[ryancab]

Help understanding "conductor" in the NEC Table 370-16(a) for Metal Boxes chart

Hi,

I bought some ceiling fans for our older house built in 1913. I am trying to figure out which fan mount setup I need to get. I'm deciding between the Raco Model 936 and the 937 retro brace mounting setup. The 936 has a 4" round box that's 1-1/2" deep and the 937 has a 4" round box that's 2-1/8" deep.

I looked at one of my current, ceiling light fixture boxes and took a photo. Please see attached.

I found this web page that has the "NEC Table 370-16(a) for Metal Boxes" table in it which tells me how many "conductors" can fit in a box, but I'm not sure what my wiring type/number is and I'm not sure what "conductors" is referring to here.

• Can someone please tell me what my wiring number is from the photo?
• By conductors, is the table referring to the "individual wires in a cable"? So, for a cable that has a black, white, and ground wire, that would be a "conductor" count of 3?

Also on that page with the information table, there were a couple other bullet points I wanted to ask about, please.

• No matter how many ground wires, they only count as one conductor in the box.

• When devices are mounted in the box, the total conductor count must be increased by two for each mounting strap.

For the first bullet above, if we're counting "conductors" as individual wires, if I have 2 cables in the box, that would be a total of 5 wires?

For the second bullet above, is that referring to the plastic wire clamps that are included with the fan mounting kit that you stick in the punch out holes to hold the wires in place? So if I used 2 of those clamps, that automatically increases my "conductor" count by a value of 4?

Thanks so much for the help!

Ryan

 

[surferdude2]

Re: Help understanding "conductor" in the NEC Table 370-16(a) for Metal Boxes chart

The box you have pictured has 7 wires, as counted by code. You do not have to add for the clamps since the box is marked for in³ by the manufacturer with that already in consideration. Since the box has 18 in³ it's good for 8 #12 wires (2.25 in³ per) and 9 #14 wires (2 in³ per).

All ground wires count as one single wire.

A strap mounted device counts as 2 wires of the largest size in the box. That doesn't apply to a fixture that mounts on the box and doesn't entend inside it.

 

[ryancab]

Re: Help understanding "conductor" in the NEC Table 370-16(a) for Metal Boxes chart

The box you have pictured has 7 wires, as counted by code. You do not have to add for the clamps since the box is marked for in³ by the manufacturer with that already in consideration. Since the box has 18 in³ it's good for 8 #12 wires (2.25 in³ per) and 9 #14 wires (2 in³ per).

All ground wires count as one single wire.

A strap mounted device counts as 2 wires of the largest size in the box. That doesn't apply to a fixture that mounts on the box and doesn't entend inside it.

Thanks so much!

Okay. I noticed in the specs that the 936 model mounting kit says that the electrical box is 15.8 cubic inches, which would just barely fit my 7 wires needing 15.75 cubic inches, so I think that I'll probably go with the larger 937 model, though then, which has a volume of 22.4 cubic inches and a little more breathing room for me.

Thanks again! I learned some new stuff today! :smile:

 

[rjniles]

Re: Help understanding "conductor" in the NEC Table 370-16(a) for Metal Boxes chart

Your wires look like #14 not #12. If true you only need 14 cu in.

 

[ryancab]

Re: Help understanding "conductor" in the NEC Table 370-16(a) for Metal Boxes chart

Your wires look like #14 not #12. If true you only need 14 cu in.

Thanks! Yeah, I went ahead and ordered the larger kits anyway because, strangely enough, they were cheaper than the smaller ones. :smile: Thank you!

Ryan