Clearance between 480V cables and dead front panel

Merry Christmas

Whangii

Member
Location
NC
Occupation
Consultant
Recently saw where a panel screw in front panel embedded itself into a cable causing an arc flash. Is there a minimum distance of cables from font of panel? I've been told there are both NEMA and NEC violations but can't find a specific standard. Panel was being reinstalled after an inspection. Thanks for any help.
 

rc/retired

Senior Member
Location
Bellevue, Colorado
Occupation
Master Electrician/Inspector retired
I don't believe there is. Common sense should kick in when making up a panel.
Confession time, I ran a screw through a wire once.
 

JGinIndy

Senior Member
Location
Indiana
Occupation
Retired Electrician currently County Inspector
408.3(A)(1) is a bit of a stretch and yes I to have ran a screw thru and hey the gfpe worked
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I wish there were a way to prevent it.
More scary is the number of times I have seen an indention in a service conductor that was caused by a cover screw.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I wish there were a way to prevent it.
More scary is the number of times I have seen an indention in a service conductor that was caused by a cover screw.
Yes, I have seen that too and it's always caused by a ugly, sloppy installation by a lousy electrician or worse, a DIYer.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Most often that happens when the original screw is lost and field replaced with something different. However it does sometimes happen with the original screw and is just a workmanship issue. Look at where you have placed the conductors before you install the cover screws.
 

Whangii

Member
Location
NC
Occupation
Consultant
I really appreciate the answers. As to the screw, it was a blunt end panel screw and the installer was using an electric drill and slot head bit to install the screw. I think the drill RPM may have had something to do with possibly melting the insulation on the 500 MCM cable. Explosion was said to have been impressive...
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
I really appreciate the answers. As to the screw, it was a blunt end panel screw and the installer was using an electric drill and slot head bit to install the screw. I think the drill RPM may have had something to do with possibly melting the insulation on the 500 MCM cable. Explosion was said to have been impressive...
That’s why I don’t allow my guys to use a drill to put covers back on! Doing it by hand has a better chance of burning the tip of the screw off and clearing, than driving a screw all the way into a cable. I hate it when people use drills to put covers on, they almost always over torque it, and strip out the clip.
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
That’s why I don’t allow my guys to use a drill to put covers back on! Doing it by hand has a better chance of burning the tip of the screw off and clearing, than driving a screw all the way into a cable. I hate it when people use drills to put covers on, they almost always over torque it, and strip out the clip.
I have been using Electric drills or impacts for putting covers on for over 20 years without one single issue. I do check that wires aren't close to the screw holes. The time it saves on large panel's is huge. I also don't over tongue them. Common sense is all that is needed.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
Yeah
I have been using Electric drills or impacts for putting covers on for over 20 years without one single issue. I do check that wires aren't close to the screw holes. The time it saves on large panel's is huge. I also don't over tongue them. Common sense is all that is needed.
Yeah, but unfortunately common sense has been dead for a while now.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I have been using Electric drills or impacts for putting covers on for over 20 years without one single issue. I do check that wires aren't close to the screw holes. The time it saves on large panel's is huge. I also don't over tongue them. Common sense is all that is needed.
Same here. Many of the commercial panels we work on can have up to 20 screws that need to be removed and reinstalled. I agree about the common sense, just check for potential problems before putting the cover back on.
 

paulengr

Senior Member
It is important to secure your wiring. That’s what cable ties are for.

When terminating cable, cut the excess off. Leave 8” like you do in receptacles. The excuse I always hear from industrial electricians is that the 6 feet of extra wire they spent 20 minutes bundling up in the panel is so they have extra to pull in case the end is damaged. I have used up a couple inches and even had to use barrel connectors to extend some wiring that was all but “grouted” in. But those same coils are still in those panels 50 years later. It’s simply laziness on the part of the installer. Your wiring should look like factory wiring.

During a fault loose cabling will jump quite far. Also vibration is a big issue in motor peckerheads. If you don’t route and secure your wiring eventually something will happen. Murphy’s law says that the sharp end of the screw exerts a powerful attractive force that will cause loose wires to move towards the screw and impale themselves on it. If there is any question cut the end off, reroute the cable, or secure something over it like a wire nut, wood, plastic, rubber...but regardless...see point 1. Secure the wiring in the first place and nothing bad will happen.

Clearances between open (unshielded) should be maintained. You can see what this is by paying attention to “open” terminals without phase barriers. With 600 V rated equipment it’s about 3:4”. In UL testing they apply a surge voltage (BIL) of 200% of the nominal voltage plus 1000 V so 600 V class wiring is subjected to 2200 V. Thus the seemingly large gap. The tiny 5 mils of PVC over #14 survives up to around 2800 V when dry and not punctured by a screw.

This falls under the “workmanship” rule that is almost unenforceable, but if electricians had as much pride in their work as they do in other things it would never be a concern.
 
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