Article 210 Branch Circuits
Article 220 Branch-Circuit, Feeder, and Service Calculations
Article 230 Services
Article 240 Overcurrent Protection
Article 250 Grounding
Article 310 Conductors for General Wiring
Article 406 Receptacles, Cord Connectors and Attachment Plugs (Caps)
Article 410 Luminaires (Lighting Fixtures), Lampholders, and Lamps
Article 517 Health Care Facilities
Meter sockets ampere ratings
Q: I have a large, one-family residence to wire that requires a 400-ampere, single-phase service. The maximum size single-phase self-contained meter base available from the utility is 320 amperes. The electrical inspector has told me the maximum service disconnect permitted with this meter base is 300 amperes. The manufacturer's representative for the meter socket manufacturer states that the 320-ampere rating for the meter socket is based on continuous loading. With this information from the manufacturer, am I allowed to use a 400-ampere service disconnect and service-entrance conductors?
A: Yes, you are allowed to use a 400-ampere disconnect and 400-ampere conductors. According to Table 310.15(B)(6), 400 Kcmil copper conductors with 75 C or 90 C insulation can be used for 400-ampere service-entrance and feeder conductors. A further reduction in the neutral conductor size is permitted by 220.61(A) and (B).
There are requirements in 210.19(A)(1), 210.20(A) and 215.2(A)(1) that indicate that 125 percent of a continuous load must be added to branch circuits and feeders that supply continuous loads. Therefore, if a 320-ampere continuous load is involved, the service-entrance conductors and service-overcurrent protection must be increased by 25 percent, and 400 amperes is the minimum ampere rating permitted.
There is a statement in 230.66 that indicates, “Individual meter socket enclosures shall not be considered service equipment.” Maybe this sentence means the meter socket ampere rating can be ignored because it is furnished by a utility not covered by the NEC under 90.2(B)(5)a. This part (5)a says: “Installations under the exclusive control of an electric utility where such installations consist of service drops or service laterals and associated metering” are not covered.
“The General Information for Electrical Equipment Directory,” 2004 edition, published by Underwriters Laboratories Inc., provides this information under the heading Meter Sockets (PJYZ): “Ratings of Listed meter sockets are limited to 600 V AC, maximum and 400 amp maximum through any meter. Meter sockets are marked with a continuous amp rating and may in addition have a maximum use (intermittent) amp rating of not more than 125 percent of the continuous amp rating.”
Based on this discussion, a 400-ampere disconnecting means with 400 amperes overcurrent protection may be installed in compliance with the National Electrical Code.
Size of service entrance conductors
Q: A new electric service to a machine shop is 600-ampere, 120/240-volt, three-phase, four-wire, delta. The total three-phase motor load is 175 amperes. The largest motor is 10 horsepower. Am I permitted by the National Electrical Code to use a 4/0 AWG copper conductor with 75 C insulation for the “B” phase (stinger or wild leg) and fuse it at 200-amperes? The other phase conductors are two parallel 500-Kcmil AWG copper conductors with 75 C insulation fused at 600-amperes. The maximum unbalanced current on the grounded-circuit conductor is calculated at 210 amperes, and a 250-Kcmil AWG copper conductor with 75 C insulation is proposed. These wires will be installed in a single 3-inch rigid metal conduit. Does this proposed service installation comply with the NEC?
A: To answer the question, some calculations are necessary. First, is the rigid metal conduit properly sized? Using Table 5 in Chapter 9 of the NEC provides the cross-sectional area of the wires. For copper conductors with Type THWN insulation, the area for 4/0 is 0.3237 square inch; for 250 Kcmil is 0.3970 square inch; and for 500 Kcmil is 0.7073 square inch. The total volume occupied by these conductors is (4 ¥ 0.7073) + 0.3237 + 0.3970 is 3.5499 square inches. And according to Table 4 in Chapter 9 for rigid metal conduit the minimum size is 31/2 inch.
Because there are more than three current-carrying conductors in the raceway, the ampacities listed in Table 310.16 must be adjusted downward to satisfy 310.15(B)(2)(a). Table 310.15(B)(2)(a) requires that the conductors be derated to 80 percent of the ampacity values shown in Table 310.16. Eighty percent of the ampacity of 4/0 AWG copper with 75 C insulation is 184 amperes (0.80 ¥ 230). The “high” leg is adequate for the connected load and may be protected by a 200-ampere fuse. This fuse size is permitted by 240.4(B). The 500-Kcmil conductors have an adjusted ampacity of 608 amperes (380 ¥ 0.8 ¥ 2). Therefore, these parallel 500-Kcmil conductors may be protected by 600-ampere fuses.
Listed fuse reducers are available and are tested by Underwriters Laboratories Inc. under the category (IZZR) and UL Standard 512 “Fuseholders.” Therefore, a 200-ampere fuse reducer with 200-ampere fuses should be acceptable to the authority having jurisdiction.
The conductor with the higher voltage to ground must be identified by an orange color or other effective means and should be connected to the center terminal in the disconnect switch.
The minimum-size, grounding-electrode conductor that connects the grounded-circuit connector in the service-disconnecting means to the underground metal water pipe is based on requirements in 250.66 and Table 250.66. Since the equivalent area of the largest ungrounded phase conductor is 1,000 Kcmil (500 Kcmil ¥ 2), the grounding-electrode conductor cannot be smaller than 2/0 AWG copper.
Redundant grounding in healthcare facilities
Q: In a hospital's critical-care areas, is it necessary to install an insulated equipment-grounding conductor in a rigid metal raceway that contains feeder conductors for a branch-circuit panelboard that supplies outlets at patient beds?
A: Part (D) of 517.19 provides three choices for grounding a panelboard used to supply patient bed location outlets in a critical-care area. Where the feeder is in a metal raceway or is type MC or MI cable, a grounding bushing and a continuous copper bonding jumper, sized in accordance with 250.122 and connected to the panel's ground bus, is acceptable. Connection of the feeder raceway or cable to threaded hubs or bosses at the panelboard is also an acceptable method of bonding or approved devices such as bonding-type locknuts or bushings may be used to bond the panelboard.
Any of these methods may be used to bond the panelboard. Redundant grounding is not required.
Laundry room receptacles
Q: Does the NEC permit more than one laundry room receptacle in the laundry area of a one-family dwelling unit?
A: Yes, it does. Part (c)(2) of 210.11 requires a 20-ampere branch circuit for laundry-receptacle outlets. This branch circuit may supply more than one receptacle, but it cannot supply any loads other than receptacles in the laundry. Part (5) of 210.52 has the statement: “In dwelling units, at least one receptacle outlet shall be installed for the laundry.” The two exceptions do not apply to single-family dwelling units.
There is a change in the 2005 edition of the National Electrical Code that requires ground-fault circuit-interrupter protection for all 15- and 20-ampere, 125-volt receptacles located within six feet of the laundry tub or sink. This change appears as item (7) of 210.8(A).
Bathtub and shower areas
Q: Is there a difference between a bathtub space and bathtub zone? The words “bathtub and shower areas” and “bathtub and shower space” are both used in different parts of the NEC and I need a clarification. The electrical inspector is questioning the installation of a GFCI-protected receptacle within three feet of the bathtub. The receptacle is located within two feet of the basin.
A: You are required to install at least one GFCI-protected receptacle within three feet of the basin, but this receptacle cannot be installed within the vertical space or directly over a bathtub. This requirement appears in 406.8(C) under the title: “Bathtub and Shower Space.”
In Bathtub and Shower Areas [410.4(D)], there are restrictions on the types of luminaires (fixtures), and ceiling-suspended paddle fans that may be installed within three feet horizontally and eight feet vertically from the top of the bathtub rim. In this zone, no cord-connected luminaries (fixtures), chain-cable or cord-suspended-luminaires (fixtures), lighting track, pendants, or ceiling-suspended (paddle) fans are permitted. These restrictions apply to certain types of luminaries (fixtures) and ceiling-mounted paddle fans. Receptacles are not mentioned; therefore, 406.8(C) applies to the locations of receptacles in a bathroom and a receptacle not installed directly above the bathtub or within the outer edge of the tub complies with 406.8(C).
Lighting Track Length on Branch Circuits
Q: What is the maximum length of lighting track on a 20-ampere branch circuit? The track is to be installed in the lobby of an office building.
A: The design or expected loading limits the maximum length of lighting track that can be connected to a 20-ampere branch circuit. There is nothing in the NEC that limits the length of track that may be connected to a single branch circuit. If the track is designed to supply 150-watt lamps on three-feet centers, the track length could be 35 feet. If the design specifies 100-watt lamps on two-feet centers, the total track length could be 40 feet.
Although the lighting track length on a branch circuit is not limited, a feeder load must be included in the calculations, which is based on the length of lighting track. According to 220.41(B), a load of 150 volt-amperes for every two feet of lighting track, or major fraction thereof, must be included when sizing the feeder and service-entrance conductors. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.254.2132.