National Electrical Installation Standards

Standards as High as Your Own

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Monday, July 6, 2020

Question:

Comment on CQD Question and Response for May 20, 2020:                                                   

Hello,

Regarding the question and answer for using FMC behind a furred out sheetrock wall posted on Wednesday 05/20/20, it should be noted that 348.20(A) states the minimum trade size for FMC allowed is 1/2". The OD of 1/2" FMC is roughly 7/8 and furring strips are only 3/4" leaving the FMC an 1/8" larger than the hollow wall space. For that reason MC cable is preferred over FMC since the OD of MC is only 3/8". Also noteworthy is 300.4(D) which comes into play should the FMC or MC be run next to a furring strip. The 300.4(D) Cables and Raceways Parallel to Framing Members and Furring Strips. In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed parallel to framing members, such as joists, rafters, or studs, or is installed parallel to furring strips, the cable or raceway shall be installed and supported so that the nearest outside surface of the cable or raceway is not less than 32 mm (11⁄4 in.) from the nearest edge of the framing member or furring strips where nails or screws are likely to penetrate. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by nails or screws by a steel plate, sleeve, or equivalent at least 1.6 mm (1⁄16 in.) thick. Exception No. 1: Steel plates, sleeves, or the equivalent shall not be required to protect rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing. Exception No. 2: For concealed work in finished buildings, or finished panels for prefabricated buildings where such supporting is impracticable, it shall be permissible to fish the cables between access points. Exception No. 3: A listed and marked steel plate less than 1.6 mm (1⁄16 in.) thick that provides equal or better protection against nail or screw penetration shall be permitted.

Richard Cressotti

A

Answer:

Hi Richard,

Thanks for the following up information. Great point about the protection requirements in Section 300.4(D). That's one of the great benefits of this forum that allows for the audience to supplement the responses with additional valuable input.

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