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Some installers routinely completely knock out the rectangular flap to create an open rectangle for passage of NM cable into a plastic box. I was under the impression that was to be left attached on one side to act as a clamp and a seal. What is the official requirement? Is this widely done?

The Carlon new work single gang blue plastic box I have in front of me (possibly 20 years old but don't actually know) has the flaps symmetrically and flimsily attached. I thought one side would be strongly attached and the other weakly. Are there better quality plastic boxes in which the flaps are asymmetrical?

EDIT It seems that only the standard small size of single gang plastic new work box allows the total removal of the flap, right? So the multi-gang new work plastic boxes require leaving the flap attached on one side? And all old work plastic boxes require some attachment of NM cable to the box? E.g., installation of old work boxes require leaving the flap as a clamp?

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    When UL approves the box, they also approve the labeling on the box, and the instruction sheet intended to go with the box. Of course, big-box stores buy by the carton, sell them loose in bins, and don't give you the instruction sheet. But it should be on the Internet. The labeling and instructions define the conditions under which UL tested and certified the device. Jan 20 at 20:22

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Products vary widely. Some have tensioned flaps that are obviously designed to retain the cable. Some are just flimsy knockouts. You can use them or not depending on your particular level of workpersonship.

The critical point may be that none are rated as complete retention devices anyway, meaning that cable must be fastened (stapled) within 12" of the box with any of them, and within 8" for boxes that don't have clamps.

Related discussion: I broke the knockouts on the blue receptacle boxes while doing electrical rough in. Now what?

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