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Old 08-21-2010, 05:52 PM   #1
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Impossible switch wiring configuration?


The answer to this appears obvious yet I find it hard to believe since it seems like this switch wiring configuration would be quite common: is it really not possible with the common 3x2x2-1/2 switch box to run the power into the switch box first and from there to the fixture to be controlled? I calculate as follows:

3x2x2-1/2 switch box capacity: 12.5 cubic inches

2 x 14-2 cable: 4 current carrying wires: 8 cubic inches.
All grounds: 2 cubic inches
switch: 4 cubic inches

So that's 14 cubic inches right there and 16 if you use an internal clamp.

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Old 08-21-2010, 06:55 PM   #2
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Impossible switch wiring configuration?


My math must be differnt than yours 3X2=6 6X2.5=15

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Old 08-21-2010, 08:03 PM   #3
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Impossible switch wiring configuration?


All the plastic switch box what you run into they will have cubic inches stamped in the box that is the size of the box then you have to count of the conductors plus ground plus switch to get the fill caluation.

It should get up near the limit but never over at all.

And that switch box you descrbing that should be a 18 cubic inches { look for the stamping on inside back of the box }

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Old 08-22-2010, 10:51 AM   #4
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Impossible switch wiring configuration?


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Originally Posted by hayewe farm View Post
My math must be differnt than yours 3X2=6 6X2.5=15
I got 12.5 cubic inches from a chart of box capacities. 3x2x2-1/2 is the nominal size. The actual size is somewhat smaller. Just to be sure I measured one and I get: 2-7/8 x 1-13/16 x 2-1/2 = 2.875 x 1.8125 x 2.5 = 13.03 cubic inches...Not too far off what the chart says.
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Old 08-22-2010, 10:59 AM   #5
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Impossible switch wiring configuration?


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Originally Posted by frenchelectrican View Post
All the plastic switch box what you run into they will have cubic inches stamped in the box that is the size of the box then you have to count of the conductors plus ground plus switch to get the fill caluation.

It should get up near the limit but never over at all.

And that switch box you descrbing that should be a 18 cubic inches { look for the stamping on inside back of the box }

Merci.
Marc
Thanks Marc. Maybe that's just for palstic boxes? These are metal.
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Old 08-22-2010, 01:30 PM   #6
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Impossible switch wiring configuration?


I mention plastic boxs but with metal boxes they will useally not stamp cuibic inches { I know that true with older box } but some newer metal box some will stamp but most case I ran into they useally not due with metal box there is too many diffrent way you can conferage them by the box and the cover { that will change a bit as well }

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Old 08-22-2010, 07:47 PM   #7
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Impossible switch wiring configuration?


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Originally Posted by Ptron View Post
The answer to this appears obvious yet I find it hard to believe since it seems like this switch wiring configuration would be quite common: is it really not possible with the common 3x2x2-1/2 switch box to run the power into the switch box first and from there to the fixture to be controlled? I calculate as follows:

3x2x2-1/2 switch box capacity: 12.5 cubic inches

2 x 14-2 cable: 4 current carrying wires: 8 cubic inches.
All grounds: 2 cubic inches
switch: 4 cubic inches

So that's 14 cubic inches right there and 16 if you use an internal clamp.
Yup, generally that's correct for an unstamped, metal box. That size box is usually only good for a switch loop or an end-of-the-run receptacle when using romex.

your example: cable in, cable out, switch, gnds., internal clamp= 16 cu in for 14awg

switch loop only: cable in, switch, gnds, internal clamp = 12 cu. in for 14awg

end-of-run receptacle: cable in, recep, gnds, internal clamps = 12 cu in for 14awg. Use a box with knockouts instead and you can put it on a 20A circuit w/ 12awg: cable, recep, gnds = 5x2.25=11.25 cu in.

If you're using conduit, you can power>switch>light by having the neutral pass thru the box so it doesn't get counted, but not very common.

What I could never figure out is how this 6.5 cu in. Steel City switch box is good for anything - unless the switch is completely mounted externally of this box...

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Old 08-23-2010, 02:27 PM   #8
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Impossible switch wiring configuration?


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Originally Posted by williswires View Post
Yup, generally that's correct for an unstamped, metal box. That size box is usually only good for a switch loop or an end-of-the-run receptacle when using romex.

your example: cable in, cable out, switch, gnds., internal clamp= 16 cu in for 14awg
Thanks for confirming. I was in the middle of hooking up a switch like this when I realized I better check the capacity. Up until that point it hadn't occured to me to check since it seemed like it would be so common to do it this way. Apparently not. Oh well. Just have to wire it as a switch loop.


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What I could never figure out is how this 6.5 cu in. Steel City switch box is good for anything - unless the switch is completely mounted externally of this box...

Saw this box at the store and wondered the same thing.
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Old 08-23-2010, 08:15 PM   #9
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Impossible switch wiring configuration?


I use the super thin junction box like that but very limited use normally I steer clear of that unless you got the pocket door frame set up { that is not my favour items at all }

If any reason if I can not find a convetal wall to set up a box then I will use it but very rarely { maybe 1 to 3 time a year I ever touch it } and they are basically no room at all due the pocket doors I work on they are only 3/4 inches plus whatever you add on the wall like drywall or whatelse you throw it on so it kinda judgement call on that one.

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Old 08-25-2010, 09:19 PM   #10
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Impossible switch wiring configuration?


Two questions on a related note:

I already removed two of the NM knockouts. I thought all holes were required to be plugged but as far as I know they don't make plugs for those. Can I still use the box?

And an even dumber question. They have 3-1/2" deep switch boxes at the store. They have plaster ears but no other mounting hardware. How were these intended to be mounted?
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Old 08-25-2010, 09:25 PM   #11
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Impossible switch wiring configuration?


Quote:
Originally Posted by Ptron View Post
Two questions on a related note:

I already removed two of the NM knockouts. I thought all holes were required to be plugged but as far as I know they don't make plugs for those. Can I still use the box?
Yeah but I will leave the NM konckout alone but you can use the electircal tape to seal up the holes { that work for short term useage that about it }

Quote:
And an even dumber question. They have 3-1/2" deep switch boxes at the store. They have plaster ears but no other mounting hardware. How were these intended to be mounted?
That is old work / remodeling box they have flange to hold angist the wall and behind there is a ears or levers to hold it together.

Few boxs I will use the old work box strips it will look like F shape and slide it in the wall between the box and wall and once you set it right and fold the tabs it will hold the box in place pretty good.

{ the Electricians will call F ears or battleship or few other trade realted words but all end up the same item }

Merci.
Marc
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Old 09-07-2010, 08:58 AM   #12
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Impossible switch wiring configuration?


Now wait a minute. Sorry to drag this thread back up but I was just looking through my wiring books for something when it hit me. Most of the example circuits shown are point to point to point, i.e. cable goes into outlet box and from there on to next outlet box, and so on. There are pictures of standard size switch/outlet boxes wired this way in both books I have. Circuit diagrams I'm looking at online show it this way. This would violate code for capacity just the same, would it not? So why is this ok?
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Old 09-07-2010, 11:13 AM   #13
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Impossible switch wiring configuration?


A lot of existing metal boxes are small compared with commonly available plastic boxes. If the sides detach by unscrewing, you can get separate side panels with a bulge to increase the space inside with.
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Old 09-07-2010, 01:47 PM   #14
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Impossible switch wiring configuration?


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A lot of existing metal boxes are small compared with commonly available plastic boxes.
Oh, that must be it. I made a bad assumption that the metal and plastic ones would be the same size. Thanks

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