Warm conduit and vibrating/ humming wires

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Another possibility is that the neutrals for A and B are connected together (perhaps within your home run box), and so they are both reading 9 amps because these neutral conductors are sharing equal portions of the total A and B current upstream of that connection.
One thing to try would be to put the clamp around both A and B neutral conductors in the panel, and see if you get 9A + 9A = 18A on the meter. If you get 18A, then it's likely that there is a connection between A and B neutrals. That's because the neutral currents on different phases will not add up in-phase, and so you should get a noticeably lower value than 18A if the neutrals are not connected together.
You could turn the breaker off, disconnect the A and B neutrals, and see if you get continuity between them or to equipment ground. Continuity to equipment ground would result in some of the neutral current flowing through the conduit, as was mentioned in previous posts.

If you had resistive LA-N and LB-N loads with their neutrals connected together, and with your A and B line currents above, the combined neutral current should be about 12.8A due to them being 120° apart. However, it could be 18A as mentioned above if the A and B currents have less overlap in time because of nonlinear loads.
 
kwired said:
Presuming loads remained steady, you have total of 32 amps on the neutrals, and 39 amps on the "hots".

If these are all properly connected two wire circuits, there is 7 amps unaccounted for, could be because of some interconnection with some other circuit, could be ground fault current through a high enough resistance to limit that current, or even some line to line current involved between the circuits in question, though that may not necessarily contribute much to the "humming". The heating mentioned may or may not be a problem depending on circumstances.

As mentioned if you clamp a ammeter around all the conductors, if you get a reading (possibly about 7 amps in this case) then it is current that is flowing outside the intended path for some reason.
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One time I there was a conduit really hot it burned me so I opened the panel to see what was going on and the new electrician thought it would look pretty if he separated all the hots and neutral and grounds lol bad idea there was a gutter box above the panel about 5 x 3/4 conduits from gutter to panel and all the neutrals in one emt all the Hots in another all the ground in another lol


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Hitschamp said:
One time I there was a conduit really hot it burned me so I opened the panel to see what was going on and the new electrician thought it would look pretty if he separated all the hots and neutral and grounds lol bad idea there was a gutter box above the panel about 5 x 3/4 conduits from gutter to panel and all the neutrals in one emt all the Hots in another all the ground in another lol
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Oy gevalt! :rolleyes:
 
Tudmutt said:
Amp reading on the netruals
A phase 9 amps netrual
A phase hot 11 amps
B phase 9 amps netrual
B phase hot 14 amps
C phase 14 amps netrual
C phase hot 14amps
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One scenario that would result in currents quite close to these measurements is the following.
A portion of the lights powered by the B phase that draws 5A is connected to the A phase neutral instead of the B phase neutral.
That leaves 14A - 5A = 9A on the B phase neutral, which is what you measured.
Because of the misconnected B phase lights, the correct A phase neutral current of 11A will sum vectorially with this 5A portion of the B phase lights' neutral current. The resulting current would be √ (112 + 52 - 2 x 11 x 5 x cos(60°) ) = 9.5A. This is quite close to the measured 9A of measured A phase neutral current.

One way to check whether this might be happening is to disconnect the A phase hot conductor from the breaker. Then if there's 5A remaining on the A phase neutral, that would likely confirm that this current is being supplied through lights powered by the B phase hot. The B phase hot could then be disconnected to provide further confirmation if it makes the measured A phase neutral current go to zero.
 
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Update: there were a few jboxes that were spliced wrong, netruals with different phases. After fixing that we still have the very warm conduit and humming. The readings seem correct now though. The netrual amperage is the same reading as the phase amperage. When I checked phase and neutral my reading is zero. I put the clamp meter over the conduit and got nothing. Tomorrow our service guy will be assisting me with this nightmare. Thank you all for your input.
 
Since it has an audible hum, you might try turning all power off, if hum still exists, it’s from an outside source. If it stops, start turning stuff back on, until the hum resumes, at least you may be able to narrow down the source. I’m leaning towards a main neutral issue, possibly on the poco side. Also make sure the service main bond is in place.
 
Tudmutt said:
After fixing that we still have the very warm conduit and humming. The readings seem correct now though. The netrual amperage is the same reading as the phase amperage. When I checked phase and neutral my reading is zero. I put the clamp meter over the conduit and got nothing.
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That leaves the conductor quantity and the source of the noise. Is there a transformer?-
 
LarryFine said:
That leaves the conductor quantity and the source of the noise. Is there a transformer?-
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No transformer, this project is a remodel job. The panel we are using is fed with a 70 amp breaker. Right now the sub panel
A phase 33amps
B phase 28 amps
C phase 23 amps
Netrual 11 amps.
I have a 3/4 with 4 circuits all #12, 3 lighting circuits and an outlet circuit.
Lighting circuit A phase 12 amps, phase to netrual. 07 amps
B phase 15 amps , phase ti netrual. 07amps
C phase 15 amps , phase to netrual. 07 amps.
Nothing reading when I put the clamp on the conduit.
Also the outlet circuit thats in that conduit is off and I'm getting a reading of .15 amps phase to neutral.
One of our smart service guys will be stopping by today to help with this headache.
 
Tudmutt said:
No transformer, this project is a remodel job. The panel we are using is fed with a 70 amp breaker. Right now the sub panel
A phase 33amps
B phase 28 amps
C phase 23 amps
Netrual 11 amps.
I have a 3/4 with 4 circuits all #12, 3 lighting circuits and an outlet circuit.
Lighting circuit A phase 12 amps, phase to netrual. 07 amps
B phase 15 amps , phase ti netrual. 07amps
C phase 15 amps , phase to netrual. 07 amps.
Nothing reading when I put the clamp on the conduit.
Also the outlet circuit thats in that conduit is off and I'm getting a reading of .15 amps phase to neutral.
One of our smart service guys will be stopping by today to help with this headache.
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Since it is a sub panel, the neutral and ground bars should be separate. I was under the impression it was the service equipment. Metal bar joists or structure? Multi-tenant building? If the neutral is bonded at the sub panel, it will make a parallel path for the other tenants services.
 
Tudmutt said:
No transformer, this project is a remodel job. The panel we are using is fed with a 70 amp breaker. Right now the sub panel
A phase 33amps
B phase 28 amps
C phase 23 amps
Netrual 11 amps.
I have a 3/4 with 4 circuits all #12, 3 lighting circuits and an outlet circuit.
Lighting circuit A phase 12 amps, phase to netrual. 07 amps
B phase 15 amps , phase ti netrual. 07amps
C phase 15 amps , phase to netrual. 07 amps.
Nothing reading when I put the clamp on the conduit.
Also the outlet circuit thats in that conduit is off and I'm getting a reading of .15 amps phase to neutral.
One of our smart service guys will be stopping by today to help with this headache.
Click to expand...
What does phase to neutral .07 amps mean? Phase to neutral would typically be a voltage measurement. A current measurement here would normally have the load in series and will be same as the phase conductor.
 
kwired said:
What does phase to neutral .07 amps mean? Phase to neutral would typically be a voltage measurement. A current measurement here would normally have the load in series and will be same as the phase conductor.
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I'm thinking he meant that the clamp was put around both the phase and the neutral conductor of a particular 2-wire circuit. But he would have to clarify that.
0.07 amps (i.e., 70mA) is not a negligible amount of common-mode current considering that a GFCI must trip above 6mA. However, most clamp meters would not be very accurate at these low levels. Even if the 0.07 amps is real, 3 x70mA x 277V = 5.8 watts, which is not likely to cause objectionable heating along a significant run of conduit. I'm assuming that the circuits are 277V, although that was not specified. It would be interesting to turn off the lights and see how the measurements change. And also shut off the 70A breaker, as was mentioned before.
 
synchro said:
I'm thinking he meant that the clamp was put around both the phase and the neutral conductor of a particular 2-wire circuit. But he would have to clarify that.
0.07 amps (i.e., 70mA) is not a negligible amount of common-mode current considering that a GFCI must trip above 6mA. However, most clamp meters would not be very accurate at these low levels. Even if the 0.07 amps is real, 3 x70mA x 277V = 5.8 watts, which is not likely to cause objectionable heating along a significant run of conduit. I'm assuming that the circuits are 277V, although that was not specified. It would be interesting to turn off the lights and see how the measurements change. And also shut off the 70A breaker, as was mentioned before.
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You can often get a tenth of an amp difference in reading just by moving the conductor location within the clamp of the meter.

I generally don't rely on accuracy of clamp on meter to any more than maybe nearest half amp at best.
 
A fascinating thread! I'd like to do a short tutorial here. A basic law in physics is Faraday's law. It says that a magnetic field will form around any current-carrying conductor. The "right-hand rule" illustrates it. If you curl your fingers and point thumb up. Thumb indicates direction of current flow and fingers point direction of circular field. Field intensity is directly proportional to current. In a two-wire circuit, at any instant in time, the load current is equal in the conductors but opposite in direction. This causes a physical force between the conductors to push them apart. Because the peak current will occur 120 times/sec (at 60 Hz), the conductors can end up forced against the inside of the conduit ... producing a buzzing sound.

An interesting experiment to illustrate of this phenomenon is to arrange the two wires of an arc-welder to be parallel and close together. When an arc is struck (and high current flows), the wires will jump to move farther apart. If they were on a friction-less floor, they would form a big circle!

The force of this magnetic repulsion can be very high - enough to pull wires out of screw-clamp terminals under fault conditions where currents can reach thousand amps. In a Georgia Tech short course, I watched a video made by Siemens in their circuit-breaker test lab that showed this - and the arc-flash that followed in slow motion.

The best cure for the buzzing that I know of is to twist the phase and neutral together before pulling them into conduit. This usually constrains the motion enough to silence them. There are other benefits to this L-N twisting in that it neutralizes small voltages magnetically induced in EGC wires as well. This voltage, although small, represents a significant source of electrical noise in electronic systems - especially professional audio systems. Anyone interested in knowing more about the magnetic fields in conduit and their effect on electronic systems of all sorts can drop me a line at engineer_bill@verizon.net
 
When I was working as a journeyman many years ago, something similar happened on a job with lighting circuits. The circuits were all electric discharge lighting (nonlinear loads) It was explained to me that higher than normal currents could be on the neutral because of the harmonic currents present. We upsized the neutral to #8 and it solved the problem. Might be something worth looking into. I was always told when weird, unexplained things happen in an electrical circuit, look for a neutral issue. Hope this helps.
 
Rick 0920 said:
When I was working as a journeyman many years ago, something similar happened on a job with lighting circuits. The circuits were all electric discharge lighting (nonlinear loads) It was explained to me that higher than normal currents could be on the neutral because of the harmonic currents present. We upsized the neutral to #8 and it solved the problem. Might be something worth looking into. I was always told when weird, unexplained things happen in an electrical circuit, look for a neutral issue. Hope this helps.
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That should only been issue with multiwire branch circuits, and even higher potential to be an issue with three phase wye MWBC as there you sharing the neutral with harmonics from three ungrounded conductors. Should also only been a problem with electronic ballasts as well, magnetic ballasts should be a linear load. Magnetic will have a displacement power factor but not a distortion power factor, though most them in later years when magnetic was common they were typically pretty high power factor.
 
It almost sounds like he pulled three neutrals, hope not! If you have a real power meter I would be interested in the power being consumed by each circuit, with a circuit diagram of where the power was measured. If the power factor is crap the current in the wire will increase and they will get hot. What Rick said make sense…
 
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Ground Loop Guru said:
The best cure for the buzzing that I know of is to twist the phase and neutral together before pulling them into conduit.
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That may work on paper....but holy hell....that would be a nightmare. It would be faster to buy twisted pair but no one in their right mind is going to do that.

This has been a good thread. And thanks for the refresher. I had forgotten about the rule of 'thumb' from college. One of them things we learnt and never used.
 
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