Standard and code basis
Wire sizing in Canada is governed by the Canadian Electrical Code (CEC), CSA C22.1-21. The two governing rules are:
- CEC Table 2 — conductor ampacity for copper and aluminum in conduit or open air, at 75°C temperature rating. This is the primary sizing reference for residential and commercial branch circuits and feeders.
- CEC Rule 8-102 — maximum allowable voltage drop. For branch circuits: 3%. For feeders: 2%. Total from service to load: 5% maximum.
The CEC is adopted by all 10 Canadian provinces and territories, with provincial amendments that do not affect the core ampacity table or voltage drop rules used in this calculator.
Your inputs — what each field means
| Input | What it represents | How it affects the result |
|---|---|---|
| Load current (amps) | The continuous current the circuit will carry | The ampacity of the selected wire must meet or exceed this value after derating |
| System voltage | 120V, 240V, 208V, 347V, or 600V | Used only for the voltage drop calculation — higher voltage means lower drop % for the same current and resistance |
| Conductor material | Copper or aluminum | Copper resistivity: 0.0172 Ω·mm²/m. Aluminum: 0.0282 Ω·mm²/m. Aluminum has 64% the conductivity of copper, so one gauge larger is typically needed. |
| Run length | One-way length of the circuit in metres | Voltage drop is proportional to run length. Circuits longer than ~30m often need upsizing for VD compliance. |
| In conduit or open air | Whether the conductor is in a raceway vs. free air | Conductors in conduit have lower ampacity because heat cannot dissipate as freely. CEC Table 2 has separate columns for each installation method. |
| Conductors in conduit | Total number of current-carrying conductors sharing the raceway | Applies CEC derating factors: 4–6 conductors = 80%; 7–9 = 70%; 10–20 = 50%; 21+ = 45%. More conductors = more heat = lower allowable ampacity per conductor. |
The calculation — step by step
The calculator runs four steps in sequence:
- Apply the derating factor. If there are 4 or more current-carrying conductors in the conduit, the CEC requires the base ampacity to be reduced. The derating factor (D) is looked up from the CEC table:
required ampacity = load current ÷ D - Find the minimum wire gauge for ampacity. CEC Table 2 is scanned from smallest to largest gauge. The first gauge whose 75°C ampacity (in conduit or open air, copper or aluminum) meets or exceeds the required ampacity is selected as the minimum code-compliant wire.
- Calculate voltage drop for that wire. The voltage drop formula from CEC Rule 8-102:
Vd = (ρ × 2 × L × I) / A
where ρ is resistivity (0.0172 for Cu, 0.0282 for Al), L is one-way length in metres, I is current in amps, and A is the conductor cross-section in mm². The factor of 2 accounts for the full circuit length (supply + return). - Upsize for voltage drop if needed. If Vd exceeds 3% of system voltage, the calculator moves to the next larger gauge and repeats the VD check. The recommended wire is the smallest gauge that satisfies both the ampacity and voltage drop requirements. If the minimum and recommended gauges differ, the result flags that the wire was upsized for voltage drop.
Reading your result
- Minimum wire (ampacity only): The smallest CEC Table 2 gauge that handles the load current after derating. This is the code-minimum for a short run.
- Recommended wire (ampacity + VD): The gauge the calculator recommends if the run length causes VD to exceed 3%. This may be the same as the minimum, or one size larger.
- Voltage drop %: The calculated VD at the recommended wire. 3% or under is CEC-compliant for branch circuits.
- Upsized for voltage drop: A flag that appears when the recommended wire is larger than the ampacity minimum, so you know exactly why a larger gauge was selected.
Assumptions and limitations
- Assumes 75°C conductor temperature rating (RW90 / THWN-2 — the most common in Canadian installations). 60°C or 90°C rated conductors have different ampacity values.
- Voltage drop calculation uses DC resistivity formula. For AC circuits, this produces a conservative result (actual AC voltage drop is slightly lower due to power factor).
- Derating is applied for conductor count only. Temperature derating (ambient above 30°C) is not applied — environments above 30°C require an additional correction factor from CEC Table 5A.
- Does not account for parallel conductors, which are used for very large services (>500 kcmil).
- Provincial amendments to the CEC do not affect Table 2 ampacity or Rule 8-102 voltage drop — this calculator is applicable in all provinces.
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