240V to 218V Buck-Boost Transformers

240V to 218V technical overview

Buzzing contactor coils, control transformers that run hot, and electronic boards that fail well short of their expected life are overvoltage symptoms, and they are predictable when 208V-class equipment is fed from a 240V supply. Stepping 240 down to 218 is the light-touch version of that correction, roughly 9%. It lands the output right at the top of the range ANSI C84.1 defines for a 208V system, which ends chronic overvoltage while keeping the maximum amount of headroom above the low end of the band.

Where 240V to 218V correction is used

This pair suits sites where the supply runs consistently high and holds steady, and where the connected load is more electronics than horsepower. A shallow correction preserves margin, so the output stays inside the band even when the service sags during peak hours.

Situations that produce it:

• Buildings whose measured service voltage sits at the top of its allowed range, often near a substation or on a short, lightly loaded service drop.
• Rack PDUs, UPS inputs and network equipment specified for 208V input in a building where 240V single phase is what is available.
• Sign and architectural lighting where LED drivers rated for 208V nominal are failing in clusters.
• Commercial refrigeration and HVAC control boards, defrost timers and coil-operated devices that keep failing while the compressors themselves look healthy.
• Equipment carrying a 208 to 240V rating that technically accepts the supply but measurably runs hotter at the top of it.

The common thread is that nothing is tripping. The building runs, the loads work, and the maintenance budget quietly absorbs a steady stream of replaced coils, boards and drivers.

Why this 240V to 218V voltage pair matters

218V is the ceiling. ANSI C84.1 sets the Range A maximum for a 208V system at 218V, so a correction from 240V to 218V moves equipment from outside its defined range to just inside it, using the smallest reduction that does the job.

That matters because a deeper correction is not automatically better. Service voltage moves through the day. If a supply that reads 240V at 6 a.m. drops to 232V under afternoon load, a deep buck follows it down, and the output can approach the 191V utilization minimum ANSI C84.1 allows for 208V equipment. A motor running that low draws more current, makes less starting torque and runs hotter, which is the original problem arriving from the other direction. Where the supply is genuinely high and stable, 240 to 218 fixes the overvoltage without spending the downside margin.

Sizing guidance

Start with phase. Single-phase supplies get a single-phase correction, three-phase supplies get a three-phase one, and the two are not interchangeable.

Then work out current. Read full-load amps off the equipment nameplate, or minimum circuit ampacity on packaged equipment that contains several internal loads. Where several devices sit behind one transformer, add their currents together. Choose the next amperage step above the total so that motor starting current and later additions do not force a change.

One characteristic of this pair is worth knowing before selecting: the smaller the percentage correction, the smaller the share of the load's power the transformer actually handles. A buck-boost processes only the difference between input and output, which is why a 9% correction like 240 to 218 supports a substantial load from a compact unit. Current, not horsepower, drives the selection.

Installation notes

Confirm the equipment's real acceptable voltage range before assuming this correction suits it. A nameplate reading 208 to 230V is telling you something different from one reading 208V, and a shallow correction only makes sense where the load genuinely needs somewhat less voltage rather than a different supply entirely.

The unit itself is an autotransformer. Input and output share a conductive path, so there is no isolation and no separately derived system. For three-phase work the standard arrangement is open delta, which alters the line-to-line voltages and produces no neutral, meaning a three-wire supply remains three-wire and any 120V requirement still needs its own transformer or panel.

Apply overcurrent protection under NEC Article 450 and size conductors from the ampacity tables. Verify the corrected voltage with the load running, and again during the busiest part of the day, because a light-touch correction leaves less margin for a supply that moves.