Voltage drop is the silent killer of electrical systems. Too much drop means motors that won't start, lights that flicker, and sensitive electronics that malfunction. The National Electrical Code (NEC) recommends keeping voltage drop within strict limits — and getting it right starts with understanding the formula.
This guide covers the voltage drop formula, NEC recommendations, copper wire sizing, and a step-by-step approach to selecting the right conductor.
What Is Voltage Drop?
Voltage drop is the reduction in voltage as electrical current flows through a conductor. Every wire has resistance, and that resistance converts some electrical energy into heat. The longer the wire and the smaller the gauge, the more voltage you lose.
NEC Article 210.19(A) Informational Note No. 4 recommends that the total voltage drop from the service entrance to the farthest outlet should not exceed 5% — with no more than 3% on the branch circuit and 3% on the feeder.
The Voltage Drop Formula
For single-phase circuits:
Vd = (2 × L × I × R) / 1000
For three-phase circuits:
Vd = (1.732 × L × I × R) / 1000
Where:
- Vd = voltage drop (volts)
- L = one-way length of the circuit (feet)
- I = current in amperes
- R = resistance of the conductor (ohms per 1000 ft) from NEC Chapter 9, Table 8
The percentage voltage drop is:
Vd% = (Vd / Vs) × 100
Where Vs is the source voltage (120V, 208V, 240V, 277V, or 480V).
NEC Copper Wire Resistance Table
Resistance values for copper conductors at 75°C (NEC Chapter 9, Table 8):
- 14 AWG — 3.14 Ω/1000 ft
- 12 AWG — 1.98 Ω/1000 ft
- 10 AWG — 1.24 Ω/1000 ft
- 8 AWG — 0.778 Ω/1000 ft
- 6 AWG — 0.491 Ω/1000 ft
- 4 AWG — 0.308 Ω/1000 ft
- 3 AWG — 0.245 Ω/1000 ft
- 2 AWG — 0.194 Ω/1000 ft
- 1 AWG — 0.154 Ω/1000 ft
- 1/0 AWG — 0.122 Ω/1000 ft
- 2/0 AWG — 0.0967 Ω/1000 ft
- 4/0 AWG — 0.0608 Ω/1000 ft
Key Factors Affecting Voltage Drop
Wire Gauge (AWG)
Larger wire = lower resistance = less voltage drop. Going from 12 AWG to 10 AWG cuts resistance by 37%. The cost of upsizing wire is almost always cheaper than the operational cost of excessive voltage drop.
Circuit Length
Voltage drop increases linearly with distance. A 200-foot run has exactly double the drop of a 100-foot run at the same current and wire size. Long runs to outbuildings, well pumps, or detached garages are the most common problem areas.
Load Current
Higher current means proportionally more voltage drop. A 20A circuit on 12 AWG wire drops 3.96V per 100 feet — already 3.3% on a 120V circuit. At 15A, the same wire drops only 2.97V (2.5%).
Step-by-Step Voltage Drop Calculation
Example: 120V single-phase, 20A load, 150-foot run on 12 AWG copper
Step 1: Look up resistance — 12 AWG copper = 1.98 Ω/1000 ft
Step 2: Apply the formula:
Vd = (2 × 150 × 20 × 1.98) / 1000 = 11.88V
Step 3: Calculate percentage:
Vd% = (11.88 / 120) × 100 = 9.9% ❌ Way over the 3% limit!
Step 4: Upsize the wire. Try 6 AWG (0.491 Ω/1000 ft):
Vd = (2 × 150 × 20 × 0.491) / 1000 = 2.95V (2.46%) ✅
Or skip the manual math and use the free Voltage Drop Calculator to find the right wire size instantly.
Common Mistakes to Avoid
- Ignoring temperature derating — wire resistance increases with temperature. Conductors in hot attics or conduit in direct sunlight may need further upsizing.
- Forgetting the return path — the "2×" in the formula accounts for the round-trip. Don't calculate one-way only.
- Using ampacity tables for voltage drop — NEC ampacity tables (Table 310.16) size wire for heat, not voltage drop. You need BOTH checks.
- Not checking at full load — calculate at the maximum expected load, not the breaker rating.
Calculate Voltage Drop Online
Enter your voltage, current, wire gauge, and circuit length — get instant voltage drop results with NEC compliance check.
→ Open the Voltage Drop Calculator
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