ECN Forum Forums Code Related Discussion NEC & other Code issues sharing neutral

I'm with you on this one Reno. I avoid shared neutrals except when I'm running 2 circuits for a distance with BX, and when I'm pushing 9 current carrying conductors in conduit. I also agree handle ties is the best safety solution.

As a courtesy measure, I twist the ungrounded conductors in the distribution panel all the way to the breakers. I also pigtail to outlets whether it's shared neutral or not.

Dave

I am still buying miles of 12/4 - 10/4.

Me and multiwire circuits are good friends and friends don't let each other down.

The distances and number of circuits I have to supply in the types of occupancies I work in make multiwire branch circuits the smart choice.

Besides, I can not supply equipment designed around 3 PH 4W supplies with 3 two wire circuits.

Scott35: You might want to take a look at the following waveform. Scroll down to figure 8 on the following web page:

http://www.zlan.com/waveforms.htm

This is actually a very severe level of harmonics, and rather typical of what I have seen a couple times I've put a scope on a switching mode power supply.. Notice that each spike is conducting for about 1/3 of the time, and 2/3 is spent not-conducting. Next, imagine superimposing the same waveform over itself for 120 degrees phase shift and 240 degrees phase shift. The spikes essentially won't overlap. And don't forget that these spikes are actually about 3 times higher in current than the peaks of a normal linear load with the same average current.

Of course if you have a mix of loads, and computers are just a small portion of those loads, then you're not really going to see much of an issue with triplens on your neutral, as long as the various overcapacity requirements in the code are followed.

However, in the case of a room full of computers (and I've worked in rooms of as many as 2500 computers), you end up with essentially no loads other than these very spiky switch mode loads. If enough computers are pushing the average current load to 12 amps on a 15 amp rated circuit, then you're going to see 36 amps average on the neutral. Those "double neutral" cable assemblies, panels, and transformers are really going to get more than just double the load.

I read a study from Square-D where an analysis suggested that in the typical office building, triplen currents would not exceed double, and rarely even reach it. But it appears that was a study of the overall office environment or building, and it appeared to have been done many years ago. In the age of the internet and a web based business presence, there will be more and more computer rooms running multiple servers and requiring dedicated circuits. A 4-wire 208Y/120 branch circuit to that room is simply going to be unsafe (36 amps on a #14 neutral). If you up the cable to have a #10 neutral, you can probably get away with it, though I think #8 should be required on the neutral for this example case.

Among the many things I do is specify data center designs for internet providers or business going online in a big way. I always include in my design a specification that all 120 volt circuits must have individual neutral conductors regardless of the system phasing. No shared neutrals should ever be allowed for this kind of load (even for single phase systems, but that involves some other more complex issues).

I've also considered connecting computers L-L at 208 volts. Most can handle it just fine. But this shifts the problem around a bit. What happens is you get 2 sets of spike currents on each line conductor instead of 1 in the L-N case. So that case of 3 sets of 12 amp average switch mode loads is going to be putting as much as 200% load on each line, instead of 173% as would be expected in the case of 3 sets of linear loads (so 15.47% more). Those 3 sets of 12 amp loads are now going to be 3 sets of 6.93 amp loads at 208 volts, so that means 13.86 amps on each line conductor (not 12). That's too much for the 80% margin of a 15 amp circuit. Up it to a 20 amp circuit (or spread the loads among more circuits) and this might work.

A good computer room design will involve UPSes. I've read that some UPSes (of the double conversion always online tyoe) will have less harmonic current than a regular switch mode power supply. That might be a way to avoid or mitigate most of these issues. But there is always the risk that someone could bypass the UPS in the event of failures (and it seems some UPSes do fail).

If the computer loads are a major portion of the whole building load (e.g. a building just for a large data center), then this issue has to be dealt with upstream, as well, likely even involving the POCO.

One way to mitigate the effect of computers is to use computers with power-factor-corrected power supplies. These pull very close to a pure sine wave from the building wiring.

Generally speaking, the power supplies that can take something like 120-240 volts in without changing a switch setting are power-factor corrected. (The power-factor-correction circuit has a side effect of allowing the "wide-mouth" input.)

I recognize that one typically has little or no influence on what computers are chosen, so this little tidbit of information is probably useful in only a few situations.

[This message has been edited by SolarPowered (edited 05-04-2006).]

Wow!! I've been busy and haven't checked this forum too much lately. I've done my job I referred to, and to some's dismay and others acknowledgement, I did use multi circuit neutrals. I did put the printers and office equipment on separate neutrals from the computers. I appreciate all the input, as it is very helpful now and down the road later. To those who would say that anyone could do our trade, they don't know what they are talking about Maybe they could "make it work", but will it still be working right down the road? Sometimes I think "painting" might be a better trade, not so much "do's and don'ts But it's in my blood now, so what do? Thanks again for all the input. Job went well, and I still have more wiring in the future for more computers when they get ready. Steve...

Roger

(IA^2+IB^2+IC^2)-(IAxIB)+(IBxIC)+(ICxIA)

Am I missing something or what? It seems to me the right hand portion of that statement should be all minus signs.

This isn't what I would call my area of expertise, but it would make sense to me if those were negative signs.

Larry, you're not missing anything, it is my mistake, I just put to many sets of brackets in the last part for the addition method I use.

It should look like this;

(IA^2+IB^2+IC^2)-(IAxIB+IBxIC+ICxIA)

This lets me add all the parts of the second side of the equation together and subtract this sum from the first part.

It can certainly be done in individual steps as you pointed out and shown as follows. (correctly )

(IA^2+IB^2+IC^2)-(IAxIB)-(IBxIC)-(ICxIA)

Roger