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Updated June 2026What 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 |
Frequently asked questions
What is the maximum voltage drop allowed in Canada?
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 the furthest outlet. On a 120V circuit, 3% equals 3.6V maximum drop. These are recommended limits — the CEC uses the word 'should' rather than 'shall', but inspectors and engineers treat them as requirements in practice.
How do I reduce voltage drop on a long run?
The most effective method is to upsize the wire. Voltage drop is inversely proportional to wire cross-section — doubling the cross-section (e.g., from 12 AWG to 10 AWG) roughly halves the voltage drop. Use the wire size calculator to find the correct upsized gauge for your run. Other options include increasing the supply voltage from 120V to 240V for applicable loads, or relocating the panel closer to the load.
Does voltage drop matter for short runs?
For runs under 15 metres, voltage drop is rarely an issue at residential current levels. It becomes significant on runs over 25–30 metres, especially at higher currents. A 20A circuit at 120V over 30 metres of 12 AWG copper drops approximately 4.7% — well above the CEC 3% limit and requiring an upsize to 10 AWG.
Is voltage drop more of a problem at 120V or 240V?
The same physical voltage loss is a larger percentage of 120V than 240V. A 5V drop on a 120V circuit is 4.2% — over the CEC limit. The same 5V drop on a 240V circuit is only 2.1% — within limits. This is why 240V circuits tolerate longer runs more easily than 120V circuits.
What happens if voltage drop is too high?
High voltage drop causes motors to run hotter and less efficiently, lights to dim or flicker, electronic devices to operate improperly, and heating elements to produce less heat than rated. For sensitive equipment, even modest voltage drop can cause malfunction or shortened equipment life.
Why use one-way run length in the formula?
The formula already accounts for the round trip by multiplying by 2 (current travels to the load and returns through the neutral/return conductor). You only need to enter the one-way distance from the panel to the outlet or device. Entering total wire length will double the calculated drop, giving a wrong answer.
Does aluminum wire have more voltage drop than copper?
Yes. Aluminum has a resistivity of 0.0282 Ω·mm²/m versus 0.0172 for copper — about 64% higher. An aluminum conductor will have approximately 64% more voltage drop than a copper conductor of the same size. For this reason, aluminum feeders are typically upsized one gauge relative to copper: 1/0 AWG aluminum instead of 2 AWG copper for a 100A feeder.
How do I calculate voltage drop for a 240V circuit?
Use the same formula but enter 240 as the voltage. For single-phase 240V circuits (dryers, ranges, EV chargers), the formula is identical — voltage drop occurs across both hot conductors, and the total run length is the one-way distance. The calculator handles this automatically.
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.