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#61817 02/02/06 10:58 PM
Joined: Jun 2005
Posts: 821
I was working with another mechanic on a job today and he insisted on running a new multi-wire branch circuit for 1 copy machine, and 3 computers. There was a bit of confusion as to who would pay for the work because the space we were working in was rented out to a tenant who needed the outlets. I suggested that we run (4) dedicated 20 amp, 120 volt receptacles, 1 for each piece of electronic equipment in the space.

To make a long story short, we got the ok to do only (2) dedicated branch circuits. The other mechanic insisted on running 12/3 NM and branching out from a j-box in the attic. I told him of the danger that exists in wiring it this way because of the non-linear load, and excessive heating of the neutral wire that could happen, but he seemed to careless and said he "never heard of that."

Do you think I was overreacting being that the load was only 3 computers and 1 copy machine? Or is this other guy lagging behind in the industry? What would you have done in this situation?

#61818 02/02/06 11:37 PM
Joined: Nov 2000
Posts: 2,148
Actual real world problems from this are rare. I would not be concerned. Did you ever notice that almost all of the articles that have been written about this "problem" were written by people with a vested economic interest in solving this "problem"?

#61819 02/03/06 12:19 AM
Joined: Apr 2004
Posts: 349
By computers, you're talking desktop PC's, right? How much current does the copier draw? Unless the copier was right near a limit, I think I'd just put the 3 computers on one circuit and the copier on the other and call it a day.

Not that PQ problems don't happen, but this seems fairly safe. If the copier's current draw is 15 amps or more, their might be a problem. But otherwise, it should be OK.


There are 10 types of people. Those who know binary, and those who don't.
#61820 02/03/06 12:21 AM
Joined: Jul 2004
Posts: 9,827
Likes: 22
This is a 3 phase problem. On single phase I have been assured it does not happen. I haven't actually pulled out my slide rule to look at triplin harmonics and phase angles but I tend to believe them.
"Super Neutral" cable is a 4 wire w/grnd. AFC would be hustling 3 wire w/grnd "Super Neutral" if there was a hint of a single phase problem.

Greg Fretwell
#61821 02/03/06 09:17 AM
Joined: Sep 2005
Posts: 39
Southwire has a type MC with an oversized neutral. From their catalog, "Applicatons affected by harmonics generated from non-linear switching loads, such as computers, variable frequency drives, electrical test equipment, and office equipment."

I'm not smart enough to know if that is just marketing or a good idea.

#61822 02/03/06 09:56 AM
Joined: Oct 2003
Posts: 156
If this is a 3-phase service, then the chances of a problem are slim to none.

If it is a single phase service, then it is not possible for a problem.

#61823 02/03/06 01:35 PM
Joined: May 2005
Posts: 178
I think this whole issue is going away quickly as obsolete computers head for the junk heaps.

The EU already mandates that all power supplies over 75 watts have PFC, and since it makes economic sense for PC manufacturers to use a single internationally-compliant design, we'll be seeing a lot fewer non-PFC computers.

+1 for not worrying about it.

#61824 02/03/06 06:26 PM
Joined: Jun 2005
Posts: 821
It was from a single phase 120/ 240 VAC service. And yes, they were PC's. Thanks for the input guys. The reason why I brought this up was because I read about it in one of my Delmar Learning electrical books. They discuss it briefly in the Harmonics section of the book. But if it's not really a big deal, why do some electrical designers insist on separate electrical panels just for computer circuits?

Also, why is it more of a problem on 3-phase services compared to single phase?

#61825 02/03/06 10:07 PM
Joined: Jan 2005
Posts: 5,390
Likes: 1
Cat Servant
Mechanic? NM in a commercial place? Why not call an electrician?

Copiers tend to use a fair amount of power. Even if this one doesn't, the next one will. So I'd run one circuit for it alone.

The three PC's can share the same, second circuit.

#61826 02/03/06 10:45 PM
Joined: Nov 2002
Posts: 794
Likes: 2
Also, why is it more of a problem on 3-phase services compared to single phase?

It's in the timing of the peaks (both positive and negative) of the various phases. In a 120/240 single phase system the positive peak of one side happens at the same point in time (with respect to a cycle of 60Hz) as the negative peak of the other side. This, assuming even loading of both sides, the neutral will not see much if any current. Computer switching power supplies only draw current at the voltage peaks (both positive and negative) of the phase they're connect to. In a 3 phase system the peaks happen at differing times over a cycle of 60Hz, so the current spikes on a neutral of a Y system will not cancel out. (overlap of the current spikes is unlikely). The neutral will see (current spikes on phase A)+(current spikes on phase B)+(current spikes on phase C). If the loads are evenly distributed the neutral will see 3x the current. New rules in Europe will be requiring these power supplies to draw current more like resistive loads (low harmonics, good power factor) so this overloaded neutral issue should ease.

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