CODE CITATIONS

Article 230            Services

Article 250            Grounding

Article 404            Switches

Article 408             Switchboards and Panelboards

Article 430            Motors, Motor Circuits and Controllers

Article 645            Information Technology Equipment

Article 680            Swimming Pools, Fountains and Similar Installations

Service disconnects for an apartment building

Q: An overhead service supplies a four gang meter base for an apartment building. Is it permissible to run four separate sets of service-entrance conductors to four disconnect switches that are accessible to each tenant or must the switches be grouped at one location that will not be accessible to all occupants?

A: The service disconnecting means for each apartment must be accessible to the occupant of that apartment. This is required by part (C) of 230.72, unless electric service and electrical maintenance are under continuous building management supervision as covered by the Exception to 230.72(C).

Where service-entrance conductors are run to each occupancy, the conductors must remain outside of the building or be encased in 2 inches of concrete within the building or be buried in at least 18 inches of earth beneath the building. Where the raceway emerges from the earth or concrete envelope, it should go directly to the service disconnecting means installed at a readily accessible location either outside or inside of the building at the nearest point of entrance of the service conductors. This requirement appears in 230.70(A)(1).

Grounding dimmer switches

Q:I have to install dimmer switches in conference rooms in a two-story office building. The switchboxes are plastic and nonmetallic faceplates will be provided for the dimmer switches. Are these switches required to be grounded even though nonmetallic faceplates are installed?

A: Yes, all dimmer switches must be grounded, and they must provide a means for grounding metal faceplates, whether or not a metal faceplate is installed. [See 404.9(B)]

Grounding electrode conductor taps

Q: The NEC permits taps to the main grounding electrode conductor where there are more than one service disconnecting means. May these taps be made with split-bolt connectors or must irreversible compression connectors or exothermic welding be used to connect the taps to the grounding electrode conductor?

A: Taps are permitted where the service consists of two to six disconnects in separate enclosures. This permission is granted under 250.64(D). The grounding electrode conductor must be sized for the largest service conductor as outlined in 250.66 and Table 250.66. Tap conductors are sized from the same table based on the largest conductor in the respective enclosure. As an example, let’s use a 400A service with 4—500 kcmil Type THWN insulated copper conductors in a wireway. From this wireway, two 100A, and three 60A disconnect switches are supplied. Each 100A switch is supplied by 2 AWG Type THWN insulated copper conductors and the 60A switches are fed from 4 AWG THWN insulated copper conductors.

According to Table 250.66, the grounding electrode conductor to the metal water pipe must be 1/0 AWG copper for the 500 kcmil conductors. This grounding electrode conductor must be unbroken (without a splice) from the grounded service conductor in the wireway to the metal water pipe. To this 1/0 AWG grounding electrode conductor, 8 AWG grounding electrode conductors are run from each disconnect to the 1/0 AWG grounding electrode conductor. Since these tap conductors are 8 AWG, they must be protected from physical damage by installation in metal raceways, rigid nonmetallic conduit, EMT or cable armor. A larger grounding electrode conductor can be used to avoid having to provide protection of the tap conductors from physical damage.

I believe that properly sized split bolt connectors could be used for these taps, although 250.30(A)(3) requires irreversible compression type connectors and exothermic welding for grounding electrode conductor taps used for separately derived alternating current systems.

If the grounding electrode conductor —the 1/0 AWG copper wire in our example — is spliced, 250.64(C) requires that the splice be made by irreversible compression type connectors or by exothermic welding.

Although irreversible compression type connectors or exothermic welding is preferred, there is nothing in 250.64 that prevents the use of properly sized split-bolt connectors for this application.

Wiring swimming pool pump motors

Q: I recently attended a Code Seminar. During one part of the program the instructor said that nonmetallic sheathed cable could not be used to supply a swimming pool pump motor. Is this true for all installations of permanently installed swimming pools?

A: The instructor is correct for all installations of swimming pools except those that are part of a one-family dwelling. This is the way part of 680.21(A) reads: “The branch circuit for pool-associated motors shall be installed in rigid metal conduit, intermediate metal conduit, or Type MC cable listed for the location. Other wiring methods and materials shall be permitted in specific locations or applications as covered in this section.”

Part (4) of 680.21 deals exclusively with one-family dwellings, and reads like this: “In the interior of one-family dwellings, or in the interior of accessory buildings associated with a one-family dwelling, any of the wiring methods recognized in Chapter 3 of this Code shall be permitted that comply with the provisions of this paragraph. Where run in a raceway, the equipment grounding conductor shall be insulated. Where run in a cable assembly, the equipment grounding conductor shall be permitted to be uninsulated, but it shall be enclosed within the outer sheath of the cable assembly.”

Where the swimming pool pump motor is in a utility room, pump room, accessory building or similar enclosed space in a one-family dwelling, 12 AWG copper or larger nonmetallic sheathed cable may be used to supply the pump motor. All of the applicable requirements in Article 334—Nonmetallic Sheathed Cable apply to this installation.

Maximum number of half-size circuit breakers in a panelboard

Q: Is there a limit to the number of “piggy-back” circuit breakers that are allowed in a 24-space panelboard?

A: Yes, a Class CTL panelboard is marked to indicate how many circuit breaker poles can be installed. Class CTL is the designation used to identify a panelboard as ‘circuit limiting.’  In these panelboards, only certain positions in the panelboard will allow the use of tandem, half-size or piggy-back circuit breakers. For example, a 125A, 3-wire, single phase, Class CTL panelboard with 20 full-size circuit breaker spaces may be marked to allow two or more tandem circuit breakers.

Class CTL panelboards are the result of a requirement that appears in 408.15. This is the sentence that requires a limit to the number of overcurrent devices in a panelboard: “A lighting and appliance panelboard shall be provided with physical means to prevent the installation of more overcurrent devices than the number for which the panelboard was designed, rated, and approved.”

Although non-CTL tandem circuit breakers are available and will fit in any space in a CTL panelboard, they are marked “For Replacement Only, Not CTL Assemblies.” These circuit breakers are for use in existing old style non-Class CTL assemblies only.

The number of piggy-back circuit breakers that is permitted in a Class CTL lighting and appliance panelboard is marked on the product, and non-CTL tandem circuit breakers are not permitted.

Size of service conductors

Q: What is the minimum size service-entrance conductors for a 15A branch circuit that supplies a school zone warning light?

A: A 15A rating for the service entrance conductors for a one-circuit installation that supplies a limited load satisfies 230.79(A). And 230.42(B) requires that service-entrance conductors have an ampacity that is not less than the ampere rating of the disconnecting means. Therefore, 14 AWG copper conductors are large enough.

If it is necessary to run overhead service drop conductors from a meter pole to the service disconnect, the exception in 230.23(B) allows this wire to be not smaller than 12 AWG hard drawn copper or equivalent. For an underground service lateral, the smallest size wire is 12 AWG copper or 10 AWG aluminum or copper clad aluminum. This information is found in 230.31(B).      

Design B energy efficient motors

Q: Are Design B Energy Efficient Motors treated the same as Design E Motors for sizing branch circuit conductors, and disconnecting means?

A: No, they are not. Although motor branch circuit conductors for energy efficient Design B and Design E motors are each sized at 125 percent of Table value of full-load current for continuous duty motors, an instantaneous trip circuit breaker that is part of a listed combination motor controller is permitted to have a trip setting of 1100 percent with permission to increase this setting to 1700 percent for Design B and E motors where it has been demonstrated that this setting is necessary. Exception No. 1 to 430.52(C)(3) permits this increase. It is my understanding the Design E motors are not being manufactured.

Use of liquid-tight flexible metal conduit in computer rooms

Q: Recently you wrote that liquid-tight flexible metal conduit could no longer be installed in air handling ceilings to supply electrical equipment in that space. Does this same prohibition apply to under raised floor spaces in a data processing room?

A: The previous question asked if liquid-tight flexible metal conduit in lengths not exceeding six feet could be used in the space above a suspended ceiling that is used for movement of environmental air. The answer is no. Liquid-tight flexible metal conduit is no longer acceptable as a wiring method in these spaces. But this restriction does not apply to wiring under a raised floor in a computer room that meets the special requirements for an Information Technology Equipment Room as outlined in 645.2. Wiring under a raised floor in this room may be installed in unlimited lengths of liquid-tight flexible metal conduit. There is no restriction on the use of this product. Other wiring methods are permissible and mentioned in 645.5(D). EC

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.