Learn About Voltage Drop
Understanding voltage drop calculations is essential for electrical installations
What is Voltage Drop?
Voltage drop is the reduction in voltage as electrical current flows through a conductor. Every conductor has resistance, and according to Ohm's Law (V = I × R), when current flows through this resistance, a voltage drop occurs.
This means the voltage available at the load (equipment) is less than the voltage at the source. Excessive voltage drop can cause:
- Motors to run hot and inefficiently
- Lights to dim
- Electronic equipment to malfunction
- Increased energy consumption and costs
CEC Rule 8-102 Requirements
3%
Maximum
Maximum voltage drop for any feeder or branch circuit
5%
Maximum Total
Maximum total voltage drop from service to point of utilization
The voltage drop calculation should be based on the connected load if known, otherwise 80% of the overcurrent device rating (per CEC Rule 8-102).
Voltage Drop Formulas
Vd = 2 × I × L × R / 1000
Factor of 2 accounts for both conductors (line and neutral)
Vd = √3 × I × L × R / 1000
√3 ≈ 1.732 is the phase factor for balanced loads
Where:
- Vd = Voltage drop in volts
- I = Current in amperes
- L = One-way conductor length in meters
- R = Conductor resistance in ohms per kilometer
Copper vs Aluminum Conductors
Copper (Cu)
- ✓ Lower resistance (better conductivity)
- ✓ Smaller size for same ampacity
- ✓ More durable connections
- ✗ Higher cost
- ✗ Heavier weight
Aluminum (Al)
- ✓ Lower cost
- ✓ Lighter weight
- ✗ Higher resistance (~1.6× copper)
- ✗ Requires larger sizes
- ✗ Special termination requirements
Tip: Aluminum typically requires about 2 wire sizes larger than copper for the same current-carrying capacity.
Temperature Rating Effects
| Rating | Insulation Types | Resistance | Ampacity |
|---|---|---|---|
| 60°C | TW, UF | Lower | Lower |
| 75°C | THWN, RW75 | Medium | Medium |
| 90°C | THHN, RW90 | Higher | Higher |
Higher temperature ratings allow more current but also have higher resistance, resulting in more voltage drop. Most terminations are rated for 75°C.
Conductor Reference Data (CEC Tables)
| Size | Cu Resistance (75°C Ω/km) | Cu Ampacity (75°C) | Al Resistance (75°C Ω/km) | Al Ampacity (75°C) |
|---|---|---|---|---|
| #14 AWG | 10.7 | 20A | 17.4 | N/A |
| #12 AWG | 6.73 | 25A | 11 | 20A |
| #10 AWG | 4.23 | 35A | 6.93 | 30A |
| #8 AWG | 2.66 | 50A | 4.37 | 40A |
| #6 AWG | 1.67 | 65A | 2.75 | 50A |
| #4 AWG | 1.05 | 85A | 1.73 | 65A |
| #3 AWG | 0.833 | 100A | 1.37 | 75A |
| #2 AWG | 0.661 | 115A | 1.09 | 90A |
| #1 AWG | 0.524 | 130A | 0.863 | 100A |
| #1/0 AWG | 0.416 | 150A | 0.685 | 120A |
| #2/0 AWG | 0.33 | 175A | 0.543 | 135A |
| #3/0 AWG | 0.261 | 200A | 0.431 | 155A |
Values based on CEC Tables D3 (resistance) and Tables 2/4 (ampacity for 1-3 conductors in raceway). Aluminum #14 AWG is not permitted per CEC Rule 4-002.