Derating Conductors, Installing Outlets in Dwelling Units and More

Article 100 Definitions

Article 210 Branch Circuits

Article 240 Overcurrent Protection

Article 250 Grounding

Article 310 Conductors for General Wiring

Article 422 Appliances

Article 680 Swimming Pools, Fountains, and Similar Installations

Derating Parallel Conductors

Q: I installed a 400-ampere, 4-wire feeder from a 208Y/120 volt service to supply a panelboard for a store in a shopping center. The calculated load is 392 amperes. The feeder consists of two sets of 3/0 AWG Type THWN copper conductors for the phase wires and two 1/0 AWG copper conductors for the neutral, all in a single 21/2-inch electrical metallic tubing. The maximum unbalanced neutral load is 165 amperes.

The electrical inspector said the feeder conductors are too small because their ampacities have to be derated to 80 percent to comply with Table 310.15(B)(2)(a). He said the adjusted ampacities of the conductors is 320 amperes. I pointed out 310.4 in the National Electrical Code that indicates parallel conductors are considered a single conductor, but he did not accept this explanation. Is the inspector's interpretation of the Code correct?

A: Yes, it is. You are also correct in your statement the 310.4 considers parallel conductors that are joined together at both ends to be a single conductor. However, for derating purposes where there are more than three current-carrying wires in a raceway, the ampacities of conductors as shown in Table 310.16 must be adjusted downward.

Let's compare the heat losses of parallel conductors with that of a single conductor carrying the total current.

Although EMT is mentioned in the question, we will use the AC resistance for the conductors in steel conduit, as given in Table 9 of Chapter 9 for comparison purposes only. We will also assume a load of 400 amperes on each phase. For the 3/0 AWG copper conductors I2R loss is (200 ¥ 200 ¥ 0.079) 3,160 watts per 1,000 feet. Since there are two conductors per phase, the loss per phase, is (2 ¥ 3,160) 6,320W. For a single 600 Kcmil copper conductor carrying 400-amperes, the loss is (400 ¥ 400 ¥ 0.025) 4,000 watts. The loss in the two parallel 3/0 AWG copper conductors is more than 1.5 times the loss in a single 600 Kcmil conductor carrying the total current.

If we derate the ampacity of a 250 Kcmil copper conductor with 75 C insulation to 80 percent of the ampacity shown in Table 310.16 (255 ¥ 0.80), which is 204, and calculate the losses of two 250 Kcmil conductors in parallel (200 ¥ 200 ¥ 0.054) 2,160W per conductor or 4,320W for two per 1,000 feet, the losses are now close to the loss in a single 600 Kcmil conductor carrying 400 amperes.

This example shows that it is necessary to derate conductors carrying current whether they are in parallel or not to prevent overheating.

The neutral conductor was not considered in these calculations because 310.15(B)(4) indicates that a neutral conductor that carries only the unbalanced current from other conductors of the same circuit is not considered to be a current-carrying conductor for derating purposes.

Receptacle Outlets in Dwelling Units

Q: Does the National Electrical Code require the installation of receptacle outlets in the walls of a large foyer in a single-family residence?

A: Foyers are not mentioned in the NEC, however, 210.52(A) uses this language for requirements for receptacle outlets in various rooms or areas in a dwelling unit: “In every kitchen, family room, dining room, living room, parlor, library, den, sun room, bedroom, recreation room, or similar room or area of dwelling units, receptacle outlets shall be installed in accordance with the general provisions specified in 210.52(A)(1) through (A)(3).” Notice that part of this sentence that reads: “ ... or similar room or area ... ”. Among other things, a foyer is defined as an entrance hall in Webster's dictionary. Part (H) of 210.52 requires a receptacle outlet in a hallway that is at least 10 feet long. Therefore, if the foyer has an unbroken (no doors or openings) length of wall that is 10 or more feet long, a receptacle outlet should be installed. By installing a receptacle in the foyer, you eliminate any discussion with the electrical inspector, and provide a convenient location for plugging in a vacuum cleaner or floor-polishing appliance.

AFCI Protection in a Bedroom

Q: Does an arc-fault circuit-interrupter protected circuit have to be provided for a wall switch in a one-family-dwelling master bedroom that supplies outdoor lighting? The switch does not control any loads in the bedroom. What about a junction box that contains only splices? The branch circuit that supplies power to the box does not supply any load in the bedroom.

A: The answer is no to both questions. Part (3) of 210.12 only applies to 15- and 20-ampere, 125V single-phase outlets. A switch is not an outlet and neither is a junction box. A definition of an outlet appears in Article 100 and reads like this: “Outlet. A point on the wiring system at which current is taken to supply utilization equipment.”

Junction boxes and switches do not qualify as outlets under this definition.

Grounding Electrode Conductor and Reinforcing Steel

Q: Where reinforcing steel in a concrete slab that is in contact with the earth is used as a grounding electrode, may the grounding electrode conductor be clamped to a rebar within the concrete or must the connection be made on the outside of the concrete?

A: Generally, the connection of the grounding electrode conductor to the grounding electrode has to be accessible, but an exception to this requirement appears in 250.68(A). “Exception. An encased or buried connection to a concrete encased, driven, or buried grounding electrode shall not be required to be accessible.”

When the connection will be encased in concrete, an inspection should be made before concrete is poured, and the ground clamp must be marked to indicate that it is suitable for direct burial. A clamp that is suitable for concrete encasement is marked “direct burial” or “DB.”

Normal practice is to have the general contractor turn up 6 to 12 inches of reinforcing steel rod on both sides of the proposed building if the location of the service is unknown to permit the electrical contractor to connect the grounding electrode conductor to the rebar when electricians arrive at the construction site. To summarize, the grounding electrode conductor may be connected to a reinforcing rod in or out of the concrete slab.

Water Heater Overcurrent Protection

Q: What is the minimum and maximum rating of the overcurrent protective device for an electric storage-tank water heater? What is the minimum branch circuit conductor size for a 4,500W, 240V single-phase water heater?

A: Let's calculate the minimum ampacity of the branch circuit conductors first. For a storage-type water heater with a capacity of 120 gallons or less, the branch circuit conductors must have an ampacity of 125 percent of the nameplate rating. This calculation (4,500 divided by 240 ¥ 1.25) indicates that the branch circuit conductors must have an ampacity of 23.4 or more. This means that 10 AWG copper conductors must be used.

The minimum size overcurrent protection for this water heater is 25 amperes. This size overcurrent device is the next larger standard size listed in 240.6. If the water heater nameplate does not list a maximum size overcurrent device, 422.11(E) allows the branch circuit overcurrent device to be increased to 150 percent of the water heater full load current which is 28 amperes (4,500 divided by 240 ¥ 1.5). Part (E) of 422.11 allows an increase to the next larger standard size overcurrent device, which is 30 amperes.

The minimum branch circuit conductor size is 10 AWG copper and the overcurrent protection is either 25 or 30 amperes.

GFCI Protection for Swimming Pool Pump Motors

Q: Where is the requirement in the 2002 edition of the NEC for GFCI protection for swimming pool pump motors that are permanently connected (not cord-and-plug) to a wiring system? This section appeared in the 1999 NEC under 680-6(d): “Motors in Other than Dwelling Units. Wiring supplying pool pump motors rated 15 and 20 amperes; 125V or 240V, single phase, whether by receptacle or direct connection, shall be provided with ground-fault circuit-interrupter protection for personnel.” Where is this rule located in the 2002 edition?

A: This requirement was removed from the 2002 edition of the National Electrical Code. Now GFCI protection for pool pump motors is only required for cord-and-plug connected motors. Parts (A)(1) and (5) of 680.22 cover receptacles around swimming pools, and Part (5) of 680.22 contains specific rules for receptacles that supply pool pump motors. This is what one sentence says: “Receptacles that supply pool pump motors and that are rated 15 or 20 amperes, 120 volts through 240 volts, single phase, shall be provided with GFCI protection.”

There were at least four proposals for the 2005 NEC to reinstate the language in Section 680-6(d) in the 1999 NEC or to require GFCI protection on all swimming pool pump motors regardless of how they were connected to the power source. All proposals were rejected, but some CMP 17 panel members did not support the panel decision.

During the public comment period, four comments were received that requested reinsertion of the requirement as it appeared in the 1999 NEC. All were rejected but the vote for rejection was not unanimous. More than two-thirds of the panel members were not convinced that a hazard exists when a swimming pool pump motor is properly installed and maintained.

Although not required by the 2002 edition of the National Electrical Code, GFCI protection for a hard-wired swimming pool pump motor is permitted and could be provided. 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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