Computer workstation load and demand load computing

[vkvgpal]

Here is an interesting topic to discuss about the bulk computer workstations usage in business/commercial/Institutional environments, (where 30, 40,....100 desktops are networked, and used everyday) and their power demand calculation. My friends and peers are telling that it is enough if we calculate the demand load per NEC's receptacle load factor calculation method. I am perplexed; comparing the actual, potential and continuous load demand, those computer systems would cause on the building power system.To my understanding /observation , in no single office/commercial environment, the computers are switched OFF until close of the day, once started in the morning. I mean to say, they are mostly continuous load on building power system, and how is it justified to calculate the computers load demand factor on par with the regular receptacle load demand, just because they are plugged into receptacles outlets ? Most of us know that a desktop system consumes somewhere between 350W to 700W including monitor power. Guess 50 or 60 systems @ an average 500W per unit working 6 to 8 hours per day! Don't we need a code amendment?

[jwelectric]

Why?
Are these not pluged into a duplex?
Was the duplex calculated in the load?
Why?

[petersonra]

most desk top computers do not use anywhere near the amount of power their power supplies are rated for. and many companies are now using lcd screens which are relatively low power.

on top of that most computers now have pwoer saving features included in them.

my guess is that realistically you are looking at more like 200W per workstation.

[wizzracing]

The Power supplies on Computers are inverted and can be as high as 180volts at the rails.

This is the reason for a IG being installed with a normal ground. I see to many just tie them in with the normal ground wires which defeats the purpose. It should be run to a ground rod seprate from the service grounding lug.

[petersonra]

Originally posted by wizzracing:
The Power supplies on Computers are inverted and can be as high as 180volts at the rails.

This is the reason for a IG being installed with a normal ground. I see to many just tie them in with the normal ground wires which defeats the purpose. It should be run to a ground rod seprate from the service grounding lug.

Really? And just how would the OCPD manage to function in such a situation as you propose were a ground fault to occur?

You seem to be avocating a position that creates a potential hazard. And a very deadly one at that.

[ August 04, 2005, 08:20 AM: Message edited by: petersonra ]

[wizzracing]

QUOTE]Really? And just how would the OCPD manage to function in such a situation as you propose were a ground fault to occur?

You seem to be avocating a position that creates a potential hazard. And a very deadly one at that. [/QB][/quote]

The extra IG is installed to stop just a situation it's used for ACP in case the power supply fails for what ever reason and returns a full 180volts back threw the neutral.

The AC is being converted to DC but can return 180volts back too the neutral or ground. Computer PS are not all the same hence the highier prices for good ones. Since most PC's are shipped with cheap ones there is a highier chance of them failing.

This is the reason for the extra IG being installed. It needs to be run to seperate grounding lugs, Then to it's own ground rod.

I seen this demonstrated and the voltage spike was 170volts on it. I know the amps are low but 1amp is all it takes to get hurt.

[petersonra]

I don't see how the IG does anything to protect anything at all in the case you are suggesting.

IGs are not run to an isolated ground rod, but back to the point at which the main bonding jumper is connected at the service point.

[charlie b]

Originally posted by vkvgpal: Don't we need a code amendment?

No. It's already covered in 90.1(B). The NEC gives us the minimum for safe operation. It does not promise that the system will be efficient or that the owner will like the results. If you perform a calculation using the 180 VA per duplex, and don't do any thinking beyond that calculation, you might end up with an unhappy owner. But that is not the NEC's fault.

[charlie b]

The only purpose of the Equipment Grounding Conductor (EGC) is related to a short circuit fault from an energized conductor to the external metal parts of a component. It will provide a low impedance path for ground fault current to return to the source, so that the breaker will trip and terminate the ground fault event.

The Isolated Ground (IG) wire would also perform that function during the same ground fault event. But there is no need for it to perform that function. Rather, it's only purpose is to provide a reference for the electronic circuits to see a "zero potential" point that is not associated with any other EGC or any other ground bar. It is run from the load back to the main panel, via any intermediate panels, without being connected to any other conductor. But it is, as petersonra has said, connected at the point within the main panel that the Neutral and Ground Buses are bonded to each other.

[vkvgpal]

Of course Charlie. This is about safety issue too. I will explain with a simple example. Suppose a design business office having 100 workstation computers, out of which at least 70 will be serving practically continuous in a typical weekday. Even if we take the minimum power of 250VA per w.stn., as suggested by Petersonra, the total load is 17,500VA. This needs a 65A feeder #6 to safely feed the load at 208volts 3phase power. (17.500VA/208x1.732 x 125%=60A). If the same 17,500VA actual, continuous demand load is calculated per NEC Receptacle demand calculation basis, you will end up with 13,750VA only.(10000VA @100%+balance@50%). That means a #8 feeder would be enough (13,750VA/208x1.732x125%=47.5A),and practically that #8 feeder will be feeding 60A load, whatever time it persists. Is not a safety issue? An average owner may be happy when you show them low installed cost,but he may not be definitely happy, when it later turns out to be either a change order or a premature failures. Will they? And then, I have still my own doubts about the power demand by individual workstations. Normally design offices, especially, graphic design offices use high end systems that consumes more power than 200W.
Regarding IG power my gusess is that IG is used more to reduce electrical noises, that affects the electronic parts and signals. Of course that protects the specified equipment through its reference ground, but not separated at source.
Pal