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Just purchased a new house and have been doing some electrical work, and I've been surprised at how many things have been crammed into single circuits.

For example, one circuit seems to serve four light fixtures and five outlets across three rooms. That feels super overcrowded! I'm considering my options with re-wiring some of them as well as strategic use of GFCI/AFCI/DFCI breakers/outlets. But I'd like to really understand what is happening before I do all that.

Is there a way to determine the order of items on a circuit? And if a circuit branches into multiple branches, is there a way to determine that?

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  • If everything is working properly, the resistance of the wire on a circuit will be extremely low. Not impossible to determine length (which can help with topology) electronically but not easy. The only practical method I know is to open up boxes, figure out what is hot (panel or earlier in circuit) and disconnect the rest and see what stops working. – manassehkatz-Moving 2 Codidact Feb 22 at 19:55
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    @dandavis That's for identifying what devices are on each circuit. OP is trying to map physical wiring layout of devices within one circuit. – manassehkatz-Moving 2 Codidact Feb 22 at 20:26
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    @manassehkatz-Moving2Codidact: ahh, I misunderstood. Within a circuit they are all in parallels, so on/off won't work, unless you're fast, like TDR fast. – dandavis Feb 22 at 20:38
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    @dandavis Yeah, TDR. I couldn't remember the term, which is why I wrote "electronically but not easy" :-) – manassehkatz-Moving 2 Codidact Feb 22 at 20:40
  • One fifteen amp circuit could be enough to have 18, 100w lights(pushing it I know). Four lights and five outlets is not that much for a circuit to handle, unless running a space heater or toaster oven on more than one outlet. – crip659 Feb 22 at 20:49
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To figure out the order of items on a circuit:

(warning, tedious and laborious, but accurate)

First, sort out which items are on the circuit - i.e. what turns off/on with the breaker.

Turn the breaker off.

Pick any device on the circuit, open it up. If there's only one cable coming into the box, close it up again, noting that it's an end.

If there are two or more cables coming into the box, note exactly what is connected to exactly where, and disconnect one cable. Turn the breaker on and see what no longer works. Note that those things are "downstream" of the item in question. Turn breaker off and repeat if there are more than two cables. Reassemble everything as it was, move to the next device location, repeat.

If you find any "backwired" connections, remove them and move that wire to the associated "side screw" when reconnecting.

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  • You are really suggesting that someone mess with each outlet in their house? – DMoore Feb 23 at 20:46
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    For fewer trips back and forth to the breaker box (and to avoid having to work with mains power at all), leave the breaker off. Find the outlet at the end of the line and short the two prongs together. Now, you should be able to use your disconnect-and-test strategy to trace the line backwards by using a voltmeter to check for continuity. Just remember to un-short that outlet before turning the breaker back on. – bta Feb 23 at 22:45
  • Thanks, this seems like the most reliable, least fussy answer. (Though yes, possibly more work.) – brentonstrine Feb 23 at 22:45
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    @bta, if you want to safely short the prongs together, plug in a lamp with an incandescent bulb and turn it on. It's show up as a resistance on the order of a few tens of ohms. – Mark Feb 23 at 23:24
  • @DMoore Insofar as that's the entire premise of the question, yes. Though it's not every outlet in the house, just the ones controlled by a single breaker. – Tim Sparkles Feb 24 at 2:31
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I think there's a way along a single branch to map out what's closest to the breaker and what's closest to the end of the line. It's somewhat dependent on wiring quality, having it consistent along the branch under test.

You need a voltmeter and a 1500W space heater.

  1. Unplug everything you know about from the branch.
  2. measure +note the voltage at each outlet on the branch.
  3. plug in the space heater and turn it on full blast.
  4. re-measure the voltages at each outlet.

If the outlet is between the heater and panel, it will have higher voltage than the heater's outlet. the heater should have basically the same voltage as all other outlets on the branch that are further away from the panel than the heater.

Highly simplified and exaggerated for clarity, here's what that looks like in a perfect sim: enter image description here

The 10 ohm resistor connecting "live and neutral" is the space heater, and the 1 ohms resistors represent the old wiring. All the slots after the heater are the same voltage, while in front of the heater the voltage drops off a little on it's way to the heater.

I think this could help you not only index the nodes, but also map out where your weakest wiring; the segments that drop the most voltage would be primae candidates for replacement.

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    How would you determine where the light bulbs are? You can't exactly plug a space heater into a light bulb socket. – nick012000 Feb 23 at 9:38
  • @nick012000 Well technically it is possible to plug a space heater into a standard E25 lighting socket, but that will exceed the Listed Rating of the outlet, and thus violate the Electric code. One can only draw 660w from a standard E25 lighting outlet. For that purpose they make and sell UL listed Edison screw to NEMA 5-15 outlet converters. So I guess you would need to temporarily install a normal non-lighting outlet, or use only a 660w space heater for those. – Kevin Cathcart Feb 23 at 14:34
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    This makes me wonder if you could a) disconnect the breaker (really important!), b) short the hot and neutral "inside" the breaker (i.e. the house wires that are now not live), and then c) measure the resistance at each outlet. In theory, with a sensitive enough meter, the resistances should approximate the length of wire from the panel to the point of measurement. But this might be incredibly stupid? (I would also take a voltage across the wires before shorting them to make sure they aren't live!) Advantage: you don't need to be drawing significant power to take readings. – Matthew Feb 23 at 14:52
  • @Matthew There are devices that do exactly that. They are used routinely to determine where a break is in a long communications (phone, network, coax TV, etc.) line. But they need to be extra sensitive to handle both the short distances in a house and the much thicker wire used for AC power distribution. There are other devices that time how long a signal takes to get from one end to the other, but they are quite expensive. – manassehkatz-Moving 2 Codidact Feb 23 at 15:06
  • Would the voltage drop from a 100W incandescent bulb be large enough to be easily measurable? – Dan Is Fiddling By Firelight Feb 23 at 16:14
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I have worked on older homes that all the lights in the entire home were on the same circuit (plus a few receptacles).

The requirement for residential pre 2021 is 1ea 15a circuit per 600 sf or 1ea 20 amp circuit for 800 sf (I haven’t looked it up to see if that has changed I doubt it because residential is still 3va per sf on the lighting calculations where everything else is smaller.

These are minimum requirements and if you wish you can have each room on its own circuit.

The only way to know for most home owners is to turn a circuit off and track what is not powered. Or this is the easier way to figure it out things branch as they get further away from the breaker panel.

You could also spend a nice chunk of change and purchase a tracer like I have CS8000 , quick check on line they are running 750$ currently. This can trace the wires through the walls but that is really not needed to figure out a home where everything is working.

There is a limit to the number of wires allowed in boxes by code this is called box fill. Things do get snug when close to the max but that snug level is not unsafe and regulated by the NEC in the US.

So turn a circuit off and track it from the panel would be your best bet.

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Dandavis has the right general idea for when you can use a high current to get a significant voltage drop on your cables. This voltage drop will be significant enough when compared with the nominal mains voltage to be distinguishable from socket to socket.

For lighting circuits, you have much less voltage drop. When measuring each socket, your meter resolution may not be enough, and small fluctuations in the incoming voltage will make a mess of trying to compare socket voltage measurements made at different times.

The answer for lighting circuits (and it makes the measurements of a power circuit more reliable) is to measure the voltage drop along a conductor. This means either measuring along the neutral (ideally) between a pair of sockets, or measuring from the neutral at your board to one socket at a time. Now the small voltage can be read on a meter scale of volts, rather than hundreds of volts.

An alternative is to remove the breaker at the board, and inject a current source into your circuit, and short-circuit the far end. This allows you to measure at each socket as before, but on a volts range rather than hundreds of volts. The last time I did this, I used a bench power supply with a constant current output as my current source.

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