![]() ![]() Informal testing shows that at this clamping force the cable has already shorted or been irreparably damaged. One traditional test has been to wiggle the wire outside the box, tightening the clamp screws until the wire no longer moves inside the box. ![]() Again under informal testing, there is no perceivable force-feedback cue to the installer that the clamping screws are sufficiently tight. The specific problem is that the margin between insufficient clamping force and excessive clamping force can be small or non-existent with these specific components. While this is excellent general guidance, it does not appear to be sufficient in the situation I have outlined above. ![]() Care should be taken when tightening clamps or driving staples – tighter is not always better. Installer awareness of “securement, without damage” is a concern with regard to the satisfactory installation of Type NM cable, regardless of the outside diameter. John Cangemi, in the UL publication THE CODE AUTHORITY®: ELECTRICAL CONNECTIONS, May 2005 issue writes: Issues with excessive clamping force on Type-NM cable are not unknown. This fault is 100% reproducible if the screws are sufficiently tightened, though there is some variability in the pressure required depending upon the alignment of this radially asymmetric cable with respect to the clamp connector. This does not appear to be an inherent fault in the products, but rather an incompatibility that may require industry cooperation for resolution.įrom informal testing it turns out that securing NM-B cable to a metal junction box through a standard metal 3/8” NM cable clamp connector creates the risk that even slight over-tightening of the clamp’s retaining screws will generate an immediate or subsequent short circuit fault between one or more conductors and box ground. I would like to bring to your attention a repeatable, hazardous fault arising from the combination of two common products UL listed for use in residential electrical wiring. Short circuit failures with 12/3 NM-B cable and NM clamp connectors ![]()
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