255V to 240V Buck-Boost Transformer

255V to 240V technical overview

255V is above what ANSI C84.1 contemplates for a nominal 240V system. Range A tops out at 252V and the wider Range B tops out at 254V, so a steady 255V reading justifies a phone call before it justifies a purchase order. When the utility confirms the service is as delivered, or when the high voltage traces back to a customer-owned transformer sitting on a boost tap, a 255V to 240V buck-boost transformer removes the 15 volts on the customer side and gives 240V-rated equipment its nameplate voltage.

Where 255V to 240V correction is used

Unusually high readings usually have a findable cause, and the cause determines whether a buck-boost is the right answer.

• Customer-owned transformers left on a raised tap. Pad-mount and dry-type units carry taps above and below nominal, and a tap chosen for a heavier load than the building now has will hold the secondary high all day.
• Lightly loaded services at the end of a rural or long distribution line, where the utility deliberately runs the head of the circuit high so the far end stays in range.
• Buildings whose load fell after a retrofit. A lighting conversion or an equipment replacement that cuts demand substantially lets the service voltage rise, and equipment that was fine for years starts reading high.
• Sites with on-site generation exporting power at the point of connection, which raises local voltage during production hours.

The equipment affected is anything nameplated 240V: single-phase and three-phase motors, resistive heating, well and booster pumps, shop equipment, EV supply equipment, and the control and electronic loads riding on the same feeder.

Why this 255V to 240V voltage pair matters

Diagnose before you correct. A 255V reading first needs to be confirmed as sustained rather than momentary, so log it across a full week if a recording meter is available, and check the tap position on any transformer the building owns. Where a tap change is available, that is the cheaper and more permanent fix, and it should be considered before a buck-boost is bought.

A buck-boost becomes the right device when the tap is already at its lowest usable setting, when the transformer also serves loads that need the higher voltage, when the source is utility-owned and the utility considers the service acceptable, or when only part of the building needs correcting.

The case for correcting is straightforward. 255V is about 6% above a 240V nameplate, which most motors tolerate, but the supply is outside the standard's own band, so it is not bounded by anything predictable and can go higher. Meanwhile core loss and magnetizing current run high continuously, coils and control electronics sit warmer than designed, and resistive loads draw more power than designed.

Sizing guidance

Size to the voltage that persists, not the worst reading you happened to catch. If 255V appears only overnight and the daytime figure is materially lower, a correction sized for 255V will undershoot when the building is loaded, because a buck-boost applies a fixed ratio and does not regulate. Log first, then choose.

For the unit itself, match phase to the supply and select an amperage rating from 10, 15, 20, 30, 40, 50 or 60 amps above the total current passing through the transformer. Build that total from nameplate full-load amps rather than breaker sizes, and include everything downstream: auxiliary motors, control transformers, heaters and any lighting or receptacle load on the corrected circuit.

The transformer handles only the 15-volt difference between input and output, never the full load power, which is why the unit stays small relative to the load and why sizing one step up is inexpensive insurance when a total lands near a boundary.

Installation notes

Document the installation clearly, including the measured input voltage and the date. This matters more here than on a routine correction: if the utility or the building owner later lowers the source voltage, the fixed-ratio transformer will keep subtracting the same proportion and the equipment will then be undersupplied. The next technician needs to know why a buck-boost is in the circuit.

Electrically the unit is an autotransformer. The 240V output is not isolated from the 255V input, and the installation creates no separately derived system, so grounding and bonding follow the existing arrangement. Three-phase corrections are wired open delta, normally two units, which adjusts the three line-to-line voltages and derives no neutral.

Line and load conductors both carry full load current, so size conductors and overcurrent protection to the load rather than to the transformer rating. Energize, then verify the corrected voltage at the equipment terminals with the load running.