Your most pressing National Electrical Code (NEC) questions answered
All questions and answers are based on the 2011 NEC.
Q. What are the grounding conductor requirements for 15A or 20A, 125V isolated ground receptacles?
A. If installed for the reduction of electrical noise, the grounding terminal of an isolated ground receptacle must be connected to an insulated equipment grounding conductor run with the circuit conductors [250.146(D)]. The circuit equipment grounding conductor is permitted to pass through panelboards [408.40 Ex], boxes, wireways, or other enclosures [250.148 Ex] without a connection to the enclosure, as long as it terminates at an equipment grounding conductor terminal of the derived system or service.
Type AC cable containing an insulated equipment grounding conductor of the wire type can be used to supply receptacles having insulated grounding terminals, because the metal armor of the cable is listed as an equipment grounding conductor [250.118(8)].
The metal armor sheath of interlocked Type MC cable containing an insulated equipment grounding conductor isn’t listed as an equipment grounding conductor. Therefore, this wiring method with a single equipment grounding conductor can’t supply an isolated ground receptacle installed in a metal box (because the box isn’t connected to an equipment grounding conductor). However, Type MC cable with two insulated equipment grounding conductors is acceptable, because one equipment grounding conductor connects to the metal box and the other to the isolated ground receptacle.
The armor assembly of interlocked Type MCAP cable with a 10 AWG bare aluminum grounding/bonding conductor running just below the metal armor is listed to serve as an equipment grounding conductor in accordance with 250.118(10)(b).
Because the grounding terminal of an isolated ground receptacle is insulated from the metal mounting yoke, a metal faceplate must not be used when an isolated ground receptacle is installed in a nonmetallic box. The reason is that the metal faceplate isn’t connected to an equipment grounding conductor [406.3(D)(2)].
When should an isolated ground receptacle be installed? How should the isolated ground system be designed? These questions are design issues and must not be answered based on the NEC alone [90.1(C)]. In most cases, using isolated ground receptacles is a waste of money. For example, IEEE Standard 1100 — Powering and Grounding Electronic Equipment (Emerald Book) states: “The results from the use of the isolated ground method range from no observable effects, the desired effects, or worse noise conditions than when standard equipment bonding configurations are used to serve electronic load equipment [18.104.22.168].”
In reality, few electrical installations require an isolated ground system. For those systems that can benefit from an isolated ground system, engineering opinions differ as to what’s a proper design. Worse, of those properly designed, few are correctly installed, and even fewer are properly maintained.
Q. What are the Code requirements for small-appliance branch circuits for dwelling units?
A. 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)], as shown in the Figure (click here to see Figure). See the definition of “Receptacle Outlet” in Art. 100.
A 15A, 125V receptacle is rated for 20A feed-through, so it can be used for this purpose [210.21(B)(3)]. Lighting outlets or receptacles located in other areas of a dwelling unit must not be connected to the small-appliance branch circuit [210.52(B)(2)]. The two 20A small-appliance branch circuits can be supplied by one 3-wire multiwire circuit or by two separate 120V circuits [210.4(A)].
Q. What are the Code requirements for the identification of a neutral conductor?
A. 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.
The use of white tape, paint, or other methods of identification isn’t permitted for grounded conductors 6 AWG or smaller. 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.
Q. How do you calculate the feeder/service calculated load for more than one range in a dwelling occupancy, if the ranges are more than 12kW and all of the same size?
A. Household cooking appliances rated more than 1.75kW 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 12kW, the maximum demand in Column C must be increased 5% for each additional kilowatt of rating, or major fraction thereof, by which the rating of individual ranges exceeds 12kW.
Here’s an example of how to apply this rule. What’s the feeder/service calculated load for three 15.60kVA ranges?
Step 1: Determine the Column C demand load for three units: 14kVA.
Step 2: Because each 15.60kVA range exceeds 12kVA by 3.60kVA, increase the Column C demand load by 5% for each kVA or major fraction of kVA in excess of 12kVA.
Step 3: Because 3.60kVA is 3kVA plus a major fraction of a kVA, increase the Column C value by 4 × 5% = 20%.
Thus, increase the Column C load (14kVA) by 20%: 14kVA × 1.20 = 16.80kVA.
Q. What is the minimum rating of the disconnecting means for a building or structure fed by a branch circuit or feeder?
A. A single disconnecting means for a building/structure must have an ampere rating not less than the calculated load as determined by Art. 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]:
- 15A for installations consisting of a single branch circuit.
- 30A for installations consisting of two 2-wire branch circuits.
- 100A, 3-wire, for a one-family dwelling.
- 60A for all other installations.
Q. Where must the disconnecting means for a building or structure fed by a feeder or branch circuit be located?
A. 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].