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Updated June 2026Full formula and methodology: How this calculator works →
What is voltage drop?
Voltage drop is the reduction in voltage that occurs as electrical current travels through a conductor. All conductors have resistance, and when current flows through resistance, voltage is consumed. The longer the run and the higher the current, the more voltage is lost before reaching the load.
Think of it like water pressure in a hose: the longer the hose and the more water flowing, the less pressure arrives at the end. For electricity, excessive voltage drop means motors run hotter, lights dim, and sensitive electronics may malfunction. It is also a sign of wasted energy — the "lost" voltage is dissipated as heat in the wire.
Voltage drop becomes a practical concern on runs to detached garages, outbuildings, barns, workshops, or any circuit longer than about 25 metres at standard residential current levels. If you're running a feeder to a subpanel in a garage or outbuilding, also use the wire size calculator to confirm ampacity — both criteria must be satisfied. It is rarely an issue for short runs inside a home.
The CEC 3% rule explained
CEC Rule 8-102 states that the voltage drop from the service entrance to any outlet should not exceed 5%, with no more than 3% on any branch circuit and 2% on any feeder. This split ensures that both the distribution system (feeder) and the final circuits (branch) are individually within acceptable limits.
In practice, this means: if your feeder already drops 1.5%, your branch circuit is limited to 3.5% before the combined 5% cap is reached. For conservative design — and to pass electrical inspection — keep each segment within its individual limit.
Note that the CEC uses the word "should" (a recommendation) rather than "shall" (a requirement) for voltage drop. However, the Authority Having Jurisdiction (AHJ) in most provinces treats the 3% and 5% limits as effectively mandatory in new construction.
Voltage drop formula
The CEC uses the resistivity method:
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 amperes
- A = wire cross-sectional area in mm² (see CEC Table 2)
VD% = (VD / V) × 100
The factor of 2 accounts for the current flowing to the load and back — the total conductor length in the circuit is twice the one-way run distance.
Worked examples
Example 1 — Basement lighting circuit (Manitoba): 15A, 120V, 12 AWG copper, 20m run.
- VD = (0.0172 × 2 × 20 × 15) / 3.31 = 3.12V
- VD% = (3.12 / 120) × 100 = 2.6% ✓ — under 3% limit
- Result: 12 AWG passes at 20 metres
Example 2 — Detached garage circuit (Saskatchewan): 20A, 120V, 12 AWG copper, 35m run.
- VD = (0.0172 × 2 × 35 × 20) / 3.31 = 7.28V
- VD% = (7.28 / 120) × 100 = 6.1% ✗ — exceeds 3% limit
- Upsize to 10 AWG: VD = (0.0172 × 2 × 35 × 20) / 5.26 = 4.58V = 3.8% — still over
- Upsize to 8 AWG: VD = (0.0172 × 2 × 35 × 20) / 8.37 = 2.88V = 2.4% ✓
- Result: 8 AWG copper required for 35m run at 20A
CEC voltage drop limits
| Circuit Type | CEC Limit | At 120V (max volts) | At 240V (max volts) | Rule |
|---|---|---|---|---|
| Branch circuit | 3% | 3.6V | 7.2V | CEC Rule 8-102 |
| Feeder | 2% | 2.4V | 4.8V | CEC Rule 8-102 |
| Total (feeder + branch) | 5% | 6.0V | 12.0V | CEC Rule 8-102 |
Maximum wire run distances — CEC 3% voltage drop limit
The table below shows the maximum one-way run length (in metres) before voltage drop exceeds the CEC 3% limit, for the most common Canadian residential circuits. Values are for copper wire at full rated circuit current.
| Circuit | Wire Size | Voltage | Max run (3% limit) | Common application |
|---|---|---|---|---|
| 15A | 14 AWG | 120V | 14.5 m (48 ft) | Lighting, general outlets |
| 20A | 12 AWG | 120V | 17 m (56 ft) | Kitchen, bathroom circuits |
| 20A (upsized) | 10 AWG | 120V | 27 m (89 ft) | Long run to outbuilding |
| 15A | 14 AWG | 240V | 29 m (95 ft) | Small 240V circuits |
| 20A | 12 AWG | 240V | 35 m (115 ft) | 240V garage circuit |
| 20A (upsized) | 10 AWG | 240V | 55 m (180 ft) | Long 240V run to workshop |
| 30A | 10 AWG | 240V | 37 m (121 ft) | Dryer, water heater |
| 50A | 8 AWG | 240V | 35 m (115 ft) | Range, hot tub |
| 60A | 6 AWG | 240V | 46 m (151 ft) | Large EV charger, subpanel |
| 100A | 1/0 AWG (Al) | 240V | 53 m (174 ft) | Garage subpanel feeder |
One-way distance from panel to load. Based on CEC Rule 8-102 (3% branch circuit limit), copper at 75°C except where noted (aluminum). Calculated using VD = (ρ × 2 × L × I) / A.
Key takeaway: A standard 20A/12 AWG circuit at 120V maxes out at 17 metres before hitting the CEC 3% limit. Runs to a detached garage (typically 20–40 metres) almost always require a wire upsize. For a 30-metre run at 20A on 120V, you need 8 AWG copper — two full gauges up from the minimum. Use the calculator above for any specific run.
Why 240V runs go farther: The same physical voltage drop is a smaller percentage of a higher voltage. A 7V drop on a 240V circuit is 2.9% — within limits. The same 7V drop on a 120V circuit is 5.8% — double the CEC limit. For any load that supports 240V (workshop tools, air compressors, EV chargers), wiring at 240V roughly doubles your permissible run distance.
Frequently asked questions
What is the maximum voltage drop allowed in Canada?
How do I reduce voltage drop on a long run?
Does voltage drop matter for short runs?
Is voltage drop more of a problem at 120V or 240V?
What happens if voltage drop is too high?
Why use one-way run length in the formula?
Does aluminum wire have more voltage drop than copper?
How do I calculate voltage drop for a 240V circuit?
Code reference
Based on CEC Rule 8-102 (voltage drop recommendations) and conductor resistivity values from the CEC Appendix. Part of CSA C22.1-21, Canadian Electrical Code, Part 1. Verify all results with a licensed electrician before installation.