CSA C22.1:24 Rule 26-256 conductor sizing for transformers. Primary at 125% rated current, secondary at rated current, with overcurrent protection per Section 26.
Rule 26-256 1)(b): sum of all primaries + 25% of largest
Primary Rated Current
—
amperes
Primary Conductor
—
A (125% of rated)
Secondary Conductor
—
A (100% of rated)
Primary O/C Rating
—
A
Secondary O/C Rating
—
A
CSA C22.1:24 Reference
Rule 26-256, Section 26
Rule 26-256 1)(a) — Single Transformer: Conductors supplying the primary of a transformer must have an ampacity of at least 125% of the rated primary current. Secondary conductors must have an ampacity not less than the rated secondary current.
Rule 26-256 1)(b) — Parallel Transformers: For a group of transformers operated in parallel on a common feeder, primary conductors must carry not less than the sum of all rated primary currents plus 25% of the largest transformer's primary current.
Current calculation: For 1-phase: I = kVA × 1000 ÷ V. For 3-phase: I = kVA × 1000 ÷ (√3 × V). The primary overcurrent device is typically sized at 150% of the rated primary current (Section 26), and the secondary overcurrent at 125% of rated secondary current.