NEC Questions and Answers – Based on the 2011 NEC
January 2012
By Mike Holt for EC&M Magazine
Here’s the follow up to Monday's newsletter. This includes all of the answers to the questions sent, so you can see how you did.
Q1. Are you required to bond isolated sections of metal water pipe that is connected to a nonmetallic water piping system?
A1. No. An isolated section of pipe is not a metal water piping system.
A metal water piping system must be bonded as required in 250.104(A)(1), (A)(2), or (A)(3). The bonding jumper must be copper where within 18 in. of the earth [250.64(A)], securely fastened to the surface on which it’s mounted [250.64(B)], and adequately protected if exposed to physical damage [250.64(B)]. In addition, all points of attachment must be accessible [250.104(A)].
- Bonding isn’t required for isolated sections of metal water piping connected to a nonmetallic water piping system.
- To remove dangerous voltage on metal parts from a ground fault, electrically conductive metal water piping systems, metal sprinkler piping, metal gas piping, as well as exposed structural steel members likely to become energized, must be connected to an effective ground-fault current path [250.4(A)(4)].
Q2. What are the Code requirements for small appliance branch circuits for dwelling units?
A2. Two or more 20A, 120V small-appliance receptacle branch circuits are required for the 15A or 20A receptacle outlets in a dwelling unit kitchen, dining room, breakfast room, pantry, or in similar dining areas as required by 210.52(B) [210.11(C)(1)].
Q3. What is counted toward box fill in an outlet box?
A3. The calculated conductor volume from the items on the following list are added together to determine the total volume of the conductors, devices, and fittings. Raceway and cable fittings, including locknuts and bushings, aren’t counted for box fill calculations [314.16(B)(2)]:
(1) Conductor Volume. Each unbroken conductor that runs through a box, and each conductor that terminates in a box, is counted as a single conductor volume in accordance with Table 314.16(B).
Each loop or coil of unbroken conductor having a length of at least twice the minimum length required for free conductors in 300.14 must be counted as two conductor volumes. Conductors that originate and terminate within the box, such as pigtails, aren’t counted at all.
Ex: Equipment grounding conductors, and up to four 16 AWG and smaller fixture wires, can be omitted from box fill calculations if they enter the box from a domed luminaire or similar canopy, such as a ceiling paddle fan canopy.
(2) Cable Clamp Volume. One or more internal cable clamps count as a single conductor volume in accordance with Table 314.16(B), based on the largest conductor that enters the box. Cable connectors that have their clamping mechanism outside of the box aren’t counted.
(3) Support Fitting Volume. Each luminaire stud or luminaire hickey counts as a single conductor volume in accordance with Table 314.16(B), based on the largest conductor that enters the box.
(4) Device Yoke Volume. Each single-gang device yoke (regardless of the ampere rating of the device) counts as two conductor volumes, based on the largest conductor that terminates on the device in accordance with Table 314.16(B).
Each multigang-device yoke counts as two conductor volumes for each gang, based on the largest conductor that terminates on the device in accordance with Table 314.16(B).
Author’s Comment: A device that’s too wide for mounting in a single-gang box, as described in Table 314.16(A), is counted based on the number of gangs required for the device.
(5) Equipment Grounding Conductor Volume. All equipment grounding conductors in a box count as a single conductor volume in accordance with Table 314.16(B), based on the largest equipment grounding conductor that enters the box. Insulated equipment grounding conductors for receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)], count as a single conductor volume in accordance with Table 314.16(B).
Q4. Is it acceptable to use a 14 AWG conductor for the bonding jumper to a receptacle on a 20A branch circuit?
A4. No. An equipment bonding jumper used to connect the grounding terminal of a receptacle to a metal box is sized in accordance with 250.122, based on the rating of the circuit overcurrent device. [250.146].
Q5. When is a grounding electrode required for a separate building that is supplied by a feeder or branch circuit?
A5. In all cases except where supplied by a single branch circuit or multiwire branch circuit.
Each building/structure supplied by a feeder or branch circuit must have its disconnect connected to an electrode of a type identified in 250.52 [250.32(A)].
Ex: A grounding electrode isn’t required where the building/structure is served with a 2-wire, 3-wire, or 4-wire multiwire branch circuit.
Q6. When is a disconnecting means required at a separate building that is supplied by a feeder or branch circuit?
A6. A disconnect is required for all conductors that enter or pass through a building/structure [225.31].
Q7. Where must the disconnecting means for a building or structure fed by a feeder or branch circuit be located?
A7. The disconnecting means for a building/structure must be installed at a readily accessible location either outside or inside nearest the point of entrance of the conductors [225.32].
Supply conductors are considered outside of a building or other structure where they’re encased or installed under not less than 2 in. of concrete or brick [230.6].
Q8. What is the minimum rating of the disconnecting means for a building or structure fed by a branch circuit or feeder?
A8. A single disconnecting means for a building/structure must have an ampere rating not less than the calculated load as determined by Article 220. If the disconnecting means consists of more than one switch or circuit breaker, the combined ratings of the circuit breakers must not be less than the calculated load as determined by Article 220. In addition, the disconnecting means must not be rated lower than [225.39]:
- For installations consisting of a single branch circuit, the disconnecting means must have a rating of not less than 15A.
- For installations consisting of two 2-wire branch circuits, the feeder disconnecting means must have a rating of not less than 30A.
- For a one-family dwelling, the feeder disconnecting means must have a rating of not less than 100A, 3-wire.
- For all other installations, the feeder or branch-circuit disconnecting means must have a rating of not less than 60A.
Q9. How do you calculate the feeder/service calculated load for more than 1 range in a dwelling occupancy, if the ranges are over 12 kW and all of the same size?
A9. Household cooking appliances rated over 1.75 kW can have the feeder/service load calculated according to the demand factors of Table 220.55.
Note 1: For identically sized ranges individually rated more than 12 kW, the maximum demand in Column C must be increased 5 percent for each additional kilowatt of rating, or major fraction thereof, by which the rating of individual ranges exceeds 12 kW.
Example: What’s the feeder/service calculated load for three 15.60 kVA ranges?
(a) 14 kVA (b) 15 kVA (c) 17 kVA (d) 21 kVA
Answer: (c) 17 kVA (closest answer)
Step 1: Determine the Column C demand load for 3 units: 14 kVA.
Step 2: Because each 15.60 kVA range exceeds 12 kVA by 3.60 kVA, increase the Column C demand load by 5% for each kVA or major fraction of kVA in excess of 12 kVA.
Step 3: Because 3.60 kVA is 3 kVA plus a major fraction of a kVA, increase the Column C value by 4 x 5% = 20%
Increase the Column C load (14 kVA) by 20%:
14 kVA x 1.20 = 16.80 kVA.
Q10. What are the Code requirements for the identification of a neutral conductor?
A10. Grounded conductors 6 AWG and smaller must be identified by one of the following means. [200.6(A)]:
- By a continuous white outer finish.
- By a continuous gray outer finish.
- By three continuous white stripes along its entire length on other than green insulation.
- Wires that have their outer covering finished to show a white or gray color but have colored tracer threads in the braid identifying the source of manufacture are considered to meet the provisions of this section.
- A single-conductor, sunlight-resistant, outdoor-rated cable used as the grounded conductor in photovoltaic power systems as permitted by 690.31(B) can be identified by distinctive white marking at all terminations.
Grounded conductors 4 AWG or larger must be identified by one of the following means [200.6(B)]:
- A continuous white outer finish along its entire length.
- A continuous gray outer finish along its entire length.
- Three continuous white stripes along its length.
- White or gray tape or markings at the terminations.
For more NEC Practice purchase Mike Holt's NEC Practice Questions book, Based on the 2011 NEC.
