Tracing multiwire circuits

[Alexis]

Hello,

I tried searching, but couldn't find clear answer to my question. Often when we update/upgrade panels we find multiple wires from one conduit where we have to identify multiwire circuits to have them ziptied (neutral and 2 hots) and have a common trip breaker installed. So far most of the cases I had to trace the whole conduit to the next jbox and find where the wiring is going which is very time consuming.

Does anyone know any easier way of locating the pairs and their neutrals? Of course if I see romex 12-3 or 14-3 that is easy. But when there is a conduit with 15 wires (lets say 10 black and 5 white ones) it becomes a bit of a problem..

Thank you!

 

[GoldDigger]

IF you can assume that you have only correctly wired MWBCs or individual hot/neutral BCs in rhe conduit, I would put a pulsed load on one load outlet at a time and use a clamp-on ammeter to find the black and white wires corresponding to that circuit.
If you have any 240V, no neutral loads, put your pulsed test load on the 240V outlet and confirm that there is no current on any of the neutrals.

 

[winnie]

To add, remember that the current on an MWBC should net to zero, so if you put a clamp around corresponding hot(s) and neutral, you should get a zero reading.

Jon

 

[ptonsparky]

IF you can assume that you have only correctly wired MWBCs or individual hot/neutral BCs in rhe conduit, I would put a pulsed load on one load outlet at a time and use a clamp-on ammeter to find the black and white wires corresponding to that circuit.
If you have any 240V, no neutral loads, put your pulsed test load on the 240V outlet and confirm that there is no current on any of the neutrals.

I don't think it can get any simpler.
Typical technique would be that the breakers are adjacent, but not always true as you have found.
I have one we did years ago that the breakers could be anywhere in the panel. Quite legal at the time and nary a handle tie. MWBC conductors were grouped but I don't think that was even a requirement then.

 

[Rick 0920]

To add, remember that the current on an MWBC should net to zero, so if you put a clamp around corresponding hot(s) and neutral, you should get a zero reading.

Jon

Jon, If circuits 1 and 3 shared a common neutral this would be a MWBC correct? If both circuits were receptacle loads and the current flowing on circuit #1 is 15 amps and the current on circuit #3 is 8 amps, wouldn't the current flowing on the neutral be the unbalanced current coming back to the source, which would be 7 amps. Not a true neutral, which would be zero. That's why they are called "grounded" conductors and technically shouldn't be called neutrals.

 

[AC\DC]

A true neutral carry’s the unbalanced current.
The “ grounded conductor” carry’s the same load as the phase associated with it.

 

[infinity]

Jon, If circuits 1 and 3 shared a common neutral this would be a MWBC correct? If both circuits were receptacle loads and the current flowing on circuit #1 is 15 amps and the current on circuit #3 is 8 amps, wouldn't the current flowing on the neutral be the unbalanced current coming back to the source, which would be 7 amps. Not a true neutral, which would be zero. That's why they are called "grounded" conductors and technically shouldn't be called neutrals.

No it's a neutral. That grounded conductor vs. neutral nonsense ended a decade or so ago when the NEC changed the definition of a neutral conductor.

Regarding your example it can depend on the system. For a 3-wire MWBC on a 120/240 volt system just subtract the smaller current from the larger and that's the neutral current.

 

[ptonsparky]

Jon, If circuits 1 and 3 shared a common neutral this would be a MWBC correct? If both circuits were receptacle loads and the current flowing on circuit #1 is 15 amps and the current on circuit #3 is 8 amps, wouldn't the current flowing on the neutral be the unbalanced current coming back to the source, which would be 7 amps. Not a true neutral, which would be zero. That's why they are called "grounded" conductors and technically shouldn't be called neutrals.

Clamping all three would net zero. If it doesn't you have another circuit tied in somewhere.

 

[winnie]

Ptonsparky beat me to it.

For any circuit the current going out on any one circuit conductor must be matched by current flowing back on another.

It doesn't matter if this is a single circuit, a single phase MWBC, a two phase MWBC on a three phase system, or a full boat. If you clamp all the conductors together at the same time, the current should net to zero. This is how a GFCI works.

All bets are off if the circuit is miswired (bootleg neutral, hots swapped with other circuits). But if the circuit is wired properly and you clamp all associated circuit conductors, you should measure net zero current.

Jon

 

[don_resqcapt19]

If there are connected loads, you can just check for contunitity between the hots and neutrals, but if there are any circuits that don't have a connected load, that won't work.

 

[mtnelect]

I will be submitting this proposed change this month for the NEC 2025.

 

[infinity]

I will be submitting this proposed change this month for the NEC 2025.

I wish you luck but circuits can get moved around in panels so the numbers out in the field could become meaningless. IMO for that reason the CMP will not want to require physical wire number markings in junction boxes.

 

[mtnelect]

I wish you luck but circuits can get moved around in panels so the numbers out in the field could become meaningless. IMO for that reason the CMP will not want to require physical wire number markings in junction boxes.

Thank you for your comments !

 

[infinity]

Thank you for your comments !

I don't want to discourage you I was just expressing my opinion as to what I think the CMP would do. I actually wrote the original proposal to allow wire markers to be used for conductor identification. Back when they first required a cable tie or tape to identify which circuit conductors went together there was no code provision to allow only the use numbers on the conductors.

In commercial work we always number each home run conductor in both the junction box and the panel so for that work I like the idea.

 

[GoldDigger]

Jon, If circuits 1 and 3 shared a common neutral this would be a MWBC correct? If both circuits were receptacle loads and the current flowing on circuit #1 is 15 amps and the current on circuit #3 is 8 amps, wouldn't the current flowing on the neutral be the unbalanced current coming back to the source, which would be 7 amps. Not a true neutral, which would be zero. That's why they are called "grounded" conductors and technically shouldn't be called neutrals.

Neutral refers to the voltage on the wire relative to the voltage on the phase conductors (2 or 3), not the current or lack thereof.

 

[don_resqcapt19]

Jon, If circuits 1 and 3 shared a common neutral this would be a MWBC correct? If both circuits were receptacle loads and the current flowing on circuit #1 is 15 amps and the current on circuit #3 is 8 amps, wouldn't the current flowing on the neutral be the unbalanced current coming back to the source, which would be 7 amps. Not a true neutral, which would be zero. That's why they are called "grounded" conductors and technically shouldn't be called neutrals.

The fact that current is flowing on the conductor does not change it from a neutral to a grounded conductor.

The term "grounded" does include neutrals, but the term neutral is defined in the NEC and has nothing to do with what current may be flowing on it.

Neutral Conductor.
The conductor connected to the neutral point of a system that is intended to carry current under normal conditions. (CMP-5)

Neutral Point.
The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct-current system. (CMP-5)

 

[GoldDigger]

The fact that current is flowing on the conductor does not change it from a neutral to a grounded conductor.

The term "grounded" does include neutrals, but the term neutral is defined in the NEC and has nothing to do with what current may be flowing on it.

Not all neutrals are grounded and not all grounded conductors are neutral. But in most cases the NEC requires that neutral conductors be grounded.

 

[infinity]

Not all neutrals are grounded and not all grounded conductors are neutral

Can you explain this? Am I wrong in assuming that according to Don's Article 100 definitions a neutral will be grounded because all of those AC systems require it?

 

[ptonsparky]

Can you explain this? Am I wrong in assuming that according to Don's Article 100 definitions a neutral will be grounded because all of those AC systems require it?

Grounded Delta. It is not a neutral.

 

[infinity]

Grounded Delta. It is not a neutral.

He stated that not all neutrals are grounded. What AC system would have an ungrounded neutral?