Thanks to
mxslick for bumping this thread back to life
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Wanted to make a short reply to it back in January, but somehow forgot to (like being busy is any excuse
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...)
Anyhow, I'll make this short and sweet - if possible
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First off, let me point out that there are a lot of valid posts made in this thread already! My contributing 2ยข is kind of meaningless, but I felt the urge to chime in and act cool!
Secondly - and most importantly why I am writing this whole thing in the first place!;
To the original poster ("OP")
sparkync,
did any of this thread assist with your installation questions???
Pretty much, the installation part is mainly up to you - as to the choice of using Multi Wire Branch Circuits, or going with an array of Two Wire Branch Circuits... with the following exceptions:
1) Are there any Specifications within Contract Documents, that exclude using Multiwire Circuitry for given equipment? - and I must exclude the typical "Boilerplate" stuff that firms toss into Contract Documents, including "Basics" in the beginning of Division 16 of the Project Manual, or Text Blocks found in the "General Notes" page of the Planset.
What I am referring to would be exclusive pages within Division 16 of the Job Manual, which cover specifics for given equipment.
2) Does the Manufacturer(s) of given equipment demand their equipment to be connected only to "Dedicated 2 Wire Circuitry", in order to hold any warrantee?
This would be an issue for you in the event of
ANY failure - the first thing they will do is pin the failure on the Circuitry, and boast this to your Client.
3) Was the project bid (proposed) to use 2 Wire Circuits for the specific loads (equipment)? If yes, than you should do as bid.
Other than this, there really is no valid reasoning either way. It's a matter of choice.
The NEC has no say in the types of circuits you _CAN_ or _SHOULD_ use for this equipment, it only mentions how to safely install oversized Common Grounded Conductors (Neutrals). This is, once again, a Design Issue, not a Safety Issue.
If you decide to use Multiwire Circuitry, feel free to keep the Common Grounded Conductor the same size as the Ungrounded Conductors, or if you feel it is needed, increase its size.
If you go the 2 Wire Circuitry route, be sure to take into consideration the fact that each 2 wire circuit will have 2 current carrying conductors; so when running multiple circuits through a single common raceway / conduit, derating the conductors will apply "faster" than with multiwire circuitry.
If you have concerns regarding excessive load current on the Common Neutral, try to limit the _Possible Loads_ to no more than 50% of the circuit's value.
Since these loads are what I would classify as "Intermittent" loads, not "Continuous" loads, if any equipment does have a real dirty level of distortion ( > 33% THD "across the board" ), it likely will only be at its peak for maybe 15 minutes of each hour - maybe 30 minutes of each hour - so there will be at least 50% of the nominal time period where the load draw is very low, thus allowing the conductors to cool down.
Lastly, in regards to this whole Harmonics / Common Conductor shared subject;
I was a victim of closed minded thoughts when it came to loads which produced harmonic distortion, and thought at first that the problem was bigger than it really is.
But think about it...
*** Viewing it as the culprit will be _Switch Mode Power Supplies_:
The Workstation PC does not draw the rated capacity _Continuously_, only in transient peaks. Most of the time, the SMPS is maybe running 20% above Idle.
So, having a Workstation with a THD of 40% being found in the 3rd, 9th, 27th and 81st Harmonic doing typical Workstation duty, how long will these high levels of current be flowing between the load equipment and the Secondary of the Transformer?
Same goes for Printers (LAN Printers, Local Printers and Print Server Stations), Copiers and Fax Machines - total peak load draw is not continuous.
File Servers may have a more continuous load draw, yet do they draw for 3 hours or more?
Intranet Servers may fall into this class - and should be figured as such when designing circuitry.
***Viewing it as the culprit will be _Lighting_:
Think back to the "Harmonics From Hell Scare" of the late 1980's; what was going on then?
This was the invent and introduction of Hybrid Component Ballastry.
Most likely, the "Primary Scare" was something _Coupled_ to the usage of these types of Ballasts.
To me it sounds like just another misquoted report or statement, which first began a frenzy of panic, then was taken advantage of by manufacturers of equipment and materials.
What I believe the scenario(or scenarios) were about was Lighting Circuits which were loaded to maximum (like 20 amp circuits loaded to 18 - 19 amps), and running for entire days - even weeks, using #12 conductors for multiwire circuitry, fed through raceways with 3 or even 4 additional multiwire circuits running through them, and were feeding Lighting Equipment with the new technology Ballastry that produced "higher than ever seen before" Harmonic Distortion back into the circuitry and the system.
I say this because of the numerous installs we had encountered during Tenant Improvement work, where these Lighting scenarios had been installed as described above.
It is not "News" that Magnetic Reactor type Ballastry produces Harmonic Distortion - sometimes in the order of as high as 20% THD, as do Induction Motors and other Reactive loads, so the issue of Harmonic Distortion has always been there in the Commercial / Industrial world.
To wrap this fricken miniseries of a post up
I still will use - and even design projects, with the concepts of utilizing 4 Wire Multiwire Circuitry wherever possible (on projects with 4 wire wye secondaries!).
If a Client feels the need to have 2 wire circuits used, I will do the normal thing and explain (as much as possible) the reasoning, ideas, myths, urban legends, pros and cons for each type of circuit (if there is a chance to do so!!!).
It is also my part to be sure the crews doing the installations do not screw up Multiwire circuitry, or any circuitry what so ever!
Sometimes it is not so easy for me to do so, as I may design a project and merely have the "fun" of playing Project Manager on it "Vicariously" (only see the job 1 or 2 days a month), but most of the jobs designed by me will also be run by me.
As to how many barbecued items I have witnessed, which were caused by Multiwire Circuitry faults or excessive Harmonic Distortion, only a handful in over 26 years.
Saw a barbecued common neutral in a Panelboard at the Bus Termination. That one turned out to be a combination of a loose screw on the Bus and some joker had 6 Lighting Circuits across the common.
Took for ever to T Shoot that one!!! (it was on a project where the EC had high turn-over employees coming and going, so things were really messed up!).
Another one which comes to mind involved fried equipment in a newly developed Commercial "Office/Warehouse" shell project, back in 1989, - where there are multiple buildings and all suites are simple shells.
This one Tenant had a Copier, Fax Machine and 2 PCs go all Smoky on him - and of course, the Property Manager calls ranting about "Fire Trucks, Flames Leaping From Buildings" and similar chaos.
I was working for an EC that just loved to smoke Pot - endlessly! At least a 1/4 Ounce every dang day!
On top of this, during the time of the french fried equipment, he decided that smoking pot wasn't quite enough - that it would also be great to smoke speed too!
Now, he wasn't the easiest to talk with when sober, so just add some pot and speed, shake well
, and you have a raging jackass!!!
Insisting that the barbecue was my fault - due to "Not Giving A Damn About Good Makeup On Neutrals", he had the entire crew open every single outlet, J. Box and etc. to verify connections.
All terminations were 100%.
I was at the Panelboard and did a "Short Circuit Test" with my screwdriver - between the Grounded Conductors Bus and the Panelboard Enclosure.
With a few minor sparks popping off from the semi conductive path, all the sudden the lighting becomes stable and bright!
Tell "Mr. Jack-Pot" that I suspect either a fault from the Utility Transformer (lost Neutral), or more likely, someone forgot to install the Jumper Bus Bars between Gear sections.
After Jack-Pot argues this and messes with other stuff for 2 hours, I go to the switchgear, open it up, and find loose carriage bolts on the last 4 meter section.
A good heads up, plus one additional rant for me!
Scott35
