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#58903 - 11/22/05 10:10 PM Current on neutral  
wraiths  Offline
Member
Joined: Apr 2005
Posts: 11
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


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#58904 - 11/23/05 12:23 AM Re: Current on neutral  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
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


#58905 - 11/23/05 09:06 AM Re: Current on neutral  
wraiths  Offline
Member
Joined: Apr 2005
Posts: 11
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


#58906 - 11/23/05 09:39 AM Re: Current on neutral  
Ron  Offline
Member
Joined: Mar 2002
Posts: 582
White Plains, NY
Are you sure the two circuits were from different phases? If same phase, then the neutral will have 2 x imbalance current.


Ron

#58907 - 11/24/05 11:53 AM Re: Current on neutral  
wraiths  Offline
Member
Joined: Apr 2005
Posts: 11
Yes they are from different phases. So can anybody tell me is this normal?


#58908 - 11/24/05 12:02 PM Re: Current on neutral  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
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.


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#58909 - 11/24/05 12:57 PM Re: Current on neutral  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
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


#58910 - 11/24/05 01:05 PM Re: Current on neutral  
earlydean  Offline
Member
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
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.)


Earl

#58911 - 11/24/05 09:49 PM Re: Current on neutral  
wraiths  Offline
Member
Joined: Apr 2005
Posts: 11
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.


#58912 - 11/24/05 11:14 PM Re: Current on neutral  
LK  Offline
Member
Joined: Jan 2003
Posts: 1,429
New Jersey
"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?


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