Limited grid power: plan a data centre in phases
A large site is not the same as a large power supply. Before arranging halls across a plot, establish what power can be delivered to the site boundary, on what date, and with what restrictions. A phased design should produce a useful first operating stage and a clear route to expansion. It should not depend on the next grid upgrade arriving early.
Separate requested capacity from usable capacity
Collect the connection offer, the agreed import limit, the supply voltage and the expected energisation date. Record whether capacity is firm or interruptible. Ask which upstream works must finish before the connection can be used. A transformer nameplate alone does not establish the permitted import. A nearby overhead line doesn't mean you can connect to it either.
Keep MW and MVA separate. MVA is apparent power; MW is real power, equal to MVA × power factor. For an initial check, a 5 MVA supply at 0.9 power factor represents 4.5 MW before downstream losses. The actual allowable load also depends on protection settings, cables, ambient conditions and the supply agreement. Record these assumptions alongside the number instead of hiding them in a spreadsheet.
Turn the power limit into an IT limit
IT load is only part of the facility demand. Cooling, pumps, fans, electrical losses and other building loads need power too. If a planning case assumes a facility-to-IT ratio of 1.5, a 6 MW import supports at most 4 MW of IT under that assumption. This is arithmetic for an early comparison, not a guarantee that the site will achieve that ratio.
PUE is an energy ratio measured over a stated boundary and period. Annual PUE does not by itself establish peak electrical demand. The detailed design needs a coincident load schedule for the hot design day, battery recharge, equipment restart and maintenance states. Ask the engineer to show both the annual energy assumptions and the peak load calculation so that a favourable annual average does not conceal an undersized connection.
Make each phase operable on its own
Choose the first phase around confirmed demand and confirmed power. A smaller opening can reduce idle equipment, but each phase still needs a complete operating chain: incoming supply, distribution, UPS, heat rejection, monitoring, fire protection and access. A partly installed chain cannot support live IT simply because racks are ready.
Draw a boundary around each phase and mark every future connection. Reserve switchboard ways, cable routes, pipe branches, plant space and lifting access. Identify which additions can happen while the first hall remains live. Where an expansion requires a shutdown, record the duration and operating consequence before the owner accepts the phasing plan. A spare space in a drawing is useful only if people can reach it later.
Don't install all the plant on day one
Installing all ultimate plant on day one can leave equipment operating at very low load. Compare staged transformers, UPS modules and cooling units against the minimum stable operating range and losses of the selected equipment. Keep room for replacement and maintenance, rather than filling every spare bay with future equipment immediately.
Redundancy should be assessed within each operating phase. Two units may provide a spare at opening load but become fully loaded duty units after expansion. Create a table that shows duty capacity, standby capacity and the largest credible single failure for every phase. Also show the state with one unit isolated for maintenance. Unit counts alone do not demonstrate that power paths remain available.
Set decision gates before ordering
Use a short set of release conditions: signed supply terms, checked site levels, agreed IT demand, confirmed interfaces and an approved test plan. Long delivery items may need early orders, but their ratings and interfaces should be fixed enough to avoid expensive changes. Treat transformer and switchgear delivery dates as supplier-confirmed dates, not generic promises.
Before energisation, test the first phase as a complete system. Prove alarms, emergency actions, cooling restart and transfer behaviour at a representative load. Keep a record of settings and capacities left for later phases. The handover pack should tell the next project team what they can connect, where they can work and what they must isolate.
Questions
Can phase 1 use all available grid power?
Only after allowing for cooling, losses and other facility loads. The engineer must also check operating margins, restart conditions and the connection agreement.
Should we buy all transformers at the start?
Not automatically. Compare staged installation with delivery risk, low-load losses, space and the work needed to add equipment beside a live phase.
Does a second feed double usable power?
Only if the supply agreement and network arrangement allow it. Two feeds may share an upstream limit or be intended as alternative supplies.
Start with your four inputs: site, IT load, available power and target date. The estimator gives a browser first pass; an engineer checks the design.
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