Wire Size Calculator — Canada (CEC Table 2)

Full formula and methodology: How this calculator works →

What is wire sizing?

Wire sizing determines the minimum gauge of electrical conductor required for a circuit. In Canada, wire size is expressed in AWG (American Wire Gauge) — a lower number means a thicker wire. Thicker wire carries more current safely and loses less voltage over distance.

Every circuit in a Canadian home or commercial building must have wire that meets two independent criteria: ampacity (the wire can carry the full load current without overheating) and voltage drop (the voltage loss over the run length stays within the CEC limit). Both must be checked — a wire may pass ampacity but fail voltage drop on a long run, requiring an upsize.

Wire sizing is one of the most common calculations performed during electrical rough-in. Undersizing wire is a fire hazard. Oversizing wastes money. Getting it right the first time means your installation passes inspection and performs safely for decades.

CEC Table 2 — ampacity rules

CEC Table 2 lists the maximum current-carrying capacity (ampacity) for each wire gauge based on the conductor material, temperature rating, installation method, and number of conductors in the same raceway.

The most commonly used column is copper, 75°C, in conduit. Even if you purchase 90°C-rated wire (such as THHN), you must use the 75°C ampacity column when the termination points — breakers, outlets, devices — are only rated to 75°C, which is standard for most residential and commercial equipment.

Key derating rules from CEC Table 5C:

• 4–6 current-carrying conductors in a raceway: multiply ampacity by 0.80
• 7–9 conductors: multiply by 0.70
• 10–20 conductors: multiply by 0.50

Aluminum wire has lower ampacity than copper at the same gauge. An 8 AWG aluminum conductor has roughly the same ampacity as 10 AWG copper. For 15A and 20A branch circuits in residential construction, aluminum is not permitted in most provinces due to connection reliability concerns at receptacles and switches.

The 3% voltage drop rule

CEC Rule 8-102 limits voltage drop to 3% on branch circuits and 2% on feeders, for a combined maximum of 5% from the service entrance to the furthest outlet. This rule is a recommendation rather than a hard requirement, but inspectors expect it to be followed in all new work.

The formula used to calculate voltage drop is:

VD (volts) = (ρ × 2 × L × I) / A

Where ρ = resistivity (0.0172 Ω·mm²/m for copper, 0.0282 for aluminum), L = one-way run length in metres, I = circuit current in amps, A = wire cross-section in mm².

VD% = (VD / V) × 100

On a 120V circuit, 3% equals 3.6V maximum drop. On a 240V circuit, 3% equals 7.2V. Voltage drop becomes significant on runs over 25 metres at higher currents — always check long runs to a garage, detached workshop, or outbuilding.

Worked examples

Example 1 — Kitchen circuit (Ontario): 20A, 120V, copper, 18m run in conduit with 3 conductors.

• CEC Table 2: 12 AWG copper = 20A ampacity ✓
• VD = (0.0172 × 2 × 18 × 20) / 3.31 = 3.74V = 3.12% — slightly over the 3% limit
• Upsize to 10 AWG: VD = (0.0172 × 2 × 18 × 20) / 5.26 = 2.35V = 1.96% ✓
• Result: 10 AWG copper required

Example 2 — Electric dryer (BC): 30A, 240V, copper, 12m run.

• CEC Table 2: 10 AWG copper = 30A ampacity ✓
• VD = (0.0172 × 2 × 12 × 30) / 5.26 = 2.36V = 0.98% ✓
• Result: 10 AWG copper — passes both ampacity and voltage drop

Example 3 — Garage subpanel (Alberta): 100A, 240V, aluminum feeder, 40m run.

• CEC Table 2: 1/0 AWG aluminum = 100A ampacity ✓
• VD = (0.0282 × 2 × 40 × 100) / 53.5 = 4.22V = 1.76% ✓
• Result: 1/0 AWG aluminum — passes both criteria

Wire size quick reference (CEC Table 2, copper, 75°C, in conduit)

Full CEC Table 2 ampacity chart (copper and aluminum)

The complete CEC Table 2 covers conductors from 14 AWG to 2000 kcmil. Below are the most common residential and light commercial sizes. Values shown are for 75°C termination rating (T90 or RW75), which applies to virtually all standard breakers, panels, and devices. Use the 90°C column only when both the conductors and all terminations are rated 90°C.

Source: CEC Table 2 (CSA C22.1-21). Values are for conductors in conduit or cable with up to 3 current-carrying conductors, in an ambient temperature of 30°C. Apply derating factors from CEC Table 5C for more conductors or higher ambient temperatures. Aluminum wire is not permitted for 15A or 20A branch circuits in most residential applications.

Canadian wire type reference:

Note: NMD90 (the Canadian equivalent of Romex) is rated 90°C but the CEC requires using the 60°C ampacity column when NMD90 is run through insulation in walls and ceilings, due to the inability to dissipate heat. This distinction is one reason the wire size calculator defaults to the 75°C column.

Frequently asked questions

Code reference

This calculator is based on the Canadian Electrical Code, Part 1 (CSA C22.1-21), Table 2 (conductor ampacity), Table 5C (conductor derating), and Rule 8-102 (voltage drop). Always verify results with a licensed electrician before installation. Local amendments may apply — check with your Authority Having Jurisdiction (AHJ).