Voltage Drop Calculator
How voltage drop is calculated
Voltage drop is the reduction in voltage along a conductor caused by the wire's resistance. Longer runs and smaller gauges produce more drop. The NEC does not set a mandatory maximum, but 210.19(A) Informational Note No. 4 recommends limiting branch circuit drop to 3% and total drop (branch + feeder) to 5%.
The formula for single-phase circuits:
Where I is the load current in amps, R is the conductor resistance in ohms per 1000 ft from NEC Chapter 9 Table 8, and L is the one-way distance in feet. The factor of 2 accounts for the round trip (hot + neutral). For three-phase circuits, replace 2 with √3 (1.732).
An alternative formula using circular mils:
Where K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C), D is the one-way distance, and CM is the conductor area in circular mils from Table 8.
Maximum distance at 3% drop
Pre-computed maximum one-way run lengths for copper conductors at common circuit loads, keeping voltage drop at or below 3%. Use these as quick field references.
240V single-phase, copper
| Wire | 15A | 20A | 30A | 40A | 50A |
|---|---|---|---|---|---|
| 14 AWG | 76 ft | 57 ft | 38 ft | 28 ft | 22 ft |
| 12 AWG | 120 ft | 90 ft | 60 ft | 45 ft | 36 ft |
| 10 AWG | 193 ft | 144 ft | 96 ft | 72 ft | 58 ft |
| 8 AWG | 309 ft | 230 ft | 153 ft | 115 ft | 92 ft |
| 6 AWG | 491 ft | 365 ft | 243 ft | 182 ft | 146 ft |
| 4 AWG | 769 ft | 588 ft | 389 ft | 291 ft | 234 ft |
| 3 AWG | 967 ft | 731 ft | 491 ft | 365 ft | 294 ft |
| 2 AWG | 1250 ft | 937 ft | 625 ft | 461 ft | 370 ft |
| 1 AWG | 1578 ft | 1153 ft | 769 ft | 588 ft | 468 ft |
| 1/0 AWG | 2000 ft | 1500 ft | 1000 ft | 731 ft | 588 ft |
| 2/0 AWG | 2500 ft | 1875 ft | 1250 ft | 937 ft | 750 ft |
| 3/0 AWG | 3000 ft | 2307 ft | 1578 ft | 1153 ft | 937 ft |
| 4/0 AWG | 3750 ft | 3000 ft | 2000 ft | 1500 ft | 1200 ft |
120V single-phase, copper
| Wire | 15A | 20A | 30A |
|---|---|---|---|
| 14 AWG | 38 ft | 28 ft | 19 ft |
| 12 AWG | 60 ft | 45 ft | 30 ft |
| 10 AWG | 96 ft | 72 ft | 48 ft |
| 8 AWG | 153 ft | 115 ft | 77 ft |
| 6 AWG | 243 ft | 182 ft | 121 ft |
| 4 AWG | 389 ft | 291 ft | 194 ft |
| 3 AWG | 491 ft | 365 ft | 243 ft |
| 2 AWG | 625 ft | 461 ft | 309 ft |
Common scenarios
| Scenario | Wire | Load | Distance | Drop | Status |
|---|---|---|---|---|---|
| Detached garage subpanel | 6 AWG | 40A @ 240V | 75 ft | 1.2% (3.0V) | OK |
| Kitchen circuit | 12 AWG | 20A @ 120V | 45 ft | 3.0% (3.6V) | OK |
| EV charger (Level 2, 40A) | 8 AWG | 40A @ 240V | 50 ft | 1.3% (3.1V) | OK |
| 200A service entrance | 4/0 AWG | 195A @ 240V | 30 ft | 0.3% (0.7V) | OK |
| Workshop 30A circuit | 10 AWG | 30A @ 240V | 100 ft | 3.1% (7.4V) | Over 3% |
| Barn/outbuilding feed | 4 AWG | 60A @ 240V | 150 ft | 2.3% (5.5V) | OK |
When to worry about voltage drop
Voltage drop matters most on long runs and high-current circuits. A 50-foot kitchen circuit rarely has issues, but a 150-foot feed to a detached shop can easily exceed 3%. Watch for these situations:
Subpanel feeds over 100 feet - the most common voltage drop problem in residential work. Size the feeder one or two gauges above the minimum ampacity requirement.
EV charger circuits - continuous load (125% rule already applies) combined with garage-to-panel distances of 50-100+ feet. Check both ampacity and voltage drop.
Outbuildings, barns, and pole lights - distances of 200+ feet are common. Aluminum feeders are cost-effective here since the larger gauge needed for aluminum also helps reduce voltage drop.
Motor loads - motors are sensitive to low voltage. Reduced voltage causes higher current draw, increased heat, and shortened motor life. Keep total drop under 5% for motor circuits.
Frequently asked questions
How do I calculate voltage drop for a wire run?
For single-phase circuits, use the formula Vd = 2 × I × R × L / 1000 where I is the load in amps, R is conductor resistance in ohms per 1000 ft from NEC Chapter 9 Table 8, and L is the one-way distance in feet. The factor of 2 accounts for the complete circuit (hot + neutral).
For three-phase, replace 2 with 1.732 (√3). The resistance value depends on wire gauge and material - for example, 12 AWG copper has a resistance of 1.98 Ω/1000 ft. The calculator at the top of this page runs this formula automatically.
What is the maximum voltage drop allowed by the NEC?
The NEC does not set a mandatory maximum. It recommends limiting voltage drop to 3% on branch circuits and 5% for the total of branch circuit plus feeder combined. These appear in 210.19(A) Informational Note No. 4 and 215.2(A)(4) Informational Note No. 2.
While exceeding these limits is not technically a code violation, it can cause dimming lights, motor overheating, and tripped breakers under load. Most inspectors and engineers treat 3% as a practical limit.
How far can I run 12 gauge wire on a 20 amp circuit?
On a 120V single-phase circuit, approximately 45 feet one-way before exceeding 3% voltage drop. On a 240V circuit, approximately 91 feet. These assume copper THHN conductor.
If you need a longer run, upsize to 10 AWG copper which allows roughly 72 ft at 120V and 144 ft at 240V for the same 20A load. See the max distance tables above for all gauge and load combinations.
What wire size do I need for an EV charger to avoid voltage drop?
Most Level 2 home EV chargers draw 32-48A continuous on a 240V circuit. For a 40A charger, 8 AWG copper handles the ampacity but can only run about 37 ft before hitting 3% drop.
For garage runs of 50-75 ft, upsize to 6 AWG copper. For detached garages over 100 ft, consider 4 AWG or aluminum feeders. Remember the 125% continuous load rule applies - a 40A charger needs a 50A breaker. See our EV charger wiring guide for brand-specific recommendations.
Does aluminum wire have more voltage drop than copper?
Yes. Aluminum has approximately 1.6× the resistance of copper at the same gauge, producing proportionally more voltage drop. However, aluminum is typically installed at a larger gauge to compensate, which partially offsets the higher resistance.
For long feeder runs to subpanels and outbuildings, aluminum is often more cost-effective even at the larger gauge needed. See our copper vs aluminum comparison for a full breakdown of cost, ampacity, and termination differences.
What is the difference between the 3% and 5% voltage drop rules?
The 3% limit applies to individual branch circuits (wire from breaker to outlet or device). The 5% limit applies to the total path from service entrance through feeder and branch circuit combined.
If your feeder drops 2%, each branch circuit off that subpanel should drop no more than 3%, keeping the total at 5%. This matters most for detached buildings with long feeder runs where the feeder itself can consume a significant portion of the 5% budget. Use the calculator above to check both your feeder and branch circuits separately.
Can I use this calculator for DC wire sizing?
Yes. This voltage drop calculator works for DC circuits. Select single-phase mode and enter your DC voltage (12V, 24V, 48V). The formula is the same: Vd = 2 × I × R × L / 1000. DC circuits are common in solar PV, battery banks, RV/marine wiring, and low-voltage lighting.
DC runs tend to be especially sensitive to voltage drop because the source voltage is low. A 12V system loses 3% at just 0.36V of drop. Use the calculator above to check your DC wire size, and consider upsizing one or two gauges for runs over 15 feet at 12V.
What wire size for solar panel runs?
Solar PV wire sizing depends on the circuit voltage, current, and distance from panels to inverter. For string inverters, the DC voltage is typically 300-600V, so voltage drop is less of a concern and 10 AWG is common for residential arrays.
For microinverters or AC modules, the output is 240V AC and standard wire sizing applies. For battery-based systems at 12V or 24V DC, wire sizing is critical because low voltage means high current. A 2000W load at 12V draws 167A. Use the calculator above with your DC voltage and one-way distance to find the right gauge. NEC Article 690 governs PV system wiring.
NEC 2023 references verified April 2026