The Formula
Voltage drop is Ohm's law applied to the wire itself: the current times the resistance of the conductors.
- Single-phase: VD = 2 × L × R × I ÷ 1,000
- Three-phase: VD = 1.732 × L × R × I ÷ 1,000
L is the one-way length in feet, I is the load in amps, and R is the resistance in ohms per 1,000 feet from Chapter 9, Table 8 (stranded, uncoated copper or aluminum at 75°C). Percent drop = VD ÷ source voltage × 100. Table 8 is DC resistance, which is accurate for the small and mid-size conductors used on most exams. For large AC feeders, Table 9 effective impedance is more precise.
Worked Example
A 120 V single-phase branch circuit carries 20 A to a load 100 ft away on 12 AWG copper. Table 8 lists 1.98 Ω/kft.
VD = 2 × 100 × 1.98 × 20 ÷ 1,000 = 7.92 V, which is 7.92 ÷ 120 = 6.6%. That is over the 3% recommendation. 10 AWG (1.24 Ω/kft) gives 4.96 V, 4.1%. 8 AWG (0.778) gives 3.11 V, 2.6%, so 8 AWG is the smallest copper size under 3%.
Three-phase check: 480 V, 100 A, 250 ft on 1/0 copper (0.122 Ω/kft). VD = 1.732 × 250 × 0.122 × 100 ÷ 1,000 = 5.28 V, or 1.1%.
Table 8 Resistance Used
| Size | Copper Ω/kft | Aluminum Ω/kft |
|---|---|---|
| 14 AWG | 3.14 | 5.17 |
| 12 AWG | 1.98 | 3.25 |
| 10 AWG | 1.24 | 2.04 |
| 8 AWG | 0.778 | 1.28 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
| 3 AWG | 0.245 | 0.403 |
| 2 AWG | 0.194 | 0.319 |
| 1 AWG | 0.154 | 0.253 |
| 1/0 | 0.122 | 0.201 |
| 2/0 | 0.0967 | 0.159 |
| 3/0 | 0.0766 | 0.126 |
| 4/0 | 0.0608 | 0.100 |
| 250 kcmil | 0.0515 | 0.0847 |
| 300 kcmil | 0.0429 | 0.0707 |
| 350 kcmil | 0.0367 | 0.0605 |
| 400 kcmil | 0.0321 | 0.0529 |
| 500 kcmil | 0.0258 | 0.0424 |
| 600 kcmil | 0.0214 | 0.0353 |
| 750 kcmil | 0.0171 | 0.0282 |
| 1000 kcmil | 0.0129 | 0.0212 |
Limits
The calculator checks voltage drop only. A wire that passes on voltage drop must still have enough ampacity under 310.16 and the terminal rules in 110.14(C). It assumes a balanced load and a 75°C conductor; a cooler wire has slightly less resistance and less drop.
Frequently Asked Questions
What is the maximum voltage drop allowed by the NEC?
Informational notes in 210.19 and 215.2 recommend no more than 3% on a branch circuit or feeder and 5% total to the farthest outlet. They are recommendations, not enforceable rules, unless a specific article or your local code makes them mandatory.
Why is the single-phase formula 2 x L?
Current flows out on one conductor and back on the other, so the total wire length is twice the one-way distance. On a balanced three-phase circuit the factor is the square root of 3, about 1.732, instead of 2.
Which K value should I use on an exam?
Many exam prep books use K = 12.9 for copper and 21.2 for aluminum with VD = 2KLI/CM. Using the exact Table 8 resistance, as this calculator does, gives nearly the same answer and is what the code table actually lists.
Sources
- NFPA 70, National Electrical Code, Chapter 9 Table 8 and informational notes to 210.19 and 215.2.
Practice code questions on our Journeyman Electrician Practice Test (NEC 2023), or size a device box with the Box Fill Calculator.