Buck-Boost Transformer Sizing (Single-Phase)
The autotransformer (buck-boost) kVA to correct a line voltage up or down for a load: boost/buck = desired minus supply voltage, and the unit is rated only for that boost times the load current, a fraction of the full load kVA (208 V to 230 V at 50 A -> a 1.10 kVA unit supports an 11.5 kVA load). Sizing a buck-boost by the full load kVA grossly oversizes it. NEC Article 450; the buck-boost selection, connection, and overcurrent protection are the manufacturer's and the AHJ's.
Formula and source
boost_v = desired_v - supply_v; load_kva = desired_v x load_a / 1000; xfmr_kva = abs(boost_v) x load_a / 1000; ratio% = xfmr_kva / load_kva x 100. The autotransformer is rated for the boost/buck voltage times the load current, a fraction of the full load kVA.
Single-phase buck-boost (autotransformer) sizing. NEC 2023 Article 450 (transformers); the autotransformer connection per the manufacturer.
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