Derating Type NM Cable, Fire Pump Power and More

Article 210 Branch Circuits

Article 250 Grounding and Bonding

Article 310 Conductors for General Wiring

Article 334 Nonmetallic-Sheathed Cable: Types NM, NMC and NMS

Article 410 Luminaires (Lighting Fixtures), Lampholders and Lamps

Article 695 Fire Pumps

Parts of the NFPA 20-2003-Standard for the Installation of Stationary Pumps for Fire Protection are also mentioned.

Derating type NM cable

Q: What is the reason for limiting the ampacity of type NM cable to the 60ºC column in Table 310.16? Where installed in raceways, 90ºC conductor ampacities are allowed to be derated from the 90ºC column in the table.

A: You must be using an old Code book. The 2005 edition of the NEC allows derating from the 90ºC ampacity column in Table 310.16.

This sentence appears in 334.80, which is part of Article 334-Nonmetallic-Sheathed Cable: Types NM, NMC and NMS: “The 90ºC (194ºF) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for 60ºC (140ºF) rated conductor.”

An exception to 336-30(b) was added to the 1996 NEC to allow derating of Type NM cable from the 90ºC ampacity column of Table 310-16, provided the derated ampacity did not exceed the ampacity for a 60ºC rated conductor.

Before the 1996 edition, derating from the 90°C column was not allowed.

On-site fire pump power supply

Q: Where backup electric power is supplied from an on-site generator to a fire pump, is this an emergency load, a legally required standby load or an optional standby load?

A Where the electric power supply to the fire pump motor is considered to be reliable by the authority having jurisdiction (AHJ), no additional power source is required. Methods and details for making the electrical connection to the fire pump motor controller are outlined in 695.3(A)(1). Qualifications for a reliable power source are mentioned in 695.3(A) of the NEC.

Where the AHJ determines that the power source is not reliable, a standby power source is required.

The on-site power source, however, must meet the following requirements.

The power source must comply with Section 6.4 and meet the requirements of Level 1, Type 10 Class X systems of NFPA 110, Standard for Emergency and Standby Power Systems.

The fuel supply for the engine driving the generator must be sufficient to provide eight hours of fire pump operation at 100 percent of rated pump capacity in addition to the fuel supply required for other loads.

Automatic sequencing of fire pumps is permitted, and the generator must have sufficient capacity to allow normal starting and running of the fire pump motor while supplying all other simultaneously operated loads.

Transfer of power to the fire pump controller between the normal supply and alternate supply must take place in the pump room. Where overcurrent protective devices are installed in the on-site power source circuit at the generator, they must allow instantaneous pickup of the pump room load.

The transfer switch must be electrically operated and mechanically held, and the transfer switch must be listed for fire pump service.

These are most of the requirements for an on-site generator and transfer switch. Additional information may be found in Chapter 10-Electric Drive Controllers and Accessories of NFPA 20 Standard for the Installation of Stationary Pumps for Fire Protection.

It does not make any difference what type of systems (emergency, legally required or optional) the on-site generator supplies, provided that the requirements of NFPA 20 and the appropriate Article (700, 701,702) in the NEC are satisfied.

GFCI laundry room receptacle

Q: Is ground-fault circuit interrupter (GFCI) protection required for a 125-volt, 15-ampere receptacle installed 18 inches above the floor and in back of a clothes washing machine in a one- family residence? The receptacle is located less than 6 feet from the laundry tub, but it is not readily accessible.

A: This is a new requirement in 2005 edition of the NEC. The 6-foot dimension extends in all directions from the outside edges of the laundry tub.

This is the way the requirement reads in 210.8(A)(7): “Laundry, utility, and wet bar sinks-when the receptacles are installed within 1.8 m (6 ft) of the outside edge of the sink.” The title for 210.8 is “Ground-Fault Circuit-Interrupter Protection for Personnel” and part (A) has the title “Dwelling Units.”

Notice there are no exceptions to this rule. It applies to all 125-volt, 15- and 20-ampere receptacles located within 6 feet of laundry and other types of sinks.

Receptacles for servicing air conditioning

Q: Is it necessary to install individual receptacles supplied from each apartment for servicing a group of 10 remote air conditioning condensing units installed outdoors, or may two receptacle outlets-one at each end of the group and less than 25 feet from the most remote condenser-be provided to satisfy the rule in 210.63?

A: Yes. It is necessary to install one receptacle from each tenant's interior wiring. Although you have satisfied the requirement in 210.63, you have not met the requirement in 210.25 that has the title: “Common Area Branch Circuits.”

Part of this paragraph has this sentence: “Branch Circuits in dwelling units shall supply loads only within that dwelling unit or loads associated only with that dwelling unit.”

Each tenant meter has to supply a receptacle next to the remote condensing unit for that apartment. The receptacle should be switched from within the apartment and should be turned off when not in use.

If there is a “house” meter for the apartment building that supplies exterior lighting of common areas, hallway lighting, fire alarm, etc., outdoor receptacles for the air conditioners may be supplied from this service, and the number of receptacles is based on the 25-foot requirement in 210.56.

In this case, two receptacles may be adequate.

Grounding electrode for remote building

Q: Does the National Electrical Code require a grounding electrode at a detached garage for a one-family dwelling that is supplied with an underground 120/240-volt, three-wire, single-phase branch circuit?

A A definition for a multiwire branch circuit appears in Article 100. A multiwire branch circuit is also permitted to be considered as multiple circuits by 210.4(A).

However, the exception to 250.32(A) allows a multiwire branch circuit to be considered as a single branch circuit in applying 250.32(A).

This is the way part of 250.32(A) reads: “(A) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system in accordance with 250.50.”

But the exception eliminates the requirement for a grounding electrode for this installation if an equipment-grounding conductor is included with the branch circuit conductors.

This is the exception: “A grounding electrode shall not be required where only a single branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the conductive non-current-carrying parts of equipment. For the purpose of this section, a multi-wire branch circuit shall be considered as a single branch circuit.”

Grounding receptacles

Q: I have to install 15-ampere, 125-volt receptacles in surface-mounted metal boxes. Where and what are the requirements for grounding these receptacles?

A: Generally, it is necessary to connect an equipment-bonding jumper to the grounding terminal of the receptacle and the grounded metal box, but there are variations to this rule.

The requirements for connecting the grounding terminal of a receptacle to a metal box are listed in 250.146(A) through (D). Part (A) applies to surface-mounted metal boxes, and allows metal-to-metal contact between the box and mounting yoke of the receptacle to serve as the equipment-bonding jumper if at least one of the insulating retaining washers on the receptacle mounting screws is removed.

The other parts of 250.146 deal with listed self-grounding receptacles, installations in floor boxes, and insulated grounding-terminal receptacles.

Ungrounded luminaires (fixtures)

Q: I have a job to replace the lighting fixtures in a single-story office building that is about 40 years old. The existing wiring method is Type NM cable without an equipment-grounding conductor. Does the NEC permit metal lighting fixtures where there is no equipment-grounding conductor?

A: Yes. Although the basic requirement is to ground exposed metal parts of luminaires (fixtures), there are exceptions.

The first exception allows an equipment-grounding conductor to be connected to the luminaire (fixture) where the other end of the equipment-grounding conductor is connected at an accessible point on the grounding-electrode system, the grounding-electrode conductor, the grounded conductor inside the service enclosure or to the grounding-terminal bar within the panelboard where the branch circuit originates.

A new exception removes the requirement for grounding replacement luminaires (fixtures) where GFCI protection is provided for the new luminaires (fixtures). Rules for grounding luminaires and the exceptions are located in 410.18. 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

Code Q&A Columnist
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 in...

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