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Order of ac outlets on branch circuit

John Frum

John Frum

Tell me your problems
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Nov 30, 2019
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Is there a way to tell the order of the oulets on typical NEC branch circuit without pulling face plates or opening walls?
 
With breakers open and confirm opened you could do a resistance check. Long wire of meter goes to hot / live wire at the panel, which for safety is disconnected during the test, then meter is taken to each outlet and resistance checked. You can also get low voltage testers where you place the test low voltage power supply across the disconnected wire and neutral and check the loop resistance by measuring the voltage at each outlet with the test load. You could also use a tone test set. Sender goes on disconnected wire at the panel end then check signal level at each outlet.

You could even use a TDR, but I think that might be getting a bit expensive.

Basically lots of ways, if you have the test gear. I'm sure there are other approved methods too.
 
I was thinking of a "ping" device. Initiator goes in the panel, responder goes in each outlet in turn to build a map.
 
Sort of like a TDR but with active receivers ? What you'd do with a TDR is walk to each outlet and test with the TDR. It'll give you a graph of the reflections on the wire from both ends and the other sockets / joints. Since you know one end is the open wire at the panel you can work out where each socket is on the wire by the number of outlets between you and the panel.
 
You could also leave the TDR at the panel end and then using a plug with a resistor across hot / neutral go to each suspected outlet and plug in then check the TDR to see how far along that socket is. The plug linking to neutral will put a bigger impedance bump at the socket you are testing.
 
gnubie said:
You could also leave the TDR at the panel end and then using a plug with a resistor across hot / neutral go to each suspected outlet and plug in then check the TDR to see how far along that socket is. The plug linking to neutral will put a bigger impedance bump at the socket you are testing.
Click to expand...
Very interesting post, guys. At the one company where I was employed we used the various techniques to test radio frequency transmission lines where the antennae were remotely located from the transmitting / receiving equipment ,
about 1km apart.
 
gnubie said:
You could also leave the TDR at the panel end and then using a plug with a resistor across hot / neutral go to each suspected outlet and plug in then check the TDR to see how far along that socket is. The plug linking to neutral will put a bigger impedance bump at the socket you are testing.
Click to expand...

I won’t pretend that I completely understand the purpose of this thread but there is a relatively new tool out in Amateur radio circles that is dirt cheap (around $50) and can be used to find impedance bumps in RF transmission lines.

Google NanoVNA and see if it’s helpful for this use case. I’m only using mine to check SWR but it’s capable of so much more.
 
The initial idea was to find the best spot to insert one of these

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into a string of outlets

Then I thought it would be useful to know the order of the outlets to do stress testing on a branch circuit.
For instance run a ~1500 watt resistive load on each outlet in turn down the chain of outlets looking for hot spots along the circuit with an infrared camera.
I don't have any experience with infrared cameras to know if this practical or not, but that is the idea.
Both @Hedges and @gnubie seem to have anticipated where I was headed with this.
 
Simpler than a camera (megapixel) is an IR thermometer (one pixel)


I was actually proposing the 1500 watt resistive load and a volt meter to accomplish the task of finding order in which outlets were wired. Plug it in, and voltage will drop progressively from breaker to heater, then remain constant at outlets beyond the heater. As an added benefit, if you can accurately determine amps, it is an ohm meter so will give you ratio of wire gauge to length.

The camera of course has better resolution to identify hot spots.
Mobile homes here used to have aluminum wiring under copper screws in outlets, so the whole outlet would get hot and insulation charred near the connection.

What makes these impractical is that sheetrock (or lath & plaster, or whatever your walls are made of) prevent you from imaging most of the wire. But bad connections are (hopefully) only at screw terminals in junction boxes, so with outlet covers removed you can find those.

NanoVNA


Dang that is cheap! Several fewer zeros than the one I use (I'll just keep mine, though)


Now all you need is to run inverse FFT, determine location of discontinuity from the S11 frequency response.

Or, DIY TDR. Send a fast rising edge through a 50 ohm resistor into the wire. With a suitably rated DC blocking capacitor, you could even do that on a live circuit. Measure reflections with a scope. Works just like SONAR/echo location, but in only one or two directions along the wire.
 
Hedges said:
NanoVNA
...
Now all you need is to run inverse FFT, determine location of discontinuity from the S11 frequency response.
Click to expand...

And by the way, obtaining time (location) domain TDR data from a swept frequency instrument like VNA is limited by the lowest frequency used.

I was provided with 20 GHz Keysight VNA having TDR software. The calibration wizard set it to operate from 20 MHz to 20 GHz (couldn't figure out how to start at 300 kHz). I used it to measure pulse-forming networks (shorted stubs that convert a fast step into a short pulse). The results it showed (zero ohms series resistance at low frequency) didn't match the components I saw by inspection (thru-hole resistor in series from input to output of DUT.)

I repeated the measurement with an actual TDR from Tektronix, which did show the resistor.

Time Domain Analysis Using a Network Analyzer

Time domain analysis is useful for impedance measurement and for evaluating a device problem. Learn how to apply the time domain display with network analyzers.
www.keysight.com www.keysight.com

The math only works as good as the data provided.
Interpolation is usually more reliable than extrapolation.

EM waves propagate at 3 x 10^8 m/s, 12" per ns in air or free space.
In a typical plastic dielectric, relative dielectric constant of 4, propagation is slowed by sqrt(4) = 2, for 6" per nanosecond.
The 50 kHz NanoVNA has 20,000 ns maximum period, so can resolve up to 10,000' round trip, 5000' one-way. Just under a mile.
So it would work for you home wiring, probably show you all the utility drops to neighbor's houses up to a transformer a mile away. But anything over a mile would alias back into the < 1 mile range. Loss in the wire, more so in the dielectric, would cause more distant reflections to be attenuated, so at some point they could be distinguished or just wouldn't be seen.
 
Trukinbear said:
If you are just going to use it once or twice, nothing beats Harbor Freight
www.harborfreight.com

Circuit Breaker Detective

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Click to expand...

That will tell me if the outlet is on the branch but I don't think it will tell me the order.
I guess I will have to take some plates off.
If I can find the last outlet I should be able to figure out the rest.
 
smoothJoey said:
That will tell me if the outlet is on the branch but I don't think it will tell me the order.
I guess I will have to take some plates off.
If I can find the last outlet I should be able to figure out the rest.
Click to expand...
You're right, I misunderstood the question - I thought you wanted to know the order in the box, not in the circuit itself.
 
smoothJoey said:
That will tell me if the outlet is on the branch but I don't think it will tell me the order.
I guess I will have to take some plates off.
If I can find the last outlet I should be able to figure out the rest.
Click to expand...

Just plug in a 1500W heater and check voltage at other outlets.
(Turn off other breakers so you don't waste time on other circuits.)

12 awg is 1.59 ohms/1000', 1.59 mohm/foot.
Put 12A through that (if you're in the 120V world) and you get 19 mV/foot. Outlets 5' apart would be 100 mV different.
Hmm, that'll be around the resolution of a 3.5 digit meter. Plug in two meters and work fast before the breaker trips?
Better yet, plug in an extension cord and measure voltage from Hot to Hot or Neutral to Neutral.
Then you're just measuring drop in the wire, not adding line voltage to the reading.
That'll give you plenty of resolution.
 
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