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ac wiring new office
#252094 08/16/07 06:49 PM
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I would think one would dedicate a couple of circuits in the panel for the computers in the office, and not use normal wall outlets in a new construction of office?

Even dedicte heavy copy machine on its own breaker ?

This being a general rule of ac wiring a new complex.

I am thinking proper, am i not ? :confused:


-TJ-
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Re: ac wiring new office
#252095 08/16/07 07:06 PM
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Not really. However if there is an excessive amount of PC they should certainly have made this clear to the architect, especially if they are concentrated in one particular area.

Re: ac wiring new office
#252096 08/17/07 05:31 AM
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If there are a lot of computers on the floor, then the recent (last 5 years)trend has been to double up the neutral conductors and put the computer equipment itself on a panel with a separate, isolated ground.

Power distribution should be based on current draw - if a device (ex. heavy duty copy machine) draws 15 Amps it should NOT be put on a 20 AMP circuit that provides convenience outlets in an office.

Convenience outlets are just that - receptacles placed arund the office to provide a convenient spot to plug in some device.

The architect should have been advised of what each specific office layout would be (PCs, printers, lamps, refrigerators, microwaves etc.). If this wasn't done, then you get a generic layout (hopefully) done as per local and National codes.

Sam


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Re: ac wiring new office
#252097 08/17/07 05:35 PM
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Sam

Isolated ground.

But still terminated on gnd. buss in panel?, or
isolated to other ground source (grd. rod ) outside of panel ?

We are referring to equipment ground?


-TJ-
Re: ac wiring new office
#252098 08/17/07 06:02 PM
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Hal or Ed will correct me if i'm wrong, but an "isolated ground outlet" is a special outlet, at least here in Canada where the ground lug on the outlet is not parallel to the neutral.

If you take your meter on a normal outlet, you'll have continuity between the ground and neutral lugs. You won't with these special outlets.

These are not normally used here except for special applications as for video lottery terminals & the like.

Dave


Scientists say that the universe is made up of Protons, Neutron & Electrons. They forgot "Morons".
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Re: ac wiring new office
#252099 08/17/07 07:12 PM
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Isolated ground (IG) receptacles have the ground contacts (the round hole) connected all by themselves to the green ground screw.

A standard receptacle would have the ground contacts connected to the metal yoke that holds the receptacle to the box. The yoke itself would then be threaded for the ground screw. If you were to use a metal box the receptacle ground is then part of the grounded structure particularly if the box is attached to metal studs.

The ground wire from an IG receptacle is supposed to run all the way back to the service entrance ground. At that point all grounds and the neutral are tied together. So Dave, if you are not reading any continuity between neutral and ground that receptacle is not grounded. The only difference between an IG and a normal grounded receptacle is the point where they are grounded. The ground will always be connected to the neutral.

That said, IG receptacles are usually a waste of time these days assuming anyone wires them correctly to begin with. Who is going to run a #12 from the third floor of an office building all the way down to the service entrance just for a receptacle? What EC is going to know this anyway? So most IG receptacles get botched and are at best the same as a normal receptacle or are a danger such as if they are wired to a ground rod.

IG receptacles used to be useful for computers in the days before ethernet when communications was via RS-232. It was necessary to maintain the same ground potential on all interconnected equipment because RS-232 cables connected the grounded chassis of one machine with the next. Any ground loop could disrupt the transmission of data.

Usually if an IG receptacle is specified today it's only because the specifier doesn't know any better.

As for the original question, dedicated circuits for PCs are a thing of the past also. Certainly if a copier has a heavy draw a dedicated circuit is advisable but unless there is a heavy concentration of PCs in one area (like a cubicle farm) there is normally no problem connecting computers in individual offices to general purpose circuits.

The problem arises when a multi wire branch circuit feeds predominantly PC type loads. The switching power supplies generate harmonics that combine and create an abnormal current draw on the neutral conductor making it necessary to oversize the neutral.

-Hal


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Re: ac wiring new office
#252100 08/17/07 08:01 PM
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Hal

Thanks so much for the direct answers to my original query.

I dont do electrical but need to know the big picture, and this seems like a very good summary.


-TJ-
Re: ac wiring new office
#252101 08/18/07 10:37 AM
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Quote
So Dave, if you are not reading any continuity between neutral and ground that receptacle is not grounded.
What I meant by this was BEFORE any wires were connected to the outlet. When testing for continuity between the ground lug & the neutral lugs, there would be no continuity. I agree with all that you said.

Here, when installing them, the code states that you must use 3/14 or 3/12 wire to each receptacle, where the ground wire must be attached directly to the box and stop there. The red conductor, which is taped green, attaches to the ground lug on the outlet.

Dave


Scientists say that the universe is made up of Protons, Neutron & Electrons. They forgot "Morons".
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Re: ac wiring new office
#252102 08/18/07 11:54 AM
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Here, when installing them, the code states that you must use 3/14 or 3/12 wire to each receptacle, where the ground wire must be attached directly to the box and stop there. The red conductor, which is taped green, attaches to the ground lug on the outlet.

No difference here. We have, and I'm sure you do too, an IG cable that has a black, white, green and a yellow/green for the IG. Common practice is to reidentify the red of a 3 wire cable as you say.

What I meant by this was BEFORE any wires were connected to the outlet. When testing for continuity between the ground lug & the neutral lugs, there would be no continuity.

As long as the other end of the cable is connected there will always be continuity between the neutral, ground and the IG if it's wired correctly. As I said above they are connected together at the service so you have to have continuity. If they didn't the ground would not be able to provide a path for any fault current and trip the breaker.

The only difference between the neutral and the ground is the neutral normally carries current, the ground does not but can if there is a fault between the hot and ground. Consider an older electric drill for instance. The green ground from the receptacle is connected through the line cord to the drill housing. Should there be a short from the hot to the housing the ground would carry the current back to the breaker panel neutral/ground junction and trip the breaker protecting the operator from being shocked.

No receptacle should never have continuity between the ground screw or yoke and either of the current carrying contacts (the silver screw and the black or brass screw) so what you say just doesn't make sense.

-Hal


CALIFORNIA PROPOSITION 65 WARNING: Some comments made by me are known to the State of California to cause irreversible brain damage and serious mental disorders leading to confinement.
Re: ac wiring new office
#252103 08/18/07 09:18 PM
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Not to mention that it is not acceptable by the current NEC edition to mark color changes for any conductor smaller than #6. Using tape to change a red to green on a #12 conductor is a code violation in the US.


Ed Vaughn, MBSWWYPBX
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