How to Calculate Voltage Drop: Complete Step-by-Step Guide
Master the voltage drop formula and learn when wire sizing matters for safe, efficient electrical installations.
Table of Contents
1. What is Voltage Drop?
Voltage drop is the reduction in electrical voltage that occurs as current flows through a wire. Every conductor has some resistance, and according to Ohm's Law (V = I × R), this resistance causes a voltage loss proportional to the current flowing through it.
Simple analogy: Think of voltage like water pressure in a pipe. A longer pipe or narrower pipe reduces the pressure at the end. Similarly, longer or thinner wires reduce voltage at the load.
If your source voltage is 120V and you lose 6V in the wire, your load only receives 114V. This 6V (or 5%) loss can cause problems with equipment operation.
2. Why Voltage Drop Matters
Excessive voltage drop causes several problems:
- Equipment malfunction: Motors may overheat, stall, or fail to start
- Reduced performance: Lights dim, heaters produce less heat
- Energy waste: Lost voltage converts to heat in the wire
- Safety hazards: Overheated wires can cause fires
- Shortened equipment life: Motors running at low voltage wear faster
Key point: Voltage drop is especially critical in low-voltage systems (12V, 24V) where even small losses represent a large percentage.
3. The Voltage Drop Formula
For Single-Phase and DC Circuits:
Where:
- Vd = Voltage drop (volts)
- I = Current (amperes)
- L = One-way length of wire (feet)
- R = Wire resistance (ohms per 1000 feet)
- 2 = Multiplier for round-trip (hot and neutral/return)
For Three-Phase Circuits:
The √3 (approximately 1.732) replaces the factor of 2 because three-phase systems have a different current return path.
Voltage Drop Percentage:
4. Step-by-Step Calculation
Step 1: Gather Your Information
- System voltage (e.g., 120V, 240V, 12V)
- Load current in amperes
- One-way wire length in feet
- Wire gauge (AWG) and material (copper/aluminum)
Step 2: Find Wire Resistance
Look up the resistance per 1000 feet for your wire gauge:
| AWG | Copper (Ω/1000ft) | Aluminum (Ω/1000ft) |
|---|---|---|
| 14 | 2.525 | 4.14 |
| 12 | 1.588 | 2.61 |
| 10 | 0.999 | 1.64 |
| 8 | 0.628 | 1.03 |
| 6 | 0.395 | 0.648 |
Step 3: Calculate Voltage Drop
Plug your values into the formula and solve.
Step 4: Check Against NEC Limits
- Branch circuits: Maximum 3% recommended
- Feeder + branch combined: Maximum 5% recommended
5. Real-World Examples
Example 1: 12V LED Lighting Run
Given: 12V system, 5A load, 50 feet one-way, 14 AWG copper
Calculation:
Vd% = (1.26 / 12) × 100 = 10.5%
Result: 10.5% drop is TOO HIGH. Need larger wire (10 AWG or bigger).
Example 2: 240V EV Charger
Given: 240V, 40A load, 75 feet one-way, 8 AWG copper
Calculation:
Vd% = (3.77 / 240) × 100 = 1.57%
Result: 1.57% is within NEC 3% limit. This wire size is acceptable.
6. NEC Voltage Drop Requirements
The National Electrical Code (NEC) provides voltage drop recommendations in Articles 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2:
- Branch Circuits: Voltage drop should not exceed 3% for reasonable efficiency of operation.
- Feeders: Voltage drop should not exceed 3%.
- Combined (Feeder + Branch): Total voltage drop should not exceed 5%.
Note: These are recommendations, not requirements. However, many jurisdictions and engineers treat them as requirements for quality installations.
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