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I would like to replace some Knob & Tube wiring in my attic.

Currently, the ceiling lights in most of our rooms are all on a single circuit and are branched using many knob & tube soldered junctions.

When I replace this lighting with NM cable, I would like to run a single cable into a junction box and then branch into three cables running to the lights in three different rooms. The junction box would be large enough to accommodate wires comfortably.

Does code allow multiple lights to be wired onto the same circuit, in a "star"type of topology? To illustrate what I mean, here is a crude wiring diagram:

One circuit, star topology, three rooms

I don't see any problems with this, and I would think this is done all the time. However I've checked 8 different home electrical books and I've looked online, and I still cannot find any example wiring diagrams showing this layout. (This is just a theoretical layout to show the star layout, not an actual plan.)

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What is your question? –  bib Jan 18 at 21:00
    
Not sure how I missed that. I added a question. –  Stefan Lasiewski Jan 19 at 1:38
    
Remember that any junction box must be accessible so this would mean an access panel in your ceiling. –  Steven Jan 19 at 1:44
    
It would be accessible in the unfinished attic space. –  Stefan Lasiewski Jan 19 at 2:06

2 Answers 2

up vote 2 down vote accepted

There's nothing wrong, or even unusual, about star topology in wiring.

  • Each junction box must remain accessible.
  • If you chain power from fixture to fixture, the continuity of ground can't depend on the fixture.
  • Each new switch needs power, neutral and ground. Your existing K&T likely only has two wires.

Set the topology to minimize wire or make things easier.

But if you're preserving the switch leg of the wiring, that could cause some real problems. First, the K&T topology is often confusing and the runs might not reach the attic. Second, the K&T soldered connections could well outlast your new Romex. Third, running ground and neutral to the switch points is likely to be awkward. And fourth a weak point of K&T where the wires enter a metal fixture box remains.

Especially with lighting getting more and more energy efficient, and the non-issue of ground for ceiling lights, replacing K&T lighting simply does not have much benefit for your time and money spent.

If you do somehow feel compelled to rip out the old wiring, at least consider upgrading to hardwire smoke alarms. See elsewhere for discussion of the necessary interconnect wire.

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I hear you, and I've considered leaving the K&T alone. But it's 70 years old and I might stay in this house for another 30 years. One of my main concerns about the K&T wiring is the age of the insulation. Also, I'd like to replace some of the lights with Recessed LED lights and a dimmer, and this requires a neutral. Since I'm up there already, running a few additional cables doesn't seem like much more work. –  Stefan Lasiewski Jan 19 at 18:27
    
K&T does not require functional insulation except where it enters metal junction boxes. Dimming an LED does not require a neutral. Modern wiring above with K&T switch legs is not recommended (which is what you seem to be headed for). See also diy.stackexchange.com/a/20279/5960 –  Bryce Jan 20 at 5:57
    
The LED doesn't require a neutral, but the switches often do. –  Stefan Lasiewski Jan 20 at 5:59

A star topology is fine, but you don't need to route packets with power. Choose a layout that uses the least amount of cable. There's no reason to do anything else. All the lights may be on the same circuit. The fact your diagram has two nearly parallel lines going to the upper lights tells me this is not the most efficient. You should carry power to the upper right light from the upper left instead of coming from the junction box.

Similarly, if the run from the upper right switch to lower right light is shorter than junction box to light, that would be a better layout. BTW, where's the switch for the left light?

You basically just need to run two conductors everywhere, and in addition a third conductor for when you need to carry power through next to a switch leg. If a light or switch is at the end of a run, you do not need the third conductor.

For example, if you were to run power from the upper right switch to lower right light as suggested, the run from upper right light to upper right switch needs to be 3 conductors to carry power through along with the switch leg. From upper right switch to lower right light is 2 conductor because no switch leg is there, The run from lower right light to lower right switch may also be 2 conductor because the switch is at the end of a run and power is not needed further on.

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To meet code (NEC 2011 404.2(C)) you'll also need to have a neutral at each switch, which means either running power to the switch first, or using a /3 wire between the switch and light. If you're not touching the wiring between the switch and light, you may get away without this, but it would be best to ask your local inspector. For more info, see this question. –  gregmac Jan 19 at 3:05

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