Outlet Spacing, Parallel Conductors and More

If you have a Code-related problem or are experiencing difficulty in understanding a National Electrical Code (NEC) requirement and are wondering why or if such a requirement exists, ask Charlie. And we will let the Code decide.

Does conduit less than 3 feet long between boxes or enclosures—e.g., from a junction box to a motor control center—need to be supported? Is there a length at which a conduit support is not needed?

Sections 342.30 (RMC), 344.30 (IMC) and 358.30 (EMT) show the requirements for securing and supporting conduit. Conduit is required to be supported within 3 feet of each outlet box or other conduit termination. Within means, conduit 3 feet or less in length requires a support. However, if the conduit is 18 inches or less, without a coupling and where oversized, concentric or eccentric knockouts are not encountered, supports are not required. This means that you cannot use short nipples and couple them together to a length 18 inches or under without providing a support.

Is it permitted to wire recessed fluorescent luminaires by feeding one from the switch and then feeding from one luminaire to another? Are the luminaires required to be identified for feed-through wiring?

Yes, luminaires can be wired from luminaire to luminaire. However, according to Section 410.21, branch-circuit wiring other than the circuit supplying the luminaire is not permitted to pass through the luminaire outlet box unless the luminaire is identified for “through wiring.”

Regarding Section 312.5(C)(Exception)(b), what would constitute a structural ceiling if drywall is not considered to be structural?

A drywall ceiling fastened to ceiling joists is considered to be a structural ceiling. An example of a nonstructural ceiling would be a suspended ceiling.

Why would you need to have a three-way switch to an unfinished basement? It seems silly that you would turn out the light while you are down there doing laundry or something. It seems to me that there should be an exception to this rule.

Section 210.70(A)(2)(c) requires where lighting outlets are installed for interior stairways, there shall be a wall switch at each floor level and landing level that includes an entryway. It is intended to control the lighting outlets where the stairway between floor levels has six risers or more. It seems the requirement is for the stairway lighting, which may or may not be the lighting necessary for what you are doing down there. It seems that maybe it might be nice to turn off an unnecessary light, and certainly, it makes sense to have control of stairway lighting from either the top or bottom of the stairway.

My question concerns outlet spacing on kitchen countertops. A designer has asked for all countertop receptacles to be located under the upper cabinets at 18 inches. My question is what horizontal spacing should I use, given my understanding the outlet is to be reached by a 2-foot appliance cord?

How long did it take you to work out the details so that the Code requirements wouldn’t work? However, there is generally only 16 inches or less between the cabinet and the countertop back-splash, so this situation will not come up. Section 210.52(C)(1) requires receptacle outlets to be installed so that no point along the wall line is more than 24 inches measured horizontally from a receptacle outlet in that space. If this doesn’t work out for you, install additional receptacles. The NEC rules are minimum requirements.

Does NEC article 240.21(C)(4) allow me to run transformer-secondary conductors without overcurrent protection at the transformer secondary if they terminate to a distribution- board main circuit breaker with any number of branch-circuit breakers?

Yes, however, according to 240.21(C)(4), the main circuit breaker must limit the load to the ampacity of the transformer- secondary conductors. This single overcurrent device is permitted to supply any number of additional overcurrent devices on its load side.

I have a question about walls that have built-in cabinets or bookshelves. Are floor receptacles required in these instances? Is it OK to locate receptacles inside the storage cabinets?

Receptacle outlets specified in Section 210.52(A)(2) are required in wall spaces. The intent is that a lamp or an appliance can be placed anywhere at a wall space and be within 6 feet from a receptacle outlet. Where built-in cabinets occupy the wall space, floor receptacles are not required. Receptacles are permitted to be installed, but are not required to be installed, inside wall cabinets.

Whenever I install parallel conductors for a three-phase service, I install two conduits: one conduit with three current-carrying conductors and a neutral and the other conduit with three current-carrying conductors and a neutral. If I were to install all service conductors in the same conduit, there would be six current-carrying conductors. Would I have to adjust the ampacity of the conductors to 80 percent per Section 310.15, or do service conductors not count for the adjustment factor?

Section 310.15(B)(2) states, where the number of current-carrying conductors exceeds three, the allowable ampacity of each conductor must be reduced as shown in Table 310.15(B)(2)(a). In your example, the six current-carrying conductors would be required to be adjusted to 80 percent. Service conductors are included in this requirement. Be sure to note the requirements of Section 310.4(A), (B), (C) and (D) for paralleling of conductors.

Is it permissible to run different voltage rating systems in the same conduit, such as 120/208/480 volts?

Yes, Section 300.3(C) permits conductors of different systems to occupy the same raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the raceway. Where the conductors of a three-phase, 4-wire, 208Y/120-volt circuit and a three-phase, 4-wire, 480Y/277-volt circuit occupy the same raceway, all conductors must be insulated for the maximum voltage, which would be 480-volts, and 600-volt insulation would be suitable.

Which receptacles in a residential kitchen should be ground-fault circuit interrupter (GFCI)-protected?

The NEC Section 210.8 contains the requirements for GFCI protection. Section 210.8(A)(6) requires ground-fault protection where this protection must be supplied for kitchens where the receptacles are installed to serve countertop surfaces.

According to Section 240.4 (B) and (C), the next higher standard overcurrent-protective device (above the ampacity of the conductors being protected) shall be permitted to be used up to 800 amps. Why were 800 amps selected and not another amperage? Why is it limited to 800 amps?

According to Section 240.6, the standard ampere ratings for conductors increases in comparatively small increments up to 800 amperes. Conductor ampacities increase in much larger segments after 800 amperes, i.e., 800 to 1,000 to 1,200 to 1,600, and it was determined that a short circuit or ground fault may not raise the current flow quickly enough to open the overcurrent device before damage to the conductor would occur.

It seems to me that the correct interpretation of 314.23 (B)(1), regarding nails and screws in and through boxes for support, would not restrict the use of screws for support as long as there are no threads exposed inside the box. The wording appears to be restricted to only the threads and not the screw heads being exposed on the inside of a box. We have inspectors in this area who believe it means you can’t use screws for support at all. What’s your take on this section?

Section 314.23(B)(1) permits the use of screws as a fastening means for a box where the screws pass through the interior of the box within inches of the back of the box. Screws are not permitted to pass through the box unless exposed threads in the box are protected using approved means to avoid damage to the conductor insulation.

TROUT answers the Code Question of the Day on the NECA Web site. He can be reached at letthecodedecide@earthlink.net.

About the Author

Charlie Trout

Code Contributor
Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s Cod...

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