Calculating kVA load for computer loads

Lets say one is sizing a transformer powering a computer lab housing 200 computers.

Let's say each work station draws 150W/230VA for computer + LCD combined with the CPU fully loaded.

The 200 workstations will add up to 30kW, but do not add up to 46kVA, because harmonic loads cancel eachother out to some degree.

When you add them all together, the answer is 46kVA, but harmonic loads cancel each other out to a certain degree so it will be less than that. When sizing a transformer, do you assume the VA rating of all loads add up?

Reply to
ITSME.ULTIMATE
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Is it going to be UPS supported? are you going to use one UPS for the room or a small one for each workstation? What about future provision for expandability?are the work stations going on ring mains or radials, make sure that you get a functional earth bar arrangement in place.

Once you have considered these a 200kVa trans and a 60kVa UPS should future proof you.

Reply to
Gavin Parsons

If all these computers/LCD monitors are the same, then why do you think the harmonics of one would cancel out the harmonics of another? You don't mention if this is a 3-phase supply and the loads are split between phases, or if it is a 120/240 'Edison' style supply.

Without particulars, it's dangerous to assume that harmonics of one computer cancel any amount of harmonics from another.

daestrom

Reply to
daestrom

the transformer is sized to the service entrance: i.e. 200 amp, 400 amp,

600 amp.
Reply to
TimPerry

I would add up the VA. While there must be some cancellation, I haven't seen any numbers to indicate how much. I wouldn't expect a whole lot. Granted, on the primary of a three phase delta wye txf there will be very little of the triplens (no zero sequence current), but that's just the primary, and just the zero seq. You are probably best to oversize stuff a little bit, so I wouldn't look to chisel on wire sizing, txf rating, etc.

Some other considerations. K-factor transformers and/or oversizing transformers for harmonic load (there is a doc in regards to supplying harmonic loads via dry type transformer), double neutrals for feeders and non-shared neutrals for branch circuits, TVSS fabricated into a panelboard. Some people are adamant on IG, others say it does nothing, others say it makes things worse.

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Reply to
operator jay

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