NEC Questions and Answers – Based on the 2011 NEC

October 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 wiring methods does the NEC allow above a suspended ceiling or other spaces used for environmental air?

A1. Electrical metallic tubing, rigid metal conduit, intermediate metal conduit, armored cable, metal-clad cable without a nonmetallic cover, and flexible metal conduit can be installed in a cavity plenum space. If accessible, surface metal raceways or metal wireways with metal covers can be installed in a cavity plenum space [300.22(C)(1)].

Author’s Comments:

• PVC conduit [Article 352], electrical nonmetallic tubing [Article 362], liquidtight flexible conduit, and nonmetallic cables aren’t permitted to be installed in spaces used for environmental air because they give off deadly toxic fumes when burned or superheated.
• Plenum-rated control, signaling, and communications cables and raceways are permitted in a cavity plenum space:

– CATV, 820.179(A)
– Communications, 800.21
– Control and Signaling, 725.154(A)
– Fire Alarm, 760.7
– Optical Fiber Cables and Raceways, 770.113(C)
– Sound Systems, 640.9(C) and 725.154(A)

• Any wiring method suitable for the condition can be used in a space not used for environmental air-handling purposes.

Metal cable tray systems can be installed to support the wiring methods and equipment permitted by this section [300.22(C)(2)].

Electrical equipment with metal enclosures is permitted to be installed in a cavity plenum space [300.22(C)(3)].

Author’s Comment: Examples of electrical equipment permitted in a cavity plenum space would be air-handlers, junction boxes, dry-type transformers; however transformers must not be rated over 50 kVA when located in hollow spaces [450.13(B)].

Q2. Where is NM cable allowed, and not allowed, to be installed by the NEC?

A2. Type NM and Type NMC cables can be used in the following [334.10]:

• One- and two-family dwellings of any height, and their attached/detached garages or storage buildings [334.10(1)]
• Multifamily dwellings permitted to be of Types III, IV, and V construction [334.10(2)].
• Other structures permitted to be of Types III, IV, and V construction, except as prohibited in 334.12. Cables must be concealed within walls, floors, or ceilings that provide a thermal barrier of material with at least a 15-minute finish rating, as identified in listings of fire-rated assemblies [334.10(3)].

Author’s Comment: See the definition of “Concealed” in Article 100.

Note 1: Building constructions are defined in NFPA 220-2006, Standard on Types of Building Construction, the applicable building code, or both.

Note 2: See Annex E for the determination of building types [NFPA 220, Table 3-1].

Types NM and NMC cable are not permitted [334.12(A)]:

In any dwelling or structure not specifically permitted in 334.10(1), (2), and (3).

Exposed in dropped or suspended ceilings in other than one- and two-family and multifamily dwellings.

• As service-entrance cable.
• In commercial garages having hazardous locations, as defined in 511.3.
• In theaters and similar locations, except where permitted in 518.4(B).
• In motion picture studios.
• In storage battery rooms.
• In hoistways, or on elevators or escalators.
• Embedded in poured cement, concrete, or aggregate.
• In any hazardous location, except where permitted by 501.10(B)(3), 502.10(B)(3), and 504.20.

Type NM cables must not be used under the following conditions, or in the following locations [334.12(B)]:

• If exposed to corrosive fumes or vapors.
• If embedded in masonry, concrete, adobe, fill, or plaster.
• In a shallow chase in masonry, concrete, or adobe and covered with plaster, adobe, or similar finish.
• In wet or damp locations.

Author’s Comment: Type NM cable isn’t permitted in ducts or cavity plenum spaces [300.22], or for wiring in patient care areas [517.13].

Q3. What Code section requires that enclosures containing overcurrent devices must be mounted vertically?

A3. Section 240.33 requires that enclosures containing overcurrent devices must be mounted in a vertical position unless this isn’t practical. Circuit-breaker enclosures can be mounted horizontally if the circuit breaker is installed in accordance with 240.81.

Author’s Comment: Section 240.81 specifies that where circuit-breaker handles are operated vertically, the “up” position of the handle must be in the “on” position. So, in effect, an enclosure that contains one row of circuit breakers can be mounted horizontally, but an enclosure that contains a panelboard with multiple circuit breakers on opposite sides of each other will have to be mounted vertically.

Q4. What is the code rule regarding tap conductors to a recessed luminaire?

A4. Fixture wires installed in accordance with Article 402 and protected against overcurrent in accordance with 240.5(B)(2), are permitted to run from the luminaire to an outlet box located at least 1 ft away from the luminaire, as long as the conductors aren’t over 6 ft long [410.117(C)].

Q5. What is the NEC requirement for a maintenance disconnect at a pool, outdoor spa, or outdoor hot tub?

A5. A maintenance disconnecting means is required for the permanently installed pool, storable pool, outdoor spa, outdoor hot tub, or fountain equipment, other than lighting for these water bodies [680.12].

The maintenance disconnecting means must be readily accessible and located within sight and at least 5 ft from the permanently installed pool, storable pool, outdoor spa, outdoor hot tub, or fountain equipment unless separated from the open water by a permanently installed barrier that provides a 5 ft reach path or greater. This horizontal distance is measured from the water’s edge along the shortest path required to reach the disconnecting means.

Author’s Comment: According to Article 100, within sight means that it's visible and not more than 50 ft from one to the other.

Q6. What is the NEC requirement for an emergency shutoff switch for a spa or hot tub?

A6. In other than a single-family dwelling, a clearly labeled emergency spa or hot tub water recirculation and jet system shutoff must be supplied. The emergency shutoff must be readily accessible to the users and located not less than 5 ft away, but adjacent to and within sight of the spa or hot tub [680.41].

Author’s Comments:

• Either the maintenance disconnecting means required by 680.12 or a pushbutton that controls a relay located in accordance with this section can be used to meet the emergency shutoff requirement.
• The purpose of the emergency shutoff is to protect users. Deaths and injuries have occurred in less than 3 ft of water because individuals became stuck to the water intake opening. This requirement applies to spas and hot tubs installed indoors as well as outdoors.

Q7. What are the rules for securing and supporting raceways? Specifically compare EMT, flexible metal conduit, and PVC.

A7. Electrical metallic tubing must be installed as a complete system in accordance with 300.18 [300.10 and 300.12], and it must be securely fastened in place and supported in accordance with 358.30(A) and (B):

• Electrical metallic tubing must generally be securely fastened within 3 ft of every box, cabinet, or termination fitting, and at intervals not exceeding 10 ft.

Author’s Comment: Fastening is required within 3 ft of termination, not within 3 ft of a coupling.

Ex 1: When structural members don’t permit the raceway to be secured within 3 ft of a box or termination fitting, an unbroken raceway can be secured within 5 ft of a box or termination fitting.

• Electrical metallic tubing installed horizontally in bored or punched holes in wood or metal framing members, or notches in wooden members, is considered supported, but the raceway must be secured within 3 ft of termination.

Flexible metal conduit must be securely fastened by a means approved by the authority having jurisdiction within 1 ft of termination, and it must be secured and supported at intervals not exceeding 4½ ft [348.30(A)].

Ex 1: Flexible metal conduit isn’t required to be securely fastened or supported where fished between access points through concealed spaces and supporting is impracticable.

Ex 2: If flexibility is necessary after installation, unsecured lengths from the last point the raceway is securely fastened must not exceed:

(1) 3 ft for trade sizes ½ through 1¼

(2) 4 ft for trade sizes 1½ through 2

(3) 5 ft for trade sizes 2½ and larger

Ex 4: FMC to a luminaire or electrical equipment within an accessible ceiling is permitted to be unsupported for not more than 6 ft from the last point where the raceway is securely fastened.

Flexible metal conduit installed horizontally in bored or punched holes in wood or metal framing members, or notches in wooden members, is considered supported, but the raceway must be secured within 1 ft of terminations [348.30(B)].

PVC conduit must be securely fastened and supported in accordance with 352.30(A) and (B):

• PVC conduit must be secured within 3 ft of every box, cabinet, or termination fitting, such as a conduit body [352.30(A)].
• PVC conduit must be supported at intervals not exceeding the values in Table 352.30, and the raceway must be fastened in a manner that permits movement from thermal expansion or contraction [352.30(B)].

PVC conduit installed horizontally in bored or punched holes in wood or metal framing members, or notches in wooden members, is considered supported, but the raceway must be secured within 3 ft of termination.

If PVC conduit is installed in a straight run between securely mounted items, such as boxes, cabinets, elbows, or other conduit terminations, expansion fittings must be provided to compensate for thermal expansion and contraction of the raceway in accordance with Table 352.44, if the length change is determined to be ¼ in. or greater [352.44].

Author’s Comment: Table 352.44 in the NEC was created based on the following formula:

Expansion/Contraction Inches = Raceway Length/100 x [(Temp Change/100) x 4.00]

Example: How much will a 25 ft run of PVC conduit contract when it’s located in an ambient temperature change of 25°F?

(a) 1 in. (b) 2 in. (c) 3 in. (d) 4 in.

Answer: (a) 1 in.

Expansion/Contraction Inches = Raceway Length/100 x ((Temp °F Change/100) x 4.00)
Expansion/Contraction Inches = (25/100) x ((25/100) x 4.00)
Expansion/Contraction Inches = 0.25 in.

Q8. What are the NEC requirements for the installation of Edison-base fuses and Type S adapters?

A8. Edison-base fuses are classified to operate at not more than 125V and have an ampere rating of not more than 30A [240.51(A)] and are permitted only for replacement in an existing installation where there’s no evidence of tampering or overfusing [240.51(B)]. Edison-base fuseholders must be used only if they’re made to accept Type S fuses by the use of adapters [240.52].

Type S adapters are designed to fit Edison-base fuseholders [240.54(A)], and must be designed for Type S fuses only [240.54(B)].

Type S adapters are designed so they can’t be removed once installed [240.54(C)], and Type S fuses, fuseholders and adapters must be designed so that tampering or shunting would be difficult [240.54(D)]

Dimensions of Type S fuses, fuseholders, and adapters shall be standardized to permit interchangeability regardless of the manufacturer [240.54(E)].

Q9. What are the Code rules regarding receptacles installed on construction sites?

A9. All receptacles used for temporary installations must be of the grounding type, and the receptacle grounding terminal must be connected to an equipment grounding conductor in accordance with 250.146 and 406.4. On a construction site, receptacles aren’t permitted to be placed on a branch circuit that supplies temporary lighting [590.4(D)(1)].

Author’s Comment: This requirement is necessary so that illumination is maintained, even when the receptacle’s GFCI-protection device opens.

All 15A and 20A receptacles installed in a wet location of a construction site must be within an enclosure that’s weatherproof when an attachment plug is inserted. For other than one- or two-family dwellings, the outlet box hood must be listed for “extra-duty” use if supported by grade, and all nonlocking-type 15A and 20A, 125V and 250V receptacles in a wet location must be listed as weather resistant [406.9(B)(1) and 590.4(D)(2)].

Author’s Comment: Exposed plastic surface material of weather-resistant receptacles must have UV resistance to ensure that deterioration from sunlight doesn’t take place, or that it’s minimal. In testing, receptacles are subjected to temperature cycling from very cold to very warm conditions, and then subjected to additional dielectric testing. The rapid transition from the cold to warm temperatures will change the relative humidity and moisture content on the device, and the dielectric test ensures that this won’t create a breakdown of the insulation properties.

Q10. What uses does the NEC permit and forbid for flexible metal conduit?

A10. According to 348.10, flexible metal conduit is permitted exposed or concealed.

The uses not permitted for FMC are much more specifically listed in 348.12:

• In wet locations.
• In hoistways, other than as permitted in 620.21(A)(1).
• In storage battery rooms.
• In any hazardous location, except as permitted by 501.10(B).
• Exposed to material having a deteriorating effect on the installed conductors.
• Underground or embedded in poured concrete.
• If subject to physical damage.

Q11. What does the NEC allow for the connection of electric-discharge luminaires and LED luminaires?

A11. Electric-discharge and LED luminaires supported independently of the outlet box must be connected to the branch circuit with a raceway, or with Types MC, AC, or NM cable. [410.24(A)].

Electric-discharge luminaires can be cord-connected if the luminaires are provided with internal adjustments to position the lamp [410.62(B)].

Electric-discharge luminaires can be cord-connected if the cord is visible for its entire length and is plugged into a receptacle, and the installation complies with 410.62(C).

When an electric-discharge luminaire or LED luminaire is surface mounted over a concealed outlet box, and not supported by the outlet box, the luminaire must be provided with suitable openings that permit access to the branch-circuit wiring within the outlet box [410.24(B)].

Q12. What is the Code requirement for the interrupting protection rating of overcurrent devices?

A12. Overcurrent devices such as circuit breakers and fuses are intended to interrupt the circuit, and they must have an interrupting rating not less than the nominal circuit voltage and the current that’s available at the line terminals of the equipment [110.9].

Author’s Comments:

• See the definition of “Interrupting Rating” in Article 100.
• Unless marked otherwise, the ampere interrupting rating for circuit breakers is 5,000A [240.83(C)], and for fuses it’s 10,000A [240.60(C)(3)].

AVAILABLE SHORT-CIRCUIT CURRENT

Available short-circuit current is the current, in amperes, available at a given point in the electrical system. This available short-circuit current is first determined at the secondary terminals of the utility transformer. Thereafter, the available short-circuit current is calculated at the terminals of service equipment, then at branch-circuit panelboards and other equipment. The available short-circuit current is different at each point of the electrical system. It’s highest at the utility transformer and lowest at the branch-circuit load.

The available short-circuit current depends on the impedance of the circuit. The greater the circuit impedance (utility transformer and the additive impedances of the circuit conductors), the lower the available short-circuit current.

The factors that affect the available short-circuit current at the utility transformer include the system voltage, the transformer kVA rating, and the circuit impedance (expressed in a percentage on the equipment nameplate). Properties that have an impact on the impedance of the circuit include the conductor material (copper versus aluminum), conductor size, conductor length, and motor-operated equipment supplied by the circuit.

Author’s Comment: Many people in the industry describe Amperes Interrupting Rating (AIR) as “Amperes Interrupting Capacity” (AIC).

Danger: Extremely high values of current flow (caused by short circuits or ground faults) produce tremendously destructive thermal and magnetic forces. Overcurrent protection devices not rated to interrupt the current at the available fault values at its listed voltage rating can explode while attempting to open the circuit overcurrent device from a short circuit or ground fault, which can cause serious injury or death, as well as property damage.

Q13. When does the NEC require a grounding electrode to have a supplemental electrode and what are the installation rules?

A13. A single rod, pipe or plate electrode must be supplemented by an additional electrode that’s bonded to one of the following [250.53(A)(2)]:

• The single rod, pipe, or plate electrode
• The grounding electrode conductor of the single electrode
• The neutral service-entrance conductor
• The nonflexible grounded service raceway
• The service enclosure

Ex: If a single rod, pipe, or plate grounding electrode has an earth contact resistance of 25 ohms or less, the supplemental electrode isn’t required.

The supplemental electrode for a single rod, pipe, or plate electrode must be installed not less than 6 ft from the single electrode [250.53(A)(3)].

Note: The efficiency of paralleling electrodes is improved by spacing them at least twice the length of the longest rod.

When an underground metal water pipe grounding electrode is present [250.52(A)(1)], it must be supplemented by one of the following electrodes [250.53(D)(2)]:

• Metal frame of the building/structure electrode [250.52(A)(2)]
• Concrete-encased electrode [250.52(A)(3)]
• Ground ring electrode [250.52(A)(4)]
• Ground rod electrode meeting the requirements of 250.52(A)(5)
• Other listed electrodes [250.52(A)(6)]
• Metal underground systems, piping systems, or underground tanks [250.52(A)(8)]

The termination of the supplemental grounding electrode conductor must be to one of the following locations:

• Grounding electrode conductor
• Service neutral conductor
• Metal service raceway
• Service equipment enclosure

Ex: The supplemental electrode is permitted to be bonded to interior metal water piping located not more than 5 ft from the point of entrance to the building/structure [250.68(C)(1)].

The grounding electrode conductor to a ground rod that serves as a supplemental electrode isn’t required to be larger than 6 AWG copper [250.53(E)]:.

Q14. What is the Code requirement in regard to installing receptacles with insulated grounding terminals in patient care areas?

A14. Receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)] aren’t permitted to be installed in patient care areas [517.16].

Q15. What is the NEC requirement about bonding around water meters?

A15. A bonding jumper must be installed around insulated joints and equipment likely to be disconnected for repairs or replacement for an underground metal water piping system used as a grounding electrode. The bonding jumper must be of sufficient length to allow the removal of such equipment while retaining the integrity of the grounding path [250.68(B)].

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