If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to firstname.lastname@example.org. Answers are based on the 2011 NEC.
Disconnecting means as service equipment
What does it mean when it requires the disconnecting means to be suitable for use as service equipment?
For a disconnecting means to be marked suitable for use as service equipment, means must be provided for the neutral bus to be bonded to the panel enclosure by a green screw or by a jumper to the ground bus in the panel. For a disconnecting means to be marked as suitable only as a service disconnecting means, the neutral bus must be permanently bonded to the panel enclosure.
What is GTO cable?
What is GTO cable, and where is it shown in the NEC?
Gas tube and oil (GTO) burner ignition cable does not appear in the NEC other than in 600.32(B) and (E). GTO cable is used principally for neon sign installations. UL has evaluated this cable and listed it as GTO-5, GTO-10 and GTO-15 to indicate the kilovolt designations of different cable subtypes. The cable generally consists of conductors that use thermoplastic insulation, enclosed in an outer thermoplastic jacket. It is a special-use cable construction and is not listed under NEC Article 310, which lists conductors for general wiring. GTO cable is identified in Annex A, which shows the product standard for this cable as UL 814 Gas-Tube-Sign Cable.
Calculating size of grounding conductor
NEC 250.122 (B)—Size of Equipment Grounding Conductors reads, “Where ungrounded conductors are increased in size, equipment grounding conductors where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.” Where 1 AWG copper conductors are installed in lieu of 3 AWG copper conductors (to compensate for voltage drop) for a circuit protected by a 100-ampere (A) overcurrent device, what size of copper equipment grounding conductor is required, and how do you calculate it?
See Table 8 in the NEC. A 3 AWG conductor has a cross-sectional area of 52,620 circular mils. You are increasing the size to a 1 AWG, which has an area of 83,690 circular mils. Dividing 83,690 by 52,620 equals an increase of 1.60 in the circular mil area. The equipment grounding conductor required by Table 250.122 for a 100A overcurrent device is an 8 AWG copper conductor, which Table 8 indicates as being 16,510 circular mils. Multiply 16,510 by 1.60 to increase the size proportionately according to the increase in size in circular mil area of the ungrounded conductors, and find 26,416 as the required circular mil area for the equipment grounding conductor. Table 8 shows that a 4 AWG conductor has a circular mil area of 41,740. A 6 AWG conductor has a cross-sectional area of 26,240 circular mils, which is close, but too small. Don’t be tempted.
Full-size grounding conductor in parallel installations
I don’t follow the need for the full-size grounding conductor in parallel installations. If the grounded conductors can be paralleled to get to the equivalent size needed to carry the supply current, then the equivalent size grounding conductors would be sufficient. They are all connected at a common point at the load and supply ends, so any fault current should flow evenly on all conductors, not just on one.
You’re correct up to where you say the conductors “are connected at a common point at the load and supply ends, so any fault current should flow evenly on all conductors.” Not exactly. While connected at a common point at the load and supply ends, the length of each of the equipment grounding conductors is different. Assume there are two parallel nonmetallic conduits, and a fault occurs at an enclosure where an ungrounded conductor in one of the parallel conduits accidentally touches the enclosure. The fault is fed by both conductors of the same phase.
The shortest path to the grounded conductor in the supply panelboard from the fault is through the equipment grounding conductor in the conduit enclosing the faulted phase conductor. The longer path through the equipment grounding conductor in the other paralleled conduit results in a higher impedance and causes a greater amount of ground-fault current to flow in the equipment grounding conductor in the conduit with the faulted conductor. For this reason, each equipment grounding conductor must be capable of carrying enough current to allow the overcurrent device to operate without the equipment grounding conductor burning open [250.122(F)].
Why doesn’t the NEC limit the number of receptacles that can be installed on a residential branch circuit?
The receptacles required by 210.52(A)(1) are convenience receptacles, i.e., they are there for the convenience of the occupant. For example, if you installed a receptacle every 2 feet around the walls in a bedroom and ended up with 20 receptacles in that bedroom, it wouldn’t mean that you had increased the load in that bedroom. It would just be more convenient for the occupant. You would be hard-pressed to reach the 3 volt-amperes (VA) per-square-foot load calculation permitted in accordance with 220.12. For example, a 10-foot-by-12-foot bedroom has 120 square feet, which multiplied by 3 VA equals 660 VA or 660 watts.
Three-way switch cables
We use two 14-2 with ground NM cables between three-way switches in the houses we wire. One cable is used for the travelers. One wire in the other cable is used for the hot, and one wire is buried (not connected). We do this to keep from having to carry a lot of different cable types on the truck. Does this comply with the Code?
Yes. When using NM cable assemblies, the NEC contains requirements for the use of conductors with white insulation when they are used to wire three-way switches. NEC Section 200.7(A) requires that a conductor with a white coloring must be used only for the grounded circuit conductor unless otherwise permitted in 200.7(C). Section 200.7(C)(1) permits, for three-way switch loops, using the conductor with a white insulation as a supply to the switch but not as a return conductor. However, in this application, the white conductor must be permanently re-identified to indicate its use by painting or other effective means at its terminations and at each location where the conductor is visible and accessible.
Why are conductors allowed to be smaller using Table 310.15(B)(7) than when using Table 310.15(B)16? Even for service laterals (residential service), the conductors can be smaller.
This permitted ampacity change for conductors used as 120/240-volt, 3-wire, single-phase service-entrance conductors service lateral conductors and feeders conductors that serve as the main power feeder to each dwelling unit of one-family, two-family and multifamily dwelling units has been in the NEC in one form or another since at least 1958. I believe this ampacity change for conductors used for these purposes came as a result of a study by the utilities that determined there was a significant difference between residential connected loads and demand loads, as shown by meter readings taken from these types of dwelling occupancies.
I have existing walls in hospital patient-care areas that need to be fished with flex, and the total length will be more than 6 feet. How would I get the redundant ground besides the ground wire?
You are referring to the requirement in 517.13(A) for redundant grounding in patient care areas. Both Type AC cable armor and Type MC cable armor are permitted to be used as equipment-grounding conductors [517.30(C)(3)(3)c].
Sink and GFCI receptacle
Two 20A duplex GFCI receptacle circuits have been installed to the left and right sides of a sink. A split duplex receptacle has been installed under the sink with two 15A feeds for dishwasher and disposal. Since the receptacle under the sink is well within 6 feet of the sink, is it required to have GFCI protection?
There is no requirement for receptacles that are within 6 feet of a kitchen sink to be GFCI-protected unless they are installed to serve the countertop surfaces [210.8(A)(6)]. However, NEC 210.8(A)(7) specifies that, for sinks located in areas other than kitchens, where receptacles are installed within 6 feet of the outside edge of a sink, these receptacles shall have ground-fault circuit interrupter protection.
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