Standard and code basis
Overcurrent protection sizing in Canada is governed by CEC Rule 14-104 (CSA C22.1-21), which states:
- The rating of an overcurrent device must not be less than the ampacity of the conductors it protects.
- For continuous loads (loads expected to run for 3 hours or more), the overcurrent device must be rated at no less than 125% of the continuous load current. This prevents the breaker from running at its thermal limit continuously, which accelerates breaker aging and can cause nuisance tripping.
- The selected breaker size must be a standard rating from the CEC-recognised series: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200A.
Your inputs — what each field means
| Input | What it represents | How it affects the result |
|---|---|---|
| Load (watts) | The nameplate wattage of the appliance or circuit | Divided by voltage to get amps. Larger loads require larger breakers. |
| Voltage (120V or 240V) | Whether this is a single-pole (120V) or double-pole (240V) circuit | amps = watts ÷ voltage. A 2,400W load at 240V draws 10A; the same load at 120V draws 20A. |
| Continuous load | Whether the load runs for 3+ hours uninterrupted | If yes, the calculated amps are multiplied by 1.25 before selecting the breaker size. HVAC compressors, EV chargers, and lighting circuits are typically continuous. |
The calculation — step by step
- Convert watts to amps:
amps = watts ÷ voltage
Example: 3,600W range hood at 240V = 15A. - Apply the continuous load factor (if applicable):
required amps = amps × 1.25 (continuous) or amps × 1.0 (non-continuous)
Example: 15A continuous load → 15 × 1.25 = 18.75A required. - Select the next standard breaker size:
The calculator scans the standard CEC breaker size series and selects the first size at or above the required amps:
15 → 20 → 25 → 30 → 35 → 40 → 45 → 50 → 60 → 70 → 80 → 90 → 100A (and above)
Example: 18.75A required → 20A breaker selected. - Determine minimum wire size:
The calculator also returns the minimum conductor size for the selected breaker, based on the CEC ampacity pairing table (e.g., 15A → 14 AWG; 20A → 12 AWG; 30A → 10 AWG). - Determine poles:
120V circuits use a 1-pole breaker. 240V circuits use a 2-pole breaker.
Reading your result
- Calculated amps: The raw watts ÷ voltage result, before the continuous factor is applied.
- Required amps: The amps after applying the 1.25 factor (if continuous). This is the threshold the breaker must exceed.
- Breaker size: The minimum standard CEC breaker size that meets or exceeds the required amps.
- Poles: 1-pole for 120V, 2-pole for 240V.
- Minimum wire (AWG): The smallest wire gauge whose 75°C ampacity supports the selected breaker.
Assumptions and limitations
- The continuous load definition (3-hour threshold) is from CEC Rule 14-104. If you are unsure whether your load qualifies, select “continuous” — it produces a conservative (larger) result.
- This calculator does not account for motor loads, which have additional starting current requirements under CEC Rules 28-200 and 28-204. Motor-driven equipment requires a separate motor protection calculation.
- GFCI and AFCI requirements (CEC Rules 26-700 and 26-724) depend on circuit location and jurisdiction — this calculator does not determine GFCI/AFCI requirements, only the breaker ampere rating.
- Power factor is not accounted for. For resistive loads (heaters, incandescent lighting) this has no effect. For inductive loads (motors, transformers) with a power factor below 0.85, consult a licensed electrician.
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