Circuit Breaker Sizing Calculator — Canada (CEC Rule 14-104)

Full formula and methodology: How this calculator works →

What is a circuit breaker?

A circuit breaker is an automatic safety switch that disconnects a circuit when the current exceeds a safe level. Unlike a fuse, a breaker can be reset after it trips. Every circuit in a Canadian home must be protected by a breaker (or fuse in older installations) sized to match the wire gauge — not the appliance.

This last point is critical: the breaker protects the wire, not the load. An oversized breaker allows the wire to overheat before tripping. A 20A breaker on 14 AWG wire is a code violation for this exact reason — 14 AWG is only rated to 15A, so a 20A breaker will allow dangerous overcurrent before acting.

Breakers come in standard sizes: 15, 20, 25, 30, 35, 40, 50, 60, 70, 100, 125, 150, 175, 200A. When a calculation produces a non-standard number, always round up to the next available size.

CEC Rule 14-104 — the 125% rule

CEC Rule 14-104 governs overcurrent protection sizing. The core formula depends on whether a load is continuous or non-continuous:

• Non-continuous load (runs less than 3 hours): breaker size ≥ load current
• Continuous load (runs 3+ hours): breaker size ≥ load current × 1.25

Most household appliances are non-continuous — they cycle on and off. Lighting circuits, EV chargers charging overnight, and commercial refrigeration are examples of continuous loads. When in doubt, apply the 125% rule — it is conservative and always compliant.

For mixed loads, add 125% of all continuous loads to 100% of all non-continuous loads, then select the next standard breaker size up.

Single pole vs. double pole breakers

Single pole breakers protect 120V circuits and occupy one slot in the panel. They interrupt one hot conductor. Typical applications: lighting, outlets, small appliances, dishwashers, microwaves.

Double pole breakers protect 240V circuits, occupy two adjacent slots, and interrupt both hot conductors simultaneously. This ensures that when the breaker trips, the full 240V circuit is de-energized — critical for safety. Typical applications: electric dryers, ranges, water heaters, EV chargers, air conditioners, subpanels.

A tandem (slim) breaker fits two 120V circuits into one slot — useful when a panel is full. Not all panels accept tandems, and there are restrictions on where they can be installed. Check your panel's labelling or consult a licensed electrician.

Worked examples

Example 1 — Level 2 EV charger (Ontario): 32A continuous load, 240V.

• Continuous load: 32A × 1.25 = 40A
• Next standard breaker: 40A ✓ (40A is a standard size)
• Required wire: 8 AWG copper (45A ampacity)
• Result: 40A double-pole breaker, 8 AWG copper

Example 2 — Electric range (BC): 50A nameplate rating, 240V, non-continuous.

• Non-continuous load: breaker ≥ 50A
• Next standard breaker: 50A ✓
• Required wire: 6 AWG copper (65A ampacity — wire must exceed breaker size)
• Result: 50A double-pole breaker, 6 AWG copper

Example 3 — Workshop subpanel (Alberta): 60A subpanel feed, 240V.

• Feeder breaker: 60A double-pole at main panel
• Required feeder wire: 4 AWG copper or 2 AWG aluminum
• Result: 60A double-pole breaker, 4 AWG copper feeder

Common appliance breaker sizes in Canada

GFCI vs AFCI — where each is required in Canada (CEC)

The CEC requires two types of protective devices in addition to standard overcurrent breakers: GFCI (Ground Fault Circuit Interrupter) and AFCI (Arc Fault Circuit Interrupter). These are different technologies that protect against different hazards.

GFCI protects people from electric shock by detecting current leaking to ground (as little as 5 milliamps). It trips in under 25 milliseconds — fast enough to prevent electrocution. GFCI protection is required where water and electricity can meet.

AFCI protects against electrical fires by detecting the characteristic waveform of an arc fault — the type of arcing that occurs in a damaged wire inside a wall, a loose connection, or a frayed cord. AFCI breakers required in newer code editions do not protect against shock — they protect against fire.

AFCI requirements for new construction apply to circuits installed under the 2018 CEC and later adoptions. Retrofit (existing home renovations) requirements vary by province. Always verify with your local Authority Having Jurisdiction (AHJ).

Combination AFCI/GFCI breakers are available and satisfy both requirements in a single device. These are commonly used in bathrooms where both are needed, or in finished basements where AFCI is required for living spaces but GFCI is also required near the laundry area.

GFCI breaker vs. GFCI outlet: Either satisfies the CEC requirement. A GFCI breaker protects the entire circuit from the panel. A GFCI outlet can protect itself and all outlets downstream on the same circuit. Breaker-level GFCI is required for hot tub and pool circuits (the disconnect must be GFCI-protected at the panel level per CEC Rule 68-064).

Frequently asked questions

Code reference

This calculator is based on CEC Rule 14-104 (overcurrent protection sizing), Rule 14-102 (continuous loads), and Rule 26-700 (GFCI requirements). Standard breaker sizes follow CSA C22.2 No. 5. Always verify with a licensed electrician and your local Authority Having Jurisdiction (AHJ).