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 voltage drop shall not exceed 3% in a feeder or branch circuit and 5% total from the supply side of the service to the point of utilization. This is not a suggestion - it is a code limit that ESA inspectors can enforce.

CEC Rule 8-102: Voltage drop shall not exceed 3% in a feeder or branch circuit and shall not exceed 5% from the supply side of the consumer's service to the point of utilization.
At the 5% overall limit, that's a maximum of 6V for a 120V circuit, 10.4V for 208V and 17.35V for 347V; a single feeder or branch circuit is limited to 3% (3.6V, 6.24V and 10.41V).

The common 3% feeder / 2% branch split is a practical design target, but the code language itself limits each feeder or branch circuit to 3% and the overall installation to 5%.

The Voltage Drop Formulas

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

Conductor Resistance Table (Copper, 75°C)

The resistance values are approximate AC resistances for the resistance-only formulas above; reactance is neglected.

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 the 3% and 5% Limits

Maximum allowed VD = 208V × 3% = 6.24V (feeder); 208V × 5% = 10.4V (overall)
Calculated VD = 13.6V - FAILS (6.5%)

Step 3: Upsize the Conductor

Try 4/0 AWG copper:

VD = 1.732 × 100 × 150 × 0.000217 = 5.64V (2.7%) - PASSES the 3% feeder limit

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
• Forgetting the return path for single-phase - use the one-way length L; 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

Frequently Asked Questions