247V to 231V Buck-Boost Transformer

247V to 231V technical overview

On a three-phase service one reading is never enough. Measure all three line-to-line pairs before deciding anything, because a 247V average can conceal a spread wide enough to damage motors by itself. Where all three legs genuinely sit near 247V and the equipment is nameplated 230V, a 247V to 231V buck-boost transformer removes 16 volts, about 6.5% of the incoming supply, and returns each line-to-line voltage to within a fraction of a percent of nameplate. The unit is an insulating transformer reconnected as an autotransformer, processing only the difference between input and output.

Where 247V to 231V correction is used

Three-phase 240V-class services that run consistently high are common in industrial parks, older manufacturing buildings and light commercial units sharing a utility transformer with neighbors whose load has declined. The service was sized for a larger tenant or an earlier era of equipment, and the voltage sits near the top of its range because very little is pulling it down.

The loads affected are the three-phase workhorses: machine shop equipment, conveyor and material-handling drives, water and wastewater lift station pumps, cooling tower and air handler motors, three-phase compressors in refrigeration and HVAC packages, commercial laundry extractors and irrigation pump sets. Most are nameplated 230V or 208-230V and most run long hours.

The reason contractors reach for this pair rather than a rounder one is that 231V is what a precise trim of a measured 247V produces. Buck-boost corrections are proportional, so the output follows from the input actually measured, and using the measured number rather than the nominal system name is what makes the correction land where it should.

Why this 247V to 231V voltage pair matters

At 247V a 230V-nameplate motor is running about 7.4% above rating, inside the plus or minus 10% NEMA MG-1 permits but close enough to the edge that anything else going wrong pushes it out. Trimming to 231V puts it within half a percent of nameplate and restores the full band as margin.

Voltage unbalance is where a high three-phase service becomes genuinely expensive. NEMA MG-1 calls for derating a motor once voltage unbalance exceeds 1% and does not recommend operation above 5%. The reason is that a small voltage unbalance produces a much larger current unbalance, commonly six to ten times as large, concentrated in one winding. That winding then runs hotter than the other two while the motor appears to be operating normally.

A buck-boost trim reduces the absolute voltage on every leg, which lowers the peak winding temperature, but it does not equalize the legs. Unbalance is a separate defect with separate causes, usually single-phase loading on one leg, a loose connection or a failing utility component, and it needs its own diagnosis.

Sizing guidance

Record all three line-to-line voltages with a true-RMS meter while the plant is running, and record the running current on each leg at the same time. Those six numbers decide both whether a buck-boost is the right device and how large it needs to be.

For sizing, use the highest of the three line currents, not the average, and base it on nameplate full-load amps where equipment is not yet running. Total everything the transformer will feed, including control transformers and small auxiliary motors that are easy to overlook on a machine line. Then pick the first amperage rating above that total from 10, 15, 20, 30, 40, 50 or 60 amps, and match the configuration to the supply, single-phase or three-phase.

A three-phase correction of this kind is normally made with two units in open delta. Because each unit handles only the 16-volt difference rather than full load power, the assembly stays small relative to the load it corrects, and stepping up one amperage rating costs little.

Installation notes

Three-phase installations have their own short list.

• Open delta is the standard arrangement, normally two units. It adjusts all three line-to-line voltages, but it derives no neutral and cannot create a 4-wire wye system from a 3-wire delta source.
• The connection is an autotransformer. The 231V output remains electrically connected to the 247V input, no isolation exists between them, and no separately derived system is created.
• Phase rotation is unchanged by a buck-boost transformer, so motor direction after the correction is the same as before it. Verify rotation anyway if conductors were disturbed during the work.
• Line and load conductors on all phases carry full load current. Size conductors and overcurrent protection accordingly, not to the transformer's own rating.
• After energizing, measure all three corrected line-to-line voltages under load, not one. A single reading will not reveal a connection error on the third leg.