Basic calc question

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#1

Originally Posted By: phinsperger
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Simple 220V, 100A service. Not much running inside the house. Measuring the service entrance conductors with an amp meter inside the main disconnect panel just for hell of it.


Hot 5.6 Amps
Hot 3.1 Amps
Neutral 4.8 Amps
Ground 0.0 Amps

Shouldn't the Neutral be giving me a reading of ~2.5 Amps (5.6-3.1)?

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#2

Originally Posted By: Thomas Ogryski
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Theoretically, you’re right. However, you are assuming that your meter is 100% accurate and that the load or supply voltage did not change at all during the time you measured the other conductors.


Some clamp on meters are not very accurate at low levels, my true RMS Fluke clamp on warns that measurements below 5 amps will not be accurate.

Tom

#3

Originally Posted By: phinsperger
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Thanks Tom. I wondered about the changes in load so I went back and forth measuring the differnt conductors about 3 times each with no change so kind of ruled that out.


I checked the documented specifications of my meter. For that range (200A) is +/-2%. In order for the above numbers to jive the accuarcy of the meter would have to +/-14.7% or more. ![icon_eek.gif](upload://yuxgmvDDEGIQPAyP9sRnK0D0CCY.gif)

Its a new meter that was at room temp but a very very inexpensive meter.

I'll get it re-calibrated and try it again on specific load amount and see what happens.

Thanks again Tom. At least I know I didn't miss something in the theory.

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#4

Originally Posted By: Greg Fretwell
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If you have a lot of inductive or electronic loads you might see some strange things on the neutral but this is usually a 3p wye thing.

#5

Originally Posted By: phinsperger
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There was a television, computer, two answering machines and two clock radios on at the time. I know what a pluming wye is but in electrical terms what is a 3p wye?

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Paul Hinsperger
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#6

Originally Posted By: Greg Fretwell
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You describe some ugly loads from a harmonic aspect. This is roughly 1000 watts so try it again with something like an assortment of space heaters, incandecent lamp and toaster type loads.


3 phase wye is when all of the phases return the unbalanced load on a common neutral. It looks like a “Y” when you draw it. (center point being the neutral) The third order harmonic will lag the current to the point where it can double the neutral load. This is not as much an issue in single phase from what I have been told but you may have some anecdotal data to dispute that. “switching” power supplies like a TV or a PC play havoc with that pretty sine wave.

#7

Originally Posted By: phinsperger
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Thanks Greg. I read that 3 times and I think I beginning to understand. Is this also why the neutral conductor… sorry grounded conductor can be reduced in size from the hots but (up here at least) there is some dispute as to just how much it can be reduced?

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Paul Hinsperger
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#8

Originally Posted By: Greg Fretwell
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They actually upsize the neutral in an office environment. “Super neutral” cable has 3 #12 phases, a #12 ground and a #8 neutral.


You have me thinking about this. I have a 20a 4 wire 240v receptacle in my garage. If I get bored someday I will wire up a little "multiwire" test jig and try a bunch of computer type loads to see what I come up with. I have a regular electrician type Fluke clamp-on and a good meter with a lab grade clamp-on lead set.