Standard and code basis
Voltage drop limits in Canada are set by CEC Rule 8-102 (CSA C22.1-21):
- Branch circuits: Maximum 3% voltage drop from the panel to the load.
- Feeders: Maximum 2% voltage drop from the service entrance to the sub-panel.
- Total (service to load): The CEC recommends a combined maximum of 5% — meaning if a feeder drops 2%, the branch circuit should drop no more than 3%.
The resistivity values used — 0.0172 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminum — are the standard conductivity values recognized by the CEC for ambient temperature conductor resistance.
Your inputs — what each field means
| Input | What it represents | Effect on result |
|---|---|---|
| Load current (amps) | The current the circuit carries under full load | Voltage drop is directly proportional to current. Doubling the current doubles the drop. |
| Wire size (AWG) | The conductor gauge being evaluated | Larger conductors (lower AWG number) have more cross-section (mm²), reducing resistance and voltage drop. |
| Conductor material | Copper or aluminum | Copper resistivity: 0.0172 Ω·mm²/m. Aluminum: 0.0282 Ω·mm²/m. Aluminum drops 64% more than copper for the same gauge. |
| One-way run length (m) | The distance from panel to load in metres | Voltage drop is proportional to run length — and the formula uses 2× the one-way length to account for both the supply and return conductors. |
| System voltage | The nominal voltage of the circuit | Drop percentage = (Vd ÷ system voltage) × 100. A 2V drop on a 240V circuit is 0.8%; on a 120V circuit it is 1.7%. |
| Circuit type | Branch circuit or feeder | Determines the applicable CEC limit: 3% for branch circuits, 2% for feeders. |
The calculation — step by step
- Look up conductor cross-section (mm²). The AWG gauge you enter is matched to its metric cross-section from CEC Table 2. For example: 12 AWG copper = 3.31 mm²; 10 AWG = 5.26 mm²; 8 AWG = 8.37 mm²; 6 AWG = 13.3 mm².
- Apply the voltage drop formula:
Vd = (ρ × 2 × L × I) / A
Where:- ρ = resistivity in Ω·mm²/m (0.0172 copper, 0.0282 aluminum)
- L = one-way run length in metres
- I = current in amps
- A = conductor cross-section in mm²
- The factor of 2 accounts for both the supply conductor and the return (neutral or ground) conductor
- Calculate drop percentage:
VD% = (Vd ÷ system voltage) × 100 - Compare against CEC limit. Branch circuit: pass if VD% ≤ 3%. Feeder: pass if VD% ≤ 2%.
- Calculate voltage at load:
voltage at load = system voltage − Vd
This is the actual voltage available at the receptacle or appliance terminals.
Reading your result
- Voltage drop (V): The absolute voltage lost across the conductor run.
- Voltage drop (%): Drop as a percentage of system voltage. This is the number compared against the CEC 3% or 2% limit.
- CEC compliant: Whether the drop is within the applicable limit for your circuit type.
- Voltage at load: The voltage available at the end of the run — useful for checking whether sensitive equipment (motors, dimmers) will receive adequate voltage.
If the result shows non-compliant, increase the wire gauge (lower AWG number) and re-run the calculation. The wire size calculator can also determine the minimum gauge required to keep both ampacity and VD within limits simultaneously.
Assumptions and limitations
- Uses DC resistivity formula. AC circuits have a slightly lower actual drop due to power factor — this calculator produces a conservative (higher) result, which is standard engineering practice.
- Assumes ambient temperature of 30°C. Higher ambient temperatures increase conductor resistance; the error is small (<2%) for typical Canadian installation temperatures.
- Does not account for conduit material effects (steel vs. PVC) on AC impedance — the difference is minor for most residential and commercial applications.
- For 3-phase circuits, the formula changes — the factor of 2 becomes √3 (approximately 1.732). This calculator is for single-phase circuits only.
Ready to calculate?
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