Article 100 Definitions
Article 210 Branch Circuit
Article 220 Branch Circuit, Feeder and Service Calculations
Article 240 Overcurrent Protection
Article 250 Grounding
Article 310 Conductors for General Wiring
Article 334 Nonmetallic Sheathed Cable; Types NM, NMC and NMS
Article 404 Switches
Article 422 Appliances
Article 517 Healthcare Facilities
Article 680 Swimming Pools, Fountains and Similar Installations;
The General Information for Electrical Equipment Directory published by Underwriters Laboratories Inc. is also referenced.
Circuit breakers on delta circuits
Q:Does the NEC permit a 3-pole circuit breaker with a 120/240V rating on a 3-phase four-wire 120/240 delta system? The circuit breaker supplies a 3-phase motor.
A:No, the circuit breaker must have a single voltage rating of 240. A slash rating such as 120/240V indicates that the breaker cannot be used where the voltage to ground on a circuit conductor exceeds 120V. The reference for this information is in the second paragraph of 240.85 and reads like this: “A circuit breaker with a slash rating such as 120/240V or 480Y/277V shall be permitted to be applied in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the values of the circuit breaker’s voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker’s voltage rating.”
Because the voltage on one conductor in a 3-phase, four-wire delta system is approximately 208V to ground, a slash rating of 120/240V is not acceptable.
Flexible metal conduit in patient care areas
Q:Is flexible metal conduit permitted as the wiring method to supply receptacles in patient bedrooms in a hospital?
A:The wiring method by itself must provide a reliable and low impedance ground-fault path. In addition, an equipment grounding conductor, properly sized, must be installed in the raceway [See 517.13(A) and (B)].
These requirements for grounding appear in 250.118—Item (5): “Flexible metal conduit provides a suitable equipment grounding path where both the conduit and fittings are listed for grounding.” According to the 2003 edition of the General Information for Electrical Equipment Directory (White Book) published by Underwriters Laboratories Inc., flexible metal conduit longer than 6 feet is not recognized as a grounding means. Therefore, the requirements in Item 6 of 250.118 apply. They are: “(a) The conduit is terminated in fittings listed for grounding; (b) The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20A or less; (c) The combined length of flexible metal conduit and flexible metal tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 6 feet; and (d) The conduit is not installed for flexibility.”
Fittings that clamp around the circumference of the conduit are considered suitable for grounding, and all flexible metal conduit fittings in sizes 3/8 through 3/4 inch are suitable for grounding where overcurrent devices protecting the contained conductors do not exceed 20A. This information was obtained from the UL White Book.
Nonmetallic sheathed cable used as a feeder
Q: What is the ampacity of 4 AWG copper NM-B cable where used as the main feeder to supply a branch circuit panelboard in a one-family residence? The service conductors are three 4 AWG copper conductors with Type THWN-2 insulation in 1-inch rigid metal conduit protected by a 100A main circuit breaker.
A:Type NM-B contains conductors that have an insulation rating of 90 C. However, 334.80 limits the ampacity to the values shown in Table 310-16 for 60 C insulation, but if the conductors must be derated, the 90 C ampacity is permitted to be used provided the final derated ampacity does not exceed the ampacity for 60 C conductors. To further confuse the issue, this sentence appears in 320/15(B)(6): “The feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors.” Based on these various NEC references, I would accept the 4 AWG copper NM-B cable as a 100A feeder. Because this is an area that is not very clear, you should check with the authority having jurisdiction before making this installation.
GFCI receptacles in a commercial bathroom
Q:May a GFCI-protected receptacle connected to a 15A branch circuit be installed in a bathroom in an office building? If the answer is yes, may a small exhaust fan be connected to the same branch circuit?
A:Although there is no requirement for a receptacle in a bathroom in other than dwelling units, if one is provided it must be GFCI protected. This requirement appears in 210.8(B)(2) of the NEC. Since there is no requirement for a receptacle in a bathroom in an office building, a 15A branch circuit is permitted. This branch circuit may also supply the exhaust fan in the bathroom. Although the location of the receptacle is not specified in the Code, it should be located near the basin for convenience of the user.
Wiring a ceiling fan and luminaire on a single circuit
Q:Does the Code allow a 3-wire 120/240V branch circuit to supply a ceiling fan and lighting fixture from a two-gang device box containing two switches? One ungrounded conductor and neutral supplies the fan and other ungrounded conductor and neutral supplies the luminaire (lighting fixture).
A: Yes, this is a multiwire branch circuit as defined in Article 100. This circuit is permitted to supply all loads that are allowed to be connected to two wire branch circuits. However, special rules often apply to multiwire branch circuits. Here are a few: In dwelling units, multiwire branch circuits that supply more than one device or equipment on the same yoke must be provided with a disconnecting means that simultaneously disconnects all ungrounded conductors. Multiwire branch circuits should not supply line-to-line loads except for a single utilization appliance, or where the ungrounded conductors are opened simultaneously by the branch circuit overcurrent device.
The snap switches for controlling the ceiling fan and luminaire are permitted to be mounted in a device box without a barrier between them by 404.8. A barrier between switches is only required where the voltage between switches exceeds 300.
Access panel for a hydromassage bathtub
Q:Is an access panel under a hydromassage bathtub that can be removed from the basement Code-compliant? The panel is secured to the basement ceiling by four screws. Is a 20A dedicated branch circuit required to supply a hydromassage bathtub?
A:The location of the access panel is not defined in the NEC. However, 680.73 requires that the electrical equipment associated with the tub be accessible without damaging the building structure or building finish. A panel that is held in place by four screws appears to meet this requirement.
A dedicated 20A branch circuit for the hydromassage bathtub is not required by the NEC, but may be required in the installation instructions provided by the manufacturer.
Receptacle outlet load calculations
Q:According to the National Electrical Code, 10 receptacle outlets are permitted and a 15A branch circuit and 13 are permitted on a 20A branch circuit in commercial buildings. How many receptacle outlets are permitted on 15 and 20A branch circuits in a dwelling occupancy?
A:There is no limit to the number of receptacle outlets permitted on 15 and 20A branch circuits in dwelling type occupancies. For other than dwelling units, part (a) of 220.3(B) requires that each single or multiple receptacle on the same yoke be calculated as a load of 180VA, or a receptacle outlet that has four or more receptacles be counted at 90VA for each receptacle. Using this load of 180VA for each single or duplex receptacles allows 10 receptacles on a 15A branch circuit and 13 on a 20A branch circuit.
In residential occupancies receptacles are required to be spaced at fixed intervals; this is not the case for commercial occupancies. In commercial, industrial, institutional, etc. occupancies receptacles are generally located only where required to supply cord-and-plug connected loads. For this reason, unit loads are assigned to receptacles in other than residential occupancies. The Code mandates spacing of receptacles in dwelling units to provide flexibility in the use of table lamps, floor lamps and other appliances and to reduce the use of exterior cords; not because there will be loads plugged into every receptacle.
In all occupancies 210.11 requires an adequate number of branch circuits for the calculated loads. And 210.11(B) require that the calculated load be evenly divided among the branch circuits.
Some inspection authorities have local laws or ordinances that limit the number of receptacle outlets on 15 and 20A branch circuits in residences. I am not in favor of such a regulation, but you should be aware of such a restriction if one exists.
Conductor size and overcurrent protection for water heater
Q:What is the maximum overcurrent protection and minimum branch circuit conductor size for a 240V, single-phase, 4,500W, 55-gallon storage-type water heater?
A:Requirements for water heaters are in Article 422—Appliances. Parts of this Article that apply are 422.10, 422.11, and 422.13. According to 422.13, branch circuit conductor ampacity must be at least 125 percent of the nameplate rating of the water heater. This means that conductor ampacity has to be at least 23A (4,500 divided by 240 times 1.25). A 25A overcurrent device protecting 10 AWG copper conductors satisfies this requirement, but this might not be the maximum overcurrent device permitted by 422.11(E). If the nameplate does not list a maximum overcurrent device rating, Part 3 of 422.11(E) allows an increase of the overcurrent protection to 150 percent of the water heater rated current. This results in an overcurrent device rating of 28.125 (1.50 times 18.75) or 30A. Finally, the minimum overcurrent device rating is 25A and the maximum is 30A. The branch circuit conductor is 10 AWG copper. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.