Derating Cables, Metal Faceplates, Grounding Parking Lot Lighting Poles and More

By George W. Flach | Oct 15, 2004
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Article 210 Branch Circuits

Article 250 Grounding

Article 310 Conductors for General Wiring

Article 320 Armored Cable: Type AC

Article 404 Switches

Derating Cables

Q: Is it necessary to derate types NM-B, AC or MC cables where these cables are bundled to enter a branch-circuit panelboard?

A: Generally, where cables are bundled for more than two feet without maintaining spacing between them, derating of the contained conductors is required.

For NM-B cables, derating factors in Table 310.15(B)(2)(a) may be applied to the ampacities shown in Table 310.16 under the 90 C column provided the adjusted ampacity does not exceed the value shown in the 60 C column. This is what part of 334.80 allows: “The 90 C (194 F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60 C (140 F) rated conductor.”

There is also an exception to 310.15(B)(2) which allows bundling of Types AC and MC cables. This exception No. 5 allows these cables to be bundled without derating, where each cable has not more than three current-carrying conductors; conductor size is 12 AWG copper; and there are not more than 20 current-carrying conductors in the bundle. Cables with four wires (three-phase wires and a neutral) may be used provided that the conductors are connected to a three-phase, four-wire wye connected panelboard, and the branch circuits supply single-phase linear loads. The point being emphasized is that the cable assembly may contain more than three wires provided that not more than three wires are current-carrying conductors.

Metal Faceplates for Switches

Q: Is there a requirement to ground metal faceplates that are used on dimmer switches, paddle-fan speed controllers and spring-wound timer switches?

A: Yes, there is. The rule for grounding switches is found in 404.9(B). This is what it says: “Grounding. Snap Switches, including dimmer and similar control switches, shall be effectively grounded and shall provide a means to ground metal faceplates, whether or not a metal faceplate is installed.” An exception allows a snap switch without a grounding connection as a replacement where there is no grounding means in the switchbox and a nonmetallic faceplate is installed.

Load Calculations for a Refrigerator

Q: How much load [volt-amperes (va)] should be added to the service calculations for a one-family dwelling where the refrigerator is connected to a dedicated 15-ampere branch circuit? Should I use 180 va or the nameplate rating on the refrigerator?

A: Don’t use either one. The load calculation of 180 volt-amperes for each receptacle outlet applies to nondwelling units. You are required to add 1,500 volt-amperes for each small appliance branch circuit to the calculations for feeder and service-entrance conductors. This is required by 210.16(A). An exception to 210.16(A) indicates no load for the refrigerator is required even though a separate circuit is provided for it.

Because the Code permits connecting the refrigerator to one of the small appliance branch circuits without adding this load to the calculations for the appliance branch circuits, a separate circuit for the refrigerator does not increase the load on the service-entrance conductors.

Supporting Type AC Cable

Q: The National Electrical Code allows AC cable to be run a maximum of six feet from the last point of support to a luminaire in an accessible ceiling. Is an AC cable connector at an outlet box or a luminaire considered to be the last point of support?

A: Part (B)(3) of 320.30 uses the words, “…from the last point of support …,” for allowing six feet of armored cable to be unsupported. These are the words: “Type AC cable shall be permitted to be unsupported where the cable: (3) Is not more than 1.8 meters (six feet) from the last point of support for connections within an accessible ceiling to luminaire(s) [(lighting fixture(s)] or equipment.”

The words “… from the last point of support …” caused interpretation problems. Does a cable connector provide any support for the cable? This is the question. A revision for the 2005 edition of the NEC clears this up. It reads like this: “For the purpose of this section, Type AC cable fittings shall be permitted as a means of cable support.”

In my opinion, a Listed, properly sized AC cable connector provides support for the cable. Therefore, I consider a connector as an acceptable means of support, and the 2005 edition will make this clear.

Grounding Parking Lot Lighting Poles

Q: An electrical contractor has a job to install 20 metal lighting poles on a car parking lot at a shopping center. The electrical designer has specified a ground rod be driven at each pole. He said an equipment-grounding conductor run with the branch circuit conductors is not necessary because of the ground rod at each pole. I do electrical plan review and have turned the job down because of the lack of an equipmentgrounding conductor. An elected official is putting pressure on me to approve the plans and specifications as submitted. Will you provide me with some information that will support my position?

A: Yes, improper grounding as described could result in serious electric shock or electrocution to anyone who contacts a lighting pole that has an internal phase-to-ground fault caused by an insulation failure on an ungrounded branch-circuit conductor. An uninsulated phase conductor in the pole or luminaire in contact with the pole or luminaire will energize the pole to line potential whenever the branch circuit is energized. The pole will remain energized until it is touched by someone, the incident is reported and repairs are made.

Let’s assume the eight-foot rod has a resistance to earth of 20 ohms. This is actually a low resistance for a single ground rod. The branch circuits supplying the parking-lot lighting are 120 volts protected by 30-ampere overcurrent devices. The maximum current will be (120 divided by 20) six amperes should a ground-fault in a lighting pole occur. Because the load is continuous, not more than 24 amperes of lighting is permitted on each circuit. Add six amperes to this figure and a total current of 30 amperes results. If 40- or 50-ampere branch circuits are used, the total current during a phase-to-ground fault while the circuit is energized would be 38 amperes on a 40-ampere branch circuit or 46 amperes on a 50-ampere branch circuit. These figures assume that the branch circuits are not loaded to more than 80 percent of their ampere ratings as required by 210.20(A).

There are at least two places in the NEC that contain these words: “The earth shall not be used as the sole equipment-grounding conductor or effective ground-fault current path.” [See 250.4(A)(5).] Part 250.54 is new in the 2002 edition and covers grounding as described in the question. The title is “Supplementary Grounding Electrodes” and points out these grounding electrodes must be connected to equipment-grounding conductors. Here again is the phrase, “… the earth shall not be used as the sole equipment-grounding conductor.”

Construction requirements for metal poles that support lighting fixtures are in 410.15(B).

Bathroom Receptacles in Hotels and Motels

Q: Is a bathroom receptacle required where part of a sleeping room is in a hotel or motel? Does a bathroom receptacle have to be protected by a GFCI? Must this receptacle be on a separate 20-ampere branch circuit?

A: All 15- and 20-ampere, 125-volt single-phase receptacles installed in bathrooms in other than dwelling units must be protected by ground-fault circuit-interrupters. This is required by 210.8(B)(1).

There is no requirement for a receptacle in the bathroom in the NEC. Therefore, the ampere rating of the branch circuit could be 15, and other loads in the guest room could be supplied by the circuit that supplies the bathroom receptacle(s).

Grounded Service Conductor for a Fire Pump Service

Q: A fire pump controller listed for use as service equipment is supplied by a 208Y/120-volt service lateral from a utility transformer. Is a neutral conductor required even though the only load is a three-phase fire pump motor?

A: Yes, four conductors are required for this service even though there is no load connected to the neutral. The neutral must be included with the phase conductors to provide a low impedance path to clear phase-to-ground faults.

The size of the grounded conductor is based on Table 250.66 until a ungrounded service-entrance conductor size exceeds 1,100 kcmil copper or 1,750 kcmil aluminum, then the grounded conductor must be at least 12.5 percent of the area of a phase conductor. This information is in 250.24(B) and (B)(1).

The grounded service-entrance conductor is terminated on the neutral bus in the controller, and a grounding-electrode conductor sized according to Table 250.66 grounds the neutral to a metal cold-water pipe and other available electrodes.

Grounding and Bonding at Service Equipment

Q: A rigid-metal conduit nipple is installed between a meter socket and a weatherproof service-disconnecting means. The conduit nipple is screwed into the top of the disconnect switch. The service neutral is grounded in the service disconnect. Since the neutral terminal in the meter socket is bonded to the enclosure, the conduit nipple and neutral conductor are in parallel. Is this a violation of the National Electrical Code?

A: Some people in the electrical industry may say this arrangement causes objectionable current flow and violates 250.6 of the NEC, but I don’t think so. The grounded circuit conductor may be used to ground equipment on the supply side of the service-disconnecting means by 250.142(A). Bonding of services is required by 250.92. Parts IV and V also contain various rules for grounding and bonding of service equipment. EC

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


About The Author

George W. Flach was a regular contributing Code editor for Electrical Contractor magazine, serving for more than 40 years. His long-running column, Code Q&A, is one of the most widely read in the magazine's history. He is a former chief electrical inspector for New Orleans and held many other prestigious positions in the electrical industry, including IAEI board of directors and executive committee. He passed away in August 2009.





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