Medium-Voltage Distribution in a Data Center – Gear and Layout
Behind the fence of any large data center sit rows of green or gray boxes. Inside the building, a long lineup of steel cabinets fills its own locked room, usually with a warning label the size of a pizza box. Those boxes and cabinets are the backbone of the facility's medium voltage distribution, which moves megawatts from the utility or the on-site plant out to dozens of smaller electrical rooms. What does medium voltage distribution do in a data center, and how is the gear laid out?
Medium voltage distribution carries power around a data center campus at roughly 4,160 volts to 34,500 volts, then steps it down near the loads to 480 volts for the UPS systems, cooling and IT equipment. At 13.8 kV, a common medium voltage (MV) choice, a feeder carries about 1/29th the current it would at 480 volts, so cables shrink and losses fall.
The main pieces are MV switchgear, shielded MV cable and unit substations (a step-down transformer joined to low-voltage switchgear). Layouts run from simple radial feeders to full dual-path designs, and the right one depends on site size, uptime target and how much maintenance must happen with the load still running.
What Counts as Medium Voltage?
Medium voltage is the band between the low voltage that equipment plugs into and the high voltage of transmission lines. Definitions vary by standard and by country. Schneider Electric's switchgear explainer puts low voltage at up to 1 kilovolt (kV) and medium voltage at roughly 1 to 36 kV. In everyday U.S. practice, people mean about 1 kV up to 35 kV, and MV switchgear is sold in voltage classes of 5, 15, 27 and 38 kV.
Inside that band, a few voltages show up over and over in North America:
- 4.16 kV: older industrial plants and large motors, such as big chiller compressors.
- 12.47 kV and 13.2 kV: very common utility distribution voltages.
- 13.8 kV: a favorite for on-site generator buses and large campus loops.
- 24.94 kV and 34.5 kV: longer campus runs and larger utility services.
Why Not Just Run Everything at 480 Volts?
Because current, not voltage, is what makes cables thick and hot. Three-phase power follows a simple formula: power = √3 × voltage × current × power factor. For the same power, higher voltage means lower current.
Here is a worked example. Say one data hall needs 2,500 kilowatts (kW) at a power factor of 0.95.
- At 480 volts: current = 2,500,000 ÷ (1.732 × 480 × 0.95) = 2,500,000 ÷ 790 ≈ 3,165 amps.
- At 13,800 volts: current = 2,500,000 ÷ (1.732 × 13,800 × 0.95) = 2,500,000 ÷ 22,707 ≈ 110 amps.
- The ratio is 13,800 ÷ 480 ≈ 29, so the MV circuit carries about 1/29th of the current.
Resistive losses in a cable scale with current squared. On the same conductor, cutting current by 29 times cuts those losses by roughly 29² ≈ 830 times. In practice, designers bank that advantage as smaller cables rather than near-zero losses, but either way MV wins once distances and loads get large. Carrying 3,000-plus amps a few hundred feet at 480 volts takes stacks of parallel cables or bus duct. Carrying 110 amps at 13.8 kV takes one set of MV cables.
What Equipment Makes Up Medium Voltage Distribution?
A typical data center MV system has five building blocks. The names vary by vendor, but the jobs do not.
Utility Service or Owner Substation
Smaller sites take service directly at a utility distribution voltage such as 12.47 kV. Large campuses often take power at transmission voltage (69 kV to 345 kV) into an owner or utility substation, where big power transformers step it down to the campus MV level. On a site with its own generation, the plant's generator bus may be the MV source instead of, or alongside, the utility.
MV Switchgear
MV switchgear is the lineup of steel cubicles that holds circuit breakers, protective relays, current and voltage transformers, and the main bus. In North America, data centers usually use metal-clad gear with drawout vacuum breakers, so a technician can rack a breaker out for service without disturbing its neighbors. If you want the broader picture of breakers, relays and gear types, our guide on what switchgear is covers it.
MV Cable
MV cable is shielded and insulated for the voltage, with factory-style terminations at each end. It usually runs in duct banks between buildings and in cable tray inside. Every new run gets tested before it is energized, typically with a very low frequency (VLF) or similar withstand test.
Unit Substations
A unit substation bolts an MV disconnect or breaker, a step-down transformer and low-voltage switchgear into one coordinated assembly. Common data center sizes are 2,000 to 3,000 kilovolt-amperes (kVA), stepping 13.8 kV down to 480Y/277 volts. The size limit is practical: a 3,000 kVA transformer at 480 volts delivers 3,000,000 ÷ (1.732 × 480) ≈ 3,600 amps, which already needs a 4,000-amp low-voltage main. Go much bigger and fault currents and bus ratings get awkward.
Protection and Controls
Protective relays watch every feeder for overcurrent and ground faults and trip the right breaker, ideally only the one closest to the fault. A supervisory system ties breaker status, relay alarms and meters back to the building management or electrical power monitoring system.
Which Medium Voltage Distribution Layouts Are Most Common?
Layout is where reliability gets designed in or left out. The table below compares the patterns you will see most often.
| Layout | How it works | Strength | Weakness |
|---|---|---|---|
| Radial | One feeder from one bus to each unit substation | Simple and cheap | Any feeder or bus failure drops the loads downstream |
| Primary selective | Each transformer can switch between two MV feeders | Survives a feeder outage | Still one transformer per load |
| Main-tie-main (secondary selective) | Two transformers feed a split low-voltage bus with a normally open tie breaker | Either transformer can carry the load during maintenance | Each transformer must be sized for the full load |
| Loop or ring | MV feeders form a loop with sectionalizing switches | Faults can be isolated and power rerouted | More switching and more complex protection |
| Dual path (2N) | Fully separate A and B systems from source to rack | Highest fault tolerance | Roughly double the equipment |
Many large data centers mix these. A campus might run a 13.8 kV loop between buildings, then use main-tie-main unit substations or a block redundant scheme inside each building, where several active blocks share one or more reserve blocks.
Where Do Generators Connect?
There are two common answers. In the classic design, each low-voltage generator (often 2 to 3 megawatts at 480 volts) backs up one unit substation through a transfer scheme on the low-voltage side. It is simple, and it keeps generator faults local.
In larger facilities, and in almost every site with prime on-site power, generators connect at medium voltage. Either the machines are built for 13.8 kV directly, or low-voltage units feed step-up transformers onto an MV paralleling bus. Paralleling lets many generators share the load and lets the site run N+1 or N+2 spares across the whole plant instead of one spare per building.
From there, the MV system feeds the unit substations, and the low-voltage side feeds the data center UPS systems that bridge the seconds between a utility loss and the generators picking up the load.
How Is MV Gear Maintained Safely?
Very carefully, and only by qualified electrical workers. An arc flash at medium voltage releases enormous energy in a fraction of a second, and MV systems can hold dangerous charge even after a breaker opens. Nobody without training and the right protective equipment should open an MV cubicle.
Professionals plan MV work around a few habits:
- An up-to-date arc flash study, with labels on every cubicle.
- Lockout and tagout, then verifying absence of voltage before any contact.
- Remote racking and arc-resistant switchgear so people stand clear during switching.
- Infrared scans, relay testing and breaker maintenance on a regular schedule.
- Switching procedures written in advance and walked through by two people.
Good layouts help here too. If the design lets you shut down one side while the other carries the load, maintenance happens in daylight on a planned day instead of never.
Medium Voltage Distribution Is a Map of Your Failure Modes
Medium voltage distribution exists because physics punishes high current. Running power at 4.16 to 34.5 kV keeps cables manageable and losses low, then unit substations bring it down to 480 volts close to the loads. The gear itself (switchgear, cable, transformers and relays) is fairly standard. The layout is where each design earns or loses its uptime.
The practical next step is to sketch the one-line diagram before the building layout is frozen, then settle two things for every feeder and unit substation: how the load survives its failure, and how it gets maintained without a shutdown. The answers will tell you whether radial is enough or whether you need a loop, main-tie-main or a full dual path.