The 5% Rule: Why Your Long Cable Runs Are Failing CEC Inspection

That 150-metre cable run to the parking garage looked fine on paper. The wire was sized for ampacity. The breaker was correct. But the ESA inspector failed it anyway — because of voltage drop. Here's the rule every engineer needs to master.

What CEC Rule 8-102 Actually Says

The Canadian Electrical Code, Rule 8-102, states that the voltage drop from the supply side of the service to the point of utilization shall not exceed 5% of the nominal circuit voltage. This is not a suggestion — it's a hard limit that ESA inspectors enforce.

CEC Rule 8-102: The voltage drop in an installation from the supply side of the consumer's service to the point of utilization shall not exceed 5%.
For a 120V circuit, that's a maximum of 6V. For a 208V circuit, it's 10.4V. For a 347V lighting circuit, it's 17.35V.

The code further recommends that feeder voltage drop not exceed 3%, leaving the remaining 2% for branch circuits. This 3%+2% split is best practice, though the inspector technically only enforces the total 5%.

The Voltage Drop Formulas

For copper conductors in steel conduit (most common commercial installation):

Conductor Resistance Table (Copper, 75°C)

Worked Example: Parking Garage EV Panel

You're feeding a 100A, 208V, 3-phase EV charging panel located 150 metres from the electrical room. The engineer specifies #1 AWG copper in steel conduit.

Step 1: Calculate Voltage Drop

VD = √3 × 100A × 150m × 0.524 Ω/km / 1000
VD = 1.732 × 100 × 150 × 0.000524
VD = 13.6V

Step 2: Check Against 5% Limit

Maximum allowed VD = 208V × 5% = 10.4V
Calculated VD = 13.6V → FAILS (6.5%)

Step 3: Upsize the Conductor

Try 3/0 AWG copper:

VD = 1.732 × 100 × 150 × 0.000269 = 6.99V (3.4%) → PASSES ✓

Notice the gap: #1 AWG has plenty of ampacity for 100A (rated 130A), but fails on voltage drop. This is the classic trap — the wire is big enough to carry the current safely, but the voltage arriving at the load is too low for equipment to operate properly.

When Voltage Drop Matters Most

• Underground parking garages: Long horizontal runs from the electrical room to the far end of the garage
• Site lighting circuits: Parking lot poles 200+ metres from the panel on 347V circuits
• Motor loads: Pumps, fans, and compressors in mechanical penthouses or remote buildings
• Construction power: Temporary panels fed by long cable runs across job sites

Common Mistakes

• Sizing wire for ampacity only — the wire may carry the current safely but deliver insufficient voltage
• Using DC resistance instead of AC resistance — AC resistance in steel conduit is higher due to skin effect and conduit losses
• Measuring one-way instead of round-trip for single-phase — the "2×" factor in the formula accounts for the return path
• Not accounting for temperature derating — resistance increases with temperature; a hot conduit run has higher voltage drop