#174798 - 02/13/08 11:39 PM
Re: IG circuits
Broom Pusher and
Loc: Anaheim, CA. USA
I would begin with the typical disclaimers:
* What is specified on the Contract Documents,
* What was written in the Proposal.
If a certain wiring configuration was specified and bid as, then install per the scope.
If these circuits are "Design Per Suggestions" (client is asking you to perform best install), then you have many options - including suggesting to NOT use IG Circuitry.
Computer equipment - such as Workstations and most Servers, really do not need the extra expense of IG Circuitry - and at times, if done incorrectly, may add problems rather than benefits.
Audio / Video Circuitry may have certain benefits from the use of IG Circuitry, in order to reduce loop currents which cause audible and visual noise.
Having said that, here are some key items regarding IG Circuitry...
1: "Intent Of Design"
The intent of the IG Circuit is to have a "Dedicated" Equipment Grounding Conductor for _EACH OUTLET_, which terminates at the "Star Point" of the System's Grounding point.
This results in the IG Conductors all terminating to a bus, where the System's Bonding Jumper, Grounded Conductor, Equipment Grounding Conductors and Grounding Electrode Conductors all terminate.
Could be at the Main Service, or at a Transformer - all depends on what System is used.
The target goal of this approach is to have _ALL_ the bonding Conductors + connections to Earth at the same point - hence the same potential; which eliminates excessive circulating current in the Conductors which are Ground Bonded.
From the description above, one may see why the NEC allows these supplementary Isolated Equipment Grounding Conductors to run through Panelboards, as necessary, to get to the "Star Point" Grounding connection - without having to be bonded to each Panelboard or other type enclosure.
2: "Per NEC Minimum"
To be NEC compliant, an IG Circuit may be as simple as any 2 Wire circuit, connected to an IG Receptacle, where the Ground terminal has a jumper bonded right to the outlet box.
That satisfies the minimum NEC requirement - as it should be a safe installation.
Article 90 clearly states the NEC cares nothing of overall performance, as these are the magic word "DESIGN ISSUES".
The installation could suck big time, as far as performance - just as long as it is installed safe.
3: "Sharing the Common Grounded Conductor"
If the Multiwire Circuit is going to be driving similar loads in the same general area (nothing with an odd running characteristic which would result in heavy noise &/or sporadic on/off switch surges), then share the Common Grounded Conductor - AKA "Neutral".
You will be "Balancing" 2 or 3 L-N Circuits across "Opposing Phases" as usual.
Unless there are some prearranged specifications which absolutely demand the use of Dedicated 2 wire Circuitry - or for some odd reason, a very noisy piece of equipment is to be connected to the Multiwire Circuit, I suggest to use a "Balanced Neutral" technique.
In your example, Circuits 1, 3 and 5 sharing a single, common Grounded Neutral Conductor.
4: "IG Circuitry - Target Goal Method #1"
Let's say there are Three (3) individual 4-plex (Double Duplex) IG Receptacles to connect, each 4-plex is driven by a "Dedicated" 20 Amp Circuit, as described below:
A: One Receptacle on Circuit #1 (Phase A),
B: One Receptacle on Circuit #3 (Phase B),
C: One Receptacle on Circuit #5 (Phase C).
To design within the "Intended Target Goal", each Duplex Receptacle would have a Dedicated IG Conductor run from it, to the Star Grounding Point.
This would result in a Homerun consisting of:
* Six (6) IG Conductors,
* Three (3) Ungrounded (Hot) Conductors,
* One (1) Grounded Neutral Conductor,
and if wanted,
* One (1) Equipment Grounding Conductor.
From a common junction box, each 4-plex Receptacle would be fed with Two IGs, One Hot, One Common Neutral, and One Equipment Ground.
5: "Landing The IGs"
As mentioned before, the Ideal location to land the IG Conductors is on a bus at the "Star Point" for the System's Grounding.
If this is not feasible, a single IG "Feeder" may be run from that Star Point, to the Panelboard which the IG Circuits are derived from, and landed to an Isolated Grounding bus in that Panelboard (Isolated from the metallic enclosure).
Then all the IG Branch Circuit Conductors terminate to that IG bus in the Panelboard
(*** "Panelboard" = "Sub Panel")
6: "If The IG Circuits Are Not Driving Critical Equipment"
If these proposed IG Circuits are just driving typical desktop Computer Workstation equipment, then you may go as far as just running a single IG Conductor to each 4-plex Receptacle - with a total of Three (3) IG Conductors, Three Hots, One Grounded Neutral, and One Equipment Ground.
Heck, if there are no specs, and $$$ is the primary motivation, go with only One IG Conductor for all 3 Circuits!
Like I said, the IG concepts really have no benefit for modern Computers, *** UNDER MOST CONDITIONS ***
There are some specific conditions, which would require IG Circuitry - these installations would have clearly defined specifications regarding the use and installation of IG Circuitry, of which it was very apparent things were Engineered and documented specifically for that project, not just some "Boiler Plate" text, or "It's Gotta Be IG" statements thrown around.
OK, Soapbox Mode = OFF
Let's hear from you - what are your thoughts?
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!