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[Mike Sokol]

The size of the wire doesn't affect the current (much) which is why we have to be careful and use over current protection devices.

However, an undersized neutral will allow the voltage on the opposite leg to jump around. For instance, I played in a club once where turning on all the PAR lights on one leg would cause the opposite leg to jump up by 10 volts. But the current would subtractive in your case, so as TJ stated, that's not the cause of your extra neutral current. Are you using a True-RMS clamp-ammeter?

[Jonathan Johnson]

At an event the other night, our small distro was served by a 250/125v 50a circuit (by way of NEMA 14-50 receptacle) that was professionally installed 15' from the 200a breaker panel.

Our amp racks are split exactly even between the two phases.  Things are low-key, and pulling ~15 amps on Phase A and ~16 amps on Phase B.  With everything evenly balanced, doesn't the opposite phases' sine waves cancel out the return current on the neutral?  Instead, at this venue I was showing approximately 22 amps on it consistantly.  I closely monitored everything for heat and got through the night with no other issues.  But still, this bothered me.

Assuming that the wiring was rated for 50A and correctly installed, the fact that there was 22 amps read on the neutral and only 15 or 16 amps on the hot/live legs, there would not have been great cause for concern in this instance. Even though the observed current on the neutral was higher than either of the hot/live legs, it was still below the circuit rating.

Of course, there still is the question of "why?". If you were to connect the same setup to a 120/240V circuit rated for only 20A, you could potentially overload the neutral. So discovering why you observed what you did is valid for future events.

[Jeffery Foster]

Are you using a True-RMS clamp-ammeter?

Yes, most certainly.  I never noticed any wild current swings on the hot legs, all the numbers, including the neutral amperage, was really consistent.  Like Jonathan said above, whereas everything seemed to be safe and within code, it was pretty baffling and I wanted to see if I was doing something incorrect.

I always appreciate everyone's input!

[Tom Bourke]

How did you measure?  What model meter?

[Jeffery Foster]

How did you measure?  What model meter?

I use the Fluke 323 meter.

I clamp it on each individual feeder cable, perpendicular to the cable, with the cable centered in the clamp jaws.

[Mike Sokol]

I use the Fluke 323 meter.

I clamp it on each individual feeder cable, perpendicular to the cable, with the cable centered in the clamp jaws.

So here's a wacky idea. If you get back into the same venue, can you try metering both feeder lines at the same time? That is, put L1 and L2 inside the ammeter jaws at the same time and measure the current. Whatever you read there should be the exact same numbers you're reading on the neutral line by itself. If you don't get the same reading, then something is REALLY crazy. Did you happen to do a current reading on the EGC Ground wire? I can't imagine how extra current could be feeding back into the neutral via the ground wire, but either the meter is being fooled or extra current is coming from somewhere.

[Cailen Waddell]

Except in *some* older systems that are 3-phase delta, in which case you have 240V any leg to any leg; one secondary is center tapped providing 120V between two main legs and neutral, and you have 208V between high-leg and neutral.

I think there could be some thing to the high leg delta theory. Was working in a building that used to be a laundry mat, and if you didn't know the incoming service was high leg delta you wouldn't of been able to tell from the panel...  It was a split phase 120/240 panel as a sub panel on the main panel in another closet.   You had to look outside and see the two big transformers and one small, and meter at the main panel...


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[Mike Sokol]

I think there could be some thing to the high leg delta theory. Was working in a building that used to be a laundry mat, and if you didn't know the incoming service was high leg delta you wouldn't of been able to tell from the panel...  It was a split phase 120/240 panel as a sub panel on the main panel in another closet.   You had to look outside and see the two big transformers and one small, and meter at the main panel...


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But in a High-Leg Delta transformer, the divided secondary acts just like a split-phase 120/240-volt system. I just don't see any way for it to feed extra current into the neutral. One test that would have been REALLY interesting is to measure the neutral for current with the amp racks powered down. Finding neutral current under that condition would hint that the extra neutral current was looping in somehow via the EGC ground. There have been instances of lost neutrals that caused imbalance currents to occur in the ground conductors.

[Cailen Waddell]

But in a High-Leg Delta transformer, the divided secondary acts just like a split-phase 120/240-volt system. I just don't see any way for it to feed extra current into the neutral. One test that would have been REALLY interesting is to measure the neutral for current with the amp racks powered down. Finding neutral current under that condition would hint that the extra neutral current was looping in somehow via the EGC ground. There have been instances of lost neutrals that caused imbalance currents to occur in the ground conductors.

Good point Mike, you are correct.   


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[Stephen Swaffer]

I don't want to come across as accusing someone of being incompetent by any means, but overlooking a decimal point-especially in low light would be easy to do-especially if you "think" you are seeing a given number.  I have been fooled by a reading of 120 mV give or take when testing a 120 volt circuit.  2.2 amps would be close enough to right for me-and human error (or even faulty LCD)  is more likely than the laws of physics being warped.

I don't see the high leg delta as a potential culprit in this case,  If you accidentally grabbed the high leg, your voltage to neutral would be off on that leg.  Keep in mind that high leg delta (as implemented) often eliminates either the AB transformer or the BC transformer-the usual reason for using it is a cost saving from eliminating that transformer.  That changes your wiring diagram a bit, but still cannot explain the neutral current.

If the distro had a misplaced/accidental ground -neutral bond and there was a missing ground-neutral bond in the panel, that could explain the mystery neutral current but it would be unusual to find 2 problems working in concert.

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