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 email@example.com. Answers are based on the 2011 NEC.
In response to a question about tamper-resistant receptacles in a detached workshop (January 2011), you cite NEC 410.12, but should it not be 406.12? Also, the person could have avoided use of the tamper-resistant receptacles had he kept them 66 inches above the floor, which is allowed by an exception.
Thanks for the correction. NEC 406.12 requires that, in all areas specified in 210.52, all 125-volt (V), 15- and 20-ampere (A) receptacles shall be listed tamper-resistant receptacles. Exception No. 1 to 406.12 permits receptacles located 5 feet above the floor to not be tamper-resistant.
Regarding luminaire and lamp ratings: we have typical 120V fixtures, suitable for their respective locations (wet, damp, dry), with medium-base screw shell lampholders and a completely enclosed lamp. These fixtures are marked “75W lamp max.” Can a medium-base, self-ballasted compact fluorescent lamp (CFL)—which is nameplate rated 42W to operate and produces lumens equivalent to a 150W incandescent lamp—be used safely and in compliance with 110.3(B)?
Yes, the CFL produces a lumen output equivalent to that of a 150W incandescent lamp but uses only 42W. The use of this lamp is in accordance with the manufacturer’s instructions.
I’m using Type MC cable to wire outlets with quick-connect style connectors. The snap-in connector has a bushing. Do I also need an anti-short on the MC cable?
An anti-short bushing is not required for Type MC cable as is required by 320.40 for Type AC cable. Type MC cable can be one or more insulated conductors in an armor of interlocking metal tape or a smooth or corrugated metallic sheath (330.2). The metallic sheath is required to be continuous and close fitting.
Arc-fault protection required?
I recently installed a service upgrade and included a combination smoke/carbon monoxide alarm in an unfinished basement area used for laundry, a small workshop and exercise on a physical fitness apparatus. I installed this new circuit on a standard circuit breaker. The inspector insisted that an arc-fault breaker be installed as his interpretation of the Code was “Any circuit that does not require a GFCI circuit does require an arc-fault breaker.” My interpretation is that the new circuit was not in a living space as described in the Code and does not require an arc-fault breaker. May I have your interpretation, please?
Based on the information you have supplied, I don’t see a requirement for the use of an arc-fault breaker. NEC 210.12 shows the dwelling unit areas that are required to have arc-fault protection on all 120V, 15- and 20A branch-circuits, and unfinished basement areas are not included.
Self-tapping screws and steel boxes
There are numerous brackets available that are prewelded to steel boxes. However, in some applications, it would just be more efficient to attach the box to the steel frame with the use of some self-tapping screws. I’ve searched the Code but have not found any obvious restrictions concerning how a steel box may be mounted to a steel frame. Are there any Code sections that I should be aware of that may prevent me from using self-tapping screws?
NEC 314.23(B)(1) contains the requirements for using nails or screws as a fastening means for supporting enclosures. Your method is Code-compliant.
Grounding vs. ground-rod electrodes
Why are grounding-electrode conductors used with underground metal water piping required to be sized according to Table 250.66, while a ground-rod electrode only requires a 6 AWG copper?
The use of this smaller conductor is based on the fact that it can never be called on to carry a current beyond its safe short-time rated capacity. This restriction is based on the theory that 42.25 centimeters (cm) of copper conductor will carry 1A of current for 5 seconds after which the conductor will destroy itself. A 6 AWG copper conductor is equal to 26,240 circular mils divided by 42.25 cm = 621A of current. However, a lightning strike or a flashover lasts only a small percentage of a second, and the 6 AWG conductor can handle the current flow in that short duration of time.
What would be the Code requirement for liquid-tight flexible metal conduit (LFMC) hanging from a cable tray to production equipment where the length would be approximately 12 feet?
A means of support for LFMC is required, according to NEC 350.30(A), at intervals not to exceed 4 feet.
Connected to earth
What does it mean to say the Code requires grounded electrical systems to be connected to earth in a manner that will stabilize the voltage-to-earth during normal operation?
An ungrounded conductor can reach any voltage within that system with respect to earth. When one conductor is grounded, a fixed-potential-to-ground is established. For example, if you ground the common conductor of a single-phase, 3-wire, 120/240V transformer secondary, the highest voltage-to-ground any conductor in that system can reach is 50 percent of the transformer secondary potential (or 120V) because the grounded conductor is tapped at midpoint of the 240V secondary winding.
Exam room wiring
I am wiring exam rooms in a clinic using the new hospital-grade MC AP Type 12 AWG stranded cable (green jacket with 10 AWG aluminum ground conductor) with approved connectors, of course. I would like to pigtail the ground wires to the hospital-grade receptacle (that has a grounding clip on the mounting screw) without also using a pigtail to the box. In the past I have installed the box pigtail but was wondering if I am adding an unneeded cost to each box since the box is bonded through the cable jacket, not the green wire. Do the grounding clips on the mounting screws do the same job that the box pigtail would? I have asked inspectors this question and get various degrees of certainty, so I always end up putting the box pigtail in to be sure.
NEC 517.13(B) requires a redundant grounding method to be installed in patient care areas. An insulated copper equipment-grounding conductor installed with the branch-circuit conductors in a metal-clad cable or a metal raceway system must be connected to the metal box.
Ceiling fan delay
At a new single-family residential job, I hung six ceiling fans in six rooms, each with a light kit. All fans are from the same manufacturer and came with 40W candelabra incandescent lamps. Two fans on one circuit and one fan on a different circuit come on late. The other three fans come on instantaneously. They are not on the same circuits that are delayed. I isolated the first fan in one circuit and connected a pigtail and 75W light to it. It came on instantaneously. When I took it out of the circuit, the remaining fan was still slow in coming on. I took down the other lights and checked for a back-feed through the neutral. I found nothing. This circuit also had a bathroom exhaust fan (no light), a three-light fixture with CFLs and a fluorescent closet light. They came on instantaneously. I did not troubleshoot the third fan on a separate circuit. I called the manufacturer and was told there was a wattage limiter in the lighting circuit, and it was normal for a one-half to one-second delay. I find it hard to believe when the supplied lamps don’t exceed the 190W that the manufacturer specified. I do not know if the three fans that are operating properly have wattage limiters. Can you give me insight into this problem, if it is a problem?
It’s hard to troubleshoot from here. From your description, I don’t believe it’s a problem. According to the manufacturer, there is a wattage limiter, and the delay is normal. I can only refer you to 110.3(B) where listed equipment shall be installed and used in accordance with the manufacturer’s instructions included in the listing.
TROUT answers the Code Question of the Day on the NECA website. He can be reached at firstname.lastname@example.org.