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For all things electrical I'd been taught that you want to make sure things are tight, and that problems generally come from something be loose - a loose opening into a box, a loose connection to a terminal, insulation being clamped into a connection with a terminal which could cause the connection with the wire to loosen over time.

Now I'm wondering about staples for runs of NM wiring along joists. To keep these from sagging, they were stapled pretty tight and at pretty close spacing, 12 to 18 inches apart depending if there was an obstacle or curve coming up. 1/2" metal staples were used for NM 10/3 and NM 12/2 on separate runs.

Is it a safety issue to have stapled NM wiring firmly against the joists? No doubt the cable was effectively hit with a hammer a few times in a bunch of places, firming down the staples and pinning the cable between the staple and joist. Looking into this it seems that this may have been a mistake.

If this is a problem, can the staples simply be pulled up and re-applied lighter (or using insulated staples instead) as long as the cable insulation looks intact (albeit probably flatter or scuffed in some places)? Given multiple layers of insulation, I'd think the cable does not need to be replaced from having been impacted a bunch of times, or I'd expect if there is a problem it would be a short circuit which the breaker would indicate once the circuit is energized.

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For many years inspectors did not look at staples at all, including spacing. In some places “Ropers” would go through and pull and staple. Many times damaged cable was a problem. This is why to even pull wire commercially you have to have a license in most states.

As far as the staple contact, barely touching or snug is best for x3 cables that are not flat barely touching as a firm staple can damage the insulation inside and on a hot day with a heavy load things start shorting out.

The rule is a cable must be flat not on end for this same reason NEC 334.30.

As far as removing staples and replacing you are possibly going to do more damage than what was done.

I staple every 3’ but code is 4-1/2, and as soon as the staple takes the wire to the board it is tight enough for me (I have never failed inspection for staples).

I do see those that want the staple to leave a mark in the outer covering but by my reading of code a mark is damaging the cable.

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  • Good to know "barely touching or snug is best". As for "on a hot day with a heavy load things start shorting out", the cables in question are for a washer and dryer (two circuits) and are very visible (run along joists not covered up) and not far from the panel, in a cool basement. Does it seem reasonable to leave as is and keep this in mind to inspect or replace in case the breaker starts tripping or receptacles feel warm?
    – cr0
    May 24 at 18:37
  • Maybe worth noting, both the 10/3 and 12/2 circuits are flat NM. So even if the cable was essentially hammered down and pinned with staples, each wire in the cable is being pinned against the joist not against each other.
    – cr0
    May 24 at 18:39

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