Buck-boost transformers

What a buck-boost transformer does

Equipment rated 230 V or 240 V turns up on 208 V services every day: rooftop air conditioners, commercial kitchen equipment, imported machines, motors rated 230/460 V. At 208 V a 230 V motor runs hot and loses torque, and a heating element delivers 18% less heat. The buck-boost transformer is the small, cheap fix: it lifts 208 V to about 229 V, or 236 V, and the equipment runs as rated. The same unit, connected the other way, lowers 240 V to 208 V, or 277 V to 240 V, or lifts 240 V to 277 V for lighting.

The trick is that it doesn’t transform the whole load. A conventional transformer between a 208 V supply and a 230 V load would have to be rated for the load’s full kVA. A buck-boost carries only the 21 V it adds, so a unit rated 1 kVA serves a 10 kVA load.

How it works: an autotransformer connection

A buck-boost is sold as an ordinary two-winding transformer: a primary rated 120 × 240 V and a secondary rated 12 × 24 V or 16 × 32 V, in sizes from 0.05 to 10 kVA. Wired as a two-winding unit it’s a small control or lighting transformer. Wired as a buck-boost, the primary goes across the supply and the secondary goes in series with the line to the load, so the secondary’s volts add to the supply (boost) or, with its leads reversed, subtract from it (buck).

Because the primary is rated 240 V and sees 208 V, the secondary gives 24 × 208 ÷ 240 = 20.8 V, and the output is 228.8 V. The current through the series winding is the load current, which is why the unit’s rating is set by the load amps and the boost volts rather than the load kVA.

Output volts = supply volts × (primary volts + secondary volts) ÷ primary volts

When to use one

Outputs for a 240 V-rated primary; the exact figure scales with the supply voltage.

The last two rows are the industrial cases: a 480 V feeder that has sagged to 450 V by the time it reaches a pump house, or a 600 V machine bought from Canada. Buck-boost units are made in 480 V and 600 V classes for these, with 24, 32, 48 and 120 V secondaries.

How to size a buck-boost transformer

The unit has to carry the load current through its series winding at the boost (or buck) voltage. So, for single-phase:

Buck-boost kVA = load amps × boost volts ÷ 1,000

A 230 V air conditioner that draws 30 A, on a 208 V supply boosted by 21 V: 30 × 21 ÷ 1,000 = 0.63 kVA, so a 0.75 kVA unit (or 1 kVA, to leave margin). A conventional transformer for the same load would be 30 × 230 ÷ 1,000 = 6.9 kVA: a 7.5 kVA unit, ten times the size, weight and price.

Three checks before ordering:

• Use the load’s actual amps at the output voltage, from its nameplate, not an estimate; a motor’s full-load amps, and a safety factor for anything with a compressor.
• Confirm on the selection table. Every manufacturer publishes a table that, for each standard input and output voltage, lists the load amps each unit can carry. The table accounts for the exact winding ratios; use it to pick the catalog number and the connection diagram.
• Protect it as an autotransformer. NEC 450.4 sets the overcurrent protection at 125% of the autotransformer’s rated input current, which is the load current, not the small two-winding rating.

Three-phase: open delta or wye

Three-phase loads are boosted with two or three single-phase units, and the choice depends on the neutral.

A 208Y/120 V supply boosted in open delta gives a 229 V three-wire output with no usable neutral; boosted in wye it gives 229Y/132 V with its neutral intact. Sizing follows the same principle (the units carry the load current at the boost voltage), but the open-delta connection loads the two units unevenly, so the manufacturers’ three-phase tables are the way to pick them.

What a buck-boost can’t do

• Regulate. It adds a fixed percentage. A supply that swings swings the output with it. A site with a voltage problem rather than a voltage mismatch needs a regulator, or the utility.
• Isolate. The input and output share a conductor. Where isolation is the point (a separately derived system, noise, a floating supply for a machine), use a two-winding transformer.
• Make a neutral. It passes the supply neutral through (wye) or leaves it out (open delta); it can’t create one. For a 240 V three-wire delta supply that needs 120 V, the answer is a 240 V delta to 208Y/120 V transformer, not a buck-boost.
• Change voltage by more than about 25%. 480 V to 240 V, 480 V to 208 V, 240 V to 120 V: these are step-down transformers.
• Serve a load bigger than its table allows. The series winding’s ampacity is the limit, and it’s far lower than the load’s kVA suggests.

Buck-boost vs an isolation transformer

For a small correction on a load with a tolerant rating, a buck-boost is cheaper, smaller and more efficient than a two-winding transformer, and it’s the standard answer. A two-winding step-up or step-down transformer is the right one when the change is large, when the load needs its own grounded neutral, when isolation matters, or when the load is big enough that a 480 V to 208Y/120 V dry-type in the right size is the ordinary thing to buy. Those we sell; for buck-boost units in the 0.25 to 10 kVA range, an electrical distributor stocks them, and a Magic Search will turn up surplus ones.