CSA C22.1:24 overcurrent protection and conductor sizing for transformers. Rules 26-250, 26-252, 26-254, and 26-256 — separated by transformer type and voltage rating as required by Section 26.
Rule 26-256 1)(b): sum of all primaries + 25% of largest
Primary Rated Current
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amperes
Secondary Rated Current
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amperes
Primary Conductor
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A (125% of rated)
Secondary Conductor
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A (125% of rated)
Overcurrent Protection
Primary O/C
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Secondary O/C
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Applicable CSA C22.1:24 Rules
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Notes
Rule 26-250 — Over 750 V: Primary overcurrent protection at ≤150% rated primary current for fuses, ≤300% for breakers. If secondary device ≤ Table 50 values or manufacturer thermal protection, individual primary O/C may be omitted (feeder must still meet Table 50).
Rule 26-252 — 750 V or less (not dry-type): Primary O/C at ≤150% rated primary current. For primary current < 9 A, 167% permitted; < 2 A, 300% permitted. If secondary O/C at ≤125% rated secondary current, no individual primary O/C needed (feeder ≤300% primary). Manufacturer thermal overload: feeder ≤6× (≤7.5% impedance) or ≤4× (>7.5% impedance).
Rule 26-254 — Dry-type 750 V or less: Primary O/C at ≤125% rated primary current (not 150%). If secondary O/C at ≤125% rated secondary current, no individual primary O/C needed (feeder ≤300% primary).
Rule 26-256 — Conductor Sizing (all types): Primary conductors ≥125% rated primary current (single) or sum + 25% of largest (parallel). Secondary conductors ≥125% rated secondary current.
Current calculation: 1-phase: I = kVA × 1000 ÷ V. 3-phase: I = kVA × 1000 ÷ (√3 × V).