Buck-Boost Transformer Sizing (Single-Phase)

The autotransformer (buck-boost) kVA to correct a line voltage up or down for a load.

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Details, formula, and sources

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.

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.

NEC is free to read at nfpa.org/freeaccess. Buck-boost selection, the connection diagram, taps, and overcurrent protection are the manufacturer's and the AHJ's; this sizes the required kVA only.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: supply_v, desired_v, load_a, boost_v, load_kva, xfmr_kva

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