NEC Questions and Answers based on 2008 NEC
September 2010
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 grounding requirements for a satellite dish?
A1 Article 810 includes the requirements for various antennas, including satellite dishes [810.1]. The antenna mast [810.15] and antenna discharge unit [810.20(C)] must be grounded as follows according to 810.21.
Grounding the lead-in antenna cables and the mast help prevent voltage surges caused by static discharge or nearby lightning strikes from reaching the center conductor of the lead-in coaxial cable. Because the satellite sits outdoors, wind creates a static charge on the antenna as well as on the cable attached to it. This charge can build up on both the antenna and the cable until it jumps across an air space, often passing through the electronics inside the low noise block down converter feed horn (LNBF) or receiver. Grounding the coaxial cable and dish to the building grounding electrode system helps to dissipate this static charge.
Nothing can prevent damage from a direct lightning strike. But grounding with proper surge protection can help reduce damage to satellite and other equipment from nearby lightning strikes.
- Material. The grounding conductor to the electrode [810.21(F)] must be copper or other corrosion-resistant conductive material, stranded or solid [810.21(A)].
- Insulation. The grounding conductor isn’t required to be insulated [810.21(B)].
- Supports. The grounding conductor must be securely fastened in place [810.21(C)].
- Mechanical Protection. The grounding conductor must be mechanically protected where subject to physical damage and where run in a metal raceway, both ends of the raceway must be bonded to the grounding conductor [810.21(D)].
Installing the grounding conductor in PVC conduit is a better practice, when the authority having jurisdiction judges that physical protection is not required.
- Run in Straight Line. The grounding conductor must be run in as straight a line as practicable [810.21(E)].
Lightning doesn’t like to travel around corners or through loops, which is why the grounding conductor should be run as straight as practicable.
(1) The grounding conductor for the antenna mast and antenna discharge unit must terminate to the intersystem bonding termination if there is one [Article 100 and 250.94].
Bonding all systems to the intersystem bonding termination helps reduce induced potential (voltage) between the power and radio and television systems during lightning events.
(2) In buildings or structures without intersystem bonding termination, the grounding conductor for the antenna mast and antenna discharge unit terminates to the nearest accessible:
(1) Building or structure grounding electrode system [250.50]
(2) Interior metal water piping system, within 5 ft from its point of entrance [250.52(A)(1)]
(3) Accessible means external to the building, as covered in 250.94
(4) Metallic service raceway
(5) Service equipment enclosure
(6) Grounding electrode conductor or the grounding electrode conductor metal enclosure
(3) In buildings or structures without a grounding means the grounding conductor for the antenna mast and antenna discharge unit terminates to:
(1) To any individual electrodes described in 250.52
(2) To structural steel grounded in accordance with 250.52(A)(2)
The grounding conductor can be run either inside or outside the building [810.21(G)], and grounding conductor must not be smaller than 10 AWG copper or 17 AWG copper-clad steel or bronze [810.21(H)].
Author’s Comment: Copper-clad steel or bronze wire (17 AWG) is often molded into the jacket of the coaxial cable to simplify the grounding of the satellite dish by eliminating the need to run a separate grounding conductor to the dish [810.21(F)(2)].
If a ground rod is installed to serve as the ground for the radio and television equipment, it must be connected to the building’s power grounding electrode system with a minimum 6 AWG conductor [810.21(J)].
Termination of the grounding conductor must be by exothermic welding, listed lug, listed pressure connector, or listed clamp. Grounding fittings that are concrete-encased or buried in the earth must be listed for direct burial [250.70] and marked “DB” [810.21(K)].
Q2 When does the Code require branch circuit conductors to be pigtailed at receptacles?
A2 Continuity of the neutral conductor of a multiwire branch circuit must not be interrupted by the removal of a wiring device. Therefore, the neutral conductors must be spliced together, and a pigtail must be provided for the wiring device according to 300.13(B).
The opening of the ungrounded conductors, or the neutral conductor of a 2-wire circuit during the replacement of a device, doesn’t cause a safety hazard, so pigtailing of these conductors isn’t required [110.14(B)].
Caution: If the continuity of the neutral conductor of a multiwire circuit is interrupted (open), the resultant over- or undervoltage could cause a fire and/or destruction of electrical equipment.
Example: A 3-wire, single-phase, 120/240V multiwire circuit supplies a 1,200W, 120V hair dryer and a 600W, 120V television. If the neutral conductor of the multiwire circuit is interrupted, it will cause the 120V television to operate at 160V and consume 1,067W of power (instead of 600W) for only a few seconds before it burns up.
Step 1: Determine the resistance of each appliance, R = E2/P.
R of the hair dryer = 120V2/1,200W
R of the hair dryer = 12 ohms
R of the television = 120V2/600W
R of the television = 24 ohms
Step 2: Determine the current of the circuit, I = E/R.
I = 240V/36 ohm (12 ohm + 24 ohm)
I = 6.7A
Step 3: Determine the operating voltage for each appliance,
E = 1 x R.
Voltage of hair dryer = 6.7A x 12 ohm
Voltage of hair dryer = 80V
Voltage of television = 6.7A x 24 ohm
Voltage of Television = 160V
Warning: Failure to terminate the ungrounded conductors to separate phases could cause the neutral conductor to become overloaded, and the insulation could be damaged or destroyed by excessive heat. Conductor overheating is known to decrease insulating material service life, which creates the potential for arcing faults in hidden locations and could ultimately lead to fires. It isn’t known just how long conductor insulation lasts, but heat does decrease its life span.
Q3 What are the grounding requirements for equipment in a patient care area?
A3 Patient care areas include patient rooms as well as examining rooms, therapy areas, examining and treatment rooms, and some patient corridors. They do not include business offices, corridors, lounges, day rooms, dining rooms, or similar areas not classified as patient care areas [517.2].
All branch circuits serving patient care areas must be provided with an effective ground-fault current path by installing circuits that serve patient care areas in a metal raceway or cable having a metallic armor or sheath that qualifies as an equipment grounding conductor in accordance with 250.118 [517.13(A)].
- The metal outer sheath of AC cable is listed as an equipment grounding conductor because it contains an internal bonding strip in direct contact with the metal sheath of the cable [250.118(8)].
- The metal outer sheath of interlocked Type MC cable is not listed as an equipment grounding conductor unless it contains a bare aluminum conductor that makes direct contact with the metal sheath of the cable [250.118(10)(a)].
In patient care areas, the grounding terminals of receptacles and conductive surfaces of fixed electrical equipment must be connected to an insulated copper equipment grounding conductor. The circuit equipment grounding conductor must be sized in accordance with 250.122 and the wiring method must meet the requirements of 517.13(A) [517.13(B)].
Metal faceplates for switches and receptacles can be connected to the equipment grounding conductor by the metal mounting screws that secure the faceplate to a metal outlet box or metal mounting yoke of switches [404.9(B)] and receptacles [406.3(C)] according to 517.13(B) Ex 1.
Luminaires located more than 7½ ft above the floor can be connected to the equipment grounding return path complying with 517.13(A), without being connected to an insulated equipment grounding conductor [517.13(B) Ex 2].
Q4 When is the neutral conductor allowed to be used as an equipment grounding conductor?
A4 The neutral conductor can be used as the circuit equipment grounding conductor for metal parts of equipment, raceways, and enclosures at the following locations [250.142(A)]:
(1) Service Equipment. On the supply side or within the enclosure of the service-disconnecting means in accordance with 250.24(B).
DANGER: Failure to connect the service neutral conductor to the service disconnect enclosure as required by 250.24(C) creates a condition where dangerous voltage from a ground fault will not be removed.
(3) Separately Derived Systems. On the supply side or within the enclosure of the system disconnecting means in accordance with 250.30(A)(1).
Danger: Failure to install the system bonding jumper as required by 250.30(A)(1) creates a condition where dangerous touch voltage from a ground fault will not be removed.
Except for service equipment and separately derived systems, the neutral conductor must not serve as an equipment grounding conductor on the load side of service equipment [250.142(B)].
In existing installations, the frames of ranges, wall-mounted ovens, counter-mounted cooking units, and clothes dryers can be connected to the grounded circuit conductor according to 250.140 Ex. [250.142(A) Ex 1].
The neutral conductor can be connected to meter socket enclosures on the load side of the service disconnecting means if [250.142(A) Ex 2]:
(1) Ground-fault protection is not provided on service equipment,
(2) Meter socket enclosures are immediately adjacent to the service disconnecting means, and
(3) The neutral conductor is sized in accordance with 250.122, based on the ampere rating of the occupancy’s feeder overcurrent device.
Q5 What is rule on using 16 AWG tap conductors for lighting fixtures supplied by a 20A branch circuit?
A5 As a general rule, branch-circuit conductors must have an ampacity sufficient for the loads served and must not be smaller than 14 AWG [210.19(A) (4)]. Luminaire taps are an exception to this rule [210.19(A) (4) Ex 1] and the overcurrent protection rules for fixture wires in 240.5(B)(2) allow fixture wires to be protected by overcurrent protection devices tapped from the following circuits:
(1) 20A–18 AWG, up to 50 ft of run length
(2) 20A–16 AWG, up to 100 ft of run length
(3) 20A–14 AWG and larger
Q6 What are the requirements for the point of attachment of a service drop?
A6 According to 230.26, the point of attachment for service-drop conductors must not be less than 10 ft above the finished grade and must be located so the minimum service conductor clearance required by 230.9 and 230.24 can be maintained.
Caution: The points of attachment for conductors might need to be raised so the overhead conductors will comply with the clearances from building openings required by 230.9 and from other areas by 230.24.
Multiconductor cables used for service drops must be attached to buildings or other structures by fittings identified for use with service conductors, and open conductors must be attached to fittings identified for use with service conductors or to noncombustible, nonabsorbent insulators securely attached to the building or other structure [230.27].
When the service mast is used as overhead conductor support, it must have adequate mechanical strength, or braces or guy wires to support it, to withstand the strain caused by the service-drop conductors [230.28]. Some local building codes require a minimum 2 in. rigid metal conduit for the service mast. In addition, many electric utilities contain specific requirements for the installation of the service mast.
Only electric utility service-drop conductors can be attached to a service mast [230.28]. Sections 810.12 and 820.44(C) specify that aerial cables for radio, TV, or CATV must not be attached to the electric service mast, and 810.12 prohibits antennas from being attached to the service mast. In addition, 800.133(B) and 830.133(B) prohibit broadband communications cables from being attached to raceways, including a service mast for power conductors.
Q7 Does the NEC require GFCI or tamperproof receptacles for replacement purposes in existing dwelling units?
A7 When receptacles are replaced in locations where GFCI protection is required, the replacement receptacles must be GFCI protected. This includes the replacement of receptacles in dwelling unit bathrooms, garages, outdoors, crawl spaces, unfinished basements, kitchen countertops, rooftops, or within 6 ft of laundry, utility, and wet bar sinks. See 210.8 for specific GFCI-protection requirements [406.3(D)(2)]. Tamperproof receptacles are not currently included in this replacement receptacle requirement.
Q8 What are the NEC requirements regarding an equipment grounding conductor installed with a feeder to a panelboard?
A8 Section 408.40 requires that metal panelboard cabinets and frames must be connected to an equipment grounding conductor of a type recognized in 250.118 [215.6 and 250.4(A)(3)].
Where the panelboard is used with nonmetallic raceways or cables, or where separate equipment grounding conductors are provided, a terminal bar for the circuit equipment grounding conductors must be bonded to the metal cabinet.
There is an exception for insulated equipment grounding conductors for receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)] which allows them to pass through the panelboard without terminating onto the equipment grounding terminal of the panelboard cabinet.
Equipment grounding conductors must not terminate on the neutral terminal bar, and neutral conductors must not terminate on the equipment grounding terminal bar, except as permitted by 250.142 for services and separately derived systems (see the definition of “Separately Derived System” in Article 100).
Caution: Most panelboards are rated as suitable for use as service equipment, which means they are supplied with a main bonding jumper [250.28]. This screw or strap must not be installed except when the panelboard is used for service equipment or on separately derived systems [250.24(A)(5)]. In addition, a panelboard marked “suitable only for use as service equipment” means the neutral bar or terminal of the panelboard has been bonded to the case at the factory, and this panelboard is restricted to being used for service equipment or on separately derived systems according to 250.142(B).
Section 215.6 also requires that feeder circuits must include or provide an equipment grounding conductor of a type listed in 250.118 and it must terminate in a manner so that branch-circuit equipment grounding conductors can be connected to it, and installed in accordance with 250.134.
Q9 What are the Code requirements for motor disconnects?
A9 Each motor must be provided with a separate disconnecting means unless the controller disconnecting means meets the necessary requirements to serve the disconnect for both.
Section 430.102(B)(1) requires a disconnecting means for each motor, and it must be located in sight from the motor location and the driven machinery location. “Within Sight” is visible and not more than 50 ft from each other [Article 100]. Or, the controller disconnecting means [430.102(A)] can serve as the disconnecting means for the motor if the disconnect is located in sight from the motor location [430.102(B)(2)].
A motor disconnecting means isn’t required under either of the following conditions if the controller disconnecting means [430.102(A)] is capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means must be installed on or at the switch or circuit breaker and remain in place with or without the lock installed [430.102(B)(1) and (2) Ex].
(a) Where locating the disconnecting means is impracticable or introduces additional or increased hazards to persons or property.
(b) In industrial installations, with written safety procedures, where conditions of maintenance and supervision ensure only qualified persons will service the equipment. This should include lockout/tagout procedures.
Some additional requirements for motor disconnects include:
- The disconnecting means for the motor controller and the motor must open all ungrounded supply conductors simultaneously and it must be designed so that it will not close automatically [430.103].
- The controller and motor disconnecting means must indicate whether they are in the “on” or “off” position [430.104].
- The disconnecting means must be legibly marked to identify its intended purpose [110.22 and 408.4], and when operated vertically, the “up” position must be the “on” position [240.81 and 404.6(C)].
- Either the controller disconnecting means or motor disconnecting means required by 430.102 must be readily accessible [430.107].
The disconnecting means for the motor controller and/or the motor must be a [430.109(A)]:
- A listed horsepower-rated motor-circuit switch.
- A listed molded case circuit breaker.
- A listed molded case switch.
- Listed manual motor controllers marked “Suitable as Motor Disconnect.”
For stationary motors of two horsepower or less, the disconnecting means is allowed to be a general-use ac snap switch, where the motor full-load current rating isn’t more than 80 percent of the ampere rating of the switch [430.109(C)].
A horsepower-rated attachment plug and receptacle having a rating not less than the motor rating is allowed as the disconnecting means for cord-and-plug-connected motors [430.109(F)].
A horsepower-rated switch or circuit breaker can serve as both a controller and disconnecting means if it opens all ungrounded conductors to the motor as required by 430.103 [430.111].
Q10 When can I “round up to the next size overcurrent device” when the conductor ampacity does not match a standard size overcurrent device?
A10 The next higher standard rating overcurrent device (above the ampacity of the ungrounded conductors being protected) is permitted, provided all of the following conditions are met [240.4(B)]:
(1) The conductors do not supply multioutlet receptacle branch circuits.
(2) The ampacity of a conductor, after ampacity adjustment and/or correction, doesn’t correspond with the standard rating of a fuse or circuit breaker in 240.6(A).
(3) The overcurrent device overcurrent device rating doesn’t exceed 800A.
Example: A 400A overcurrent device overcurrent device can protect 500 kcmil conductors, where each conductor has an ampacity of 380A at 75°C, in accordance with Table 310.16.
This rule “next size up” doesn’t apply to feeder tap conductors [240.21(B)], or secondary transformer conductors [240.21(C)].
If the circuit’s overcurrent device exceeds 800A, the conductor ampacity, after ampacity adjustment and/or correction, must have a rating not less than the rating of the overcurrent device [240.4(C)].
Example: A 1,200A overcurrent device overcurrent device can protect three sets of 600 kcmil conductors per phase, where each conductor has an ampacity of 420A at 75°C, in accordance with Table 310.16.
Q11 When can I use Table 310.15(B)(6)?
A11 For individual dwelling units of one-family, two-family, and multifamily dwellings, Table 310.15(B)(6) can be used to size 3-wire, single-phase, 120/240V service or feeder conductors that supply all loads that are part of or associated with the dwelling unit.
120/208V, single-phase feeders from a 120/208V, three-phase service are sometimes used to supply single-family dwelling units in a multifamily building, and this table can't be used when that is done. Table 310.16 must be used when that is the case.
Feeder conductors are not required to have an ampacity rating more than the service conductors [215.2(A)(3)].
Warning: Table 310.15(B)(6) doesn’t apply to 3-wire, single-phase, 120/208V systems, because the neutral conductor in these systems carries neutral current even when the load on the phases is balanced [310.15(B)(4)(6)]. For more information on this topic, see 220.61(C)(1).
Question: What size service conductors (120/240V, single-phase) are required when the calculated load for a dwelling unit equals 195A and the neutral load is 100A?
(a) 1/0 AWG and 6 AWG (b) 2/0 AWG and 4 AWG
(c) 3/0 AWG and 2 AWG (d) 4/0 AWG and 1 AWG
Answer: (b) 2/0 AWG and 4 AWG
Service Conductor: 2/0 AWG rated 200A [Table 310.15(B)(6)]
Neutral Conductor: 4 AWG is rated 100A in accordance with Table 310.15(B)(6). In addition, 250.24(C) requires the neutral conductor to be sized no smaller than 4 AWG, based on 2/0 AWG service conductors in accordance with Table 250.66.
Q12 What is the rule for adjusting the ampacity of conductors based on the number of conductors in the raceway?
A12 Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 in., the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(a).
Each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor.
Question: What’s the ampacity of 10 THHN when four current-carrying conductors are installed in a raceway or cable in a dry location?
(a) 20A (b) 24A (c) 29A (d) 32A
Answer: (d) 32A
Adjusted Ampacity = Table 310.16 Ampacity x Bundled Ampacity Adjustment Factor from Table 310.15(B)(2)(a)
Table 310.16 ampacity for 10 THHN is 40A at 90ºC.
Bundled adjustment factor for four current-carrying conductors is 0.80.
Adjusted Ampacity = 40A x 0.80 = 32A
Q13 What is the requirement for the location of receptacle outlets in the bathroom of a dwelling unit?
A13 In dwelling units, not less than one 15A or 20A, 125V receptacle outlet must be installed within 3 ft from the outside edge of each bathroom basin [210.52(D)]. The receptacle outlet must be located on a wall or partition adjacent to the basin counter surface, or on the side or face of the basin cabinet not more than 12 in. below the countertop [210.11(C)(3)].
• One receptacle outlet can be located between two basins to meet the requirement, but only if the receptacle outlet is located within 3 ft of the outside edge of each basin.
• The bathroom receptacles must be GFCI-protected [210.8(A)(1)].
Q14 Does the NEC allow switched receptacle outlets above a suspended ceiling for cord and plug connection of equipment?
A14 Flexible cords must not be concealed by walls, floors, or ceilings, or located above suspended or dropped ceilings [400.8(5)].
Receptacles are permitted above a suspended ceiling, but a flexible cord is not. Why install a receptacle above a ceiling if the flexible cord is not permitted in this space? Because the receptacle could be used for portable tools; it just can’t be used for cord-and-plug equipment fastened in place.
Q15 What are the GFCI protection requirements for agricultural buildings wiring?
A15 GFCI protection is required for all 15A and 20A, 125V, general-purpose receptacles (circuits are rated 120V and receptacles are rated 125V) located [547.5(G)]:
- In areas having an equipotential plane in accordance with 547.10(A).
- Outdoors.
- In damp or wet locations.
- In dirt confinement areas for livestock.
GFCI protection is not required for a receptacle supplying a dedicated load where a 15A or 20A, 125V, GFCI-protected receptacle is located within 3 ft of the receptacle for the dedicated load. Dedicated loads include brooders, incubators, feed mixers, feed grinders, feed conveyors, and the like.
Q16 What are the GFCI protection requirements for construction jobsites?
A16 Ground-fault protection for personnel is required for all temporary wiring used for construction, remodeling, maintenance, repair, or demolition of buildings, structures, or equipment, from power derived from an electric utility company or from an on-site generated power source [590.6].
GFCI protection is required for all 15A, 20A, and 30A, 125V receptacles used by personnel during construction, remodeling, maintenance, repair, or demolition of buildings, structures, equipment, or similar activities [590.6(A)].
Circuit breakers, receptacles, cord sets, or adapters that incorporate listed GFCI protection can be used.
Receptacles rated other than 15A, 20A, or 30A, 125V that supply temporary power used by personnel during construction, remodeling, maintenance, repair, or demolition of buildings, structures, equipment, or similar activities must be GFCI protected [590.6(B)].
Q17 What are the vertical clearance requirements for overhead conductors over a roof?
A17 In general, service-drop conductor spans must maintain a minimum of 8 ft vertical clearance above the surface of a roof for a minimum distance of 3 ft in all directions from the edge of the roof [230.24(A)]. However, there are some specific exceptions to this rule. If the slope of the roof exceeds 4 in. for every 12 in, 120/208V or 120/240V conductor clearances can be reduced to 3 ft over the roof 230.24(A) Ex 2], and if no more than 6 ft of conductors pass over no more than 4 ft of roof, 120/208V or 120/240V conductor clearances over the roof overhang can be reduced to 18 in. [230.24(A) Ex 3].
The 3 ft vertical clearance that extends from the roof doesn’t apply when the point of attachment is on the side of the building (below the roof) . [230.24(A) Ex 4].
Q18 Can I install 120V conductors with 277V conductors together, and if yes what restrictions apply?
A18 Power conductors of ac and dc 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) ].
• Control, signal, and communications wiring must be separated from power and lighting circuits so the higher-voltage conductors do not 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 1 circuits of Class 2 and Class 3 circuits that have been reclassified as a Class 1 circuit [725.130(A) Ex 2] can be run 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)].
Q19 What are the GFCI protection requirements for swimming pool pump motors?
A19 Outlets supplying pool pump motors for a permanently installed pool, outdoor spa, or outdoor hot tub from branch circuits rated 15A or 20A, 120V or 240V must be GFCI-protected [680.22(B)].
Q20 When I add a GFCI protected receptacle in an existing circuit that does not have an equipment grounding conductor, can I add additional grounding type receptacles on the load side of it?
A20 Where no equipment grounding conductor exists in the outlet box for the receptacle, such as old 2-wire Type NM cable without an equipment grounding conductor, existing nongrounding-type receptacles can be replaced with one of the following [406.3(D)(3)(1), (2), and (3)]:
- Another nongrounding-type receptacle.
- A GFCI-type receptacle marked “No Equipment Ground.”
- A grounding-type receptacle, if GFCI-protected and marked “GFCI-Protected” and “No Equipment Ground.”
GFCI protection functions properly on a 2-wire circuit without an equipment grounding conductor, because the circuit equipment grounding conductor serves no role in the operation of the GFCI-protection device.
Caution: The permission to replace nongrounding-type receptacles with GFCI-protected grounding-type receptacles doesn’t apply to new receptacle outlets that extend from an existing ungrounded outlet box. Once you add a receptacle outlet (branch-circuit extension), the receptacle must be of the grounding type and it must have its grounding terminal connected to an equipment grounding conductor of a type recognized in 250.118, in accordance with 250.130(C).
