datacenter build out and sizing

I am not an electrician, but I would really like to get a good understanding of some stuff so I can better estimate the costs
involved in building out a private datacenter.
Here's what I know... I'll need the utility company to provide 3-phase high voltage. I'll then need to provide a transformer and everything else.
My first question is how to size the transformer.
My power requirements are pretty basic. 90% of the equipment in the datacenter is 115V single phase, and occasionally you have a 230V single phase piece of computer equipment. Some lighting, some office wall outlets, and the AC units.
Everything except for the AC units will be behind a 3-phase UPS. Most models I price out are 460V 3-phase input.

Not sure about AC units, but figure a 10-ton unit, or 2x 5-ton units.
If the UPS balances my 115V equipment over all three phases, then I dont need a 50kVA transformer for 50kVA worth of single phase equipment, right? The transformer is rated in kVA for all three phases, right? So I could have 70kVA of 115V stuff, and be okay.
My other question is how do you bring down the voltage coming out of the UPS. The UPS has 460V 3-phase input and output. So you can grab individual phases on output, but wouldn't they be at 460V? How to do bring it down to 115V?
How finally, how does the power company bill usage over 3-phase. Is it billed per phase, or all three. Say I am charged 7 cents per kW-hr, and I have a 115V device running at 1000 watts. So on one phase out of the three is in use. Do I pay 7 cents, or one third of 7 cents?
Thanks John
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Europe doesn't use 115V. You will find 230V single phase and 400V 3-phase. If you want to take a medium voltage feed, this will vary by country -- in the UK it would be 11kV 3-phase.

That load would probably not justify a medium voltage feed. At least in the UK, you would probably be looking to use at least 1MVA before it's worth considering a medium voltage feed and your own transformer. With a load of the order 100kVA, you can share a transformer with something like 10 similar customers. This probably depends on local infrastructure too, i.e. if your're in the middle of nowhere, or in the middle of an industrial estate full of light commercial loads.

You buy a UPS with the correct output voltage.

This will vary by country, supplier, and tarrif. Larger commercial customers will often have to pay a penalty for low power factor and/or high harmonic current content too (probably not at the power levels you are looking at though).
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Andrew Gabriel
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One other thing -- make sure you engage a local engineer familiar with local supply infrastructure and customs to do the design.
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Andrew Gabriel
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| I am not an electrician, but I would really like to get a good | understanding of some stuff so I can better estimate the costs | involved in building out a private datacenter. | | Here's what I know... I'll need the utility company to provide 3-phase | high voltage. I'll then need to provide a transformer and everything | else. | | My first question is how to size the transformer. | | My power requirements are pretty basic. 90% of the equipment in the | datacenter is 115V single phase, and occasionally you have a 230V | single phase piece of computer equipment. Some lighting, some office | wall outlets, and the AC units.
Quite a lot of computing equipment is switchable between 115V and 230V and generally does not care about the frequency. Most power suplies will run fine on 100 to 240 volts, 50 or 60 Hz. So saying "90% of the equipment in the datacenter is 115V ..." doesn't really ring true to me. Do you have that much old equipment than only works on a single voltage?
| Everything except for the AC units will be behind a 3-phase UPS. Most | models I price out are 460V 3-phase input.
So you'll have your transformer to convert 460V to utilization voltage following the UPS.
|>From an equipment standpoint, my 115V stuff will probably be 50kVA. | Not sure about AC units, but figure a 10-ton unit, or 2x 5-ton units. | | If the UPS balances my 115V equipment over all three phases, then I | dont need a 50kVA transformer for 50kVA worth of single phase | equipment, right? The transformer is rated in kVA for all three | phases, right? So I could have 70kVA of 115V stuff, and be okay.
Three phase transformer VA ratings give the total for all three phases assuming three phase balanced utilization. A 75kVA transformer will be rated 25kVA on each of the three phases. To select the transformer size you need, figure the worst case out of balance scenario you might ever have and multiply the worst phase by three. Then add a margin for growth (usually recommend doubling, but datacenters might need even more than that). If I had a datacenter starting out with 50kVA of load I'd be looking at 112.5kVA or larger transformer (and UPS).
| My other question is how do you bring down the voltage coming out of | the UPS. The UPS has 460V 3-phase input and output. So you can grab | individual phases on output, but wouldn't they be at 460V? How to do | bring it down to 115V?
That's what the transformer does. You'll need a separate transformer for the non-UPS loads.
| How finally, how does the power company bill usage over 3-phase. Is it | billed per phase, or all three. Say I am charged 7 cents per kW-hr, | and I have a 115V device running at 1000 watts. So on one phase out of | the three is in use. Do I pay 7 cents, or one third of 7 cents?
7 cents. There's no freebie. Energy is energy, regardless of how you split it out into phases. If you add a 2nd device, you double the power being used, whether you put it on the same phase as the 1st device or on a different phase. There will be very slight variances in loss based on higher current in wiring if combined on the same phase.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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