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Calculating kVA load for computer loads

I

ITSME.ULTIMATE

Jan 1, 1970
0
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?
 
G

Gavin Parsons

Jan 1, 1970
0
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.
 
D

daestrom

Jan 1, 1970
0
ITSME.ULTIMATE said:
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?

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
 
T

TimPerry

Jan 1, 1970
0
ITSME.ULTIMATE said:
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?

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

operator jay

Jan 1, 1970
0
ITSME.ULTIMATE said:
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?

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.

http://webstore.ansi.org/ansidocstore/product.asp?sku=C57.110-1998
 
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