Last year I ran a subpanel and all wiring to an addition on a house and passed the underwriters inspection. In the process, I learned about calculating the number of wires and switches or outlet's that can be placed in a receptacle box. I was told (by the underwriter) that when you look at a receptacle box, the numbers written in the box work as follows:

If I have a 20.3 cu. in. box, it has numbers in it that read "10/14, 9/12, 8/10" which I understand means 10-14 gauge wires, 9-12 gauge wires, etc... I was told that to calculate the usage for the box that the switch or outlet count as 1.

Following this, if I ran 14 gauge wire to that box and a switch, a 14/2 supply feed, a 14/2 load feed and a switch would add up like this: 2 14/2 feeds = 4 points. Ground wires = 1 point. Switch = 1 point. That would be a total of 6 points and well within the 10 point total.

Someone told me that now you have to count the switch as 3 points. I would still be fine in my scenario, but the real question I have is "does a switch count as 1 or 3 now?"

Also, the electric dept. guy at home depot used a different method. He seemed to know how many cu. in. each piece required and added those up.

First, is the source breaker a 15 Amp or 20 Amp feed? I need that answer.

Second, 14/2 is saying that the two 14 gauge conductors and a bare ground wire inside of a sheath.

The contents of the box are counted based on the cubic inch value for the conductor, as in T.314.16.(B). In this case 2.0 cubic inches. For example, if you use 14 AWG conductors, you would have (2X2.0) + each cable clamp = 1 (2.0 value) + Each yoke mounted item (a switch or receptacle = 2X2.0) + 1 X 2.0 for or more EGC conductors.

I cannot find a device type box matching your 20.3 cu. inches, however your box does give its volume so we are fine.

2 14 AWG Conductors = 2X2.0 = 4.0
2 clamps = (1X2) X (2.0 ) = 4.0
1 Yoke = 2X2.0 = 4.0
total = 12.0 cu. inch volume

Thanks Don. My example was just a sample. I am wiring an entire house so there will be many different scenarios. All new wire. Romex 12/2, 12/3, 14/2, 14/3 and some 240 (10/3 and possibly 8/3).

My question was mainly about what the actual switch or outlet counts as. Any 14 gauge will be run to a 15 amp breaker. Any 12 gauge will be run to a 20 amp breaker. All lighting will use 14 gauge. All Kitchen, dining room, bedroom, bathroom outlets will be 12 gauge.

The boxes I'm using are from Home Depot. They are new work blue plastic nail in boxes by Carlon. The single gang boxes say 20.3 cu in and the double gang boxes say 32 cu in.

I want to run 14/3 and 12/3 home runs where possible to minimize the supply pulls and pigtail in the boxes so that each home run can supply two branch circuits. My fear is that I will over run the capacity of each box if outlets and switches now require more space.

I'm not sure I understand your math. What do you mean by "Clamp" and "Yoke"? As I mentioned in my 1st post, I was told a different way of calculating than by cu. in. but it sounds like your way is the way licensed electricians do these calculations.

Thanks again,

A clamp is the cable holder that the wire slides into and then is clamped down down to hold the wire in place.

A "Yoke" is a device that holds a switch or receptacle in place. In other words, your switch mounts into a Yoke (that's the way it comes in the package) the Yoke mounts onto your box. Each yoke mounted device is calculated as 2 X ( the largest conductor's volume) for 14 gauge, it would be 2 X 2.0 = 4.0 for one yoke or 4 X 2.0 = 8.0 Volume for 2 yokes.

For 12 gauge, it would be 2 X 2.25 = 4.5 cu. inches volume or 4 X 2.25 = 9 cu. inches. T 314.16.A gives the mimium volume of the boxes and the maximum number of conductors by gauge. T 314.16.B gives you the volume for each gauge size used.

Just curious, are you an electrician? If you are in fact re-wiring an entire home, you would be wise to have an electrician do it. There are all manner of codes that you have to be aware of. Plus, you more than likely need to get a permit from your AHJ. If you are planning to change out your main panel or up your service, you need to work with your Power Company. Good luck :)

I understand the clamp (that is what I thought you were referring to). These boxes have no clamps; they are plastic with what I would describe as a pressure clamp built in. There are small tabs that bend and let you slide the wire in. The plastic edge prevents the wire from pulling back out.

The Yoke is still unclear to me. If I buy a light switch, it comes with two screws at the top and bottom of the switch. Those screws go directly into the plastic boxes (there are screw holes at the top and bottom). Maybe you are referring to a metal box that has metal pieces (yoke?) screwed into the metal box where the switch screws mount?

Regardless, you don't seem to be counting the switch (or outlet fixture) itself in the calculation of cu inches used.

Lastly, I'm glad you are concerned about my qualifications to perform electric work. I'm sure there are many home owners who attempt to perform electric work and pay no attention to code or safety. I myself saw a lot of this type of work in my last home. I am not an electrician but have been doing small electric work since I was young. I have a good understanding of the basics, but do not know everything I need to know about the code. I actually almost purchased the 2008 code book on Wednesday and would have gladly spent the $75 but I found it difficult to locate items in there since it appears to be arranged in order of article # and I think I might spend more time digging for a code than finding. Anyway, I'm not an electrician however the town where I'm doing the electric work allows home owners to perform electric (I have a permit) so long as it is inspected by a licensed underwriter. I just don't want to do something that I believe to be to code and find out I wasted a lot of time and materials at the inspection. In addition, I have a friend who is a licensed electrician and he is coming over tomorrow afternoon to go over the entire installation with me. I suspect that I will hire him to help me with some of the major tasks (I think I need to install a new service mast, change out the meter, and replace the entire electric panel).

Thanks again for your assistance.

There are several study guides available as a companion to the 2008 code book. I'm using the one authored by Ray C. Mullin. I picked both up at Barnes and Noble.

Also, the is a hardback Illustrated code book for 2008 that comes with a DVD or CD. I believe it sells for $130.

Thanks again for your help.