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Joined: Aug 2003
Posts: 99
richard Offline OP
for computers, is is neesserary, or desireable, to have 1 hot 1 neutral, or is there little effect using 1 neutral with 3 120 volt circuits(on differt phases,3 phase system)

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Joined: Nov 2002
Posts: 793
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Most computers use switching power supplies. These supplies draw a large spike of current when the line is at max voltage during a cycle of the 60Hz AC waveform, and nothing in between. As most of these supplies use a bridge rectifier connected to the line, there will be a spike of current at max positive voltage, and another at max negative voltage of the AC waveform. Current spikes at 120Hz rate. These current spikes are rich in harmonics. This isn't a big deal for a 2 wire system (one hot and neutral). The hot and neutral see the spikes at the same point in time of the AC waveform. Now imagine a Y system, 3 hots, each peaking 1/3 cycle after another. When the power supply on say phase A is drawing a current spike, the supplies on phase B and phase C are not drawing any current. So the neutral wire has to handle the return current spike. The neutral will see 3 times as many spikes as any one hot wire will see. And thus the neutral will get rather hot. So you'd need a super neutral.

There is a possible work around to having to use a super neutral. Most computer power supplies can accept line voltages from 100V (Japan) to 125V (USA), and 200V to 250V (Europe and elsewhere) all AC. These power supplies convert the input AC to a non-isolated high voltage DC, and then to a high frequency AC and finally to isolated low voltage DC. Check the specs on the nameplates of the power supplies inside the computers. There is usually a powerline voltage selector switch on the power supply, usually marked "110V-220V". This switch changes the line rectifier from a voltage doubler mode (100-125V) to a straight rectifer mode (200-250V) to produce the non-isolated high voltage DC mentioned above. Switch the power supplies to 220V and connect the computers to 208V. The supplies will draw about half as large of a current spike from the hots, and the neutral sees nothing.

Computer monitors can usually accept 100V to
250V AC without the 110V-220V selector switch, again check the nameplates.

Joined: Apr 2001
Posts: 518
Strongly reccommend separate neutrals for computer circuits. This eliminates some of the things that can go wrong. Indeed, I often also use "isolated ground" circuits for the same reason.

Joined: Aug 2003
Posts: 99
richard Offline OP
Thank you for the informative replies. Computers are sensitive to surges, so I have a batt backup on my personal pc, but always wondered about circuits sharing the neutral. I agree that it is better to have a clean return for each circuit, but one of the bosses try to save money and time by what I consider to be short-changing the customer. Oh well.

Joined: Jan 2003
Posts: 4,391
The replies above make good sense and I am not disagreeing with them.

But I will say it is very common in buildings with office cubes (modular office partitions) to use 3 hots to a neutral.

It all depends on how the "cubes" where ordered.

When they are wired with common neutral, is a "super neutral" 10 awg as apposed to 12 awg.

The IG IMO is a waste of time and energy in most applications.

Often it gets corrupted down stream anyway, as an example, when a PC is networked to equipment on other circuits with a coax.

Bob Badger
Construction & Maintenance Electrician
Joined: Aug 2002
Posts: 1,081
Where's the link to the article about UPSs, etc? I think it was posted by C-H a few months ago. Something about polarity, etc., a bit controversial.

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