Is this normal voltage drop?

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  #1  
Old 04-03-15, 06:08 AM
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Is this normal voltage drop?

I have a multiwire circuit.

Theres a refrigerator running on one of the two.

Both measure 120 volts.

I plugged in a coffee pot and that side of the multiwire circuit dropped to 118V. While the other side remained at 120.

Remove the coffee pot load and it returns to 120.

Is the 2 volts normal voltage drop from a resistant load (coffee pot).? I did this same test with a similar sized toaster and it produces the same effect.

Also: On one side I have a power hungry microwave.... same test as before just microwave running.

With microwave running, the voltage drops on that side from 120 to 117/118. However, what is different here, I notice that the other side rises from 120 to 120.9. Less than a volt.... should I consider that normal or that a neutral issue?
 

Last edited by mikerss; 04-03-15 at 07:18 AM.
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  #2  
Old 04-03-15, 07:35 AM
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All well within normal limits. Consider about +/- 5% to be worth investigating and +/- 10% to be a sign of a problem.
 
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Old 04-03-15, 07:44 AM
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Thanks... I was thinking it was +/- 3%.

So the rise though on the other leg of the MWBC with the microwave running.... its less than a volt but do you think that is indicative of a high resistant neutral?
 
  #4  
Old 04-03-15, 01:56 PM
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another question I thought of...

when measuring with a multimeter, do readings of less than a volt mean anything?

In my specific case, I don't know if I should be concerned with a .9 increase on the opposite leg of that mwbc or if that is considered insignificant.
 
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Old 04-03-15, 03:13 PM
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I would consider it insignificant, probably within the tolerance of the meter anyway. Yes the reading does mean the voltage is fluctuating a small amount, but that is part of the normal operation of the system.

The 3% figure is a non-enforceable recommendation in the code book to use when designing the feeders between panels. The voltage isn't really out of spec until you're beyond the +/- 10% level, and even then it depends on what you're powering. Resistance heaters don't have any problem with low voltage. Motors can be damaged by low voltage. Some electronics it could be a problem, but these days most have a very wide acceptable voltage range on the power supply to accommodate international use.
 
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Old 04-03-15, 03:30 PM
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Yes the reading does mean the voltage is fluctuating a small amount, but that is part of the normal operation of the system.
Even in the case of a multi-wire branch circuit as I describe?
 
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Old 04-03-15, 05:04 PM
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An imbalance in a multiwire branch circuit will cause more of the load to be carried by the neutral. A drop of only a volt or two with a heavy load means that the wire run between that receptacle and the panel is relatively short.

The further the receptacle from the panel.... the more voltage drop you'll experience.
 
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Old 04-03-15, 06:59 PM
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An imbalance in a multiwire branch circuit will cause more of the load to be carried by the neutral.
I think you guys may be missing my main concern or I am having trouble understanding or explaining it.....

When the microwave was running on one leg of the mwbc the voltage dropped on that leg. I understand that. However, the other leg went UP by that .9 volts like I described. (This was checking hot to neutral on an outlet). That is what I do not understand.

I feared this was indication of a loose or high resistant neutral connection...
 
  #9  
Old 04-06-15, 06:34 AM
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Im still quite confused and concerned about this... hopefully someone familiar with MWBC's can comment:

With a microwave running on one leg of the mwbc, the other leg went up .9 volts.

Does that indicate a neutral problem?
Or is that small reading insignificant?

Thank you
 
  #10  
Old 04-06-15, 06:40 AM
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There is no reason to think what you are seeing is significant. If it makes you feel better redo all connections and replace all wire nuts.
 
  #11  
Old 04-06-15, 11:54 AM
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Yes what you saw is normal operation of a MWBC. Small variations up or down in voltage is perfectly acceptable.
 
  #12  
Old 04-07-15, 03:30 PM
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With a microwave running on one leg of the mwbc, the other leg went up .9 volts.
Yes, in a multibranch, an increase in load on one side will increase the voltage in the other leg. The resistance in the neutral leg causes a reduction in voltage on the hi-current device and an increase of voltage on the low current leg. This is because the Leg 1 and Leg 2 voltages are out of phase. This effect is the opposite in a DC system.
 
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