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.
Boat dock conduit
I am installing polyvinyl conduit (PVC) on a boat dock. The conduit run is in a straight line and is 90 feet long. Do I need PVC expansion joints?
Expansion fittings for rigid PVC must be provided to compensate for thermal expansion and contraction where the length change, in accordance with Table 352.44, is expected to be 1/4-inch or greater in a straight run between securely mounted items or other conduit terminations. You can use the tables, interpolating for the length of your run. If your area could reasonably be expected to change from 30°F to 90°F for your 90-foot run, you would use ⁄ (90%) of the 100-foot figure shown corresponding to the temperature change expected in your area. A temperature change of 60° would result in a length change of 2.43 inches per 100 feet or 2.43 0.90 = 2.19 inches. This adjustment exceeds 1/4-inch because the tables only show the length change of PVC conduit in the inch change per 100 feet, which doesn’t mean any run less than 100 feet won’t need expansion fittings to compensate for length change.
Safety in mind
I have a buyer that wishes to purchase a 1929 home in Burlington, N.C. The home inspector noted that multiple ground-fault circuit interrupter (GFCI) receptacles located inside the house do not have an operating ground, and multiple three-prong receptacles have an open ground. As part of the repair negotiations, the sellers were asked to ground all GFCI receptacles. Their electrician stated that the outlets were not required to be grounded, just tagged/labeled, and that is all he did. He supplied a paragraph from his NEC book, “(C) Ungrounded Conductors.” Is his statement that the [three-prong outlets, GFCIs] can have an open ground or nonoperating ground acceptable in the NEC guidelines? Both my first-time homebuyer and I need to know that they will be safe. My clients want to know that this home is up to Code. Are GFCI receptacles that have a nonoperating ground and open ground receptacle outlets acceptable and to Code?
Based on the information you have supplied, the electrician is correct in his interpretation of the 2011 NEC requirements. Section 406.4(D)(2)(c) requires the marking the electrician provided. Basically, this means nongrounding-type receptacles are permitted to be replaced with grounding-type receptacles where supplied through a GFCI. These grounding--type receptacles supplied through a GFCI must be marked “GFCI protected” and “No equipment ground.” An equipment-grounding conductor must not be connected between these grounding-type receptacles. You could check with the local authority having jurisdiction at the municipal building department for further verification.
Heating cable in pool deck
I have a customer who wants me to install heating cable in the deck around a pool. Is this permitted?
No, no, no! NEC 680.27(C)(3) does not permit radiant heating cables to be embedded in or below a pool deck within 20 feet from the inside walls of the pool. Doing so might make the pool deck an involuntary dance floor!
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 is based on the theory that 42.25 circular mils (cm) of copper conductor will carry 1 ampere (A) of current for 5 seconds, after which the conductor will destroy itself. A 6 AWG copper conductor is equal to 26,240 cm divided by 42.25 cm = 621A of current. A lightning stroke 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.
At a farm in Arizona, there was an irrigation pump rated three-phase, 480 volts (V), 125 horsepower, part-winding motor. The main service, a 200A switch, had a 1 AWG bare conductor landed from the mechanical ground (box) to the L2 lug on top of the service. The center leg is totally grounded. With a meter, I read as follows: L1 to L2 = 495V, L2 to L3 = 496V, L1 to L3 = 496V, L1 to ground = 279V, L2 to ground = L3 to ground = 280V. What’s the name of this type of service? I have never heard of this way of grounding on an electrical system. Where can I obtain more information about this service?
The type of service you are describing is a 480V corner-grounded delta. The phase-to-phase voltage readings you show are correct nominal voltages for that system. You don’t indicate where you took your readings to get the voltages--to-ground you show, but I have to assume they were taken at a reduced voltage starter or a part-winding contactor. Information relating to part-winding motors can be found in NEC 430.4. You can also get some good information regarding part-winding motors in my book, “Essentials of Electric Motors and Controls,” published by Jones and Bartlett for the National Fire Protection Association.
How much free conductor?
Am I required to leave 6 inches of conductor extending from a box where I am just pulling through the box? In other words, I’m not terminating at a device installed in that box.
NEC 300.14 requires at least 6 inches of free conductor, measured from the point in the box where it emerges from the raceway or cable sheath, to be left at each outlet box and be long enough to extend at least 3 inches outside the box opening. If the conductors are not terminated or spliced at the outlet box, there’s an exception to this requirement. This exception permits that these conductors can be pulled in tightly. However, I always followed the “better safe than sorry” philosophy and left enough slack so I could use the conductors in the future if necessary. NEC 314.16(B)(1) permits each conductor that passes through the box without termination or splice to be counted once but doesn’t restrict the length of the conductor loop.
Stabilizing the voltage-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.
What is ‘it’?
Why does 230.91 call “it” the service overcurrent device if “it” doesn’t protect the service-entrance conductors or service conductors, whatever you call them, from short circuit or ground fault?
Because “it” is an overcurrent device that acts as an overload device for the service-entrance conductors. However, it does provide a double function since what is called the service overcurrent device actually serves as overcurrent protection for feeder or branch circuits on the load side of the service equipment.
Temperature rating and ampacity
If we connect to equipment that specifies 75 degree-rated terminals and internal wiring, can we use 90-degree rated conductors at comparable amperage rating? An example would be 500 MCM wire at 75 degrees is 380A versus 400 MCM wire at 90 degrees is 380A, or do we use 500 MCM wired 90 degrees derated to the 75 degree rate? Why or why not? What is the logic behind this?
If you have equipment rated at 75°C, you are permitted to use 90°C rated conductors provided the ampacity of the conductors is based on the 75°C ampacity of the conductor size used. The logic is that the temperature rating associated with the ampacity of a conductor shall be selected so as not to exceed the temperature rating of the connected termination. This information may be found in 110.14(C)(1)(a)(3) or 110.14(C)(1)(b)(2).
TROUT answers the Code Question of the Day on the NECA website. He can be reached at firstname.lastname@example.org.