Don,
The PC's power supply, since it is a switching power supply, causes the load to be non-linear. That not only creates a power factor - less than unity - but also brings in conserns of Harmonic Distortion.
The thing is though, even with the older Reactor core ballasts and linear power supplies [with step down transformers, bridge rectifiers and filters all running at the frequency of the power system - 60 Hz -], the problem of Harmonics was still present. Things have evolved to the point that nearly all loads in Commercial are now non-linear and have a possible >30% THD. Non-linear loads would be Reactive loads, where as linear ones would be more like Resistive loads. This is the reasoning behind why it is not allowed to derate the grounded conductor [AKA "Neutral"] on 120/240 V 1 phase 3 wire systems' feeders anymore. This would apply to 1 phase 3 wire systems derived from a single transformer, or from the 1 phase 3 wire transformer on a 4 wire Delta.
Just a little techno babble FYI
Electrician02125:
How high are these branch circuits loaded? Have you metered the RMS load current on these circuits with the barbecued grounded conductors?
Some of the older Ballasts that were "Hybrids" had as much of a Harmonic Distortion problem that some of the High Frequency Ballasts used currently do. Also, older Ballasts might not have been PF corrected, so only 1/2 as many fixtures could be on a circuit, due to twice as much line current.
I know this sounds like a load of bull, but not every non-linear load has a tremendously inheret THD problem. Mix-matching lamps and ballasts would most likely increase the THD above the Manufacturers' listed calcs.
Make sure that no one has mistakenly put two circuits from the same phase in with these multiwire branch circuits. It's not too clear what type of power system is being used for the multiwire circuits - 1 phase 3 wire [120/240 volt], or 3 phase 4 wire [208Y/120 volt low voltage, 480Y/277 volt high voltage], so if 1 phase 3 wire, the possibility of an overloaded grounded conductor might be that someone mistakenly tried balancing three lines to the common.
On the other hand, if it's 3 phase 4 wire, someone might have tried to balance "Two Phase A" circuits to the common. I only say this because I have troubleshot this one a lot when barbequed commons in sub panels turned up. The circuits had been wired this way maybe since the project was new. The loads were so low before hand that there wasn't an excessive load on the common. After several remodels and upgrades, the circuits became highly loaded - which brought up the hidded problem.
If you can, give the most delicate Computer equipment Dedicated - Isolated ground circuits. Dedicated meaning Dedicated Line and Common [two wire], plus Dedicated IG conductor, per circuit. Only having the Computer and related equipment on the circuit. Most desktop PC's that are Commercially used for business can get away with a balanced 4 wire branch, but remember to calculate the KVA loads as to their MAX draw [don't forget to figure the Laser Printers!!].
Not too sure of how many LANs you have wired [both Power and Data], so I'll leave things at this point. Feel free to reply to this thread and add some comments. If you need a few suggestions, let me know. Been doing LAN power systems for Banks for a good 15 years and have seen things change over the years. Only been doing in depth com/data cable setups for 6 years, where as previously, we would install the cable [IBM Type 2 and Triplex for 10base-2], then an IBM certified vendor would terminate cables. We did a few 10B-2 triplex LAN terminations [with soldered BNC ends, made by Amphenol].
Just a little "Retrothinking"
Scott "S.E.T."