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I'm rewiring 3 bedrooms on the second floor of an old (1920s) house. All knob and tube has been disconnected. I've got lighting done, but now doing bedroom general outlet circuits. I'm putting each bedroom on its own 20amp circuit.

The house has plaster walls that are in very good shape, and I want to keep opening up these walls to a minimum. My plan is to make the home runs from the basement to an outlet in each room, through an exterior balloon framed wall. Then, run up to the attic/crawlspace and back down to the next outlet, and so on. Each room has 5-6 outlets, so that means a cable coming down, and another going back up in the same stud bay. I had a couple people advise that this was a good way to do it, including a commercial electrician.

I already roughed in one of the bedrooms this way, but then looking at other examples I saw online, I'm second guessing myself. Many people seem to be cutting a horizontal channel out of the walls to then make horizontal runs between outlets. Another option I came across was one central junction box in the attic for each circuit with wires dropping down to each outlet.

Is there anything wrong with my method (not just from a code perspective)? The extra wire doesn't bother me, I just don't want to make any compromises in safety.

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Any of the above are fine. I would not be a fan of the "make a channel horizontally" method because of the destructive factor.

The "star" topology, one large junction box in the ceiling, is ideal because it keeps wiring length the shortest, just panel to box and box to receptacle. It is also the easiest if down the road you want to increase power capacity in there to run portable heaters, gaming PC, what have you.

The box needs to be large. You will need 4.5 cubic inches, plus another 4.5 cubic inches for each cable coming into it including supply. My go-to is a 4-11/16" steel box, which has 42 c.i. good for 8 cables. A proper electrical supply will have them for $2-something, expect to overpay at a big-box store, which is ironic since you'd think they'd specialize in, well, big boxes.

However, the attic junction box must remain accessible. It can't be "improved over" or if you do, you have to put a hatch there to allow access without disassembling any part of the building. Also don't bury the box in insulation, it needs air contact to carry away any internal heat.

  • Thank you for the response! All of my outlets connected inline as suggested total around 130ft of 12awg cable. With the last outlet in the chain loaded at 16amps I'm calculating a voltage drop of 5.5%. Thats unlikely, but I want to make this installation foolproof. Maybe your suggestion of using a star design is the right idea. I've been told, perhaps incorrectly, that J boxes should be used sparingly as they can be a fire hazard with big continuous loads such as a space heater or a/c unit. As long as the box is large enough, are there any other risk factors here? – Uncle Woody Oct 8 '18 at 16:42
  • Also, the crawlspace doesn't have a great place for an accessible j box (loose fill insulation over the joists), and is very hot. Needs better ventilation, but that's another project. The basement is much cooler, and easier to work in. Shouldn't really affect the length using crawlspace or basement. I'll probably use the basement, but what do most people do to mount j boxes in attic crawlspaces? Can you mount on the sides of rafters? – Uncle Woody Oct 8 '18 at 16:47
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    Oh dear, yes, 130' is way too much for a 120V circuit. You could upsize a wire size to #10. A hot attic might be cause for that same derate. I don't see how junction boxes could start a fire unless they're non-metallic, so use metal. Don't bury it in insulation.If I was that worried about fire, I'd mount it centered on a sheet of steel in clear air, between the steel box and sheet that's a lot of heat dissipation in case there is arcing in there. An AFCi breaker (now Code for bedrooms) will put the kibbosh on any arcing regardless. – Harper Oct 8 '18 at 16:56
  • Thanks for your help with this one and my last post. 130' was a liberal estimate, but at the very least it's near 100'. I was able to get the boxes you recommend from the local electrical supply. Good call. They were $2 each. Going to mount between floor joists in the basement with a steel bracket. I'm probably being overcautious, but I'd rather be redundant and above and beyond code. Thanks again. – Uncle Woody Oct 9 '18 at 0:51

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