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Circuits for cubicle farm

15K views 10 replies 6 participants last post by  BrightLight 
#1 ·
Hey,

I have a request for a bid on a gig. The client is in a very modern office building, three phase of course.

He has a cube farm of 30 cubicles in three rows. He called me a while back for a power outage, I restored the breaker and sent a modest bill. The rows have ten cubicles each. The electrical spine of the cubicles permits as many as four circuits, but each row is served by three circuits, with one 12 ga neutral (worrisome if he gets a high current due to a third harmonic). The circuit that tripped had four cubicles on it. I am now rating each cubicle at 6 amps, so that is a limit of three per 20 amp circuit. (Each cube has a fast cpu and two flat screen monitors).

But he wants more cubes in this area. He wants four rows of twelve. Two of the circuits come up thru the floor, and will be in the walk way in the new arrangement, so just disconnecting them and pushing them back down their conduit, and likewise disconnecting them at the panel and winding them up and taping them off does not bother me too much :whistling2:.

I think I am going to bring him four runs of conduit in the ceiling, with four circuits each. Black, red, blue, pink, (the color code in the whip from the cubicles) with two 10ga neutrals, and an isolated ground, allowing the conduit to be the chassis ground.

Six current carrying conductors requires a 20% derating, 12 ga thhn with 90 degree insulation derates to 24 amps, so the hots should hold up ok, but what can I expect on the 10 ga neutrals that are going to return the current from two three phase circuits each? Will they hold up?
 
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#2 ·
.............Six current carrying conductors requires a 20% derating, 12 ga thhn with 90 degree insulation derates to 24 amps, so the hots should hold up ok, but what can I expect on the 10 ga neutrals that are going to return the current from two three phase circuits each? Will they hold up?

No need for a 10 neutral. 12 is just fine.

And it's the grounding conductor, not chassis ground. Keep the automotive terms in the garage.
 
#3 ·
Chassis ground

Yeah, I was just looking for a way to distinguish the ground that bonds the chassis of the cubicles from the iso ground that serves the computers.

I have always preferred a #10 neutral, in fact the whip that is part of the cubicle system has four #12 hots and two #10 neutrals. I'm thinking the manufacturer knows something.

It is my understanding that computer loads in a three phase environment can create a harmonic that drives a current higher than that protected on the hots, possibly as much as twice the current as might be found on the hots. Hence the #10.

Now I am thinking about a neutral returning the current from two circuits in a three phase enviroment feeding computers, and I am wondering if I could conceivably cook a neutral and over volt a mess of computers and go bankrupt paying for the damage.
 
#4 ·
Sounds lie Herman Miller cubes.
All the cubes we have have the upsized neutrals. Most of the time I feed them with mc cable with a #12, and so far never had a prblem.

Are yoiu deriving the 4th circuit from the same panel?
We tried that and relized it was a waste of time and money, as the people in the cubes wired anything anywhere they saw an open receptacle.

I run up to 10 cubes per boat, but all of our cubes are now 6x6 and people can not havbe as many devices as before, so no problems with overloads.
 
#5 ·
"Sounds lie Herman Miller cubes.

I feed them with mc cable with a #12,

Are yoiu deriving the 4th circuit from the same panel?

I run up to 10 cubes per boat,"

:001_huh:I would like very much to wire them with 12/3mc, as that would be the cheapest and fastest route, but am still concerned that I would cook the neutral.

Yes, the 4th circuit is from the same panel. Black, red, blue, pink, red, blue, black, pink, from one panel, and the same from another.

I did find a space heater plugged into the cubicles. I told the IT wiz that f he was going to tolerate that then I could not make any promises that his circuits were going to hold, and if people lost data (lots of money!) that is not my fault.

He wants to go to twelve per row, so I want to give him four circuits per.
 
#9 ·
No, I really don't.

Yes, I am aware of a "super" neutral, although I have never heard it so named. I've read articles about third harmonics and having seen what can happen when a common neutral gets dropped, I would very much not like to be responsible for such a melt down.
 
#8 ·
I think I am going to bring him four runs of conduit in the ceiling, with four circuits each. Black, red, blue, pink, (the color code in the whip from the cubicles) with two 10ga neutrals, and an isolated ground, allowing the conduit to be the chassis ground.
We would just run MC cable this.

We generally use AFC home run cable for this and I bet some equivalent is available in your area.

You can get it with conductors that match the furniture whips.
 
#10 ·
That's a good tip. I hate to think what the price differential is on 250' of 12-4/10-2 and just pulling into some 3/4" flex would be. Flex and individual wires costs $1.08 before taxes. The savings on pulling time and effort would be worth? another ten cents per foot? Were I wiring from scratch I certainly would want to use this stuff.

I'll see if I can find it. Thanks.
 
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