Standard Ratings
The rating steps you pick from when sizing protection, transformers and motors.
Fuse and inverse-time breaker ampere ratings
Standard ratings rise in 5 A steps from 15 A to 50 A (15, 20, 25, 30, 35, 40, 45, 50), then in 10 A steps to 110 A (60, 70, 80, 90, 100, 110). Above that they run 125, 150, 175, 200, 225 and 250, then 300, 350, 400, 450, 500 and 600, then 700, 800, 1000, 1200, 1600 and 2000, and finally 2500, 3000, 4000, 5000 and 6000 A. Fuses add 1, 3, 6, 10 and 601 A.
NEC 2023 240.6(A) — confirm against the edition in force.
Common circuit breaker frame sizes
Molded-case breakers are commonly built on 100, 150, 225, 250, 400, 600, 800 and 1200 A frames; insulated-case and low-voltage power breakers on 800, 1200, 1600, 2000, 2500, 3000, 4000 and 5000 A frames. Frame sizes and the trip ratings available in each vary by manufacturer.
Manufacturer catalogs; UL 489 and UL 1066 / IEEE C37.13 describe the breaker classes.
Single-phase distribution transformer kVA
Common ratings are 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333 and 500 kVA. Pole-mounted units usually stop at 167 kVA; pad-mounted single-phase units go higher.
IEEE Std C57.12.00 and C57.12.20 / C57.12.38 describe preferred ratings.
Three-phase transformer kVA
Common ratings are 15, 30, 45, 75, 112.5, 150, 225, 300 and 500 kVA for dry-type and small pad-mounted units, then 750, 1000, 1500, 2000, 2500, 3000, 3750 and 5000 kVA and upward for substation and large pad-mounted units.
IEEE Std C57.12.00, C57.12.01 (dry-type) and C57.12.34 (pad-mounted) describe preferred ratings.
Standard motor horsepower
Low-voltage induction motors are commonly built at 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100, 125, 150, 200 and 250 HP, with larger ratings, often at medium voltage, at 300, 350, 400, 450, 500 HP and up.
NEMA MG 1 describes standard ratings.
Sources are listed with each paragraph. © RokBench.