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.
Plumber’s compound approval
When connecting polyvinyl chloride (PVC) terminal adapters to any hubs—be it for a meter enclosure, a 3R load center, or a Meyers type of hub—we are required to have plumber’s compound installed on the threads of the terminal adapter. This is to prevent water leakage, but the requirement does not apply when using the same hubs with rigid steel conduit. I feel that plumber’s compound is not an approved material for electrical work.
PVC terminal adapters are designed for connection to boxes or hubs using a suitable solvent-type cement. The manufacturer’s instructions [110.3(B)] must be followed for the proper use of this cement. I believe the plumber’s cement compound used with PVC is the same and is suitable for use with the PVC used as electrical raceways. When using ferrous metal conduit, electrical conductivity must be ensured, and 300.6 requires, where threaded in the field, the threads must be coated with an approved electrically conductive, corrosion-resistant compound. Thomas & Betts’ Kopr-Shield is commonly used for this purpose.
Car wash coin meter boxes
I was very interested in this question and answer in the October 2010 Code FAQs section. I have worked on many self-serve car washes and have occasionally seen these coin meter boxes become energized, usually 24 volts (V) but a few times with 120V. This has been caused by various circumstances, including, but not limited to, spare conductors “touching other higher voltage terminals in the pump control units, and the other end was touching the metal self service box, causing it to be energized.” Since your answer states that the boxes are not likely to become energized unless there is a problem covered by 110.12—Mechanical Execution of Work, would you give some examples of a part that is required to be grounded that only contains Class 2 circuits but is likely to be energized by a higher voltage source? Or to put it another way, what are examples of parts that 250.4(B)(4) would require me to bond?
Class 2 circuits have a power source, usually a listed Class 2 transformer. The transformer has a primary voltage generally rated at 120V. NEC 250.4(B)(4) relates to ungrounded systems and, similar to 250.4(A)(5), requires connecting (bonding) of noncurrent--carrying electrical equipment in a manner that establishes an effective ground-fault current path. Noncurrent-carrying electrically conductive materials that are “likely” to become energized must be bonded together and to the electrical supply source in a manner that establishes an effective ground-fault current path. I believe the use of the term “likely” is meant to show that where there are electrically energized conductors in the vicinity of electrically conductive materials, they could come in contact with each other, and the electrically conductive materials must be bonded together to establish an effective ground-fault current path. Examples of parts, which you are requesting, “that would require me to bond” are these electrically conductive materials in the vicinity of electrically energized conductors.
VSD, VFD clarification
In the November 2010 issue, you define a variable speed drive (VSD) as controlling a direct current (DC) motor by changing the voltage to the motor. In the next paragraph, last sentence, you write that a VSD can control the speed by the output frequency of the drive unit. Is this a misprint?
A VSD is a variable speed drive. This terminology can apply to both alternating current (AC) and DC motors. A VSD controls the speed of a DC motor by altering the voltage to the motor. A VSD controls the speed of an AC motor by varying the frequency to the motor and is more specifically referred to as a variable frequency drive (VFD).
Grounding in hot tubs
I cannot get a straight answer on what to do with the new Code rule of having 9 square inches of metal in contact with the water to bond to the equipment ground. Some people say this is ridiculous, inspectors cannot give a straight answer, and hot tub manufacturers have no idea. My inspectors said that either the manufacturer must have a metal strip in contact with the water, or we need to install a metal plate. Then I asked the inspectors how to go about this, and they had no idea. I also asked if they have ever inspected one of these jobs, and they replied that there are several open permits on hot tubs because no one knows what to do.
NEC 680.26(C), “Pool Water,” requires an intentional bond of a minimum conductive surface area of 9 square inches to be installed in contact with the pool water. This new requirement (2008 NEC) does not include hot tubs. The requirement for pools can consist of parts required to be bonded, such as ladders or underwater luminaires. If these parts are not available, there are devices that have been specifically listed for this purpose.
Knob and tube wiring
In Hawaii, we have many houses with knob and tube wiring installed and still in use. These houses are normally of the post-and-beam style and are about 2 to 3 feet above the ground. Now we are required to use Type UF cable when rewiring under the house area due to the presence of moisture. It was common practice for many years to use Romex Type NM-B cable. If moisture is a problem, the whole house should be rewired with UF-type cable. In Hawaii, the humidity level averages about 70 percent year-round. I can see places around the exterior of the house should use UF cable where water can get to the wire but not under the house.
NEC 334.12(B)(4) does not permit Type NM cable to be used in a wet or damp location. The area under the house is exposed to the weather, so Type NM cable is not permitted. If you feel the whole house should be wired with Type UF cable, the NEC does not prohibit it. The NEC contains minimum requirements for safety and does not prohibit using more stringent requirements.
Concealed knob-and-tube wiring is shown in Article 394 and is permitted for extensions of existing installations.
When installing a residential overhead service-entrance conduit, we used an offset nipple. This was rejected because of dissimilar metals between the aluminum offset nipple and the rigid-steel conduit being used. Is this correct?
NEC 344.14 permits aluminum fittings to be used with steel rigid metal conduit. According to the NEC Handbook, tests have shown that the galvanic corrosion at steel and aluminum interfaces is minor compared to the natural corrosion on the combination of steel and steel or aluminum and aluminum.
The disconnects you desire
We have an existing 500 kilovolt-ampere (kVA), 480V primary, 120/208V secondary transformer. The primary FLA is 602 amps (A); it is protected with a single disconnect fused at 600A and is drawing well under that. The secondary FLA is 1,388A and is protected with six disconnects, each fused at 200A for a total of 1,200A. We need to add a seventh 200A fused disconnect on the secondary for more equipment, so the secondary total would now be 1,400A. The primary load would still be under 80 percent of the 600A primary fuses. According to Table 450.3(B), since the primary is protected at less than 125 percent, secondary protection is not required with currents of 9A or more, so Note 2, which limits the secondary to six overcurrent devices, does not apply. Also, since the transformer is a separately derived system and not a service, the requirements of 230.71 limiting it to six disconnects would not apply. Of course, we need overcurrent protection on the secondary to satisfy 240.21, since it is a tap. Can we add that seventh disconnect or even an eighth as long as we watch the loads?
There is nothing to prohibit adding as many disconnects as you desire, as long as you provide transformer secondary protection in accordance with 240.21(C).
Microwaves and continuous loads
I have a question in regard to your response to the branch circuit for microwave oven question in the October 2010 issue. NEC 210.21(B)(2) limits the loads for two or more receptacles or outlets. What if you used a 15A circuit with a single receptacle for this microwave? Would you consider this a continuous load, in which case you would be limited to 12A for a 15A circuit?
A continuous load is “a load where the maximum current is expected to continue for three hours or more.” I don’t believe a microwave oven would operate for three hours. Check the manufacturer’s listing requirements for proper installation.
TROUT answers the Code Question of the Day on the NECA website. He can be reached at email@example.com.