Voltage Drop
Single-phase and three-phase voltage drop formulas, lookup tables, and max distance charts.
5 reference items · 4 sections
Voltage Drop Formulas
Single-Phase Voltage Drop
VD = (2 × K × I × D) ÷ CM
Single-Phase Voltage Drop
| Variable | Description | Unit |
|---|---|---|
| VD | Voltage drop | Volts |
| K | Resistivity constant | 12.9 (Cu) / 21.2 (Al) |
| I | Load current | Amperes |
| D | One-way distance | Feet |
| CM | Wire area | Circular mils |
Variations:Find CM: CM = (2×K×I×D) ÷ VD% VD: %VD = (VD ÷ V_source) × 100
K = 12.9 for copper at 75°C. K = 21.2 for aluminum at 75°C. The factor of 2 accounts for the round-trip distance.
Three-Phase Voltage Drop
VD = (1.732 × K × I × D) ÷ CM
Three-Phase Voltage Drop
| Variable | Description | Unit |
|---|---|---|
| VD | Voltage drop | Volts |
| 1.732 | √3 | — |
| K | Resistivity constant | 12.9 (Cu) / 21.2 (Al) |
| I | Line current | Amperes |
| D | One-way distance | Feet |
| CM | Wire area | Circular mils |
Variations:Find CM: CM = (1.732×K×I×D) ÷ VD
Uses 1.732 instead of 2 for three-phase circuits.
Single-Phase Voltage Drop Tables
Voltage Drop — 120V Copper (per 100 ft, per amp)
NEC 210.19(A) Info Note 4
Voltage Drop per Amp per 100 ft — 120V 1Φ Copper
| AWG | VD/A/100ft | Max Amps @3% | Max Amps @5% |
|---|---|---|---|
| 14 | 0.63 | 5.7 | 9.5 |
| 12 | 0.40 | 9.0 | 15.0 |
| 10 | 0.25 | 14.4 | 24.0 |
| 8 | 0.16 | 22.5 | 37.5 |
| 6 | 0.10 | 36.0 | 60.0 |
| 4 | 0.06 | 60.0 | 100.0 |
| 2 | 0.04 | 90.0 | 150.0 |
| 1/0 | 0.02 | 180.0 | 300.0 |
1.NEC recommends max 3% VD for branch circuits
2.NEC recommends max 5% total VD (feeder + branch)
3.Values based on K = 12.9 for copper at 75°C
NEC 210.19(A) Informational Note No. 4: Branch circuits should not exceed 3% voltage drop. The total of feeder and branch circuit drops should not exceed 5%.
Three-Phase Voltage Drop Tables
Voltage Drop — 480V 3Φ Copper (per 100 ft, per amp)
NEC 215.2(A)(4) Info Note 2
Voltage Drop per Amp per 100 ft — 480V 3Φ Copper
| AWG | VD/A/100ft | Max Amps @3% | Max Amps @5% |
|---|---|---|---|
| 14 | 0.54 | 26.5 | 44.1 |
| 12 | 0.34 | 42.1 | 70.1 |
| 10 | 0.22 | 66.9 | 111.5 |
| 8 | 0.14 | 106.4 | 177.4 |
| 6 | 0.085 | 169.1 | 281.8 |
| 4 | 0.054 | 269.0 | 448.3 |
| 2 | 0.034 | 427.6 | 712.7 |
| 1/0 | 0.021 | 680.2 | 1133.7 |
1.VD/A/100ft = (1.732 × K × 100) ÷ CM
2.Values based on K = 12.9 for copper at 75°C
3.3% of 480V = 14.4V; 5% of 480V = 24V
Quick Lookup: Max Distance at 3% Drop
Max One-Way Distance at 3% VD — 120V
NEC 210.19(A) Info Note 4
Max Distance (ft) at 3% Voltage Drop — 120V 1Φ Copper
| AWG | 15A | 20A | 30A | 40A | 50A |
|---|---|---|---|---|---|
| 14 | 38 | 29 | 19 | — | — |
| 12 | 61 | 46 | 30 | 23 | — |
| 10 | 97 | 72 | 48 | 36 | 29 |
| 8 | 154 | 115 | 77 | 58 | 46 |
| 6 | 244 | 183 | 122 | 92 | 73 |
| 4 | 388 | 291 | 194 | 146 | 116 |
| 2 | 617 | 463 | 309 | 231 | 185 |
1.D = (VD × CM) ÷ (2 × K × I) where VD = 3.6V (3% of 120V)
2.— = wire not rated for this amperage
3.Based on copper conductors, K = 12.9 at 75°C
4.For 240V, multiply distances by 2
Formula: Max distance (ft) = (3.6 × CM) ÷ (2 × 12.9 × I) = (3.6 × CM) ÷ (25.8 × I)
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