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Updated June 2026What 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)
| Wire Size | Cross-section (mm²) | Max Ampacity | Typical Use |
|---|---|---|---|
| 14 AWG | 2.08 | 15A | Lighting, outlets |
| 12 AWG | 3.31 | 20A | Kitchen, bathroom circuits |
| 10 AWG | 5.26 | 30A | Dryers, water heaters |
| 8 AWG | 8.37 | 45A | Ranges, EV chargers, hot tubs |
| 6 AWG | 13.3 | 65A | Subpanels, large EV chargers |
| 4 AWG | 21.1 | 85A | Large subpanels, workshops |
| 2 AWG | 33.6 | 115A | Service entrances, feeders |
| 1/0 AWG | 53.5 | 150A | 200A panel service entrance |
| 3/0 AWG | 85.0 | 200A | 400A service, large feeders |
Frequently asked questions
What wire size do I need for a 20-amp circuit in Canada?
12 AWG copper minimum per CEC Table 2. If the run exceeds approximately 20–25 metres, upsize to 10 AWG to keep voltage drop under the CEC 3% limit. In Ontario, BC, and Alberta — all following the same CEC — 12 AWG for a 20A circuit is universal.
What wire gauge for an electric dryer in Canada?
Electric dryers require a 30A, 240V dedicated circuit. The minimum wire size is 10 AWG copper (or 8 AWG aluminum) per CEC Table 2. Use a 4-conductor cable (two hots, neutral, ground) for modern 4-prong dryer connections.
What wire size for a Level 2 EV charger in Canada?
A Level 2 EV charger running at 32A requires 8 AWG copper on a 40A breaker (125% rule). A 48A charger needs 6 AWG copper on a 60A breaker. Always run a dedicated circuit — no other loads on the same circuit.
Can I use aluminum wire for residential wiring in Canada?
Yes, for circuits 30A and above. Aluminum is not permitted for 15A or 20A branch circuits in most residential applications due to connection reliability concerns. For larger feeders and service entrances, aluminum is standard and cost-effective. Always use aluminum-rated connectors and anti-oxidant compound.
What is the maximum voltage drop allowed by the Canadian Electrical Code?
CEC Rule 8-102 recommends no more than 3% on any branch circuit and 2% on feeders, for a combined maximum of 5% from service entrance to final outlet. These are recommendations, not hard requirements — but following them is considered good engineering practice.
What wire size do I need for a 100-amp subpanel?
A 100A subpanel requires 2 AWG copper or 1/0 AWG aluminum for the feeder conductors, protected by a 100A breaker at the main panel. For a 60A subpanel, use 4 AWG copper or 2 AWG aluminum.
How does conduit installation affect wire sizing?
Wire in conduit with 4 or more current-carrying conductors must be derated. With 4–6 conductors, apply a 0.80 derating factor to the ampacity from CEC Table 2. This means a wire rated at 20A in open air may only carry 16A in a conduit with multiple conductors.
What size wire for a hot tub in Canada?
Most hot tubs draw 40–60A at 240V. A 50A hot tub circuit requires 6 AWG copper on a 50A GFCI double-pole breaker. A 60A circuit needs 4 AWG copper. The CEC requires GFCI protection and a disconnect within sight of the hot tub.
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).