CSA C22.1:24 Table 29 / Rule 28-200 overcurrent protection and Rule 28-106 conductor sizing for motor branch circuits. Full-load current from Table 44 (3-phase) or Table 45 (1-phase), conductor ampacity at 125%, and fuse/breaker ratings by motor type.
Full Load Current (FLC)
—
amperes
Min Conductor Ampacity
—
A (125% of FLC)
Recommended Conductor
—
—
Max OC Device Rating
—
A
OC Multiplier
—
% of FLC
Overload Protection (Rule 28-306)
— A (115% of FLC)
CSA C22.1:24 Reference
Table 44/45, Rule 28-106, 28-200, Table 29
Step 1 — Full Load Current (Table 44 / Table 45): The motor's FLC is looked up from CSA C22.1:24 Table 44 (3-phase) or Table 45 (1-phase) based on horsepower, voltage, and phase. Table 44 provides standard FLC values for 3-phase induction motors (squirrel-cage and wound rotor) and synchronous motors. Table 45 provides values for single-phase AC motors.
Step 2 — Conductor Ampacity (Rule 28-106): Branch circuit conductors must have an ampacity not less than 125% of the motor FLC. The conductor size is then selected from Table 2 (copper, 75°C column for < 100A, 90°C for ≥ 100A).
Step 3 — Overcurrent Protection (Rule 28-200, Table 29): The maximum rating or setting of the branch circuit overcurrent device depends on the motor type and device type. Time-delay fuses: 175% of FLC. Non-time-delay fuses: 300% of FLC. Inverse-time circuit breakers: 250% of FLC. When the calculated rating doesn't match a standard size, the next lower standard size may be used.
Step 4 — Overload Protection (Rule 28-306): Motor overload protection is typically set at 115% of FLC (for motors with service factor ≥ 1.15) or 125% (for service factor < 1.15). This protects the motor from sustained overcurrent while allowing for starting inrush current.
Rating or setting of overcurrent devices for motor branch circuits. See Rules 28-200, 28-206, 28-208, 28-308 and Table 44.
| Type of motor | Time-delay fuses (%) | Non-time-delay fuses (%) | Inverse-time CB (%) |
|---|---|---|---|
| AC single-phase, all types | 175 | 300 | 250 |
| Squirrel-cage & sync — full-voltage/resistor/reactor starting | 175 | 300 | 250 |
| Auto-transformer & star-delta — ≤30 A | 175 | 250 | 200 |
| Auto-transformer & star-delta — >30 A | 175 | 200 | 200 |
| Wound rotor | 150 | 150 | 150 |
| Direct current | 150 | 150 | 150 |
* Includes time-delay "D" fuses referred to in Rule 14-200. Note 1: Low-torque, low-speed synchronous motors (≤450 rpm) that start unloaded do not require >200% FLC. Note 2: For instantaneous-trip circuit breakers, see Rule 28-210.