Voltage Drop Over Long Wire Runs NEC Calculator & Guide 2026
Voltage drop is the reduction in voltage as current flows through wire over distance. NEC 210.19 Informational Note No. 4 recommends keeping voltage drop under 3% for branch circuits and 5% for feeder + branch combined. While not a hard NEC requirement, exceeding these limits causes dimming lights, reduced motor efficiency, and potential equipment damage. For long wire runs, upsizing wire is the primary solution.
The Voltage Drop Formula
The standard voltage drop formula for single-phase circuits is: VD = (2 × L × I × K) / CMA where L = one-way length in feet, I = current in amps, K = conductor resistivity (10.37 for copper, 17.02 for aluminum), and CMA = wire cross-sectional area in circular mils. To get VD as a percentage: VD% = VD / V × 100. For example, a 20A load on 12 AWG copper (6,530 CMA) at 100 feet on a 120V circuit: VD = (2 × 100 × 20 × 10.37) / 6,530 = 6.35V → 6.35/120 × 100 = 5.3% (exceeds 3%). You would need to upsize to 10 AWG.
Maximum Distance Tables
These tables show the maximum one-way wire run at 3% voltage drop for common circuits. 120V, 20A: 14 AWG = 36 ft, 12 AWG = 57 ft, 10 AWG = 90 ft, 8 AWG = 143 ft. 240V, 20A: 14 AWG = 72 ft, 12 AWG = 114 ft, 10 AWG = 181 ft, 8 AWG = 288 ft. Notice that 240V circuits allow double the distance of 120V for the same percentage drop — this is why long runs should use 240V when possible.
When to Upsize Wire
Upsize when: the run exceeds the maximum distance for the standard wire size, equipment requires voltage within a tight range (motors, compressors, sensitive electronics), or you want to reduce energy waste from resistive losses. Rule of thumb: for every doubling of distance beyond the standard maximum, go up one wire gauge. For critical equipment like well pumps or air handlers at the end of long runs, consider upsizing two gauges for safety margin.
240V vs 120V for Long Runs
For the same power delivery, 240V draws half the current of 120V (P = V × I). Half the current means half the voltage drop in absolute terms, and when expressed as a percentage of 240V, it is one-quarter the drop percentage. This is why long-distance runs for workshops, outbuildings, and well pumps should always use 240V. Example: A 2,400W load at 120V = 20A. At 240V = 10A. The 240V circuit has 1/4 the VD% — dramatic savings on wire cost.
| Wire Size | 15A/120V | 20A/120V | 20A/240V | 50A/240V |
|---|---|---|---|---|
| 14 AWG | 48 ft | 36 ft | 72 ft | N/A |
| 12 AWG | 76 ft | 57 ft | 114 ft | N/A |
| 10 AWG | 121 ft | 90 ft | 181 ft | 72 ft |
| 8 AWG | 192 ft | 143 ft | 288 ft | 115 ft |
| 6 AWG | 305 ft | 228 ft | 457 ft | 183 ft |
| 4 AWG | 485 ft | 363 ft | 726 ft | 291 ft |
Practical Tips for Reducing Voltage Drop
1. Use 240V whenever possible for long runs. 2. Upsize wire — the next gauge up is the cheapest voltage drop solution. 3. Use copper instead of aluminum (38% lower resistivity). 4. Shorten the run — route wire more directly, even if it means more work. 5. Install a sub-panel at the remote location and run one large feeder instead of multiple long branch circuits. 6. Calculate before you pull wire — use our Voltage Drop Calculator to verify your design before purchasing materials.
Disclaimer: For educational reference only. Consult a licensed professional.