NEC Questions and Answers – Based on the 2011 NEC
May 2011
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 basic working space requirements for a dwelling unit?
A1. The same rules apply to dwelling units as for any other occupancy. According to 110.26, access and working space must be provided about all electrical equipment, for the purpose of safe operation and maintenance of equipment.
(A) Working Space. Equipment that may need examination, adjustment, servicing, or maintenance while energized must have working space provided in accordance with (1), (2), and (3):
(1) Depth of Working Space. The working space, which is measured from the enclosure front, must not be less than the distances contained in Table 110.26(A)(1). For dwelling units, the minimum depth required from this table is 3 ft, since the voltage to ground is not over 150V in dwelling units.
(2) Width of Working Space. The width of the working space must be a minimum of 30 in., but in no case less than the width of the equipment.
In all cases, the working space must be of sufficient width, depth, and height to permit all equipment doors to open 90 degrees.
(3) Height of Working Space (Headroom). The height of the working space in front of equipment must not be less than 6½ ft, measured from the grade, floor, platform, or the equipment height, whichever is greater.
Equipment such as raceways, cables, wireways, cabinets, panels, and so on, can be located above or below electrical equipment, but must not extend more than 6 in. into the equipment’s working space.
Ex 1: The minimum headroom requirement doesn’t apply to service equipment or panelboards rated 200A or less located in an existing dwelling unit.
Q2. What are the grounding requirements for a portable generator?
A2. According to 250.34(A), the frame of a portable generator isn’t required to be grounded (connected to the earth) if:
(1) The generator only supplies equipment or receptacles mounted on the generator, and
(2) The metal parts of the generator and the receptacle grounding terminal are connected to the generator frame.
The frame of a vehicle-mounted generator isn’t required to be grounded (connected to the earth) according to 250.34(B) if:
(1) The generator frame is bonded to the vehicle frame,
(2) The generator only supplies equipment or receptacles mounted on the vehicle or generator, and
(3) The metal parts of the generator and the receptacle grounding terminal are connected to the generator frame.
A portable or vehicle-mounted generator used as a separately derived system to supply equipment or receptacles mounted on the vehicle or generator must have the neutral conductor connected to the generator frame [250.34(C)].
Note: A portable or vehicle-mounted generator supplying fixed wiring for a premises must be grounded (connected to the earth) and bonded in accordance with 250.30 for separately derived systems and 250.35 for nonseparately derived systems.
Q3. What are the receptacle spacing requirements for a dwelling unit, in other than kitchens or bathrooms?
A3. Section 210.52 provides requirements for 15A and 20A, 125V receptacle outlets in dwelling units. The receptacles required by this section are in addition to any receptacle that is:
(1) Part of a luminaire or appliance,
(2) Controlled by a wall switch in accordance with 210.70(A)(1), Ex 1.
(3) Located within cabinets or cupboards, or
(4) Located more than 5½ ft above the floor.
(A) General Requirements—Dwelling Unit. A receptacle outlet must be installed in every kitchen, family room, dining room, living room, sunroom, parlor, library, den, bedroom, recreation room, and similar room or area in accordance with (1), (2), and (3):
(1) Receptacle Placement. A receptacle outlet must be installed so that no point along the floor line of any wall is more than 6 ft, measured horizontally along the floor line, from a receptacle outlet.
(2) Definition of Wall Space.
(1) Any space 2 ft or more in width, unbroken along the floor line by doorways and similar openings, fireplaces, and fixed cabinets.
(2) The space occupied by fixed panels in exterior walls.
(3) The space occupied by fixed room dividers, such as freestanding bar-type counters or guard rails.
(3) Floor Receptacle Outlets. Floor receptacle outlets aren’t counted as the required receptacle wall outlet if they’re located more than 18 in. from the wall.
(4) Countertop Receptacles. Receptacles installed for countertop surfaces as required by 210.52(C), can’t be used to meet the receptacle requirements for wall space as required by 210.52(A).
Ex 1: The 20A, 120V small-appliance branch circuit can be used to supply a receptacle for an electric clock.
Ex 2: A receptacle can be connected to the small-appliance branch circuit to supply a gas-fired range, oven, or counter-mounted cooking unit.
Q4. What are the overcurrent protection and conductor sizing requirements for a transformer with a primary voltage of under 600V? Can you give an example for a 45 kVA 480V, three-phase transformer?
A4. Overcurrent Protection for Transformers Not Over 600V is covered by 450.3(B). The primary winding of a transformer must be protected against overcurrent in accordance with the percentages listed in Table 450.3(B) and all applicable notes.
Table 450.3(B) Primary Protection Only
|
Primary Current Rating
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Maximum Protection
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9A or More
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125%, see Note 1
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Less Than 9A
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167%
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Less Than 2A
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300%
|
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)].
Q5. Are anti-short bushings required for both Type AC and Type MC Cable terminations?
A5. Type AC cable must terminate in boxes or fittings specifically listed for Type AC cable to protect the conductors from abrasion [300.15 and 320.40].
An insulating anti-short bushing, sometimes called a “redhead,” must be installed at all Type AC cable terminations. The termination fitting must permit the visual inspection of the anti-short bushing once the cable has been installed [320.40].
Fittings used to secure Type MC cable to boxes or other enclosures must be listed and identified for such use [300.15 and 330.40].
Q6. What are the GFCI protection requirements for a 15 or 20A, 125V receptacle outlet on the dwelling unit laundry circuit?
A6. For other than kitchen sinks, GFCI protection is required for all 15A and 20A, 125V receptacles located within an arc measurement of 6 ft from the outside edge of the sink [210.8(A)(7)]. If the laundry receptacle is located beyond this 6 ft arc, there are no other Code requirements that it be GFCI protected.
Q7. Is it allowable to install a cable wiring method inside a circular raceway for a short distance for support or protection?
A7. The general rule covering the mechanical continuity of raceways requires that raceways and cable sheaths must be mechanically continuous between boxes, cabinets, and fittings [300.12].
However, there are exceptions:
Ex 1: Short sections of raceways used to provide support or protection of cable from physical damage aren’t required to be mechanically continuous [250.86 Ex 2 and 300.10 Ex 1].
Ex 2: Raceways at the bottom of open-bottom equipment, such as switchboards, motor control centers, and transformers, aren’t required to be mechanically secured to the equipment.
Q8. Is it permissible to lengthen the conductors by splicing them inside the panelboard when changing out a service?
A8. Section 312.8 allows cabinets, cutout boxes, and meter socket enclosures to 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:
(1) The total area of the conductors at any cross section doesn’t exceed 40 percent of the cross-sectional area of the space.
(2) 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.
(3) A warning label on the enclosure identifies the disconnecting means for feed-through conductors.
Q9. When low voltage thermostat cable is used for connecting the photo eye used as a sensor to reverse a garage door opener in a commercial occupancy, must it be installed in a raceway?
A9. Only those sections contained in Article 300 specifically referenced in 725.3 apply to Class 1, 2, and 3 circuits [725.3].
If installed in a raceway, 725.3(A) requires that the number and size of conductors or cables in a raceway be limited in accordance with 300.17. Raceways must be large enough to permit the installation and removal of conductors without damaging conductor insulation.
Article 725 does not require that Class 2 or 3 conductors be installed in a Chapter 3 wiring method, such as a raceway.
Equipment and cabling must be installed in a neat and workmanlike manner according to 725.24. Exposed cables must be supported by the structural components of the building so that the cable won’t be damaged by normal building use. Such cables must be secured by straps, staples, hangers, cable ties, or similar fittings designed and installed in a manner that won’t damage the cable.
In summary, Class 2 and 3 conductors are not required to be installed in a raceway unless it is for protection or if specific requirements of a location, such as a hazardous classified location, would require the use of a raceway.
Q10. Are receptacle outlets allowed above a suspended lay-in tile ceiling for cord-connected loads such as temporary holiday lighting displays?
A10. Not if the lighting makes use of flexible cords. According to 400.8(5), flexible cords must not be concealed by walls, floors, or ceilings, or located above suspended or dropped ceilings.
Q11. Are communication and computer networking cables required to be listed for use in wet locations per 310.10(C) when installed in an underground raceway as covered by 300.5(B)?
A11. The requirements in 310.10(C) that conductors installed in underground raceways must be listed for wet locations does not apply to underground communications cables entering buildings [800.47]. Remember that communications systems aren’t subject to the general requirements contained in Chapters 1 through 4 or the special requirements of Chapters 5 through 7, except where a Chapter 8 rule specifically refers to one of those chapters [90.3]. Also, installations of communications equipment under the exclusive control of communications utilities located outdoors, or in building spaces used exclusively for such installations, are exempt from the NEC [90.2(B)(4)].
Q12. Are 240V receptacles installed in a dwelling unit garage required to have GFCI protection?
A12. GFCI protection for dwelling units is required for all 15A and 20A, 125V receptacles located in the locations listed in 210.8(A), which includes garages and accessory buildings [210.8(A)(2)]. There is no requirement for GFCI protection in these locations for receptacles rated over 125V, such as 240V receptacles.
(2) Garages and Accessory Buildings. GFCI protection is required for all 15A and 20A, 125V receptacles in garages, and in grade-level portions of accessory buildings used for storage or work areas of a dwelling unit.
Q13. Is GFCI protection required for electric hand dryers in a public restroom?
A13. GFCI protection is required for all 15A and 20A, 125V receptacles installed in the “other than dwelling units” locations listed in 210.8(B). This includes the bathroom receptacles mentioned in 210.8(B)(1), but does not include any hard-wired equipment such as hand dryers. Only if the hand dryer is cord and plug connected to a 125V rated 15A or 20A receptacle does it require GFCI protection.
(1) Bathrooms. All 15A and 20A, 125V receptacles installed in commercial or industrial bathrooms must be GFCI protected.
Q14. Is it acceptable to install communication cable in the same lighting pole which is being used as a raceway for lighting conductors?
A14. Only if there is a barrier or listed divider between the communication cable and the power conductors.
Communications conductors must not be placed in any raceway, compartment, outlet box, junction box, or similar fitting with conductors of electric light, power or Class 1 circuits [800.133(A)(1)(d)].
Ex 1: Communications circuits can be within the same enclosure with conductors of electric light, power and Class 1 circuits, where separated by a permanent barrier or listed divider.
Q15. In a commercial occupancy, does the Code require that each office must have its own separate light switch to control the luminaires in that office?
A15. In non-dwelling locations, the installation of light switches is not generally specified by the Code, although 210.70(C) addresses the requirements for switches to control lighting where the illumination is needed to service equipment.
(C) Other Than Dwelling Units. At least one lighting outlet that contains a switch or is controlled by a wall switch must be installed in attics and underfloor spaces containing equipment that requires servicing. The switch must be located at the usual point of entry to these spaces, and the lighting outlet must be located at or near the equipment requiring servicing.
In dwelling units, there are more specific requirements in regard to lighting outlets and switch control for them:
210.70 Lighting Outlet Requirements.
(A) Dwelling Unit Lighting Outlets. Lighting outlets must be installed in:
(1) Habitable Rooms. At least one wall switch-controlled lighting outlet must be installed in every habitable room and bathroom of a dwelling unit.
Ex 1: In other than kitchens and bathrooms, a receptacle controlled by a wall switch can be used instead of a lighting outlet.
Ex 2: Lighting outlets can be controlled by occupancy sensors equipped with a manual override that permits the sensor to function as a wall switch.
(2) Other Areas.
(a) Hallways, Stairways, and Garages. In dwelling units, not less than one wall switch-controlled lighting outlet must be installed in hallways, stairways, attached garages, and detached garages with electric power.
(b) Exterior Entrances. At least one wall switch-controlled lighting outlet must provide illumination on the exterior side of outdoor entrances or exits of dwelling units with grade-level access.
(c) Stairway. If the stairway between floor levels has six risers or more, a wall switch must be located at each floor level and at each landing level that includes an entryway to control the illumination for the stairway.
Ex to (a), (b), and (c): Lighting outlets for hallways, stairways, and outdoor entrances can be switched by a remote, central, or automatic control device.
(3) Storage and Equipment Rooms. At least one lighting outlet that contains a switch or is controlled by a wall switch must be installed in attics, underfloor spaces, utility rooms, and basements used for storage or containing equipment that requires servicing. The switch must be located at the usual point of entry to these spaces, and the lighting outlet must be located at or near the equipment that requires servicing.
(B) Guest Rooms or Guest Suites. At least one wall switch-controlled lighting outlet must be installed in every habitable room and bathroom of a guest room or guest suite of hotels, motels, and similar occupancies.
Ex 1: In other than bathrooms and kitchens, a receptacle controlled by a wall switch is permitted in lieu of lighting outlets.
Ex 2: Lighting outlets can be controlled by occupancy sensors equipped with a manual override that permits the sensor to function as a wall switch.
