Buck-Boost Transformer Calculator

Find the voltage you have and the voltage the load needs, then the load current at the load voltage. Pick a connection that lands within the load's allowed voltage range, and multiply the load amps by the volts on the series winding. That is the VA the unit must supply. Round up to the next unit size. For 20 A at 208 V to 240 V, the 16/32 V unit connected to give 240.0 V needs 20 x 36.9 = 738 VA, so a 0.75 kVA unit.

Boost it. A 120 x 240 V primary with a 16/32 V secondary can be wired in two ways that suit 208 V to 240 V. Adding the 32 V winding to the 208 V line gives about 235.7 V (-1.8%). Putting the load on the 240 V winding and the 208 V line across it with the 32 V winding subtracted gives 240.0 V. The second needs a slightly larger unit. This tool lists both, with the unit size for your load.

A unit can be wired several ways, and each gives a slightly different output voltage and needs a different kVA for the same load. The closest voltage is listed first. If your equipment tolerates a few percent, a lower voltage may fit on a smaller, cheaper unit. The tolerance under More detail sets how far from the voltage you asked for an option may land. It starts at 5%, the default used by the Federal Pacific selector.

The load amps, through the series winding. The unit only carries the voltage it adds or removes, so a buck-boost unit is much smaller than a transformer for the same load. The source current is different from the load current, because power is the same on both sides: a boost draws more current than it delivers. The result shows both.

NEC 2023 450.4(A) limits the overcurrent device on the input to 125% of the rated input current, and allows the next higher standard rating when that is not a standard size and the input is 9 A or more. Below 9 A the limit is 167% and the rating is not rounded up. The 0.75 kVA unit in the example has a rated input of 23.4 A, so the limit is 29.3 A and the next standard rating is 30 A. The device goes in series with each ungrounded input conductor, never across the common winding. The tool also shows this for the unit you pick.

Yes, on the same number of phases it serves. Two units make an open delta, which gives three wires and no neutral. Three units make a wye and need a four-wire source with the neutral tied to the common point. The calculator sizes each unit for the line current. It does not change single phase to three phase, and it does not make a neutral on a delta system.

No. It is wired as an autotransformer, so the load stays connected to the supply, and if the supply voltage moves, the output moves with it by about the same percentage. It does not regulate. Eaton lists limits for 60 Hz only and says not to use these units on control circuits or on an unstable line voltage.

It works from the common ratings of 120 x 240 V primaries with 12/24 V or 16/32 V secondaries, which Eaton and Federal Pacific publish. The tool applies the winding voltage and VA relations in those guides and was checked against the Eaton tables and the Federal Pacific selector for 208 V to 240 V. Not every cell of the Eaton tables is reproduced, so confirm the final unit against the manufacturer table. It does not return a manufacturer part number, and kVA sizes follow the Eaton list, so another maker may offer different ones. KVCalc has no connection to any of these companies. Terminal marks and wiring diagrams differ between makers, so wire from the diagram that comes with your unit. Schneider Electric describes the same method for 208 V to 240 V: 32 V times the load current gives the VA.

It does not cover 240 x 480 V units or any 480 V class system, so 480 V, 480Y/277 V and 600 V are outside it. It does not derate for altitude or ambient temperature, which Eaton does for high altitude and high ambient. It does not cover loads above the largest listed unit, 7.5 kVA. It does not check the load's allowed voltage range, motor starting current, or whether the equipment is rated to run from a non-isolated source.

NEC 2023 210.9 and 215.11 say branch circuits and feeders shall not be derived from an autotransformer unless the circuit (210.9) or system (215.11) supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer. Exception No. 1 allows an autotransformer without that connection when transforming from a nominal 208 V to a nominal 240 V supply, or from 240 V to 208 V. Exception No. 2 covers 480 V and 600 V in industrial occupancies with qualified maintenance and supervision. For any other pair, check with your inspector. 450.4(B) also says a transformer field-connected as an autotransformer shall be identified for use at elevated voltage.