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.
Grounding well casings
The inspector made me ground a well casing to the grounding conductor. It seems to me that the casing is grounded already. Is he right?
Section 250.112(M) requires that, “where a submersible pump is used in a metal well casing, the well casing shall be bonded to the pump circuit equipment-grounding conductor.” If the well casing were not bonded to the equipment grounding conductor and the ungrounded pump circuit conductor were to accidentally energize the well casing, the only path for the ground-fault current would be through the earth to either or both the building grounding electrode conductor and the grounding electrode conductor at the utility transformer location. Both 250.4(5) and 250.54 state, “The earth shall not be continued as an effective ground-fault current path.” The impedance of the earth is too great to permit a sufficient current flow to open the circuit overcurrent device. Please note that while there is not sufficient current flow to open the circuit overcurrent device, there is sufficient current flow to cause severe electric shock or electrocution to a person who becomes part of the load by touching the energized well casing and the earth simultaneously.
An electrical inspector told me that a dry-type transformer could be hung (with all-thread and Unistrut) as long as the rating was under 50 kilovolt-amperes (kVA). The inspector stated that, if the transformer were more than 50 kVA, it would have to be mounted on the ground or on a structure similar to a catwalk system. My current project has prints showing all transformers 75 kVA and under to be hung from the ceiling. I have searched the NEC and transformer books and have come up empty-handed regarding transformer-mounting requirements. I would also like to know of other transformer-mounting requirements that you know of. For instance, when does a transformer need to be mounted on a 4-inch concrete housekeeping pad?
Transformer installation requirements, covered in Article 450 of the NEC, must be studied carefully. General requirements for mounting and cooling of electrical equipment are found in Section 110.13. Dry-type transformers 600 volts (V) or less can be located in the open on walls, columns or structures and are not required to be readily accessible [450.13(A)]. This means that ladders, lifts or other portable means can be used to access the transformer (see “readily accessible” in Article 100—Definitions). Dry-type transformers 600V or less and not exceeding 50 kVA are permitted to be installed in hollow spaces of buildings, such as above dropped ceilings, and are not required to be readily accessible [450.13(B)]. The 75 kVA transformers you reference can be hung in the open on walls, columns or structures and are not required to be readily accessible, but they cannot be installed in hollow spaces of buildings [450.13(B)]. If you have an NEC Handbook—and you should—check out the commentary on 450.13 for information regarding hollow spaces used for environmental air and other pertinent information. Housekeeping pads are a very workmanlike procedure and, in my opinion, should be included in every floor-mounted installation of dry-type transformers, but this is neither a safety-oriented nor an NEC requirement.
Conductor rating requirements
If we connect to equipment that specifies 75°C-rated terminals and internal wiring, can we use 90°C-rated conductors at comparable amperage rating? For example, 500 kcmil wire at 75°C is 380A versus 400 kcmil wire at 90°C is 380 amperes (A). Or do we use 500 kcmil wired 90°C derated to the 75°C 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).
When upgrading an older kitchen with existing 15A countertop receptacles, adding a couple of new ones, do they have to be 20A, or can I just extend the 15A?
If you are upgrading the kitchen appliance circuits you must use 20A-rated conductors, and you are permitted to use 15A-rated receptacles.
Sharing neutral conductors
In regard to your answer on page 29 of the February 2013 issue, when I read Code Section 210.4, it seems to me the contractor will need either a neutral for each circuit or to tie the handles together. I hope you are correct, but I am looking for the section or exception that allows this.
I believe you are referring to the section on “Sharing Neutral Conductors.” If you share a neutral with two ungrounded conductors, 210.4(B) requires you to provide a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates. An informational note directs you to 240.15(B) for information on the use of single-pole circuit breakers as the disconnecting means. NEC 240.15(B)(1) permits the use of individual single-pole circuit breakers with identified handle-ties as the protection for each ungrounded conductor of multiwire branch circuits.
Storage battery power
If I use a storage battery for supplying emergency lighting and power, how long must the battery be capable of maintaining adequate power?
NEC 700.12(A) requires that a storage battery supplying emergency lighting and power shall maintain not less than 87½ percent of full voltage for 90 minutes. Section 701.12(A) has the same requirement for legally required standby systems.
Panelboard vs. load center
What is the difference between a panelboard and a “load center”?
A panelboard is a panel or assembly with automatic overcurrent devices designed to be installed in a cabinet or cutout box mounted in or against a wall with accessibility from the front only. The panelboard is the interior assembly that is mounted in an enclosure. They may be furnished to the job together, or the enclosures may be shipped separately to the job for installation during the rough-in with the panelboard being shipped for installation during the trim stage. Load centers are not defined in the NEC and are not listed as load centers. A load center listing is the same as that of a panelboard except that they are assembled and shipped as one unit. The name “load center” is a manufacturer’s designation for that type of a panelboard. The NEC requirements for load centers are the same as for panelboards and panelboard enclosures.
NM box requirement
Are nonmetallic boxes required when using type NM cable?
No; they are permitted but not required. NEC 334.40(A) permits nonmetallic outlet boxes as provided by 314.3.
20A bathroom circuit
Can I connect other equipment in a bathroom, such as lighting or an exhaust fan, to the bathroom receptacle circuit?
Yes; there is an exception to 210.11(C)(3), which states, if the bathroom circuit supplies only one bathroom, other equipment within the same bathroom may be connected to the 20A bathroom circuit.
Overcurrent protection correction
On page 30 of the April 2013 edition, is the reference to Section 408.16(D) a typo?
Good catch. The reference should be 408.36(B) where overcurrent protection for a panelboard supplied through a transformer shall be located on the secondary side of the transformer.