Voltage Drop Calculation in Canada — CEC Rule 8-102 Explained

What is voltage drop and why the CEC limits it

Voltage drop is the reduction in voltage that occurs as current flows through the resistance of a wire. Every conductor has resistance, and the longer and thinner the wire, the greater the resistance — and the greater the voltage drop along its length.

Excessive voltage drop causes real problems: motors run hotter and wear out faster, LED drivers flicker or fail, and sensitive electronics may malfunction. A circuit that loses too much voltage at the load end is effectively delivering less power than it was designed to provide.

The Canadian Electrical Code addresses this in Rule 8-102, which sets mandatory limits on how much voltage a circuit is permitted to drop. Unlike the US National Electrical Code, which only recommends limits in a non-enforceable fine-print note, CEC Rule 8-102 is mandatory — installations that exceed the limits do not pass inspection.

CEC Rule 8-102: the 3% and 5% limits

Rule 8-102 sets two voltage drop limits that both must be met:

• Branch circuit: Maximum 3% voltage drop from the panel to the load. On a 120V circuit this is 3.6V maximum. On a 240V circuit this is 7.2V maximum.
• Total (feeder + branch circuit combined): Maximum 5% voltage drop from the utility service entrance to the load. If your feeder already drops 2%, your branch circuits can only drop an additional 3%.

In practice, for a residential circuit on a modern 200A panel close to the meter, the feeder drop is negligible and the 3% branch circuit limit is the binding constraint.

The voltage drop formula

The standard formula for single-phase AC circuits (also used for DC):

• L = one-way run length in metres
• I = current in amps (use 80% of breaker rating per CEC Rule 8-104, or actual load if known)
• R = resistance of the wire in mΩ/m from CEC Table 2
• The factor of 2 accounts for the complete circuit (hot wire + neutral/return)

To convert to a percentage:

Worked example: 20A circuit, 25-metre run, 120V system, 12 AWG wire (R = 5.31 mΩ/m).
VD = (2 × 25 × 20 × 5.31) / 1000 = 5,310 / 1000 = 5.31V
VD% = (5.31 / 120) × 100 = 4.4% — exceeds the 3% CEC limit.
Solution: upsize to 10 AWG (R = 3.33 mΩ/m): VD = (2 × 25 × 20 × 3.33) / 1000 = 3.33V = 2.8% ✓

Voltage drop by wire gauge — 120V circuit, 30-metre run

The table below shows the voltage drop and percentage for a 30-metre one-way run at two common circuit sizes. Values exceeding the 3% CEC limit are marked.

Load = 80% of breaker rating per CEC Rule 8-104. Resistance from CEC Table 2 (copper). Red = exceeds 3% CEC Rule 8-102 limit.

CEC vs NEC: why US tools give wrong answers in Canada

The most common error Canadian electricians encounter when using US-built voltage drop calculators is that the US tools treat the 3% limit as a recommendation, not a hard requirement. A US calculator may show a result of 3.5% and not flag it as non-compliant — because in the US, it technically isn't.

In Canada, any branch circuit exceeding 3% under CEC Rule 8-102 is non-compliant and must be corrected before the installation will pass inspection. The standard is mandatory, not advisory.

US tools also commonly use NEC wire ampacity tables, which differ from CEC Table 2 in their temperature correction factors and some conductor ratings. Always use a CEC-based calculator for work done in Canada.