Motor branch circuit — sizing reference

Cable, protection & starting
current calculator

Select a supply voltage and motor size to size the branch-circuit short-circuit protection, overload relay, starting-current behaviour, feeder cable, conduit and disconnect switch — built from a standard 35 °C-ambient motor protection reference table (CSA C22.1 / Canadian Electrical Code Part I basis).

⚠ Engineering reference only. Values follow CEC Rule 28‑200 (Table 29), 28‑106 and 28‑306 concepts as commonly applied — always verify final selections, standard device ratings and local amendments against the current edition of CSA C22.1 and the equipment nameplate before installation.

§1Motor & supply data— voltage, size, and (optional) actual PF / efficiency

Common presets

Supply voltage / phase

Motor rating (HP)

Power factor (optional)

0 – 1. Leave blank to use table FLA.

Efficiency % (optional)

Leave blank to use table FLA.

§2Short-circuit protection — fuse & breaker— CEC Rule 28‑200 / Table 29

Table 29 caps the branch-circuit short-circuit device at a multiple of full-load current (FLC), depending on device type. Time-delay (dual-element) fuse: 175% FLA. Non-time-delay fuse: 300% FLA. Inverse-time circuit breaker: 250% FLA. Instantaneous-trip breaker: up to 1300% FLA (CEC) — used mainly ahead of VFDs / soft starters, sized per the drive manufacturer's data.

CEC 28‑200 / Tbl 29The rating is calculated from the motor's full-load current, then rounded up to the next standard overcurrent device size (CEC Table 13 / Rule 14‑104). If the device won't allow the motor to start, Rule 28‑200(d) permits a time-delay fuse up to 225% FLA. Instantaneous-trip breakers feeding a VFD or soft starter are normally sized from the drive's own input-current data rather than Table 29 directly.

§3Overload protection— CEC Rule 28‑306

Nameplate service factor / temp. rise

Or enter a custom multiplier

% of FLA. Overrides the dropdown if filled in.

Overload relay setting

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CEC 28‑306Overload devices protect the motor from sustained overheating, not from short-circuit current. A motor with a nameplate service factor of 1.15 or greater (or a temperature rise of 40 °C or less) is set at 125% of FLA; otherwise the relay is limited to 115% of FLA. This is a separate device from the branch-circuit fuse/breaker in §2.

§4Starting method & starting current

Direct-on-line starting connects the motor straight across full line voltage. Starting (locked-rotor) current is estimated from the motor's nameplate Code Letter, which states the locked-rotor kVA per HP (NEMA MG‑1 / CEC Table 45‑equivalent code letter table).

Nameplate code letter

Locked-rotor / starting current

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Starting current as multiple of FLA

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Star-delta (wye-delta) starting connects the motor windings in star (Y) for the start — dropping the voltage across each winding to 1/√3 (≈58%) of line voltage — then switches to delta (Δ) for the run. Because starting current scales with voltage on a constant-impedance locked rotor, the line starting current drops to about 1/3 of the DOL value; starting torque also drops to about 1/3.

Wiring — star / delta contactors

Current vs. time

A solid-state soft starter uses back-to-back SCRs to ramp the applied voltage smoothly, holding starting current to a set current limit instead of a fixed step. The adjustable range is usually about 200–400% FLA — lightly loaded, low-inertia loads (e.g. pumps, fans) can often be set as low as ~200%, while high-inertia loads need closer to 400%. Starting torque ramps with it, giving a smoother mechanical start than DOL or Y‑Δ.

Current-limit setting

Limited starting current

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A variable-frequency drive starts the motor at low frequency/voltage with full closed-loop current control, so line-side inrush stays close to (typically 100–150%) rated current — no locked-rotor spike is presented to the supply. Protection ahead of the VFD is selected from the drive's own input-current / let-through data, not Table 29.

Current-limit setting

Limited starting current

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§5Cable, conduit & disconnect— copper conductors, 75 °C termination, 35 °C ambient, ≤3 CCC/conduit

Conduit sizes shown are minimum trade size for the conductor set listed, in RW90 and RWU90 (wet-location) insulation. Aluminum conductor sizing is not yet included in this table — copper only for now.