Box Fill

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Rick 0920

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Jacksonville, FL
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Electrical Instructor
I need some clarification please. Art. 314.16 (2) clamp fill. Would someone explain the portion of this section that says: "Conductors that originate within the clamp assembly shall be included in the conductor fill calculations covered in 314.16 (B) (1) as though they entered from outside the box" I don't know if I've ever seen a conductor that originated from a clamp assembly??? If I'm missing something obvious here I'll feel pretty dumb! :)
 
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Take a box such as this one. If you use NM cable in one of the clamps you must count the clamp as one conductor against the box fill.

If you use NM cable in both clamps you must count each clamp as one conductor against the box fill.

If you use any the standard KO's in the box with "external clamp type connectors" you don't have any clamp for that entry that counts against the box fill.
 
kwired said:
View attachment 2559595
Take a box such as this one. If you use NM cable in one of the clamps you must count the clamp as one conductor against the box fill.

If you use NM cable in both clamps you must count each clamp as one conductor against the box fill.

If you use any the standard KO's in the box with "external clamp type connectors" you don't have any clamp for that entry that counts against the box fill.
Click to expand...
I thought all the clamps counted as a single (meaning one) volume allowance. Whether you had 1 or 10, counts as one volume allowance based on the largest conductor present in the box? My question was, what conductors originate from a clamp assembly?
 
The full text in question from 2020 NEC 314.16(B)(2), second paragraph:

A clamp assembly that incorporates a cable termination for the cable conductors shall be listed and marked for use with specific nonmetallic boxes. Conductors that originate within the clamp assembly shall be included in conductor fill calculations covered in 314.16(B)(1) as though they entered from outside the box. The clamp assembly shall not require a fill allowance, but the volume of the portion of the assembly that remains within the box after installation shall be excluded from the box volume as marked in 314.16(A)(2).
Click to expand...

Cheers, Wayne
 
Rick 0920 said:
I thought all the clamps counted as a single (meaning one) volume allowance. Whether you had 1 or 10, counts as one volume allowance based on the largest conductor present in the box? My question was, what conductors originate from a clamp assembly?
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That may be correct.

If you don't use the clamp assembly at all I'm pretty sure you don't have to count it though.
 
kwired said:
That may be correct.

If you don't use the clamp assembly at all I'm pretty sure you don't have to count it though.
Click to expand...
That has been my understanding through the years. You count internal cable clamps as ONE volume allowance no matter how many are present.
We usually throw away the clamps that we don't use. The NEC verbiage has me confused on this one.
 
Rick 0920 said:
Thanks Wayne. What does this mean? Are there cable clamps out there that conductors originate from within them?
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I would presume it means if cables/conductors enter the box through said clamp...

Therefore if no cables/conductors enter through an internal clamp you don't have to count the clamp(s).
 
No, 314.16(B)(2) is talking about something more exotic. It mentions a "clamp assembly that incorporates a cable termination for the cable conductors". I'm thinking some of the cable conductors don't actually enter the box proper, they are terminated in the clamp assembly. So then you don't count those conductors. If they terminate to a pigtail that does enter the box, you count the pigtail (unlike a "conductor, no part of which leaves the box" per 314.16(B)(1)).

That's certainly what's being described. I have no idea if there are any commercial products that match the description.

Cheers, Wayne
 
What about the plastic thingies in plastic boxes that you push the romex through. Are the considered a clamp and counted?
 
qcroanoke said:
What about the plastic thingies in plastic boxes that you push the romex through. Are the considered a clamp and counted?
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I would say no. They project into a box no more than a locknut would, and a locknut isn't counted. In other words, I would not consider them "internal" clamps.

Cheers, Wayne
 
qcroanoke said:
What about the plastic thingies in plastic boxes that you push the romex through. Are the considered a clamp and counted?
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I would count them just to be safe because it states if the clamping mechanism is outside of the box, you don't need to count it. It's a gray area with those, but I see the clamping mechanism inside the box.
 
wwhitney said:
No, 314.16(B)(2) is talking about something more exotic. It mentions a "clamp assembly that incorporates a cable termination for the cable conductors". I'm thinking some of the cable conductors don't actually enter the box proper, they are terminated in the clamp assembly. So then you don't count those conductors. If they terminate to a pigtail that does enter the box, you count the pigtail (unlike a "conductor, no part of which leaves the box" per 314.16(B)(1)).

That's certainly what's being described. I have no idea if there are any commercial products that match the description.

Cheers, Wayne
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Best guess I've heard yet! Thanks Wayne. Hopefully someone in here has used or has knowledge of such clamps and can shed some light on it.
 
wwhitney said:
No, 314.16(B)(2) is talking about something more exotic. It mentions a "clamp assembly that incorporates a cable termination for the cable conductors". I'm thinking some of the cable conductors don't actually enter the box proper, they are terminated in the clamp assembly. So then you don't count those conductors. If they terminate to a pigtail that does enter the box, you count the pigtail (unlike a "conductor, no part of which leaves the box" per 314.16(B)(1)).

That's certainly what's being described. I have no idea if there are any commercial products that match the description.

Cheers, Wayne
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Sounds like you are describing a receptacle - which if in the box is counted - as two conductors.

If not that maybe a molded plug assembly - but that is not so common with chapter 3 wiring and more so common within appliances that are covered by listing standards instead of code.
 
wwhitney said:
It mentions a "clamp assembly that incorporates a cable termination for the cable conductors"
Click to expand...

Possibly a mobil or prefab home type receptacle?
kwired said:
If you don't use the clamp assembly at all I'm pretty sure you don't have to count it though.
Click to expand...

A clamp takes up space. My understanding always was that whether you use one or both sides of an internal clamp it counts as one of the largest size conductors in the box. If an unused clamp is left in place, it still counts as one of the largest conductors. So, remove those unused clamps!

-Hal
 
hbiss said:
Possibly a mobil or prefab home type receptacle?


A clamp takes up space. My understanding always was that whether you use one or both sides of an internal clamp it counts as one of the largest size conductors in the box. If an unused clamp is left in place, it still counts as one of the largest conductors. So, remove those unused clamps!

-Hal
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I don't think the mobile or prefab home type receptacles fit into art 314 or that you can introduce any conductors to them that aren't covered in their listing/instructions, most cases one cable in, one cable out will likely be the limitations.
 
My world just turned upside down :oops:

I ALWAYS thought each metal clamp was to be counted as a single conductor. BUT apparently 314.16(B)(2) states other wise, one or more = single volume allowance.
Still trying to find the allowance for internal PVC box clamps... that I never count.


Maybe conductors originate within clamp assembly is when you look in the box and see the ground or other wire cut right back up by the clamp because they didn't know what to do with it (kidding).
 
My guess is the module type setup that comes with some fixtures. Under the canopy you mount a receptacle and the fixture tails have a cap that plugs into the device in the box
 
ritelec said:
My world just turned upside down :oops:

I ALWAYS thought each metal clamp was to be counted as a single conductor. BUT apparently 314.16(B)(2) states other wise, one or more = single volume allowance.
Still trying to find the allowance for internal PVC box clamps... that I never count.


Maybe conductors originate within clamp assembly is when you look in the box and see the ground or other wire cut right back up by the clamp because they didn't know what to do with it (kidding).
Click to expand...
I have been taught and always assumed those non metallic boxes with clamps for NM cable are marked with clamp deduction(s) built into the marked volume. All you need to do is count the grounds as one and devices as two conductors toward the fill.

That could be wrong, if so it only effects your conductor count by one of the largest conductors in the box though.
 
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