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Hi, i was just wanting some people to take a loot at this diagram i threw together to see if this system would work? and if so do i need to put each area that i tie the main 12/2 together in a box of some kind?

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Why 12AWG 20A? For most residential lighting, you won't get close to the max load ever, and a little voltage drop is not a problem. 14AWG 15A is cheaper, easier to work with, and sufficient for lighting-only circuits. (One exception may be solar systems, where the cost of voltage drop is higher.) –  Jay Bazuzi Nov 23 '12 at 19:49
it will be two outside motion lights, a ceiling fan and a chandelier most likely. and i was also wanting to run 6 outlets too. i wasnt sure if i could run all that on a single breaker so i was gunna have 2 separate breakers. one for lights one for outlets. but if i can run it all on a single 20A breaker then let me know lol it would just be less work for me. –  Kevin Nov 23 '12 at 20:10
Given the questions you're asking, I have to recommend that you hire an electrician for this job. It's too easy to overload a circuit, incorrectly run wires through the walls, or create an illegal junction that would result in a safety hazard. –  BMitch Nov 25 '12 at 22:15
What type of details are you looking for in a perfect answer? Your question is fairly broad, as asked. Perhaps including exactly what you want to know in the question would result in more detailed answers. As it stands, the answers to your questions are yes, and yes. –  Tester101 Nov 26 '12 at 16:31
Your diagram, as drawn, will work. But you need to wrap the white wire entering the J-box for the switch in black electrical tape to indicate that it is a hot line. –  Matthew Nov 26 '12 at 16:32

4 Answers 4

up vote 6 down vote accepted


This is what your wiring diagram should look like.

Wiring diagram

Notice the black wire is used to feed the switch, while the white wire is markered and used to feed the light from the switch. This is because the black wire going to the switch will always be hot, but the white wire "returning" from the switch is only hot when the switch is in the ON position. The white "switched" wire can be marked simply by wrapping a bit of black electrical tape at each end of the wire.

All the grounding conductors are tied together, and connected to the box (if a metal box is used).

Box Fill Calculations

According to section 314.16 of the National Electrical Code (NEC), you'll have to count each conductor that enters the box once, all the grounds as one, any luminaire studs, hickeys, or fixture straps as one, each single yoke device is 2, and internal clamps are 1. Once you have this number, you'll multiply it by the value in Table 314.16(B) according to the largest conductor used. This will give you the total minimum box size, in cubic inches (cubic centimeters).

Ceiling Fixture Boxes

For each ceiling fixture box (excluding the last), the box fill will be as follows:

3 12/2 cables connected to box = 6 (13.5cu.in.).
All grounding conductors = 1 (2.25cu.in).
A luminaire stud, hickey or strap (to support the light) = 1 (2.25cu.in.).
Fixture wires less than 14 AWG = 0 (see 314.16(B)(1)ex 1.).
Total Box Fill = 8 (18cu.in.).

Assuming you'll be using 12 AWG wires, Table 314.16(A) tells us you'll need a 4" x 2 1/8" round/octagonal box for the first 3 boxes (or a box at least 18 cubic inches).

octagonal box

The last fixture box fill will be as follows:

2 12/2 cables connected to box = 4 (9cu.in.).
All grounding conductors = 1 (2.25cu.in).
A luminaire stud, hickey or strap (to support the light) = 1 (2.25cu.in).
Fixture wires less than 14 AWG = 0 (see 314.16(B)(1)ex 1.).
Total Box Fill = 6 (13.5cu.in.).

So you'll need at least a 4" x 1 1/2" round/octagonal box, for the last box.

octagonal box

Switch Boxes

1 12/2 cable connected to box = 2 (4.5cu.in.).
All grounding conductors = 1 (2.25cu.in.).
Switch = 2 (4.5cu.in.).
Total Box Fill = 5 (11.25cu.in.).

This means you'll be able to use any device box larger than 11.25 cubic inches, for the switches.

device box

Final Diagram

final wiring diagram

While 12 AWG wire may be a bit overkill for this installation; if that's the size wire you have, there is no problems using it.

According to section 300.15 of the National Electrical Code, all junctions must be made in a box or conduit body. Section 314.29, adds that the box must be "accessible without removing any part of the building".

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He should also verify that the circuit has excess capacity for 4 more fixtures. –  BMitch Nov 26 '12 at 17:19

Yes, that should work with some notes:

1) The lights should be grounded as well.

2) Yes, any area in which you have a junction needs to be accessible. In your case, you would typically have the light fixture and the junction from the incoming and outgoing main cable in the same box.

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are you saying have the wires that are coming in, the light wires, the switch wires, and the wires running to the next light all inside the same box? as in the circular ceiling box? –  Kevin Nov 23 '12 at 20:13
Right, all in the same box, assuming it's not a flat pancake box. The electric code has a specification for how many connections you can make given wire size and box volume. You need to reverse the white and black between light and switch. With the white between switch and light marked at both ends with black tape or permanent marker. –  bcworkz Nov 23 '12 at 23:44
reverse the white and black between light and switch?? but why? –  Kevin Nov 24 '12 at 17:54
@Kevin because the line coming into the switch is always hot and should be black, but going out of the switch is a switched hot, so you use the white with black tape as an indication that it's switched. –  BMitch Nov 25 '12 at 22:09

That'll work, but...

Let start with the basics. A light turns on when 120/240 VAC is applied on one side and neutral is applied to the other side. For safety reasons you want the casing of the light to be grounded, there is typically a screw on the light dedicated for this purpose.

Of course you will also want the ability to turn this light on and off. The way this is done is by placing a switch in-line with 120/240 VAC line. I assume you have these basics under control.

Now for the logistics of making this happen. First off, all connections have to occur inside of an accessible junction box. You don't want some wires dangling around open to who knows what messing with them. But you do want them accessible, this way you just have to remove a couple of screws if a wire comes loose instead of having to dig through Sheetrock remembering where you put that connection at. The accessible junction box can be the box in the ceiling, the switch box, or can even be a box with a blank face plate who's only purpose is to protect a junction.

The next thing to worry about is those who mess with your work after you. You want them to know what is power and what is neutral. Connecting a black and white wire together makes this very difficult to determine whats what. As a result, you need to find a way to make this more clear. What you can do is connect all of the black wires together, and then put a black mark on the white wire. This will let people know that despite it being a white wire (typically neutral) it is not actually a neutral wire.

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Your diagram is a bit crude, but it will "work" as I read it.

A few things to note:

  1. All wire junctions must be inside of an accessible junction box. If these are recessed lights then they will likely have an integrated junction box where you can make the connection for both the switched wires and the wires to the next light.
  2. Using 12/2 is overkill for most residential lighting. It's thick, and expensive. If you can use 14/2, I would do so.
  3. Your wiring to the switch must have the white wire wrapped in black electrical tape to indicate that it is hot. Also, at the junction box you should attach the black wire to the line, and the white wire (now taped black) to the load. This is because the black wire is always hot, and the taped black wire is switched.
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