How to size a UPS for a CT scanner

A CT scanner is not a steady load, and its nameplate alone does not fully describe what the supply has to deliver. Between patients the machine draws housekeeping power. When the tube fires, demand jumps for a few seconds and drops again. Size the UPS from the nameplate alone and you get one of two outcomes: it trips during the first busy clinic, or you pay for capacity the scanner never asks for.

What the supply actually sees

Three things happen on a CT feeder, and they have to be separated before any capacity figure means anything.

Between scans the scanner is quiet: cooling, electronics, table and displays. This continuous demand is useful, but it does not describe the scanner's maximum electrical requirement.

During scanning, power demand can rise sharply for a short period. What matters for UPS sizing is the maximum demand, how long it lasts and how frequently it repeats.

Around the scanner sits everything else in the room: the contrast injector, the operator workstation, the reconstruction and PACS terminals, sometimes the reading room, the room lighting and air handling. That is a scope decision rather than an electrical one, and it moves the answer further than the scanner's own rating does.

Start from three numbers, not from the label alone

Sizing runs from three figures: real power in kW, apparent power in kVA, and peak current. They answer different questions, and a nameplate kVA alone is not enough to size the UPS. The same UPS may be rated 120 kVA / 96 kW, with the two figures describing apparent-power and real-power capacity respectively. Both can be too small for a scanner whose average sits far below them.

What you want is that maximum demand either measured or stated. Log it if the site is already running. If it is a new installation, the scanner manufacturer's installation planning document gives the supply requirements, including the maximum instantaneous demand and the permitted voltage dip — use that document rather than a sales sheet.

Overload capability decides before capacity does

This is the part that gets skipped. Two quotes for the same kVA are not the same product, because overload capability is stated as a percentage and a duration, and the pairing is the whole specification.

The overload curve of this 120 kVA low-frequency UPS. The scanner's maximum demand and its duration must both remain inside the applicable overload region.

Read the table with the scan pattern beside it. An exposure is a few seconds of high demand, repeated with a gap long enough for the battery and inverter to recover. A short duration does not by itself place the demand inside the one-minute step — the height of the demand has to sit there too. Both numbers go against the table: what the scanner asks for, and how long the step can be held. A unit sold on its continuous rating alone says nothing about either.

Two questions to put to any supplier. First, is the overload rating stated at the output terminals with the battery carrying the load, or only while mains is present? Second, what happens at the end of the stated time — transfer to bypass, or shutdown? An overload percentage without the seconds beside it is not a specification, and a rating that ends in a transfer is not the same as one that ends in a clean ride-through.

Runtime: what has to be finished before the batteries are empty

Runtime is a duration someone has to justify, and the justification for a CT room is usually one of three things.

Finish the study in progress. A scan interrupted during acquisition may have to be repeated, resulting in additional radiation exposure and workflow disruption. The runtime that matters is long enough to complete a study and shut the system down cleanly.

Hand over to a generator. Where the hospital has standby generation, the battery covers the gap until it takes the load — generator start, transfer and settling. Ask what that actually takes on this site rather than assuming a number.

Ride out the disturbance. Where short interruptions and voltage sags are more common than prolonged outages, battery cycling can matter as much as nominal backup time — the UPS spends its life moving in and out of battery, and the bank is sized for cycle life as much as for minutes.

In the Cameroon installation, the 32 × 12 V 100 Ah bank was designed for roughly ten minutes. That was the figure for that hospital's scanner and that site's handover arrangement, not a rule. Your number follows from what has to finish, and from the DC path — battery voltage, maximum discharge current and the one-way distance to the battery cabinet all change what the bank actually delivers.

Isolation, earthing and leakage

Imaging equipment brings its own limits: leakage current, earthing and the isolation requirements set by the equipment manufacturer or the site design. Where those requirements call for it, an isolation transformer becomes part of the UPS specification rather than an accessory, and the UPS and the transformer get considered together rather than bought separately.

That requirement is also the argument for a transformer-based low-frequency topology. The output transformer provides galvanic isolation between the UPS and the scanner and can help reduce certain common-mode disturbances, while the inverter, DC stage, transformer and protection system together determine short-time overload capability. It is not an answer to every power-quality problem — it will not hold up a sagging feeder or fix a poor earth — and where nothing calls for isolation, it is weight and floor area carried for nothing. The limits that apply come from the room classification and the code the hospital is built to — a general X-ray room and a cardiac catheterisation lab do not sit under the same requirements, and the scanner maker's installation document usually states what it needs.

Transfer behaviour matters here too. A double-conversion unit holds the output through the move to battery, so the scanner sees no break when the input fails. What still needs checking is the transfer to and from static bypass, which is a separate event with its own timing.

Does a CT need a low-frequency UPS?

Not automatically.

CT rooms are often paired with transformer-based low-frequency units because these designs can combine strong short-duration overload capability with galvanic isolation where the installation requires it.

That is a reason, not a rule. A high-frequency unit whose electrical characteristics meet the scanner manufacturer's requirements is a legitimate answer, and on a site with no isolation requirement and a modest peak it can be the better one — smaller, lighter and far easier to get through the door.

Let the numbers decide rather than the topology:

• maximum scanner demand, and how long it holds
• the overload curve, as a percentage with its duration
• output voltage stability under a step load
• short-circuit current, and how the protection clears it
• input and output configuration — phases, neutral, earth
• bypass behaviour, including transfer to and from it
• whether isolation is required at all
• the upstream supply: transformer, generator, or both
• the electrical design rules the site is built to

Where the scanner has high short-duration demand and the project also requires galvanic isolation, a transformer-based low-frequency UPS is often a practical option. Where neither requirement applies, a lighter high-frequency design may offer a more efficient use of space and installation cost. The two are compared in detail in our guide to low-frequency and high-frequency UPS designs.

Decide the scope before you size

Each ring changes the answer. Sizing for the scanner and then discovering the injector and the reading room are on the same UPS is the most common way this calculation goes wrong.

Write the list down before you ask for a price. Scanner alone is one calculation. Scanner plus injector, workstation and displays is another. Add the reading room, PACS and the next modality and the machine size, the battery and the distribution all move.

Two answers change as well. If the room also needs voltage regulation because the supply is unstable, a UPS is not a substitute for a stabilizer on a bad feeder. If the scanner is part of a department that cannot go dark, redundancy enters the picture and the choice between N+1 and 2N is decided by whether the load has one input cord or two.

The room takes the machine, or it doesn't

A 120 kVA low-frequency cabinet is 635 × 975 × 1326 mm and around 650 kg. That is not a question of electrical engineering; it is a question of whether it fits through the door, whether the floor carries it, and whether there is a route from the loading bay to the plant room.

Decide this before the order, not after delivery. A unit that has to be craned in, or a floor that needs strengthening, costs more than the difference between two quotes. Leave the maintenance bypass accessible, leave room to pull a battery cabinet, and check the heat the room has to reject. Projects at this size slip on logistics far more often than they slip on capacity. A correctly sized UPS that cannot be moved into the room is still the wrong answer.

Parallel operation is worth mentioning here for the same reason. TAFENG has supplied single-unit UPS systems for multiple CT projects, including 120 kVA and 100 kVA installations, and TAFENG builds single low-frequency units up to 800 kVA, so sites that run machines in parallel are usually buying maintenance continuity or redundancy rather than capacity they ran out of.

Where this fits

Sizing runs in one order: the load, then the topology, then the numbers on the datasheet, then the application, then redundancy, then the battery, then the room. This page covers one application inside that sequence. For the full order, see How to size and select an industrial or medical UPS.

FAQ

What size UPS does a CT scanner need? There is no standard figure. The CT projects published here were supplied at 100 kVA for a Fujifilm Supria and at 120 kVA for a Fujifilm CT in Guatemala and for the Cameroon installation above. Each was sized from that scanner's peak and that room's scope, so a different scanner with a different peak lands somewhere else. Start from the manufacturer's installation document rather than from a fixed CT-to-kVA rule.

Can I size from the scanner's nameplate rating? Not on its own. The nameplate gives apparent power under stated conditions. Demand during scanning rises well above the figure between scans, so the maximum demand and how often it repeats matter more than the steady figure, and the room equipment on the same UPS has to be counted.

How much overload capability is enough for a CT? Compare the scanner's maximum demand and duration directly with the UPS overload curve. A 150% / 1-minute rating is only sufficient if the actual scanner demand stays within 150% of the UPS rated capacity for that period. A percentage with no time attached tells you nothing.

How much runtime does a CT room need? Enough to finish the study in progress and shut down cleanly, or long enough to hand over to standby generation. Ten minutes was right for one hospital's scanner and handover arrangement. Your figure follows from what has to finish on your site.

Does a CT UPS need an isolation transformer? Where the equipment manufacturer or the site design calls for isolation, it becomes part of the specification rather than an option. Low-frequency transformer-based units have it built in, which is convenient — but the requirement decides the topology, not the other way round.

Can one UPS cover the scanner and the reading room? It can, and it changes the size, the battery and the distribution. Write the scope down before you ask for a price — the most common sizing error is quoting for the scanner and then finding the rest of the room on the same feeder.

What to send a UPS supplier

Send the inputs rather than a kVA figure:

• CT make and model
• the electrical requirements from the manufacturer's site-planning document
• input voltage, phases and earthing arrangement
• maximum demand, how long it holds, and how often it repeats
• continuous load between exposures
• everything else that has to stay on the same UPS
• required backup time, and what has to finish inside it
• generator rating, start time and transfer arrangement
• whether isolation is required
• bypass requirements, including maintenance bypass
• battery-cabinet location and the distance to it
• room layout, access route and installation restrictions

With that list, a supplier can check continuous capacity, the overload envelope, the battery bank and whether the cabinet reaches the plant room. TAFENG engineers work from these inputs rather than from a CT-to-kVA table — the load, the peak, the runtime that has to be justified, and the route the cabinet has to take. Start with the load list.