Understanding wire size and ampacity
How This Calculation Works
This calculator compares the selected breaker with a baseline ampacity table for copper NMD90 conductors. Ampacity is the maximum current a conductor can carry continuously under stated conditions without exceeding its permitted temperature. The tool chooses the smallest conductor in its reference table whose ampacity meets or exceeds the breaker rating. For example, a 20-amp branch circuit commonly maps to 12 AWG copper in this simplified table. The result is a starting point, not a complete circuit design: the actual conductor must also suit the insulation, terminals, wiring method, temperature, number of current-carrying conductors, run length, and equipment instructions.
Applicable Code Rules & Standards
Canadian installations are governed by the current Canadian Electrical Code, Part I, as adopted and amended by the province or territory. Ontario work also falls under the Ontario Electrical Safety Code and ESA processes. Conductor ampacity is typically checked against the applicable CEC table and then adjusted under the rules for correction and adjustment factors. The breaker protects the conductor; it is not a way to make undersized wire safe. Confirm the 60°C or 75°C column based on the conductor and termination ratings, and check whether a separate rule applies to small conductors, motors, HVAC equipment, continuous loads, or aluminum wiring.
Also verify the circuit voltage, calculated load, equipment nameplate, and required disconnecting means. A general-purpose receptacle circuit is not evaluated the same way as a fixed heater, range, air conditioner, welder, or EV supply. Continuous loads may need additional capacity, while motors and compressors have starting characteristics that affect overcurrent protection. The code edition and local inspection authority control when a quick online estimate must be replaced by a complete design.
Common DIY/Installation Errors
- Choosing wire by physical diameter or colour instead of its marked gauge and material.
- Installing a larger breaker because a smaller breaker trips.
- Ignoring voltage drop on long runs or high-draw equipment.
- Forgetting derating for bundled cables, hot spaces, conduit fill, or more than three current-carrying conductors.
- Using a cable type, connector, or termination that is not approved for the location.
When to Hire a Licensed Professional
Hire a Licensed Electrical Contractor for a new circuit, panel or service work, feeders, concealed wiring, EV charging, generators, pools, spas, or any project that requires a permit or inspection. A professional can verify the load calculation, route, protection, grounding and bonding, terminations, and local requirements before the circuit is energized. If you see heat, discolouration, buzzing, exposed copper, or repeated trips, stop using the circuit and arrange an inspection rather than changing the breaker.