Shared Neutral Current Additive 120/208?

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charlie b said:
I think we are overthinking this.
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English only please.

charlie b said:
Forget the phasor additions and the issue of how close is "about" right. The measurements were 1 and 2 amps, but exactly how accurate were those measurements? Can a clamp on ammeter really distinguish 0.9 amps from 1.1 amps, so that we can say with confidence that the actual current was, in fact, 1.0 amps and not 1.1? We don't know how the measurements were taken, nor with what instrument, and we don't know the degree to which the loads were balanced on the panel as a whole (i.e., we don't know phase angles).
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Can't forget the phasors as they are the answer.

Let's make a wild guess and say there is a refrigerator pulling about 1.14 amp on one phase and a spinning unloaded 3/8" drill on the other phase pulling about 2.14 amps. I do not have a drill with me to measure so assign a 0.1 pf to the drill and a 0.8 pf to the refrigerator. Now the neutral current is 2.71 amps.

I'll buy the power factor and the reading inaccuracies combined but not the readings alone because the phasors are key factors, IMO.
 
mivey said:
I'll buy that as a contributing factor but not the whole reason.
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I have no idea how you can make that call with the info given.

My Fluke 336 which is true RMS will show 1 or 2 amps sitting on the table. Move it around an open panel and the reading will be bouncing all around.
 
mivey said:
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I'll buy the power factor and the reading inaccuracies combined but not the readings alone because the phasors are key factors, IMO.
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I agree.

With purely resistive loads, the current on the neutral would be greater than the lesser current and equal or less than the higher current.

Enter reactive loads and that changes to from zero to both currents summed arithmetically.
 
Just want to point out it's happening again, this thread is going technical without waiting for the OP to jump back in.
 
iwire said:
Just want to point out it's happening again, this thread is going technical without waiting for the OP to jump back in.
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For the record, my post wasn't anything close to technical. But also for the record, it's hard to explain how neutral current on a 120/208V system can be greater than either line load without going technical...
 
iwire said:
I have no idea how you can make that call with the info given.
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Deductive reasoning.

iwire said:
My Fluke 336 which is true RMS will show 1 or 2 amps sitting on the table. Move it around an open panel and the reading will be bouncing all around.
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Your meter has a 600.0 A scale and accurate to 2% of reading +/- 0.5 amps so would not be the meter to use for a current reading of an amp or so.

FWIW, a meter with adjustable scales is more versatile. For example one of my meters has a 40.00 A, 400.0 A, and 1000 A selectable scale. At the 1000 A scale my clamp meter is good for 1% of reading +/- 5 amps but at the 40.00 A scale it is good for 1% of reading +/- 0.05 amps.

Even if you did not know the accuracy, I would assume most people would know, like you do, that a meter that jumps around an amp or two would not be used for measuring an amp or two.

I use them on everything from instrument transformers up to MV systems with no problem.
 
Smart $ said:
For the record, my post wasn't anything close to technical. But also for the record, it's hard to explain how neutral current on a 120/208V system can be greater than either line load without going technical...
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I will agree it is hard to explain without some technicalities. But some explanations are maybe more complex than need be for a somewhat simple answer.

How about if we just say since the two phases are not 180 degrees apart, they do not completely cancel each other in a multiwire circuit using only two phases and the neutral of a wye system like they do in a single phase system or in a delta system with one phase grounded at the mid point?
 
iwire said:
Just want to point out it's happening again, this thread is going technical without waiting for the OP to jump back in.
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What else are we going to do if he won't come out and play with us? :D
 
kwired said:
I will agree it is hard to explain without some technicalities. But some explanations are maybe more complex than need be for a somewhat simple answer.

How about if we just say since the two phases are not 180 degrees apart, they do not completely cancel each other in a multiwire circuit using only two phases and the neutral of a wye system like they do in a single phase system or in a delta system with one phase grounded at the mid point?
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We can say that... and I believe you just did :p
 
iwire said:
Still having trouble with the word 'about', it's OK I know engineers hate imprecision. ;)
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Well, with only one significant digit in each of the numbers, 1+2=3 has a lot of room for error.

But at the same time the only way adding 1 amp from A to two amps from B and getting more than 2A in the neutral would be if only one of the loads had a low power factor (phase displacement angle greater than 30 degrees.)

I think the OP also needs to verify the voltage between the two phase wires in his MWBC just to confirm he has different phases.
 
kwired said:
But some explanations are maybe more complex than need be for a somewhat simple answer.

How about if we just say since the two phases are not 180 degrees apart, they do not completely cancel each other in a multiwire circuit using only two phases and the neutral of a wye system like they do in a single phase system or in a delta system with one phase grounded at the mid point?
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Because the OP recognizes that the simple answer is not enough and a more technical answer is needed:

donniet1977 said:
...I expected to see something less than 2 amps on the neutral but was surprised to see the full 3 amps.
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The OP evidently already knows the 120/208 service will result in neutral currents even for balanced cases and it is not just a matter of simple subtraction. The OP correctly recognizes that the neutral current should be less than 2 amps for normally expected loading. That indicates he already understands the simple things like there not being complete current cancellation in the neutral.

Knowing the simple things, the OP wants to understand why he got 3 amps and the simple answer simply won't do.
 
IMHO, that leaves either harmonics (which the OP doubts) or phase angle issues.
It would help to know what the loads are.
BTW, non linear loads like rectifier banks or uncorrected input VFDs could also cause this result.

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Update

Update

After more thorough testing I am even more troubled.

Quick recap: My service 120/208, hot to hot is 208.

With an 8.5 amp toaster oven on one circuit and 11 amps on the other circuit which feeds the fridge (about 1 amp) and space heater (temp) my neutral current reads 19.2 amps. I'm using an Amprobe ACD-15 Pro clamp on ammeter. This condo has been wired with mostly MWBCs ever since it was built from what I can tell. Have the neutrals been overloaded the whole time?! I'd really like to get on the feed to my panel which is just a subpanel but I just don't have the room.

Lol, someone caught me. I did post this on 2 or 3 other forums. I figure that enough quantity is bound to generate some quality.

Thanks,

Don
 
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