Voltage Drop Calculator
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Results
6.34 V drop (5.28%) on 100 ft of 12 AWG copper at 16 A, 120 V
| Size | Area | Ω/1,000 ft | Drop | Drop % | ≤ 3% |
|---|---|---|---|---|---|
| 18 AWG | 0.82 mm² | 7.95 | 25.44 V | 21.2% | No |
| 16 AWG | 1.31 mm² | 4.99 | 15.97 V | 13.31% | No |
| 14 AWG | 2.08 mm² | 3.14 | 10.05 V | 8.37% | No |
| 12 AWG | 3.31 mm² | 1.98 | 6.34 V | 5.28% | No |
| 10 AWG | 5.26 mm² | 1.24 | 3.97 V | 3.31% | No |
| 8 AWG | 8.37 mm² | 0.778 | 2.49 V | 2.07% | Yes |
| 6 AWG | 13.3 mm² | 0.491 | 1.57 V | 1.31% | Yes |
| 4 AWG | 21.2 mm² | 0.308 | 0.99 V | 0.82% | Yes |
| 3 AWG | 26.7 mm² | 0.245 | 0.78 V | 0.65% | Yes |
| 2 AWG | 33.6 mm² | 0.194 | 0.62 V | 0.52% | Yes |
The drop is over your 3% target. Use 8 AWG or larger, or shorten the run.
Informational notes in NEC Articles 210 and 215 suggest no more than 3% drop on a branch circuit or feeder and 5% in total to the farthest outlet. They are recommendations unless your local code adopts them.
This checks voltage drop only. The wire must also be rated for the current (ampacity) and the breaker, and installed to your local electrical code. Have a licensed electrician design and inspect real installations.
How to use the voltage drop calculator
- Choose copper or aluminum and the wire size, in AWG and kcmil or in metric mm².
- Pick the system: single-phase AC, three-phase AC or DC, and enter the source voltage and the load current in amps.
- Enter the one-way length from the panel or source to the load. The calculator accounts for the return path itself.
- Set your target maximum drop, often 3% for a branch circuit, to see the smallest size that meets it and the longest run for the size you chose.
Voltage drop formula
Three-phase AC: VD = √3 × L × R × I
Percentage drop = VD ÷ source voltage × 100 · Voltage at load = source − VD
L = one-way length · R = conductor resistance per unit length · I = current (A)
Current flows out on one conductor and back on another, so a single-phase or DC circuit has twice the one-way length of wire in the loop. In a balanced three-phase circuit the factor is √3 (about 1.732) for the line-to-line voltage. The resistance values for AWG and kcmil sizes are the DC resistances of stranded, uncoated copper and of aluminum at 75 °C from NEC Chapter 9, Table 8, as reproduced in Michigan State University's Electrical Tech Note 212. Metric sizes are not in Table 8, so their resistance is calculated from the nominal area and the 75 °C resistivity given in the same note (0.0214 Ω·mm²/m for copper, 0.0352 for aluminum). Using DC resistance is the usual simplification; for large AC conductors, the impedance in Table 9 is more exact.
Worked example: a 120 V circuit 100 ft long
A 16 A load sits 100 ft from the panel on 12 AWG copper, which has 1.98 Ω per 1,000 ft. One conductor over 100 ft is 0.198 Ω, so the drop is 2 × 0.198 × 16 = 6.34 V, or 5.28% of 120 V, leaving 113.7 V at the load. Moving up to 10 AWG gives 3.97 V (3.31%), still over 3%; the smallest size that meets 3% is 8 AWG, at 2.07%. The same 12 AWG run on a 240 V circuit drops the same 6.34 V, but that is only 2.64% of the higher voltage. For a three-phase example, 50 A on 200 ft of 4 AWG copper at 480 V drops 5.33 V, 1.11%.
Longest run for a 3% drop at 120 V
One-way length in feet, copper, single-phase. At 240 V the distances double. These limits are for voltage drop only; check the wire's ampacity and breaker size separately.
| Wire size | 10 A load | 15 A load | 20 A load | 30 A load |
|---|---|---|---|---|
| 14 AWG | 57 ft | 38 ft | 29 ft | 19 ft |
| 12 AWG | 91 ft | 61 ft | 45 ft | 30 ft |
| 10 AWG | 145 ft | 97 ft | 73 ft | 48 ft |
| 8 AWG | 231 ft | 154 ft | 116 ft | 77 ft |
| 6 AWG | 367 ft | 244 ft | 183 ft | 122 ft |
| 4 AWG | 584 ft | 390 ft | 292 ft | 195 ft |
Conductor resistance at 75 °C (NEC Chapter 9, Table 8)
| Size | Area | Copper, Ω/1,000 ft | Aluminum, Ω/1,000 ft |
|---|---|---|---|
| 18 AWG | 0.82 mm² | 7.95 | 13.1 |
| 16 AWG | 1.31 mm² | 4.99 | 8.21 |
| 14 AWG | 2.08 mm² | 3.14 | 5.17 |
| 12 AWG | 3.31 mm² | 1.98 | 3.25 |
| 10 AWG | 5.26 mm² | 1.24 | 2.04 |
| 8 AWG | 8.37 mm² | 0.778 | 1.28 |
| 6 AWG | 13.3 mm² | 0.491 | 0.808 |
| 4 AWG | 21.2 mm² | 0.308 | 0.508 |
| 3 AWG | 26.7 mm² | 0.245 | 0.403 |
| 2 AWG | 33.6 mm² | 0.194 | 0.319 |
| 1 AWG | 42.4 mm² | 0.154 | 0.253 |
| 1/0 AWG | 53.5 mm² | 0.122 | 0.201 |
| 2/0 AWG | 67.4 mm² | 0.0967 | 0.159 |
| 3/0 AWG | 85 mm² | 0.0766 | 0.126 |
| 4/0 AWG | 107 mm² | 0.0608 | 0.1 |
| 250 kcmil | 127 mm² | 0.0515 | 0.0847 |
| 300 kcmil | 152 mm² | 0.0429 | 0.0707 |
| 350 kcmil | 177 mm² | 0.0367 | 0.0605 |
| 400 kcmil | 203 mm² | 0.0321 | 0.0529 |
| 500 kcmil | 253 mm² | 0.0258 | 0.0424 |
What voltage drop is acceptable?
Informational notes in NEC Articles 210 (branch circuits) and 215 (feeders) suggest sizing conductors so the drop is no more than 3% on a branch circuit or feeder and no more than 5% in total from the service to the farthest outlet. Informational notes are advice rather than enforceable rules, but some jurisdictions and equipment makers set their own limits. Motors, pumps and compressors in particular can overheat or fail to start on low voltage.
- Measure the real route. Cable runs up walls, across ceilings and around obstacles; the straight-line distance is always shorter.
- Use the actual load current, or the breaker rating for a worst case.
- Low-voltage DC is unforgiving. A 0.7 V drop is trivial at 120 V but nearly 6% of a 12 V system, so solar, RV and landscape-lighting runs need larger wire.
- Safety first. Electrical work must follow your local code and be done by a qualified person where required. Use a licensed electrician for real installations.
Related tools: the Ohm's law calculator for voltage, current, resistance and power, and the electricity cost calculator for what the load costs to run.
Frequently asked questions
How do you calculate voltage drop?
For single-phase AC or DC, multiply 2 × one-way length × resistance per unit length × current. A 16 A load 100 ft away on 12 AWG copper (1.98 Ω per 1,000 ft) drops 2 × 0.198 × 16 ≈ 6.34 V, which is 5.28% of 120 V. For three-phase, use √3 instead of 2.
What is the maximum acceptable voltage drop?
Informational notes in the National Electrical Code suggest no more than 3% on a branch circuit or feeder and 5% in total to the farthest outlet. They are recommendations, not enforceable rules, unless your local code or the equipment maker requires them.
What size wire do I need for 20 A at 100 ft on 120 V?
To stay within 3%, the wire needs no more than 0.9 Ω per 1,000 ft, which means 8 AWG copper (0.778 Ω): it drops about 3.11 V, or 2.59%. 10 AWG would drop about 4.13%. The wire must also meet the ampacity and breaker rules, so confirm the size with an electrician.
How does aluminum compare with copper?
Aluminum has about 1.64 times the resistance of copper of the same size (12 AWG: 3.25 versus 1.98 Ω per 1,000 ft at 75 °C), so it drops about 64% more voltage. Going up roughly two AWG sizes in aluminum gives about the same drop as copper: 10 AWG aluminum is 2.04 Ω per 1,000 ft.
Why does three-phase use √3 instead of 2?
In a balanced three-phase circuit the currents in the three lines cancel at the neutral point, so there is no return current to double the length. The drop measured between two lines is √3 (about 1.732) times the drop in one conductor, which is why three-phase feeders lose less voltage for the same wire.