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NEC Questions and Answers - Based on 2011 NEC
June 2013
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. If circuit conductors are installed in parallel in separate raceways, is an equipment grounding conductor required in each raceway and how do I size it?
A1. If circuit conductors are installed in parallel in separate raceways as permitted by 310.10(H), an equipment grounding conductor must be installed for each parallel conductor set [250.122(F)]. Where conductors are installed in parallel in the same raceway or cable tray, a single equipment grounding conductor is permitted.
Each equipment grounding conductor must be sized in accordance with Table 250.122, based on the rating of the circuit overcurrent device, but it’s not required to be larger than the circuit conductors [250.122(A)].
Example: A 1,200A disconnect supplies feeder conductors paralleled in 3 PVC raceways. What size copper equipment grounding conductor is required for the feeder circuit? Figure 250-s122F0 01
- A 4 AWG equipment grounding conductor in each raceway
- A 3/0 AWG equipment grounding conductor in each raceway
- A 400 kcmil equipment grounding conductor in each raceway
- A single 350 kcmil equipment grounding conductor in any one of the raceway
Answer: (b) 3/0 AWG in each raceway, Table 250.122, based on the 1,200A feeder protection device [250.122(F)].
Q2. When is a common neutral counted as a current carrying conductor in a conduit for calculating conductor ampacity?
A2. According to 310.15(B)(5), the following rules determine when a neutral is considered current carrying for purposes of applying the adjustment factors of 310.15(B)(2)(a):
(a) The neutral conductor of a 3-wire, single-phase, 120/240V system, or 4-wire, three-phase, 120/208V or 277/480V wye-connected system, isn’t considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) [310.15(B)(5)(a)].
(b) The neutral conductor of a 3-wire circuit from a 4-wire, three-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) [310.15(B)(5)(b)].
Author’s Comment: When a 3-wire circuit is supplied from a 4-wire, three-phase, 120/208V or 277/480V wye-connected system, the neutral conductor carries approximately the same current as the ungrounded conductors.
(c) The neutral conductor of a 4-wire, three-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) if more than 50 percent of the neutral load consists of nonlinear loads [310.15(B)(5)(c)].
Author’s Comment: Nonlinear loads supplied by a 4-wire, three-phase, 120/208V or 277/480V wye-connected system can produce unwanted and potentially hazardous odd triplen harmonic currents (3rd, 9th, 15th, and so on) that can add on the neutral conductor. To prevent fire or equipment damage from excessive harmonic neutral current, the designer should consider increasing the size of the neutral conductor or installing a separate neutral for each phase. For more information, visit www.MikeHolt.com, click on the “Technical” link, then the “Power Quality” link. Also see 210.4(A) Note, 220.61 Note 2, and 450.3 Note 2.
Q3. Is GFCI protection required for a sump pump installed in an unfinished basement of a dwelling?
A3. GFCI protection is required for all 15A and 20A, 125V receptacles located in the unfinished portion of a basement not intended as a habitable room and limited to storage and work areas [210.8(A)(5)].
Ex: A receptacle supplying only a permanently installed fire alarm or burglar alarm system isn’t required to be GFCI protected [760.41(B) and 760.121(B)].
Author’s Comment: A receptacle outlet is required in each unfinished portion of a dwelling unit basement [210.52(G)].
Q4. How do I size the conductor and overcurrent protection for an air-conditioner?
A4. The short-circuit and ground-fault protective device for a single motor-compressor must not be more than 175 percent of the motor-compressor current rating. If the protective device sized at 175 percent isn’t capable of carrying the starting current of the motor-compressor, the next size larger protective device can be used, but in no case can it exceed 225 percent of the motor-compressor current rating [440.22(A)].
Question: What size conductor and protection are required for a 24A motor-compressor connected to a 240V circuit?
(a) 10 AWG, 40A (b) 10 AWG, 60A
(c) a or b (d) 10 AWG, 90A
Answer: (a) 10 AWG, 40A
Step 1: Determine the branch-circuit conductor [Table 310.15(B)(16) and 440.32]:
24A x 1.25 = 30A, 10 AWG, rated 30A at 75°C [Table 310.15(B)(16)]
Step 2: Determine the branch-circuit protection [240.6(A) and 440.22(A)]:
24A x 1.75 = 42A, next size down = 40A
If the 40A short-circuit and ground-fault protective device isn’t capable of carrying the starting current, then the protective device can be sized up to 225 percent of the equipment load current rating. 24A x 2.25 = 54A, next size down 50A
Q5. When a building has more than one service as allowed by 230.2 is there a requirement to ground them to a common grounding electrode be used for both?
A5. Where an ac system is connected to a grounding electrode in or at a building or structure, the same grounding electrode must be used. If separate services, feeders, or branch circuits supply a building, the same grounding electrode must be used [250.58].
Two or more grounding electrodes that are bonded together will be considered as a single grounding electrode system in this sense.
Author’s Comment: Metal parts of the electrical installation are grounded (connected to the earth) to reduce induced voltage on the metal parts from lightning so as to prevent fires from a surface arc within the building/structure. Grounding electrical equipment doesn’t serve the purpose of providing a low-impedance fault current path to open the circuit overcurrent device in the event of a ground fault.
Caution: Potentially dangerous objectionable neutral current flows on the metal parts when multiple service disconnecting means are connected to the same electrode. This is because neutral current from each service can return to the utility via the common grounding electrode and its conductors. This is especially a problem if a service neutral conductor is opened.
Q6. What are the rules regarding grounding receptacles, lights, and equipment in wiring methods for patient care areas?
A6. Wiring in patient care areas must comply with 517.13 (A) and (B):
Author’s Comment: Patient care areas include patient rooms as well as examining rooms, therapy areas, treatment rooms, and some patient corridors. They don’t 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)].
Author’s Comments:
• 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 isn’t listed as an equipment grounding conductor unless it contains an uninsulated equipment grounding/bonding conductor and it’s listed and identified as an equipment grounding conductor. Once the bare aluminum grounding/bonding conductor exits the cable, it can be cut off because it no longer serves any purpose. In addition, the effective ground-fault current path must be maintained by the use of fittings specifically listed for the cable [330.40]. See 300.12, 300.15, and 330.100.
Additionally, the following must be directly connected to an insulated copper equipment grounding conductor that’s installed with the branch-circuit conductors in wiring methods required in 517.13(A) [517.13(B)(1)].
Ex: An insulated equipment bonding jumper from a receptacle that directly connects to the equipment grounding conductor is permitted.
(1) The grounding terminals of all receptacles.
(2) Metal enclosures containing receptacles.
(3) Fixed electrical equipment operating at over 100V.
Ex 1: 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 [406.4(C)].
Ex 2: 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.
(2) Equipment grounding conductors and equipment bonding jumpers must be sized in accordance with 250.122.
Q7. What is the rule regarding receptacles with insulated grounding terminals (isolated ground) in patient care areas?
A7. Receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)] aren’t permitted to be installed in patient care areas [517.16].
For more NEC Practice, purchase Mike Holt's NEC Exam Practice Questions book based on the 2011 NEC.
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