In short. Colocation generator planning reconciles three numbers: the load measured today, the capacity contracted to customers, and the capacity installed and commissioned. Backup power is sized and reserved against contracted capacity, while the shared plant behind many customer environments must be maintainable without exposing any one of them. The operator must know the reach of every shared dependency, keep a live capacity register, and run work in the occupied facility under written isolation plans.
Three capacities, not one
A colocation hall with 2 MW of installed UPS and generator capacity may be measuring 900 kW of IT load while having 1,700 kW under contract. Each number answers a different question. The measured load says what the plant is doing now and drives fuel consumption and set loading. The contracted capacity says what customers are entitled to draw and is what the generator plant must be able to carry, with the agreed redundancy, at any moment. The installed and commissioned capacity says what exists and has been proven. The gap between measured and contracted load is where most colocation power problems hide: a facility that has quietly sold 95 percent of its capacity while running at 45 percent still has to survive a customer migration that doubles the load in a week.
| Figure | What it governs | How it is kept current |
|---|---|---|
| Measured load | Set loading, fuel burn, wet-stacking risk, cooling demand | Metering per hall and per customer, trended monthly |
| Contracted capacity | Generator and UPS reservation, redundancy arithmetic, sales limit | Capacity register updated at every contract change |
| Installed and commissioned capacity | What can actually be relied on | Commissioning records, excluding anything untested or isolated |
The generator plant is sized and reserved against contracted capacity, derated by the diversity the operator is prepared to defend in a customer dispute, plus the facility’s own cooling and services, with redundancy applied on top. The sizing method itself is covered in data centre power; what follows is specific to the shared, multi-tenant case.
Shared infrastructure and the reach of a failure
Behind the cage walls they share generators, paralleling switchgear, UPS systems, chilled water, control systems and often the same transfer switches. A fault or a maintenance error on any shared element reaches every customer behind it. The operator therefore needs a dependency map that answers, for each shared element, which customers lose redundancy if it is isolated and which lose supply if it fails. The analysis method is set out in single-point-of-failure elimination.
Maintainability is a customer commitment
A colocation operator promises availability in a service level agreement and usually promises it during maintenance as well. That requires concurrent maintainability: any generator, switchboard section, UPS module or transfer switch can be removed from service while the contracted load remains protected at the agreed level. In generator terms it means N+1 at least, with the spare set available while any one set is on its service interval, and it means isolation points, bus sections and bypass arrangements in the switchgear so that work can be done without a shutdown. It also means that the maintenance rotation is planned against the capacity register, so that two sets are never down together in a hall where the arithmetic only allows one. The design requirements are developed in concurrent maintainability engineering.
Expansion inside a live facility
Colocation grows by adding halls, adding sets to a paralleling bus, or replacing UPS and switchgear while customers remain online. Each of these is a project with the same shape. A survey establishes the as-built condition, which frequently differs from the drawings. A written method statement defines the work boundaries, the isolations, the capacity and redundancy that will be reduced during each step, the customers affected, the rollback at each hold point, and the handback tests. Temporary generation and distribution are brought in where the work would otherwise leave the contracted load below its agreed redundancy. Worked example (illustrative). A hall with three 1,250 kVA sets in N+1 for a 2,000 kW contracted load must have one set’s controller and switchgear section replaced. During the work the hall is at N with no spare. A 1,250 kVA rental set connected through a temporary paralleling panel restores N+1 for the six weeks of the work, and the method statement records when it is connected, how it is synchronised and how it is removed. The rental side is described under data centre rental power.
Paralleling bus growth and protection
Adding a fourth set to a bus designed for three raises the fault level at the bus, changes the load-sharing configuration, and may exceed the switchgear’s short-circuit rating or disturb the protection grading. Every addition is checked against the bus rating, the relay settings and the controller configuration, and the paralleling plant is re-tested as a whole, including sharing across all sets and the loss of one set under load. Many colocation sites also move from a single paralleling bus to two independent buses as they grow, which improves fault containment at the cost of a more complex control hierarchy. These checks belong to the multi-DG capacity and N+1 studies.
Testing with customers watching
Colocation customers increasingly ask to witness tests and to see records. The operator needs a testing programme that proves the plant without putting customer load at risk: loaded runs with load banks on the generator bus, periodic integrated tests in which the utility is dropped under controlled conditions with customers notified, and records that show per-set sharing and UPS behaviour rather than a plant total. A test that is run only on the shared plant and never through the full chain to a customer’s distribution is partial, and it should be described as partial in the record. The load-bank testing service supports this programme.
Keeping the records that run the facility
A colocation facility accumulates change: new contracts, new halls, replaced equipment, revised settings. Four records must stay current or the engineering decisions above cannot be made. The capacity register links contracted, measured and installed capacity per hall and per shared element. The dependency map links shared elements to customers. The configuration record holds every controller’s settings and firmware, every protection setting and the as-built single-line diagram. The test record shows what was proven, when, and to what boundary. Handover of any project includes updating all four.
Common mistakes
- Sizing and reserving backup against measured load rather than contracted capacity. A customer migration arrives and the hall is over-subscribed on the generator plant with no notice.
- Assuming redundancy because there are spare sets, without a dependency map. A shared fuel pump or control PLC removes several sets at once and every customer behind them.
- Taking two shared elements out of service in the same window. The hall drops below its agreed redundancy and a single further fault becomes a customer outage.
- Connecting temporary generation casually because it is temporary. Earthing, protection and synchronising errors on rental plant carry the same consequences as on permanent plant.
What to send us
Send the capacity register or at least contracted, measured and installed capacity per hall, the single-line diagram of the generator and UPS plant, the maintenance rotation, the nameplates and controller types of the sets, and the scope of any expansion or replacement you are planning, with the redundancy you must hold during the work. If temporary power is needed for a project window, include the dates and the connection point. Use the application engineering enquiry.
Questions engineers ask
Should a colocation generator plant be sized on measured or contracted load?
Contracted, with whatever diversity the operator is prepared to defend, plus the facility’s own cooling and services, plus redundancy. Measured load is useful for fuel and loading trends but customers are entitled to draw their contracted capacity without warning.
How do I maintain a generator without reducing a customer’s redundancy?
Design and operate to concurrent maintainability: N+1 or better on sets, with isolation points and bus sections that allow one set or switchboard section to be worked on while the rest carry the contracted load at the agreed level. Where the plant cannot meet that during a specific job, bring in temporary generation for the window.
Can a generator be added to an existing paralleling bus?
Usually, after checking the bus and switchgear fault rating, revising the protection grading, configuring the new set into the load-sharing and master control scheme, and re-testing the plant as a whole. The addition is a project with a method statement, not a plug-in.
What should a customer ask to see about the backup power?
The capacity register for their hall, the dependency map showing what they share with others, the maintenance rotation, the latest integrated test record including per-set load sharing and UPS behaviour, and the fuel autonomy with the refuelling arrangement. An operator who can produce these promptly has an engineered facility.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.