ID'ing circuits on shared neutral

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  #1  
Old 09-22-06, 05:37 PM
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ID'ing circuits on shared neutral

I've got 4 old K&T circuits coming into my box, and, of course, only two neutrals. No big deal, but nothing is marked (of course). I'd like to find out which hots go with which neutrals, as to make sure they are not on the same phase.

My thinking was to disconnect one of the neutrals from the neutral bar and wire nut it off. Turn the power back on, and test the outlets/lights on the four circuits. It shouldn't be too difficult to find out which neutral goes with which pair (hopefully it's a pair and not 3 and 1). For the lighting it would be easy, but for the outlets I believe I would need a plug in tester, as a pen tester would pick up the hot. If there is an easier/better/safer way to do this, please let me know.


Thanks.
 
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  #2  
Old 09-22-06, 07:58 PM
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No need to replace them. I've inspected the wires and they look fine. I also do not wish to turn my walls into swiss cheese in the process; most of the wiring is on outside walls.

The circuits are overloaded, so I will be removing everything I can get to and wire them with Romex. So what is left should be well within the capabilities of the wire. Of course all the K&T will be on 15A breakers, as I think they always have been (as far and I can tell).

Thanks for the advice on the testing; I'll confirm the hot receptacles for each of the circuits, then disconnect a neutral and test again.
 
  #3  
Old 09-23-06, 05:52 AM
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I would make sure that no heavy loads were on when testing this way. If those two neutrals are joined anywhere else on the circuits they are sharing the neutral loads. If you remove one of them, the other will carry all of the un-balanced current. You could overload it.
Just a thought
steve

Edit to add: A better way to ID the neutrals may be to use a amp meter to measure the load on each neutral. Turn each circuit on one at a time and make sure that there is a load plugged in to that circuit. You will be able to see which neutral is for that circuit by the current reading on the corresponding neutral..
 
  #4  
Old 09-23-06, 06:04 AM
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Hey Wait. This could be very unsafe.

If you forget to unplug one appliance or turn off one switch you could cause serious damage.

With the neutral disconnected in the panel any loads that are on different legs will be running in series on a 240 volt circuit.
 
  #5  
Old 09-23-06, 06:17 AM
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Originally Posted by jwhite
Hey Wait. This could be very unsafe.

If you forget to unplug one appliance or turn off one switch you could cause serious damage.

With the neutral disconnected in the panel any loads that are on different legs will be running in series on a 240 volt circuit.
I know (but the other readers may not) that you're not talking about my suggestion to use a amp meter to check neutral current. It is a good way to ID neutrals.
Just remember, you're working in a live panel. ELECTRICITY CAN KILL! QUICK!
You're right, disconnecting a neutral can have un-expected consequences.
 
  #6  
Old 09-23-06, 06:34 AM
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You are right hillbilly. I did not mean your way. In fact I like the idea. Get a friend with a drill, or buy a christmas light flasher and amprobe the neutral is the best way to go. IMHO.

I was saying that disconnecting the neutral on a MWBC is not safe, unless you are absolutly sure that there is no load on the circuit. And that someone will not come in and flip a switch while you are working.
 
  #7  
Old 09-23-06, 07:41 AM
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thanks guys. Here is what I was going to do. Let me know if you think it's safe.

All lighting on these circuits would be off. No loads would be on these circuits. Appliances would be plugged in (except for the refrigerator), but they would not be operating. No chance of anyone going into the panel while I'm working, either.

So then I disconnect on of the neutrals, and turn the main back on.

For the lighting circuits, I would simply turn on one of the lights on each circuit to test them. For the appliance circuit, simply look at the clock on the gas oven or microwave; if it's off, then the neutral goes with that circuit.

For the receptacle circuit, I am going to use one of those nightlights with the sensor. If the light illuminates when I put my finger over the sensor, then the neutral does not go with that circuit.

Essentially there would be very very little, if any measurable load going thru these circuits at the time of the test.

Thoughts?
 
  #8  
Old 09-23-06, 07:57 AM
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That is exactly what I was saying is extreemlly unsafe.

When the neutral is open, and you start turing on and off things, you have no way to insure that the loads are ballanced.

You will be forcing two devices to run in series on a 240 volt system. Someone posted the math here just reciently.

Take a pen and paper and draw a MWBC. Use a 60 watt bulb on A phase then a say .25 amp preheater like on a tv or clock on a range for the B phase.
As long as the neutral is there no problem. The circuit will share the .25 amps and the extra .25 amps will return on the neutral. The imbalance.

Now remove the neutral. You have forced the two devices to run in series on a 240 volt system.

You will have 240 ohms on the bulb and 480 ohms on the clock for a total of 720 ohms. That is about .33 amps in series. The light wants .5 amps and will burn dimm. The clock only wants .25 amps and will burn up.

Different values of what is turned on and off will have different results. And I think that .25 was way to high an amperage for a clock on the stove, so even with my example the results would be more extreem.
 
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Old 09-23-06, 08:04 AM
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well I certainly don't want to burn up my lights or appliances. Thanks for the advice.

So is the amp meter the only way to go here? I don't have one and have never used one, so any help with that would be appreciated also. I have a fluke multimeter, but I think a different lead is needed to measure amps, and I don't have it.
 
  #10  
Old 09-23-06, 08:16 AM
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Do you have a simple continuity tester?

If I had forgot my amp meter and had to get the job done, I would turn off the main breaker. Then I would rig light bulb to a cord cap. The bulb will read Zero ohms when cold.

I would unplug everything in the house, turn off all the lights, then turn off power. Main and sub breakers.

Next I would remove one neutral at a time and test for continuity. I would use the light bulb rig to short out the outlets one at a time. The wall switchs can be used for the lighitng loads. Use tape and a numbering system, or buy a book of wire numbers. When a neutral is identified to a circuit number mark the hot and neutrral wire. When you are done, neutrals with more than one wire number share.

You could speed the process up by making a map of the rec and light locations and finding out what ones are on what breaker before you start. This can be done by just turning off one breaker at a time and seeing what does not work.
 
  #11  
Old 09-23-06, 08:25 AM
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actually I just checked; the fluke does read amps and does have an amp clip.

So I just clip onto the neutral, then touch hot #1 to get a reading; then flip the breaker and see if I get a drop...then move on to the others?

Would it be better to place loads on the circuits for a more measureable drop? I guess I'll find out when/if the drop is so low it's not clear which circuit goes with which hot.
 
  #12  
Old 09-23-06, 08:38 AM
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Originally Posted by fuente
actually I just checked; the fluke does read amps and does have an amp clip.

So I just clip onto the neutral, then touch hot #1 to get a reading
Not sure what you mean by "clip": Clamp? That clamps around a wire with no metal-to-metal contact? That's an inductive measurement.

Or clip as in alligator clip? If that's the case the ammeter must be in series with the load.

I would do it jwhite's way.
 
  #13  
Old 09-23-06, 08:39 AM
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Watch OUT, That fluke may only be good for 10Amps!
And you will blow the fuse in the meter.

If you go with Jwites method, wich I also will endorse (delete my 1st Post,(stupid me)).
Make double sure EVERY THING is unplugged and off.
Quickly, with main off and all others off, do a quick continuity check from ckts to neut bus.
Any resistance, something is still connected.
 
  #14  
Old 09-23-06, 08:40 AM
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You need a load on the circuit or you get no results. zero minus zero is still zero.

A small load, like a clock may not be enough to notice.

Also, it is not good to keep switching on and off compressors, like the fridge, or electronics like the tv.

This is why above I suggested the hand drill. Have a helper plug the drill in, then pull the trigger and let go, pull stop, pull stop. about once every three seconds or so. You will see the amperage go up and down easily.
 
  #15  
Old 09-23-06, 08:51 AM
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yes, the clip is an alligator clip, not a clamp.

And yes, the meter is only rated for 10A max.

I shut/disconnect everything, and do the drill method for the two outlet circuits. For the two lighting circuits, would simply turning, say, a switch to 8 can lights off and then on do the trick? Well below the 10A max for the meter, and it should give me a reading, I would think.
 
  #16  
Old 09-23-06, 08:57 AM
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With a clip on meter as apposed to a clamp arround meter.

the load must be in series with the meter. this means that you will have to disconnect the neutral from the buss and hook it to one meter lead, then hook the other meter lead to the neutral buss.

IMHO the portable lamp and continuity test is a better way to go.
 
  #17  
Old 09-23-06, 09:07 AM
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ok Jwhite. Thanks.

I've already mapped out every outlet/light in the house, and I know which circuits/breakers they are on. I"m familiar with using wire numbers so no problem there.

Can you walk me thru this process of using the light bulb method? I've re-read your instructions but something is still not sticking.

Thanks so much.
 
  #18  
Old 09-23-06, 09:14 AM
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If you take a table lamp or rig a light bulb to a cord and test resistance from hot to neutral it will read near zero when there is no power to the bulb.

So, if you plug it into an outlet and check at the panel you should read near zero resistance from hot to neutral.

If you use the table lamp, you can switch the bulb on and off to verify you are on the correct circuit.

Just move the lamp to the outlet you want to check next.
 
  #19  
Old 09-23-06, 09:20 AM
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I think I get it. Forgive me as I'm not really up to speed.

When everything is off/powered down, the resistance between the hot and neutral will be 0 ohms. When I cycle the light on and off (either the light in the receptacle or the light swich for the lighting circuits) I should see the resistance jump up, if the neutral goes with the hot. If I don't see it, then I could simply touch the leads to the other neutral (or other hots) until I have the right combination.

I'm assuming my fluke will read resistance...again forgive me but physics is not my strongsuit.

Thanks !
 
  #20  
Old 09-23-06, 09:27 AM
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I think I get it. Forgive me as I'm not really up to speed.

When everything is off/powered down, the resistance between the hot and neutral will be 0 ohms. When I cycle the light on and off (either the light in the receptacle or the light swich for the lighting circuits) I should see the resistance jump up, if the neutral goes with the hot. If I don't see it, then I could simply touch the leads to the other neutral (or other hots) until I have the right combination.

I'm assuming my fluke will read resistance...again forgive me but physics is not my strongsuit.

Thanks !
 
  #21  
Old 09-23-06, 09:29 AM
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First all of the breakers for individual circuits need to be off or the fuses pulled. If not you will read through the panel guts.

If you have the wrong neutral you meter will show OL (open load)

When you get the right neutral you will get a resistance value near zero.
 
  #22  
Old 09-23-06, 09:33 AM
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ok the so the breakers are off during the test? Now I'm really confused...!


I'm clear on the OL and zero reading...this shows continuity. And I'm taking these measurements at the panel, correct?

So where does the lamp come into play?
 
  #23  
Old 09-23-06, 09:40 AM
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Yes you are taking the measurements at the panel.

One test lead on the removed neutral and the other on the circuit wire. Breakers off.

Move the lamp from outlet to outlet that you want to test.

If you have the right circuit you will read continuity through the lamp bulb.

Do a trial run. Get a bulb out of the cupboard and test center pin to screw shell with your tester. you will see what I mean.
 
  #24  
Old 09-23-06, 09:47 AM
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ok, I'm reading 17-18....whatever the unit is.

and obviously if the leads are not connected, I read OL.

Ok, I think I understand. So if the neutral goes with the hot, then I will read something other than OL when the lamp is plugged into the circuit.

Now what about the lighting circuits? Would I just switch on a light switch, and the continuity would read thru the lights?

I'm assuming the switch would need to be on as to complete the circuit, which would complete the continuity?
 
  #25  
Old 09-23-06, 09:54 AM
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Yes. since lamps are already installed on those circuits you will not need to drag your lamp over there and hook it up.



I think you got it now.

Let us know how you make out.
 
  #26  
Old 09-23-06, 09:59 AM
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beautiful. So I'll simply disconnect one of the neutrals, then move the lamp or turn on the lights until I figure out which neutral goes with which two hots. Then number them. Double check the other neutral/hots, and number them also.

Thanks so much all. As always this has been a very valuable learning experience.

I should say that the electrician that installed the panel probably made sure that everything was wired correctly, but unforunately he did not label anything. And he is on vacation for two weeks, and I want to check to be absolutely sure.

I'll update you with the results.

Thanks again !
 
  #27  
Old 12-22-06, 08:57 PM
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A quick update:

I couldn't get the resistance method to work. Either my fluke is faulty or I'm doing something wrong. No matter what combination of hot/neutral I use, I get 'OL'.

Back to the original idea, and of course correct me if I am wrong. I have 4 circuits to test. 3 general receptacle and 1 lighting. I know exactly what is on these circuits.

I disconnect one of the two neutrals. Now, I go to each of the receptacle circuits, one at a time, and plug in a single nightlight. If it lights up, then the neutral is shared with that circuit. If it doesn't then it isn't. I do all 3 general receptacle circuits this way. By process of elimination, I don't even need to test the lighting circuit; I can get all the info I need from the 3 other circuits.

I understand that if I disconnect a neutral when I have two seperate loads, but with only one load on the entire circuit, I am failing to see why this is unsafe. Any advice, or just an explanation of what I may be missing would be appreciated.

BTW 3 of the hots are on the same phase, so there is definately a neutral being shared. Luckily, I have isolated these circuits an done the calcs to make sure the total load does not exceed 12A total (The breakers are 15A).
 
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