NEC Questions and Answers Based on the 2008 NEC
January Part 1 of 2
Here is the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent yesterday so you can see how you did.
By Mike Holt for EC&M Magazine
Q1. Is PVC conduit permitted to be used to supply circuits in a health care facility?
A1. Yes and no. Yes PVC conduit can be used to supply feeder circuits in health care facilities, but not for branch circuits. Branch circuits serving patient care areas must be provided with an effective ground-fault current path by installing the circuits 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)].
Q2. Are switchboards, panelboards, and transformers permitted within a room that serves as a return air space to the air handler?
A2. Yes. The requirement that limits wiring methods in return air space doesn’t apply to habitable rooms or areas of buildings, the prime purpose of which isn’t air handling [300.22(C)].
Q3. Section 110.14(C) requires the use of the 60°C column for 100A or less and the 75°C column for above 100A for conductor sizing. What is the value of THHN rated 90°C if the ampacity can not be used for conductor sizing?
A3. The advantage of 90°C conductor insulation only appears when the conductor ampacity must be adjusted because of conductor bundling or elevated ambient temperatures as per 310.15. When adjusting conductor ampacity, the ampacity is based on the temperature insulation rating of the conductor as listed in Table 310.16, not the temperature rating of the terminal [110.14(C)]. For example: What is the adjusted ampacity of nine current carrying 12 AWG THHN conductors in a raceway?
(a) 18A (b) 20A (c) 21A (d) 25A
Answer: (c) 21A
Table 310.16 ampacity if 12 AWG THHN is 30A
Adjustment Factor [Table 310.15(B)(2)(a)] = 0.70
Adjusted Ampacity = 30A x 0.70
Adjusted Ampacity = 21A, this conductor is still suitable to be used on a 20A circuit.
Q4. What are the rules on spacing receptacles in a dwelling unit??
A4. 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 so no point along the wall space is more than 6 ft, measured horizontally along the floor line, from a receptacle outlet [210.52(A)].
In addition, a receptacle outlet must be installed for each kitchen and dining area countertop wall space 1 ft or wider, and receptacles must be placed so no point along the countertop wall space is more than 2 ft, measured horizontally from a receptacle outlet [210.52(C)(1)].
But honestly there are so many rules on locating receptacles in dwelling units, that I suggest you review all of 210.52.
Q5. When a single generator is used to provide standby power for emergency and nonemergency loads, can one transfer switch be used?
Q6. What are some of the new GFCI protection requirements in the 2008 NEC, and must existing facilities be updated to comply with the new requirements?
A6. GFCI protection is required for all 15A and 20A, 125V receptacles located in dwelling unit garages and unfinished basement [210.8(A)(2) and 210.8(A)(5)].
In other than dwelling occupancies, all 15A and 20A, 125V receptacles installed outdoors [210.8(B)(4)], within 6 ft of the outside edge of a sink must be GFCI-protected [210.8(B)(5), and electric drinking fountains must be GFCI protected [422.52].
When receptacles are replaced in locations where GFCI protection is required the replacement receptacles must be GFCI protected [406.3(D)(2)], but the Code doesn’t require you to update your facilities every time a new edition is written.
Q7. Is ground-fault protection required for a 1200A, 277/480V, three-phase service disconnect switch if it contains a 900A fuse?
A7. Yes, ground-fault protection of equipment is required for each service disconnecting means rated 1,000A or more supplied by a 4-wire, three-phase, 277/480V wye-connected system. The rating of the service disconnecting means is considered to be the rating of the largest fuse that can be installed or the highest continuous current trip setting of a circuit breaker.
Q8. How many 12-2 NM cables can be installed in an 18 cu in. plastic box with a duplex receptacle?
A8. Two 12-2 NM cables. Each device yoke counts as two conductor volumes based on the largest conductor that terminates on the device in accordance with Table 314.16(B) [314.16(B)(4)], and each conductor that terminates in a box is counted as a single conductor volume [314.16(B)(1)]. Equipment grounding conductors in a box count as a single conductor volume [314.16(B)(5)]. The conductor volume for 12 AWG conductors is 2.25 cubic inches each [Table 314.16(B)].
Step 1: Determine the number and size of conductor volumes for the device and the equipment grounding conductors.
One receptacle 2 – 12 AWG conductor volumes
One equipment grounding conductor 1 – 12 AWG conductor volumes
Total 3 – 12 AWG conductors volumes
Step 2: Determine the volume of those conductor volumes [Table 314.16(B)].
12 AWG conductor = 2.25 cu in. x 3 wires = 6.75 cu in.
Step 3: Determine the space remaining for conductors.
Total space = 18 cu in.
Remaining space = 18 cu in. – 6.75 cu in.
Remaining space = 11.25 cu in.
Step 4: Determine the number of 12 AWG conductors permitted in the spare space.
Conductors added = Remaining space/added conductors’ volume
Conductors added = 11.25 cu in./2.25 cu in. = 5 conductors
An 18 cu in box will hold two 12-2 NM cables with a duplex receptacle.
Q9. Are light switches for exterior lighting required immediately adjacent to each entrance of a single family dwelling?
A9. No. The specifies the location of the lighting outlet in a dwelling unit [210.70(A)(2)(b)], but it doesn’t specify the location of the switch.
Q10. When PVC conduit is used, does the Code require an equipment grounding conductor between the meter enclosure and the service disconnect?
A10. No, the neutral conductor can be used as the circuit equipment grounding conductor for metal parts of equipment, raceways, and enclosures at service equipment [250.142(A)(1)]. This is because the service neutral conductor provides the effective ground-fault current path to the power supply to ensure that dangerous voltage from a ground fault will be quickly removed by opening the overcurrent device [250.4(A)(3) and 250.4(A)(5)].
Q11. When five feet of conductor passes through a higher ambient temperature location, must the entire circuit still be subjected to ampacity correction?
A11. It depends on the circuit length. When different ampacities apply to a length of conductor, the higher ampacity is permitted for the entire circuit if the reduced ampacity length doesn’t exceed 10 ft and its length doesn’t exceed 10 percent of the length of the higher ampacity [310.15(A)(2) Exception].
Q12. When a cable tray enters through the outside wall of a metal building, is the opening required to be fireproofed or just weatherproofed?
A12. The locally adopted building code would give you the answer for this. If the exterior wall is required to be of fire resistance rated construction, it would have to be firestopped.
Q13. I have two duplex receptacles in the same two gang box, each on a separate circuit. Are these circuits required to be fed from a double pole circuit breaker?
A13. No, but if the circuits share a neutral (multiwire branch circuit) they require a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates [210.4(B)], such as a two pole breaker or single pole breakers with an identified handle tie [240.15(B)(1)].
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