The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!


2017 NEC and Related
2017 NEC
Now Available!

Recent Posts
International Wire Colour Codes
by Tjia1981
Today at 09:09 PM
Son of Sparky
by HotLine1
Yesterday at 07:43 PM
Speaking of Plugmold ...
by gfretwell
10/17/16 02:37 PM
Broken battery charger? Check for cobwebs!
by gfretwell
10/17/16 02:30 PM
230 or 345 kV transmission lines?
by annemarie1
10/12/16 01:23 PM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 11
HotLine1 6
renosteinke 6
ghost307 5
Potseal 4
Who's Online
0 registered (), 139 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#86473 - 11/04/03 03:14 PM shared neutral on computer circuits 3 hots 1 neutral
richard Offline

Registered: 08/07/03
Posts: 84
Loc: L.I. New York
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)

2014 / 2011 NEC & Related Books and Study Guides
#86474 - 11/04/03 03:51 PM Re: shared neutral on computer circuits 3 hots 1 neutral
wa2ise Offline

Registered: 11/29/02
Posts: 769
Loc: Oradell NJ USA
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.

#86475 - 11/04/03 05:23 PM Re: shared neutral on computer circuits 3 hots 1 neutral
John Steinke Offline

Registered: 04/03/01
Posts: 509
Loc: Reno,Nv., USA
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.

#86476 - 11/04/03 07:06 PM Re: shared neutral on computer circuits 3 hots 1 neutral
richard Offline

Registered: 08/07/03
Posts: 84
Loc: L.I. New York
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.

#86477 - 11/05/03 12:14 AM Re: shared neutral on computer circuits 3 hots 1 neutral
iwire Offline

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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

#86478 - 11/05/03 04:14 PM Re: shared neutral on computer circuits 3 hots 1 neutral
ThinkGood Offline

Registered: 08/07/02
Posts: 1084
Loc: Milwaukee, WI
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.


ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals