247V to 220V Buck-Boost Transformer

247V to 220V technical overview

Equipment that ran without complaint for years in one building often starts tripping the week it is set down in another, and the reason is usually that nothing about the machine changed while everything about its supply did. On a 247V service a 220V nameplate is running roughly 12 percent high. A 247V to 220V buck-boost transformer, an autotransformer whose windings are shared between input and output, subtracts 27 volts so the relocated machine sees the voltage it saw in its old home.

Where 247V to 220V correction is used

This correction follows used machinery. Auction purchases, plant consolidations, lease returns, downsizing and second-hand dealers all move working machines into buildings that were never designed around them, and the voltage they meet at the new site is whatever that service happens to deliver.

Machines that travel well and carry 220V nameplates include packaging and filling lines, case erectors, cartoners, labelers and shrink tunnels, slitters, rewinders and converting equipment, woodworking machinery, textile and sewing plant, food processing equipment, and older imported machine tools sold on after a plant closure.

The buyers are contract packagers taking on a new run, small manufacturers scaling up on second-hand equipment, riggers and millwrights doing the install, and dealers who want a machine to start cleanly on a customer's floor. All of them are working from a machine's history rather than its documentation, which is why supply voltage at the new location tends to get measured late, and often only after the first fault.

Why this 247V to 220V voltage pair matters

The reason a relocated machine behaves differently is that voltage drop had been doing part of the work. The NEC addresses voltage drop only in informational notes, which recommend rather than require roughly 3 percent on a branch circuit and about 5 percent across feeder and branch circuit combined, and a machine sitting at the end of a long run in an old building was quietly receiving the benefit of that drop. Set the same machine thirty feet from a new panel and the drop disappears, so its terminals now see close to full service voltage.

On a 247V service that puts a 220V nameplate about 12 percent over, past the plus or minus 10 percent NEMA MG-1 allows, which caps a 220V motor at 242V. The consequences arrive in a familiar order. Motors run hotter and draw more current. Overload relays set years ago in a different building begin tripping at times that make no sense. Contactor coils and control transformers fail early, and whatever electronics were added to the machine across its life run permanently warm.

Sizing guidance

Used equipment defeats catalog sizing, because the data plate is often painted over, missing, written in another language, or describing a machine that has since been rebuilt. Measure instead.

Clamp each supply conductor with the machine running its actual production duty rather than idling, and record the highest sustained current instead of the starting peak. Confirm phase from the machine's own disconnect and motor connections rather than assuming, and if the machine was ever rewired for a different voltage, verify how the motors are connected now before ordering anything. Single-phase and three-phase corrections are available across 10, 15, 20, 30, 40, 50 and 60 amp steps.

Select on that measured current, not on the machine's connected kVA. A buck-boost handles only the 27-volt difference between supply and nameplate, so its own kVA rating is small next to the load it serves, and sizing from load kVA produces a unit several times larger than the correction actually requires.

Installation notes

Measure in the new building before ordering. Readings from the machine's previous location say nothing about this service, and one reading says little about a service that moves through the day, so take line-to-line measurements at the intended location at more than one time.

A buck-boost transformer is an autotransformer, so its output is not isolated from its input and no separately derived system is created. Three-phase corrections are wired open delta, typically with two units, which corrects the line-to-line voltages without producing a neutral. That catches out relocated European machinery in particular: where a machine expects a 4-wire supply with a neutral, a buck-boost cannot make one from a 3-wire source.

Size conductors and overcurrent protection on both sides per NEC. Once the correction is in, revisit the machine's overload relay settings and control transformer taps, both of which were chosen for the voltage the machine used to see.