Wired a garage today an put in 8 8 foot lights. I split the lights on 2 circuits. After hooking up the power I checked the amps on the phases an was pulling 7 amps on each phase. I also checked the neutral an it was pulling 7 amps too. Is this normal? I plugged in a drill an ran it to see if it made a difference and the amps on the neutral dropped to like 3 amps. Please explain this to me.Thanks

You don't say if this is a single phase or a three phase system. From the use of 8 foot lights I am guessing a commercial occupancy with three phase power.

When you use three phase wye circuits, you need all three phases to carry equal load for the neutral current to balance out. If you take away any one of these phases, then the current on the neutral would equal the current on the missing phase. So with equal loads on two of the three phases, you expect the full load current on the neutral as well.

If this is a single phase system, then we'll need to dig more deeply for an explaination.

-Jon

Sorry yes this is a single phase system. Have 120v on each phase to ground , 120v on each phase to neutral , and 240 between phases. Everything works fine an can't find anything wrong but have never really checked the neutral current before so seeing the current there kinda threw me. Any help is greatly appreciated. Thanks

Are you sure the two circuits were from different phases? If same phase, then the neutral will have 2 x imbalance current.

Yes they are from different phases. So can anybody tell me is this normal?

If it is really 240/120 single phase and the conductors are indeed on opposite legs it does not sound normal to me.

One thing that might explain it is if this building is served single phase 208/120 from a 208Y/120 3 phase system.

It _might_ be 'even order harmonics', though these are generally much lower in magnitude than the odd order harmonics that cause problems on three phase systems.

In a three phase system, any multiple of 3rd harmonic current will be 'in phase', and _add up_ on the neutral even when the fundamental currents balance out. Non-linear loads create odd harmonics. In a non-linear load the current flowing is not proportional to the instantaneous applied voltage, so that the current flow waveform has a different shape than the applied voltage waveform.

In a single phase center tapped system, any multiple of 2nd harmonic current will be 'in phase' and add up on the neutral even when the fundamental currents balance out. In order to generate even order harmonics, a load must be both non-linear and show 'hysteresis' (see http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html for an example in magnetic materials). Hysteresis means that the current flow for a particular applied voltage depends upon the _history_ of the current flow, so that the wave shape going up is different from the wave shape going down.

If you can get a scope, look at the shape of the current flow waveform on the phases and on the neutral. If you see 60 Hz on the neutral with balanced loads then something very strange is happening or you have 2 phases of a wye system as iwire suggests. If you see 120 Hz then you have 2nd harmonic. If you see 60 Hz, and you confirm that this is a single phase system (and not a pair of hots tapped from a three phase system), then I would start investigating for something like a neutral to ground fault with one circuit not actually returning to neutral.

-Jon

It might also have been a 3-phase 208/120 volt network system, where only two hots and a neutral are brought to each apartment. (Sometimes used in cities too, for single and multiple family homes--- requires the fifth jaw in the meter can.)

Yes its a 120/240 volt service. Like I said I've never checked the neutral current before an it kinda threw me. In the garage the only load is the 8 8foot lights so I was wondering if it was because of the flourescent lighting. Everything is working correctly an no problems just after i hooked everything up I was checking the amps an just happened to check the neutral an seen this.

"It might also have been a 3-phase 208/120 volt network system, where only two hots and a neutral are brought to each apartment. (Sometimes used in cities too, for single and multiple family homes--- requires the fifth jaw in the meter can.)"

We have this 208/120 on our main street, and 120/240 on the side streets, so we have to take care when working on homes or business near the main street network, but you keep saying you have 240, are you sure?

Yes positive out in the country everythings 120/240. So could it be because of the flourescent lighting or something else. Or is it normal because the lights are the only load ?

As I said above, it is _possible_ that the lighting is producing enough even harmonics to cause this current reading. However I don't think that it is particularly likely that the lighting would cause quite this much harmonic current flow on the neutral, and would be inclined to check for another problem.

You could try metering all of the currents in all of the wires (ungrounded conductors, grounded conductor, and _equipment ground_ conductor) with different combinations of load, eg. no lights on, then 1 bank on, then the first off and the second on, then all the lights on. How the currents change with load might provide a good clue as to what is happening.

-Jon

When you find the problem/answer please post your findings, I for one would be interested to see what did cause this phenom.

Jon,
I don't think so. If they are fed from the two hots, the same order harmonic from one hot will always cancel out the that harmonic on from the other hot on a true single phase system. The two hots come from opposite ends of the transformer winding are "in effect" 180 degrees out of phase. All of the harmonics, no matter what order they are, will also be 180 degrees out and cancel in the neutral.
Don

Don,

That is not correct. The phase displacement of the harmonics is equal to the phase displacement of the fundamental multiplied by the harmonic order.

In three phase systems, you take a fundamental phase displacement of 120 degrees, and any triplen harmonic will produce a phase displacement that is a multiple of 360 degrees.

In single phase systems, _even_ harmonics will take the phase displacement of 180 degrees in the fundamental, and produce a phase displacement of that is a multiple of 360 degrees, or _0_ degrees. As far as I know, people have not been seeing even order harmonics as a problem, they do, however exist.

-Jon

Do the fixtures have electronic ballast or coil ballast ?
Are the tubes "watt miser" type.
Need more information to make an educated guess.
Alan--

edited, guess I didn't read the orignal post very well Now I see both legs were at 7 amps

[This message has been edited by walrus (edited 11-25-2005).]

Wraiths,
Before we get into the possible quantum physics involved, did you check the current on the neutral after you turned either bank of lights off?
my suggestion would be to look at this the same way you would trouble-shoot a tripping breaker.
Turn each bank off one at a time, taking readings on the neutral after each time.
Is the neutral current only apparent with one of those circuits or is it additive between both?
Are the lights on both circuits the same make, model, amperage, etc?