5-wire circuits ?

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We are wiring an office space with approx. 100 computer workstations, arranged in groups of 6.

The PE is calling for 3 20A circuits at each of these groupings, 2 for PCs and
1 convenience circuit. He is calling for these circuits to be wired as follows:

"...Provide number of phase wires as shown (3) with two #10 neutral and two #12 equip. ground for each grouping."

This is unusual to me. Is he paralleling the neutral because he's cautious of a non-linear load ? Wouldn't it be more straightforward to simply run three 2-wire circuits to each grouping?

So far I have just seen his specs. I will be speaking with him next week.
Does anybody have any thoughts on this ?
 
Perhaps the intention is to provide a dedicated neutral for the computer circuits seperate from the convenience circuit to account for the non-linear loading of the switching power supplies in the computer equipment. Although, why both neutrals would need to be sized at #10 is not clear to me. I can see it for the computer circuits but not for the convenience circuit. I'll be interested in hearing what the PE has to say on this.
 
we have run into this before and what had happened was the pe took the information from the modular furniture manufacturer's electrical detail. they sometimes include the detail on the pigtail that comes with the furniture... this pigtail can have supersized neutral #10 and separated equipment grounds for different systems. in this case it sounds to me like you really need three dedicated circuits with a common equipment ground -- but i'm not a p.e.! sad part about all this special wiring is the secretary will remove this and that and before you know it she'll have her laser printer and space heater on the same circuit as her computer!!!!!!!!!!!!!!!!!!
 
.....before you know it she'll have her laser printer and space heater on the same circuit as her computer!!!!!!!!!!!!!!!!!!
____________________________________________________

That's a definite !

At least computers are more robust than they used to be.
(except when you need them most)
 
My first reaction is that three phase conductors plus two neutrals plus two EGCs is seven, not five. ;)

My second reaction is that 310.4 does not allow for paralleling conductors smaller than 1/0 (with a couple exceptions that do not apply here). So I can't figure out what the engineer has in mind. If there is a concern with harmonics, then install a 1000 MCM neutral, and I will only laugh. But if you install a pair of #10 neutrals and connect them in parallel, then I would cite this as a violation.

My third reaction is that installing two EGCs is patently absurd. An EGC has absolutely no purpose in life, unless there is a fault, and in that case it fulfills it purpose quickly and is done. Having two EGCs does not improve its ability to do its job. Paralleling two #12s to create a larger EGC violates 310.4.
 
I kind of agree with 5Spot.

(2) computer circuits may be run to panelboards fed from K-rated transformers so you would need (2) phase, (1) neutral, (1) Ground

The (1) normal circuit may be run to panelboard not fed from K-rated transformers so you would need (1) phase, (1) neutral, (1) Ground

Which brings us to the total.

If this is the case then I would say V.E. the installation and save the owner some money.
-Ed
 
I don't get the 2 oversized neutrals but isn't the second ground for isolated recepticals normaly used for today computers??
Thanks for the input
 
I have a cut sheet about the wires in modular furnitures from one furniture agent. There are 6 circuits for a cubical group. They are three convenient circuits ((L1,N1,G1),(L2,N1,G1),(L3,N1,G1)), and three dedicated circuits ((L1,N2,G2),(L2,N2,G2),(L3,N2,G2)). L1,L2,and L3 are three phase lines (Black, Red, Blue), N1 and N2 are two neutral lines (the cut sheet requires 10 AWG for the neutrals, one for convenient circuits and one for dedicated circuits) G1 is ground (Green, for convenient circuits), and G2 is isolated ground (Green/Yellow, for dedicated circuits). That is why the PE specifies to install 7 wires circuits with two #10 AWG neutral wires and two grounding wires.
 
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charlie b said:
An EGC has absolutely no purpose in life, unless there is a fault, and in that case it fulfills it purpose quickly and is done.
Click to expand...

My guitar knows when a EGC isn't present. My amp makes a loud hum sound when there isn't one. IMO it does serve a purpose. :)

I think Duhualin has the winning numbers.
 
The furniture we use has the following. L1/L2/L2 with an oversize netural and a ground. It then has an isolated circuit L1/N1 and ground. The netural on this circuit is also oversized. We never use the isolated circuit
 
Thanks everybody for your enlightening comments. :)

What the PE has called for would make more sense if he had phrased it
as Ed Downey has (L1, L2, N1, G1 to one panel and L3, N2, G2 to another) In fact, he has it all going to the same panel, but this arrangement still makes more sense than what he seems to have called for: L1, L2, L3, N1, N2, G1, G2 as 3 circuits with paralleled neutrals, which Charlie B pointed out as a violation under 310.4.

In an arrangement L1, L2, L3, N1 how much might the neutral conceivably be asked to carry, considering harmonic loading? #10 should be fine for that neutral, shouldn't it ? (And for the 2-wire convenience circuit I would think it's overkill)

Thanks again.
 
I have to wonder if you are projecting the whole story here. You have depicted the connections for one workstation. What about the possibility of the convenience receptacle circuit being shared amongst two or more workstations (it would be called a dedicated receptacle/circuit if it supplied power to only one appliance/device)? Perhaps L3 is only an indication that it is not the same branch circuit as L1 or L2, not necessarily that it is a C-phase branch circuit, and substantiate the required second neutral.

I'm also inclined to believe G1 is an IG rather than an EGC. However, an IG does not negate the requirement for an EGC unless the wiring method thereon is totally non-metallic (which I doubt). However, G2 is then most likely an EGC and will serve the purpose if integral to all device wiring within the workstation.

BTW, the two neutrals would only be parallel conductors if they are electrically bonded to each other somewhere other than at the panel. I can't tell from here :rolleyes:

#10 should be fine for multiwire 20-Amp branch circuits, provided the non-grounded conductors (#12's) do not have to be derated to #10 or larger for whatever reason.
 
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As per PE's panel directory, L3 is indeed a C phase conductor.

6 workstations will share 2 PC circuits, and 1 "convenience" circuit.
There is no IG called for, only EGC, from the PE's vague instructions with which I initiated this thread.

If I were laying it out, I would choose three separate 2 wire circuits
L1N1 L2N2 L3N3 for these purposes. I know someone will say then we
lose the benefit of phases cancelling each other @ neutral. However,
we lose the risk of bad stuff happening if a neutral is ever loose or
compromised.

I don't mean to beat a dead horse with this topic. I'll be talking
to the PE this week.

Thanks again everyone for your input.
 
Sys furniture - Isolated grounds

Sys furniture - Isolated grounds

If we agree that the isolated ground conductor is a waste of money, and we run a single EGC conductor and tie in the sys furniture ground conductors to it, would this be a code violation?

I think it would be a violation if the sys furniture vendor provides isolated ground receptacles with the triangle notation. By installing one EGC, we are not wiring the IG correctly. If we can convince the sys furniture mfr to just regular receptacles and still allow him to use the same 7-wire scheme , then we should be ok by tying the 2 ground conductors to a single EGC.
 
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