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What Is Voltage Drop?
Every conductor has resistance. When current flows through that resistance, some voltage is consumed — this is voltage drop. By Ohm's Law (V = I × R): if a #12 AWG copper wire in a 100-ft run has 0.2 Ω of round-trip resistance and carries 15A, the voltage drop is 15 × 0.2 = 3V lost — 2.5% on a 120V circuit.
The load at the end of the wire receives less voltage than the source provides. Equipment rated for 120V that receives 108V (10% drop) will malfunction, run hot, or fail prematurely.
The NEC 3% and 5% Rules
These limits appear as Informational Notes in NEC 210.19(A) and 215.2(A) — they are recommendations, not mandatory code requirements. However, they are the universally accepted engineering standard and are adopted as requirements by many local jurisdictions and engineering specifications.
Why 3%? Most sensitive equipment (motors, electronics, HVAC) is rated for ±10% voltage tolerance. A 3% branch circuit drop combined with 3% feeder drop = 6% — still within equipment tolerance with a small margin. Going above 5% combined risks being outside equipment ratings at voltage fluctuations.
Voltage Drop Formula
Worked Example — 20A, 120V, 75 ft run, #12 AWG copper
- K = 12.9 (copper) | I = 20A | L = 75 ft | CM = 6,530 (for #12 AWG)
- VD = 2 × 12.9 × 20 × 75 / 6,530 = 5.93V
- VD% = 5.93 / 120 × 100 = 4.94% — over the 3% recommendation
- Fix: use #10 AWG (CM = 10,380) → VD = 2 × 12.9 × 20 × 75 / 10,380 = 3.73V = 3.1% ✅
The Voltage Drop Calculator automates this for any AWG, material, length, and voltage.
Maximum Run Lengths Before 3% Drop — Quick Reference
| AWG | 15A @ 120V (ft) | 20A @ 120V (ft) | 30A @ 240V (ft) | 50A @ 240V (ft) |
|---|---|---|---|---|
| #14 Cu | 50 ft | — | — | — |
| #12 Cu | 80 ft | 60 ft | — | — |
| #10 Cu | 125 ft | 95 ft | 125 ft | — |
| #8 Cu | 200 ft | 150 ft | 200 ft | 120 ft |
| #6 Cu | 315 ft | 236 ft | 315 ft | 190 ft |
Approximate one-way maximum distances at 3% drop. Copper, 75°C. See the full Voltage Drop Reference Table for more combinations.
What Happens With Too Much Voltage Drop
- Motors: Run hotter, draw more current, fail earlier. A motor designed for 240V receiving 228V (5% drop) draws ~10% more current.
- HVAC: Compressors struggle at low voltage, causing hard starts and reduced capacity. Most manufacturers specify ±10% voltage tolerance.
- Lighting: Incandescent bulbs dim visibly at 5% drop. LEDs are more tolerant but may flicker or fail to regulate color temperature properly.
- Electronics: Switching power supplies compensate for voltage drop, but with reduced efficiency and more heat dissipation.
- EV chargers: Many EVSE units throttle charging rate at low voltage — a 5% voltage drop on a 240V circuit delivers 228V instead of 240V, potentially reducing charge rate.
Frequently Asked Questions
Voltage drop is voltage lost as current flows through a conductor's resistance (V = I × R). The wire's resistance consumes some voltage, so the load at the end receives less than the source voltage.
No — the 3% and 5% limits are informational notes (recommendations) in NEC 210.19 and 215.2, not mandatory code. However, they are standard engineering practice, and some local jurisdictions adopt them as requirements. Most design and inspection follows the 3/5 rule.
Single-phase: VD = (2 × K × I × L) / CM, where K=12.9 (copper), I=amps, L=one-way feet, CM=circular mils of conductor. Or simply: VD = I × R_roundtrip using NEC Table 9 resistance values.
Upsize the wire gauge (biggest impact), shorten the run, increase voltage (e.g. 240V instead of 120V has half the current for same power = half the drop), or reduce the load. Upsizing from #12 to #10 AWG copper reduces resistance by ~37%.
Yes. Aluminum has higher resistivity — K=21.2 vs K=12.9 for copper. The same AWG aluminum wire has ~64% higher resistance than copper, so it produces more voltage drop for the same current and run length. You typically need to upsize two AWG sizes in aluminum to match copper's voltage drop.