applies only to dwellings: 120V branch circuits in dwelling units supplying outlets in family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms or areas must be protected by a listed AFCI device of the combination type [210.12].

Author’s Comment: The 120V circuit limitation means AFCI protection isn’t required for equipment rated 230V, such as a baseboard heater or room air conditioner. For more information, visit www.MikeHolt.com, click on the “Search” link, and search for “AFCI.”

Note 3: See 760.41(B) and 760.121(B) for power-supply requirements for fire alarm systems.

Author’s Comment: Smoke alarms connected to a 15A or 20A circuit of a dwelling unit must be AFCI protected if the smoke alarm is located in one of the areas specified in 210.12(A). The exemption from AFCI protection for the “fire alarm circuit” contained in 760.41(B) and 760.121(B) doesn’t apply to the single- or multiple-station smoke alarm circuit typically installed in dwelling unit bedroom areas. This is because a smoke alarm circuit isn’t a fire alarm circuit as defined in NFPA 72, National Fire Alarm Code. Unlike single- or multiple-station smoke alarms, fire alarm systems are managed by a fire alarm control panel.

Q12. What are the requirements for installing receptacles at a dwelling unit kitchen island?

A12. At least one receptacle outlet must be installed at each island countertop space with a long dimension of 2 ft or more, and a short dimension of 1 ft or more [210.52(C)(2)].

Author’s Comment: The Code doesn’t require more than one receptacle outlet in an island or peninsular countertop space, regardless of the length of the countertop, unless the countertop is broken as described in 210.52(C)(4).

Q13. How are branch circuits sized for motors?

A13. Conductors to a single motor must be sized not less than 125 percent of the motor FLC rating as listed in Table 430.247 Direct-Current Motors, Table 430.248 Single-Phase Motors, or Table 430.250 Three-Phase Motors [430.22].

Question: What size branch-circuit conductor is required for a 7½ hp, 230V, three-phase motor?

(a) 14 AWG (b) 12 AWG (c) 10 AWG (d) 8 AWG

Answer: (c) 10 AWG

Motor FLC = 22A [Table 430.250]

Conductor’s Size = 22A x 1.25

Conductor’s Size = 27.50A, 10 AWG, rated 30A at 75°C [Table 310.15(B)(16)]

Note: The branch-circuit short-circuit and ground-fault protection device using an inverse time breaker is sized at 60A according to 430.52(C)(1) Ex 1:

Circuit Protection = 22A x 2.50

Circuit Protection = 55A, next size up 60A [240.6(A)]

Q14. Must all conductors [ungrounded and neutral] of a circuit be run in the same raceway?

A14. All conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray, except as permitted by (1) through (4) [300.3(B)].

Conductors installed in parallel in accordance with 310.10(H) must have all circuit conductors within the same raceway, cable tray, trench, or cable [300.3(B)(1)].

Ex: Parallel conductors run underground can be installed in different raceways (Phase A in raceway 1, Phase B in raceway 2, and so forth) if, in order to reduce or eliminate inductive heating, if the raceway is nonmetallic or nonmagnetic and the installation complies with 300.20(B). See 300.5(I) Ex 2.

Author’s Comment: All conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray to minimize induction of the heating of ferrous metal raceways and enclosures, and to maintain a low-impedance ground-fault current path [250.4(A)(3)].

Q15. What is the Code rule regarding neutral conductors at switches?

A15. Switches controlling line-to-neutral lighting loads must have a neutral provided at the switch location [404.2(C)]

Ex: The neutral conductor isn’t required at the switch location if:

(1) The conductors for switches enter the device box through a raceway that has sufficient cross-sectional area to accommodate a neutral conductor.

(2) Cable assemblies for switches enter the box through a framing cavity that’s open at the top or bottom on the same floor level, or switches enter the box through a wall, floor, or ceiling that’s unfinished on one side.

404.2.(C) Note: The purpose of the neutral conductor is to complete a circuit path for electronic lighting control devices.

Q16. Is a neutral conductor allowed to be switched for 3-way or 4-way switching?

A16. Wiring for 3-way and 4-way switching must be done so that only the ungrounded conductors are switched [404.2(A).

Author’s Comment: In other words, the neutral conductor must not be switched. The white insulated conductor within a cable assembly can be used for single-pole, 3-way, or 4-way switch loops if it’s permanently reidentified to indicate its use as an ungrounded conductor at each location where the conductor is visible and accessible [200.7(C)(2)].

If a metal raceway or metal-clad cable contains the ungrounded conductors for switches, the wiring must be arranged to avoid heating the surrounding metal by induction. This is accomplished by installing all circuit conductors in the same raceway in accordance with 300.3(B) and 300.20(A), or ensuring that they’re all within the same cable.

For more NEC Practice purchase Mike Holt's NEC Practice Questions book, Based on the 2011 NEC.