box fill and travellers

[Energy-Miser]

Greetings,
How does code treat the traveller wires for 3-way switches, as far as box fill is concerned? Any code references would be appreciated. Thanks, E/M.

 

[chris kennedy]

Greetings,
How does code treat the traveller wires for 3-way switches, as far as box fill is concerned? Any code references would be appreciated. Thanks, E/M.

Each one counts. Current carrying conductors in conduit may be a different story.

314.16(B)(1) Conductor Fill. Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once, and each conductor that passes through the box without splice or termination shall be counted once. Each loop or coil of unbroken conductor not less than twice the minimum length required for free conductors in 300.14 shall be counted twice. The conductor fill shall be calculated using Table 314.16(B). A conductor, no part of which leaves the box, shall not be counted.
Exception: An equipment grounding conductor or conductors or not over four fixture wires smaller than 14 AWG, or both, shall be permitted to be omitted from the calculations where they enter a box from a domed luminaire or similar canopy and terminate within that box.

 

[Energy-Miser]

Each one counts. Current carrying conductors in conduit may be a different story.

Thanks, yes that's what I was afraid of. I was hoping that the two of them would count as one, since they can only be active one at a time, but that makes more sense for derating in conduits as you referenced. Thanks again, E/M.

 

[Dennis Alwon]

Thanks, yes that's what I was afraid of. I was hoping that the two of them would count as one, since they can only be active one at a time, but that makes more sense for derating in conduits as you referenced. Thanks again, E/M.

Box fill is very different than conduit fill . If I were to figure the # of CCC in a conduit I would only count 2 of the 3 wires of a 3 way. As you said only two of the 3 wires can carry current at the same time.

 

[celtic]

Box fill is very different than conduit fill . If I were to figure the # of CCC in a conduit I would only count 2 of the 3 wires of a 3 way. As you said only two of the 3 wires can carry current at the same time.

....and still count the travelers as fill in said conduit.

 

[Dennis Alwon]

....and still count the travelers as fill in said conduit.

But of course-- I was referring to the derating issue. Thanks for clarifying for others.

 

[celtic]

But of course-- I was referring to the derating issue. Thanks for clarifying for others.

I knew what you meant :smile: ...but like you said ~ the casual reader may not follow

 

[wwhitney]

Box fill is very different than conduit fill.

OK, so here's a question about the theory behind the NEC provisions:

Derating for multiple conductors in a confined space (wires in conduit, NM cable in thermal insulation, etc.) is based on the need to safely dissipate all the heat generated without exceeding the temperature rating of the conductor insulation.

Conduit fill limitations are based on the mechanics of pulling the wires into the conduit and avoiding damage to the conductor insulation.

What are the box fill limitations based on? My understanding was that the primary factor was again being able to dissipate the heat generated by the wiring and the device safely. From this point of view, why aren't two travelers counted as just one conductor for box fill? Note that all the EGCs count as only one conductor for box fill, which makes some sense from the heat dissipation point of view.

Cheers, Wayne

 

[JohnJ0906]

OK, so here's a question about the theory behind the NEC provisions:

Derating for multiple conductors in a confined space (wires in conduit, NM cable in thermal insulation, etc.) is based on the need to safely dissipate all the heat generated without exceeding the temperature rating of the conductor insulation.

Conduit fill limitations are based on the mechanics of pulling the wires into the conduit and avoiding damage to the conductor insulation.

What are the box fill limitations based on? My understanding was that the primary factor was again being able to dissipate the heat generated by the wiring and the device safely. From this point of view, why aren't two travelers counted as just one conductor for box fill? Note that all the EGCs count as only one conductor for box fill, which makes some sense from the heat dissipation point of view.

Cheers, Wayne

There is also the physical size needed for conductors, which, (IMO) is why clamps, hickeys and the like have to be counted towards box fill.

 

[Dennis Alwon]

I don't even pretend to have the answers but I disagree with a few things here. Conduit fill is certain concerned with damage to wire when pull in the conduit but I believe heat dissipation is also a part of it. The same is true in a JB. Although the travelers may not have current flowing thru them at the same time they are taking up space in the box. The question would be is why an egc is only counted as one. Perhaps part of the reason is that there is no current flowing thru them.

The code does not really give us the reason esp. for box fill. The pipe fill it does consider both fill space and CCC. I also think that writing the code to cover every scenario is too difficult so they covered the bases the best they could.

 

[infinity]

The pipe fill it does consider both fill space and CCC.

Dennis, does pipe fill really care about CCC's? If so wouldn't a 4" conduit with 6 CCC's require less derating than a 1/2" conduit due to the huge amount of extra airspace within the conduit?

IMO conduit fill is only about installing the conductors without damaging them. Derating is a completely separate issue.

 

[Dennis Alwon]

Dennis, does pipe fill really care about CCC's? If so wouldn't a 4" conduit with 6 CCC's require less derating than a 1/2" conduit due to the huge amount of extra airspace within the conduit?

IMO conduit fill is only about installing the conductors without damaging them. Derating is a completely separate issue.

You may be correct but as I said before that trying to cover all the scenarios is too difficult. I really believe that is why we can only fill 40%. I could install a lot more conductors than the 40% fill without damage to conductor esp. in straight pulls etc. There are many variables and I think it is possible that CCC is one that the code panel had considered.