Voltage Drop Calculator — Free Online Calculator
Calculate voltage drop across any wire run. Enter wire gauge, distance, load, and voltage to find exact voltage loss and percentage.
How to Use This Calculator
Select the wire gauge, enter the load current in amps, the one-way wire run distance in feet, and the source voltage. Choose copper or aluminum and single or three-phase. The calculator computes the exact voltage drop across the wire run and tells you whether it meets NEC recommendations.
The Formula Explained
Voltage drop is calculated using the circular mil method: VD = (K × I × D × 2) / CM, where K is the resistivity constant (10.37 for copper at 75°C, 17.02 for aluminum), I is the current in amps, D is the one-way distance in feet, and CM is the circular mil area of the conductor. For three-phase circuits, the factor 2 is replaced by √3 (1.732). The result tells you how many volts are lost in the wire, what percentage of the source voltage that represents, and the actual voltage delivered to the load.
Why Voltage Drop Matters More Than You Think
Voltage drop is invisible until it isn't. A 5% drop on a 240V circuit means your equipment sees 228V instead of 240V — a difference small enough that nothing obvious fails, but large enough that motors run hotter, incandescent bulbs dim noticeably, and resistive heating elements deliver 10% less heat (power scales with voltage squared). Over years, those small inefficiencies add up to shortened equipment life, higher electricity bills, and intermittent problems that defy diagnosis because nothing is truly broken.
The NEC treats voltage drop as a recommendation rather than a strict requirement for most installations. The language in 210.19 and 215.2 is "should not exceed" rather than "shall not exceed." This matters legally — an inspector can't fail a job strictly for voltage drop unless it's for a specific installation type that has a hard limit (fire pumps per NEC 695.7 require 15% max during starting, 5% running; sensitive electronic loads have their own rules). But professionally, the 3% branch / 5% total rule is treated as mandatory because exceeding it causes real problems that eventually come back as warranty calls.
Worked Example: Outdoor Lighting at 200 Feet
Low-voltage outdoor landscape lighting, 300 watts total, 12V transformer output. The 12V system is brutally sensitive to voltage drop because the absolute drop is large relative to the source voltage. At 300 watts and 12V, the current is 25 amps. Using 12 AWG copper cable (typical landscape wire), the formula gives: Vdrop = (2 × 10.37 × 25 × 200) / 6,530 = 103,700 / 6,530 = 15.9 volts. That's catastrophic — you'd deliver negative voltage, meaning the lights simply wouldn't work at the far end.
The fix is either much larger wire (8 AWG at 15.9 / (16,510/6,530) = 6.3V drop, still too much), splitting the run into multiple shorter home runs, or using a higher voltage system (24V or 120V line voltage). This is why modern landscape lighting often uses 120V LED fixtures with individual drivers — the voltage drop math simply doesn't work at long distances with low voltage.
Worked Example: 50-Amp RV Hookup at 100 Feet
A 50-amp RV outlet is actually two 50A 120V legs sharing a neutral — it's a 240V split-phase circuit. The RV can draw up to 50A on each leg. At 100 feet using 6 AWG copper: Vdrop = (2 × 10.37 × 50 × 100) / 26,240 = 103,700 / 26,240 = 3.95V, which is 1.65% of 240V line-to-line or 3.3% on either 120V leg. The 120V leg drop is the one that matters — most RV appliances are 120V internally. At 3.3%, you're right at the NEC recommended limit.
If the RV parks even farther — say 150 feet — the drop on the 120V leg climbs to 5%, exceeding the recommendation. The fix is upsize to 4 AWG copper, which cuts the drop to about 3.1% at 150 feet. Many professional RV pad installs use 4 AWG as the default for runs over 75 feet precisely because owners notice the problem immediately — air conditioners struggle to start, microwaves run slow, and electronics act flaky.
Five Voltage Drop Mistakes to Avoid
1. Measuring one-way distance wrong. The formula uses one-way distance and multiplies by 2 for single-phase (one hot and one neutral) or by √3 for three-phase. If you enter the round-trip distance and also multiply by 2, you've double-counted and will oversize wire by a gauge or two. Measure from the panel to the load, not there and back.
2. Using nominal voltage instead of actual. The formula assumes 120V or 240V at the source, but real utility voltage varies from 114V to 126V (ANSI C84.1 range A). If you're at the low end of the range and have a 4% drop, your load sees about 109V — below the minimum for many appliances. When in doubt, measure the actual source voltage and target tighter drop limits.
3. Ignoring the neutral on unbalanced loads. On split-phase circuits, a heavily unbalanced load means current flows through the neutral, causing additional voltage drop that asymmetrically affects one leg. This is why subpanels to shops or additions need careful balance across the two hot legs.
4. Forgetting that AC has reactance too. The simplified voltage drop formula only considers resistance. For larger wire sizes (1/0 AWG and larger) and higher power factors below 0.9, the reactance of the wire contributes meaningfully. NEC Chapter 9 Table 9 includes both resistance and reactance values for more accurate calculations on feeder circuits.
5. Using DC formulas for AC at long distances. Short runs — the simplification is fine. But on long runs with large wire, ignoring reactance can underestimate drop by 10-15%. For anything over 500 feet or wire larger than 1 AWG, use the full AC formula with power factor.
Professional Voltage Drop Strategies
Prefer 240V over 120V whenever possible. At the same power level, 240V draws half the current of 120V, and voltage drop scales with current — so 240V circuits have one quarter the voltage drop for the same wire gauge over the same distance. That's why 240V-capable appliances (dryers, water heaters, EV chargers) use 240V in North America — the wire savings alone justify the extra switching cost.
Use larger wire than the minimum ampacity requires. A 20-amp circuit with 12 AWG is minimum code. Using 10 AWG costs maybe 50% more in wire, but cuts voltage drop in half and adds future headroom.
Split long runs into home runs. Instead of daisy-chaining three outdoor outlets off one circuit over 200 feet, run three separate branches from the panel. Each branch sees much less current and dramatically less voltage drop.
Consider power factor on motor circuits. Motors typically run at 0.85 power factor or worse at startup. The effective voltage drop for motor starting is higher than the calculator suggests — design for 2% steady-state on any motor circuit so you have margin for inrush.
NEC and IEEE Voltage Drop References
NEC 210.19(A) Informational Note No. 4 is the primary reference for branch circuit voltage drop: maximum 3% to the farthest outlet. 215.2(A)(1) Informational Note No. 2 gives the 5% total rule (feeder + branch). NEC Chapter 9 Table 9 provides AC resistance and reactance values for conductors in PVC, aluminum, and steel conduit — the values you need for precise calculations on circuits with significant reactive component.
For specialized applications: NEC 695.7 covers fire pump voltage drop (15% max during starting). IEEE 141 (Red Book) has detailed industrial voltage drop guidance. ANSI C84.1 defines acceptable utilization voltage ranges (Range A: 114–126V for 120V systems; Range B, used during abnormal conditions: 110–127V).
Voltage drop: what it is, when it matters, and how the math works
Voltage drop is the loss of voltage along a conductor caused by the conductor own resistance. On short runs (under 50 feet) at normal current, it is usually negligible. On long runs, high current, or both, the drop can push equipment outside its tolerance band and cause hot connections, motor overheating, premature LED failure, and dim outlets.
The NEC does not enforce a voltage-drop limit. Informational notes in 210.19(A)(1) and 215.2(A)(1) recommend 3 percent on a single branch or feeder and 5 percent combined. Most inspectors treat these as enforceable on long pulls because IEEE 141 and equipment listings assume operation within those bounds.
The formula and what it does
Vd is the round-trip voltage drop in volts. K is the resistivity (12.9 for copper, 21.2 for aluminum). I is the current in amps. L is the one-way length in feet. CM is the conductor circular mils from NEC Chapter 9 Table 8. For three-phase, replace 2 with 1.732 (sqrt 3) because line-to-line voltage drop is lower than line-to-neutral round trip.
Worked example
Scenario: 100-amp subpanel feeder, 125 feet from main panel, 240 V single-phase, 4 AWG copper. Is the drop acceptable?
4 AWG has 41,740 CM. Vd = (2 x 12.9 x 100 x 125) / 41,740 = 7.73 V. As a percentage of 240 V: 3.22 percent. That just edges past the 3 percent guideline. You have three choices: live with it (many inspectors will sign off), bump to 3 AWG (52,620 CM, drop falls to 2.55 percent), or specify higher-voltage equipment if available. For a continuous feeder, I would size up.
Common mistakes to avoid
Confusing one-way and round-trip drop. The factor of 2 in the formula accounts for current flowing out the hot and back through the neutral. Drop is round trip.
Ignoring inductive reactance on large AC feeders. Above about 1/0 AWG and at higher currents, conductor reactance adds non-trivially to total impedance. NEC Chapter 9 Table 9 gives effective Z for AC. The calculator uses pure resistance, which is accurate for residential branch circuits but slightly optimistic on big commercial feeders.
Using nominal voltage instead of actual. If your utility delivers 117 V rather than 120 V, your equipment sees the drop on top of an already low supply. Build in margin.
Frequently asked questions
Why does voltage drop matter if my breaker still holds?
Because the load sees less voltage than the source provides. Motors draw more current to maintain torque, heating windings. Resistance heaters produce less heat. LEDs flicker. Power supplies run hot. Long-term, all of these shorten equipment life.
Is 5 percent voltage drop really a problem?
For incandescent lighting or pure resistance heat, you lose proportional output and not much else. For motors, electronics, and any modern equipment with a switching power supply, 5 percent is the edge of acceptable. Above that you start seeing real failures.
Does going to higher voltage reduce voltage drop?
Yes, dramatically. The same load at 240 V draws half the current of 120 V, and voltage drop scales with current squared in some configurations. A 100 A 120 V circuit and a 50 A 240 V circuit deliver the same power but the 240 V version has roughly a quarter of the drop on the same conductor.
How does temperature affect voltage drop?
Conductor resistance rises with temperature, about 0.4 percent per degree C. A run in a hot attic at 50 C has 8 percent more drop than the same run at 20 C. NEC Chapter 9 Table 9 includes temperature correction.
Do I include the neutral length in voltage drop?
For single-phase circuits, yes. The formula factor of 2 covers the round trip. For balanced three-phase loads, the neutral carries zero current and is ignored. For unbalanced three-phase, the neutral matters.
Are voltage drop limits in the NEC enforceable?
The 3 percent and 5 percent figures live in Informational Notes, which by NEC 90.5(C) are not enforceable code. However, many inspectors and AHJs treat them as enforceable based on equipment listings and IEEE standards. Always check with your local AHJ.
How do I reduce voltage drop without resizing the conductor?
Move the load closer to the source (shorter run), increase the supply voltage if possible, or split the load across two smaller circuits each at lower current. On extreme runs (1000+ feet) you sometimes use medium-voltage distribution and a step-down transformer at the load.
Does the calculator account for power factor?
Not directly. For resistive loads (heaters, incandescent, EVs charging) power factor is near unity and resistance-only math is accurate. For motors and HVAC at lower PF, real drop will be slightly higher; pad an additional 5-10 percent for safety.