What Is Voltage Drop and How Does This Voltage Drop Calculator Work?
Voltage drop is the reduction in electrical voltage that occurs as current travels through the resistance of a wire, caused by the conductor's length, gauge, and material. A voltage drop calculator uses these three factors, plus the load current, to predict how many volts — and what percentage of the source voltage — are lost by the time power reaches the load.
The standard circular-mil voltage drop formula is: Vdrop = (2 × K × I × L) ÷ CM for a single-phase or DC circuit, and Vdrop = (1.732 × K × I × L) ÷ CM for three-phase. Here, K is the resistivity constant (12.9 ohm-circular-mils per foot for copper, 21.2 for aluminum), I is the load current in amps, L is the one-way conductor length in feet, and CM is the conductor's cross-sectional area in circular mils.
According to the National Electrical Code (NEC), voltage drop should not exceed 3% on a branch circuit or feeder, and no more than 5% total from the service entrance to the farthest outlet, to keep equipment operating efficiently and prevent overheating.
How to Use This Voltage Drop Calculator
Select your circuit type and wire specs, enter the electrical values, and the voltage drop updates instantly. Here's what each field means:
- Circuit Type: Single-phase or DC circuits use a factor of 2 (current travels out and back); three-phase circuits use 1.732 (√3).
- Wire Material: Copper has lower resistance than aluminum (K = 12.9 vs 21.2), so a copper conductor of the same gauge carries current with less voltage drop.
- Wire Gauge: The American Wire Gauge (AWG) or kcmil size of the conductor — larger numbers are thinner wire (14 AWG) and larger kcmil values are thicker (500 kcmil).
- Source Voltage: The nominal voltage of the circuit, such as 120V or 240V in North America, or 12V/24V for low-voltage DC systems.
- Load Current: The current in amps the connected device or circuit draws, found on the equipment's nameplate or breaker rating.
- One-Way Distance: The distance from the power source to the load, in feet or meters — the calculator automatically doubles it for the round-trip path.
This calculator works with both feet and meters, and covers voltage systems used worldwide — from 120V/240V North American branch circuits to 12V and 24V DC systems used in solar, RV, marine, and automotive wiring.
What a High Voltage Drop Calculator Result Means
A voltage drop above the NEC's 3% guideline means the load receives noticeably less voltage than the source supplies — dimming lights, sluggish motors, and appliances running hotter and less efficiently because they draw more current to compensate. The fix is almost always a larger wire gauge, since doubling the circular-mil area roughly halves the voltage drop for the same current and length.
| Voltage Drop | NEC Status | Typical Effect |
|---|---|---|
| Under 3% | Compliant | Negligible — equipment runs at rated performance |
| 3% – 5% | Marginal / feeder limit | Minor dimming, slightly reduced motor torque |
| Above 5% | Non-compliant | Visible dimming, motor overheating, upsize the wire |
Frequently Asked Questions
What is an acceptable voltage drop for a circuit?
The NEC recommends keeping voltage drop at or under 3% for a branch circuit or feeder alone, and no more than 5% combined from the service entrance to the farthest outlet. These are recommendations rather than hard code violations in most jurisdictions, but exceeding them risks dim lighting, motor overheating, and wasted energy as heat.
How accurate is this voltage drop calculator?
This calculator applies the standard NEC circular-mil formula using nominal AWG and kcmil conductor areas, which matches the results published by wire manufacturers such as Southwire and Cerrowire for DC and low-frequency AC circuits. It does not account for AC skin effect at high frequencies, conduit fill derating, or ambient temperature above 30°C — for a code-critical installation, verify with a licensed electrician.
What is the difference between voltage drop and ampacity?
Ampacity is the maximum current a conductor can safely carry without overheating its insulation, while voltage drop measures how much voltage is lost over the length of the run at a given current. A wire can be rated for enough ampacity to carry the load safely and still drop too much voltage over a long distance, so both must be checked — the NEC requires using whichever calculation results in the larger wire size.
How do I reduce voltage drop on a long wire run?
Use a larger wire gauge (lower AWG number or higher kcmil rating), shorten the distance between the source and the load, switch from aluminum to copper conductors, or raise the system voltage where possible, since voltage drop percentage falls as source voltage rises for the same load and length.