Sizes an automatic or manual transfer switch (ATS/MTS) in amperes from the connected standby load, applying the same continuous-load sizing practice used for other emergency/legally-required/optional-standby system equipment under NEC Articles 700–702 (continuous loads at 125%, non-continuous at 100% — the same relation as general service/feeder equipment under 215.2(A)(1)/230.42(A)(1)), then rounding up to the next standard UL 1008-listed switch ampere rating.
Standards followed
NEC Art. 700/701/702 — emergency, legally required standby, and optional standby systemsNEC 215.2(A)(1) / 230.42(A)(1) — 125% continuous-load sizing convention (applied here to ATS/MTS equipment)UL 1008 — standard transfer switch ampere ratings
What it can do
- Load entry directly in amps, or from kW/kVA with voltage, phase count and power factor.
- Splits the load into continuous and non-continuous fractions and applies the 125%/100% sizing factors separately.
- Rounds up to the next standard UL 1008 transfer switch ampere rating.
Example problem
250kW, 480V three-phase, PF=0.9, fully continuous load.
- I = (250/0.9 × 1000) / (√3 × 480) ≈ 334.1 A
- Min switch = 1.25 × 334.1 ≈ 417.6 A
- Next standard UL 1008 rating ≥ 417.6A → 600A
≈334.1A load current, ≈417.6A minimum, 600A recommended standard switch rating.
This sizes the switch's continuous current rating only — actual ATS/MTS selection must also confirm withstand and closing rating (WCR) against available fault current, number of poles/transition type (open/closed/delayed), and any load-shed or in-phase monitor requirements for the specific application.