Entries into Raintight Enclosures, Derating NM-B C

Cables in metal framing member

Q: Sections 320.17 and 330.17 require that Types AC and MC cables be installed to comply with 300.4. Among other things, Part (B) of 300.4 requires the installation of listed bushings or grommets in all holes or slots in metal framing members through which the cables pass. This same requirement does not appear in Article 348—Flexible Metal Conduit: Type FMC, and Article 350-—Liquidtight Flexible Metal Conduit: Type LFMC. Is it necessary to install grommets in metal framing members where flexible metal conduit or liquidtight flexible metal conduit is pulled through metal framing members?

A: Grommets or bushings are not required for any of these wiring methods. A change in 320.17 and 330.17 was made to clarify that bushings are not required in holes in metal framing members where these cables are pulled through them. This is the way the requirement appears in 320.17: “Through or Parallel to Framing Members. Type AC cable shall be protected in accordance with 300.4(A), (C), and (D) where installed through or parallel to framing members.”

Notice that Part (B) of 300.4 does not appear in 320.17. A similar change was made in 330.17. Because the requirements in 300.4 generally apply to cable wiring methods, there is no reference to this section in the articles that apply to flexible metal conduit or liquidtight flexible metal conduit, which are raceway type wiring methods, and do not contain conductors when the raceways are installed.

Optional dwelling standby generator

Q: A homeowner wants a 15 kVA, 120/240-volt, single-phase generator connected to the wiring in his home through an automatic or manual transfer switch so that he can select the loads to be served during a power outage. The service is 200 amperes, single-phase. The transfer switch will be installed between the service disconnect switch and distribution panel. May the transfer switch be sized for the full-load output current of the generator or does it have to be 200 amperes? Is it necessary to provide automatic selective load pickup and load shedding when operating in the optional standby mode?

A: Where the transfer switch is installed as indicated, it must be rated 200 amperes. Automatic load shedding must be provided if an automatic transfer switch is used to switch the load from the utility source to the on-site generator because the generator can only deliver 62.5 amperes at 240-volts without overloading.

A manual transfer switch consisting of a multipole double throw switch, slide interlocked circuit breakers or molded case switches may be used along with a circuit schedule that will prevent overloading the generator. A 200-ampere multipole double throw fusible switch with 70-ampere fuses in fuse reducers provides overcurrent protection for the generator.

The loads are divided into three groups. All loads to be energized simultaneously are marked A, B or C. These letters should appear on the panelboard circuit directory opposite the circuits involved. Loads (usually motor loads) that are to be energized first should be marked A-1, the next motor load A-2, etc.

Branch circuits that are to be energized continuously may be marked A, B or C. These branch circuits could include receptacle circuits that supply television sets, table or floor lamps, the refrigerator circuit, water pump, and similar loads.

The user should be instructed to disconnect all branch circuits when an outage occurs, start the generator and connect all loads in one group in the order indicated. When it is time to switch groups, turn off all loads before energizing another group. This sequence should provide an installation that is safe and will prevent opening the generator overcurrent device.

Face-down ceiling receptacles

Q: Does the National Electrical Code (NEC) have any specific requirements for the installation of receptacles installed face down on a ceiling?

A: None that I am aware of. All listed receptacles must pass a pull-out test. A blade is inserted in the receptacle openings and the minimum force to remove the blade is recorded.

If the pull-out force is lower than required by the standard, the product is not listed. Hospital-grade receptacles must withstand a higher pull-out force. Therefore, they may be used for a more reliable installation. Also, twist-lock receptacles should be considered for this application.

Provisions for guest suite cooking

Q: A guest suite in a motel has an area with an undercabinet refrigerator, sink and built-in microwave oven. Is the built-in microwave oven considered to provide permanent cooking facilities?

A: The words in 210.60 are “... permanent provisions for cooking ... ” And, since there is no explanation or definition for “permanent provisions for cooking,” it is up to the electrical inspector to make a decision. In my opinion, the built-in microwave oven provides permanent cooking provisions and the requirements in 210.52 apply. This space should be treated the same as a kitchen in a dwelling unit.

Listed for direct sunlight exposure

Q: “Conductors exposed to direct sunlight must be listed or listed and marked as sunlight resistant.” These words are in 310.8(B)(1). What types of cables are sunlight resistant but not marked?

A: The 2005 edition of the “General Information for Electrical Equipment Directory (White Book)” published by Underwriters Laboratories Inc. provides this information under the title, Service Entrance Cable (TYLZ): “Type SE cable contains Type RHW, RHW-2, XHHW, XHHW-2, THWN, THWN-2 Conductors. Type USE cable contains conductors with insulation equivalent to RHW or XHHW. Type USE-2 contains insulation equivalent to RHW-2 or XHHW-2 and is rated 90ºC wet or dry.

“Types SE—indicates cable for aboveground installation. Both the insulated conductors and the outer jacket or finish of Type SE are suitable for use where exposed to sun.”

“Types USE and USE-2—indicates cable for underground installation including direct burial in the earth... . Both the insulation and the outer covering, when used, on single and multiconductor Types USE and USE-2 are suitable for use where exposed to sun.”

These types of Service Entrance and Underground Service Entrance cables are sunlight resistant although not marked “sunlight resistant.”

Entries into raintight enclosures

Q: What are proper methods for making conduit entries in the sides and back of a panelboard in a raintight enclosure? Are Meyers Hubs required or may weatherproof locknuts and bushings be used? Where are ordinary locknuts and bushings acceptable?

A: Where penetrations in the enclosure are made above a horizontal plane that includes electrical components such as busbars and overcurrent devices, the integrity of the enclosure must be maintained.

Generally, any holes that are made in the sides or back of the panelboard that are above the prepunched knockouts that are part of the enclosure must be sealed to prevent the entrance of rain, snow or sleet.

Sealing locknuts may be used where the wiring method is rigid metal conduit or intermediate metal conduit. One sealing locknut is required either inside or outside of the enclosure.

An ordinary locknut can be used on the opposite side of the enclosure or another sealing locknut may be provided. A bushing inside the enclosure completes the conduit entry.

Derating NM-B cables

Q: Is derating required for three NM-B cables containing 3 to 12 AWG copper conductors that pass through a single hole in wood framing members that are draft-fire stopped? The cables supply 120-volt loads. If they must be derated, what is their final ampacity?

A: It appears that they must be derated to comply with the change in the 2005 edition of the NEC. This new paragraph was added to 334.80: “Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through wood framing that is to be fire- or draft-stopped using thermal insulation or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).”

It is assumed that there are six current-carrying conductors in the bundle, because each cable supplies three-wire 120/240-volt loads, and the neutral carries unbalanced current only.

Therefore, the derating factor according to Table 310.15(B)(2)(a) is 0.8. Multiplying 0.8 by the 90ºC ampacity for 12 AWG copper in Table 310.16 results in a corrected ampacity of (0.8 x 30)=24. Although 12 AWG copper has an ampacity of 25 in Table 310.16, the note at the bottom of the table restricts the overcurrent protection to 20 amperes by reference to 240.4(D).

This additional requirement for derating does not reduce the ampacities of the conductors in this installation.

Delta systems

Q: In a 120/240-volt, three-phase, four-wire delta-connected panel-board, are single voltage rated two-pole circuit breakers (240-volts) required where connected to any of the phase buses, or are 240-volt, two- or three-pole circuit breakers only required where one pole is connected to the high leg (B phase)? Are 120/240-volt (slash-rated) two-pole circuit breakers permitted to be connected to the A and C phases?

A: If there is no restriction on the use of slash-rated circuit breakers by the manufacturer of the panelboard, they can be used as suggested in the question. Here are two sentences in 240.85 that apply: “A circuit breaker with a straight voltage-rating, such as 240V or 480V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker’s voltage rating.”

This sentence in 240.85 also appears: “A circuit breaker with a slash rating, such as 120/240V or 480/277V, 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 two 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.”    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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