Voltage Drop Calculator
Calculate voltage drop and percentage over a wire run for a given load and gauge
Results
Step-by-Step Calculation
Understanding Voltage Drop
Voltage drop is the reduction in voltage as electrical current travels through a conductor. Every wire has resistance, and longer runs or higher currents cause more of the source voltage to be lost as heat before it reaches the load. Excessive voltage drop can cause lights to dim, motors to overheat, and electronics to malfunction.
Circular-Mils Formula (Single-Phase):
- Voltage Drop: VD = (2 x K x I x L) / CM
- Drop Percentage: % = (VD / Source Voltage) x 100
- Voltage at Load: Source Voltage - VD
Where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way wire length in feet, and CM is the conductor area in circular mils. The factor of 2 accounts for the round-trip distance the current travels.
Rule of Thumb: The National Electrical Code recommends keeping voltage drop at or below 3% for branch circuits and 5% for the combined feeder and branch circuit.
Wire Gauge Reference (AWG)
| AWG Size | Circular Mils | Typical Use |
|---|---|---|
| 14 AWG | 4,110 | 15 A lighting circuits |
| 12 AWG | 6,530 | 20 A general outlets |
| 10 AWG | 10,380 | 30 A dryers, water heaters |
| 8 AWG | 16,510 | 40 A ranges, subpanels |
| 6 AWG | 26,240 | 55 A feeders, AC units |
| 4 AWG | 41,740 | 70 A subpanels |
| 2 AWG | 66,360 | 95 A service feeders |
Tip: If your calculated voltage drop exceeds 3%, choose a larger wire gauge (more circular mils) to reduce resistance and bring the drop within acceptable limits.
How to Reduce Voltage Drop
1. Increase Wire Size
A larger conductor has more circular mils and less resistance. Going up one or two gauge sizes is the most common way to bring voltage drop within limits on long runs.
2. Shorten the Run
Voltage drop is directly proportional to length. Routing the circuit more directly or relocating a subpanel closer to the load can dramatically reduce losses.
3. Use Copper Instead of Aluminum
Copper has a lower resistivity constant (12.9 vs. 21.2), so for the same gauge a copper conductor produces about 40% less voltage drop than aluminum.
4. Raise the System Voltage
At higher voltages the same power is delivered at lower current, which reduces both voltage drop and the percentage of voltage lost. This is why long runs often use 240 V instead of 120 V.
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Note: This calculator provides estimates for single-phase circuits using the simplified circular-mils method and does not account for conductor temperature, power factor, or conduit fill. For critical installations, consult a licensed electrician and your local electrical code.