You’ve sized the rack, picked the servers and switches, maybe added a UPS, and now you’re on the last piece: the power distribution unit (PDU). Then the product listings start throwing amps, kW, single-phase and three-phase at you, and it’s not immediately obvious which of those numbers actually apply to your kit.
Outages are slowly becoming less common, but roughly one in ten still cause serious or severe disruption, and an overloaded circuit is one of the more avoidable ways to cause one. Size your PDU properly, and you sidestep two problems at once: the tripped breaker that drops your rack and the money wasted on capacity you’ll never touch.
The Two Ways You Can Get PDU Load Wrong
Get the load wrong in either direction, and it costs you in different ways.
Undersize it – and you’re running the power distribution unit too close to its limit. Push past that limit and the breaker trips, the rack drops, and you’re into unplanned downtime with your hardware potentially at risk. That’s the expensive mistake and the one worth designing out.
Oversize it – and you’ve swung the other way: paid for a high-density PDU and a heavier feed you’ll never come close to using. Less dramatic, but it’s money that could have gone into the rack itself.
Racks tend to carry more than they used to, so it’s worth sizing for where yours is heading rather than only considering what’s in it today. Work out the real draw, match a server rack PDU to it, and IT power distribution stops being guesswork.
Single-Phase vs Three-Phase: Which One You Actually Need
Before you add anything up, work out which type of supply your rack is on, because it sets the ceiling for everything that follows.
For most UK setups, the answer is single-phase. An office comms room, a small server room, an education rack: they’re almost always fed from a single-phase supply, and the PDU takes that one feed and splits it out. In practice that usually means a 16A PDU or a 32A PDU, depending on how much the rack draws.
Three-phase comes in higher up. Instead of one live line, the load is spread across three, which lets a rack carry a lot more power without overloading any single feed. It’s what dense or fast-growing racks move to when single-phase runs out of room.
A simple way to place yourself:
Single-phase if you’re running a handful of switches, a firewall, a NAS and a server or two. This covers the majority of small and mid-size racks.
Three-phase if the rack’s total draw is climbing past what a single feed comfortably handles. You’ll usually know, because both the load figures and your electrician will point that way.
Get this settled first, and the rest of the sizing is just arithmetic against the right rack power distribution unit.
The Simple Calculation: What Your Rack Draws vs What the PDU Can Give
Once you know your supply, sizing the PDU is mostly an addition. Follow these four steps:
Add up the draw: List every device in the rack and its power figure. Use the rated (nameplate) figure and not the idle draw, since that’s the worst case the PDU has to cope with. Figures come in watts or in amps on the label.
Get everything into the same unit: The simple version is watts = volts x amps. On a UK 230V single-phase feed, that puts a 16A PDU at roughly 3.6 kW at the very top and a 32A PDU at around 7.3 kW.
Compare the total against the PDU’s rating: Your combined draw needs to sit under the unit’s rated capacity, with room to spare rather than right up against it. More on that headroom in a moment.
Match the outlets, not just the capacity: Count your devices and check the connector type. Kit with UK plugs wants UK sockets; rack-native servers and switches usually come with IEC leads, so an IEC PDU with C13 or C19 outlets is the tidier fit. Leave a couple of outlets spare for whatever gets added next.
As a worked example, a small rack pulling around 2.2 kW sits comfortably on a single-phase 16A unit, using a little over half its ceiling. For a look at a straightforward unit at this scale, our 4-way PDU with UK sockets unboxing shows the format most small racks actually use.
Headroom: Why You Don’t Run a PDU at 100%
Here’s the catch with the number you just worked out: it isn’t the number you buy to.
A PDU running flat out for hours on end is a PDU under constant stress. Components and breakers aren’t designed to sit pinned at their maximum, and there’s rarely any slack for the things that eat into capacity in the real world. Equipment draws a surge at startup. Someone adds another switch six months in. A rack that was calculated to the exact limit has nowhere to absorb any of it.
The accepted way round this is to leave yourself headroom. As a rule of thumb, load a PDU to around 80% of its rated capacity and keep the top fifth in reserve.
Put that against the earlier example. That 16A unit with a ceiling near 3.6 kW isn’t really 3.6 kW of usable power; it’s closer to 2.9 kW once you leave sensible headroom. The rack pulling 2.2 kW still fits, but now you can see the honest margin you’ve actually got, rather than the one on the box.
When to Step Up to a Metered PDU
Everything so far relies on nameplate figures, which are deliberately worst-case estimates. That’s fine for sizing, but it means you’re always working from what your kit might draw rather than what it actually does.
A metered PDU closes that gap by showing you the real load in real time, so you can see exactly how much headroom you’ve got left, catch a rack creeping toward its limit before a breaker ever trips, and size the next one from hard data instead of estimates. A switched PDU goes a step further, letting you power individual outlets on and off remotely, which is genuinely useful when the rack lives somewhere you’d rather not drive to.
It’s the sensible move once a rack is busy, growing, or running anything you can’t afford to lose, or when a high-density PDU has tightened your margins to the point where guessing isn’t good enough. For a look at one of these, our 4-way switched PDU unboxing shows the format up close.
Find the Right PDU for Your Rack
Sizing a PDU comes down to three honest questions:
What does the rack really draw?
What type of supply is it on?
Have you left yourself room to grow?
Answer those and you avoid both the tripped breaker and the wasted spend.
When you’re ready to choose, you’ll find single-phase and three-phase options, from basic strips to metered and switched units, across our full Power Distribution Units (PDU) range. Not sure which fits your setup? Use the Express Cabinet/PDU finder to narrow it down, or get in touch and our team will size it with you.
It’s worth a look at our server racks & cabinets and UPS ranges while you’re planning too, since the right enclosure and proper power protection go hand in hand with the PDU you land on. And for a quick look before you buy, watch our 4-way rack PDU unboxing, then head over to the Comms Express YouTube channel for more product guides.
