Question on electrical box fill

[us3rnotfound]

I am helping out with remodeling a kitchen and one desire is to add a receptacle for a wine cooler under the kitchen counter. One good looking option for feeding power to this new receptacle is simply tapping into an existing receptacle right on the other side of the wall. That existing receptacle is in a standard single gang steel 2.5" deep device box with two 12/2 NM already filled with a 15A receptacle. To tap into this box I plan to add one 12/2 NM and pigtail the hot and neutrals and neatly fold the wiring back into the box. The device box by the way has 4 punch-outs, two on top (already occupied) and two on bottom (will use one).

The twist is I've already done the work. This is where the phrase 'knows enough to be dangerous' hits home like a brick or two. It wasn't too painful really to fit all of the wiring into the box after neatly tieing the hots and neutrals and strapping the earth ground to a steel clamp screw. However, I'm realizing, as a rude awakening while doing the box fill calculations post-work (yeah, yeah), that the only size single gang box that can legally fit 3 12/2 NM conductors, when taking into account the ground, clamp, and receptacle, that the only size box on the market to achieve this is a 22.5 cu in old work box. I am 99% certain the box I worked in is not 22.5 cu in, but rather more likely 18-20, max. But again the work was relatively easy and I'm sure is done all the time. I'm thinking I should go back and hacksaw out the old box and replace it with the deepest single gang old work box I can find, but I wouldn't be stubborn to resist this chore with advice leading otherwise.

Has anyone else done this and what did you buy to get this goal reached of tapping into an electrical box already servicing a non-GFCI receptacle? This cannot be an uncommon thing, and the thing is most receptacle boxes will already be serviced by 2 sets of Romex cables since they are daisy chained. Thanks is advance.

 

[NetWareHead]

I have a junction box in my basement where 5 different wires converge (a powered wire from the switch and 4 wires leading to flourescent lighting fixtures). My electrician confirmed it is too small for all the wiring; I dont remember the cubic inch sizing etc...

Yes I could have replaced the box with a deeper box that would have acommodated all the wires and met the spec. But when I opened it up, everythign was made up properly and no exposed copper that would short anywhere so I decided to leave it be. Replacing the box for a larger one would have entailed possibly replacing some wiring as there is just enough slack left in the box to tie together the conductors and make proper splices. The box is crammed but IMO safe. It is just impossible to work with should I need to get in there.

There are some standards/rules that I won't violate and others that I'm more comfortable breaking. If I were a proper electrician with a license, working on a customer's house and expecting an electrical inspection then no question about it, I would do it to spec. In my own house and following all other pertinent safety standards, I'd probably avoid the hassle and leave it.

 

[us3rnotfound]

I have a junction box in my basement where 5 different wires converge (a powered wire from the switch and 4 wires leading to flourescent lighting fixtures). My electrician confirmed it is too small for all the wiring; I dont remember the cubic inch sizing etc...

Yes I could have replaced the box with a deeper box that would have acommodated all the wires and met the spec. But when I opened it up, everythign was made up properly and no exposed copper that would short anywhere so I decided to leave it be. Replacing the box for a larger one would have entailed possibly replacing some wiring as there is just enough slack left in the box to tie together the conductors and make proper splices. The box is crammed but IMO safe. It is just impossible to work with should I need to get in there.

There are some standards/rules that I won't violate and others that I'm more comfortable breaking. If I were a proper electrician with a license, working on a customer's house and expecting an electrical inspection then no question about it, I would do it to spec. In my own house and following all other pertinent safety standards, I'd probably avoid the hassle and leave it.

That box in your basement sounds pretty safe to me, box fill aside, because you will always have the same load demand from the fluorescent lights. With a receptacle it will vary

Now, the code specifies box fill mostly because each connection creates some heat, which will accumulate inside the box. But my question is, how much hear are we talking here? I know it will depend on how well the connections are made and the wire nuts used. I made damn sure to strip the appropriate length and tie them together with wire nuts rated to conductor size and quantity. So if your wires were for some reason to shift and cause arcing that would cause heat to build up quickly.

 

[NetWareHead]

Now, the code specifies box fill mostly because each connection creates some heat, which will accumulate inside the box. But my question is, how much hear are we talking here? I know it will depend on how well the connections are made and the wire nuts used. I made damn sure to strip the appropriate length and tie them together with wire nuts rated to conductor size and quantity. So if your wires were for some reason to shift and cause arcing that would cause heat to build up quickly.

I might be getting beyond my own expertise here but I always thought the heat was generated to due to natural resistance in wire of an operating circuit. Shifting wires and arcing AFAIK are not anticipated heat sources. RE: the shifting wires, each wire should be secured by a tightening clamp before it enters the box (metal and plastic boxes each with their own design) to prevent movement so the splices themselves do not bear any load in securing the wire.

 

[natto fire]

I'm assuming you used NEC article 314.16 (B) to do your calculations? Did you read (5) about EGCs?

From the 2014 code:

"Where one or more equipment grounding conductors or equipment bonding jumpers enter a box, a single volume allowance in accordance with Table 314.16(B) shall be made based on the largest equipment grounding conductor or equipment bonding jumper present in the box..."

Basically, you only have to count the ground once, as it is not a current carrying conductor.

As for your second question, I see overstuffed boxes all the time, and the only time I have seen heat damage is when a splice was made poorly, or some hack overloaded a neutral. I mainly do commercial work, which is changed out more often residential. I suppose heat damage from overfill is more likely in a residential setting, with older 60° or lower wire, and some houses having branch wiring much older than your average commercial space.

 

[stormkroe]

In the CEC, it's 12-3034 and Table 23. At 2.5" (12.5 cubic inches such as iberville 2104) deep device box is allowed 7 #12 wires.
2 Wirenuts count for 1 conductor, a flush mounted device counts for 2, bond wires aren't counted, hot and neutral are 1 each. So, 3 12-2 nm wires in that case would be a count of 9.
Table 23 says you would need a 16 cubic inch box, such as iberville 3104 (I prefer model 2004 for large needs like GFIs, which is 18.5 cubic inches).