Voltage Drop Calculator
Voltage Drop Calculator
Voltage drop is the reduction in voltage that occurs as electric current flows through a conductor due to its inherent resistance. All wires and cables have some resistance, and when current passes through them, a portion of the supply voltage is lost along the path from the source to the load. This loss can cause equipment to operate inefficiently, lights to dim, motors to overheat, and sensitive electronics to malfunction if not properly accounted for.
The National Electrical Code (NEC) recommends that the total voltage drop in a circuit should not exceed 5% of the supply voltage, with a maximum of 3% allowed on any individual branch circuit. For example, in a 120V system, the total voltage drop should be no more than 6V (5%), and the drop on any single branch circuit should not exceed 3.6V (3%).
Proper wire sizing based on voltage drop calculations is critical for electrical safety and energy efficiency. Undersized wires can overheat and create fire hazards, while oversized wires are unnecessarily expensive. This calculator helps determine expected voltage drop and verify that a chosen wire size meets NEC guidelines.
The consequences of excessive voltage drop extend beyond simple equipment malfunction. In lighting circuits, voltage drop causes visible dimming and color temperature shifts, particularly noticeable with LED lighting where a 5% voltage drop can cause a perceptible reduction in light output. In motor circuits, undervoltage causes motors to draw more current to maintain torque output, leading to overheating and reduced motor life. Electronic equipment with switching power supplies can tolerate wider voltage variations (typically +/- 10%) but digital equipment and precision instruments may malfunction or produce errors with even small voltage deviations.
Wire sizing involves balancing competing considerations: larger wires reduce voltage drop but cost more and are harder to install in tight spaces. The NEC provides ampacity tables (how much current each wire size can safely carry) based on the wire material, insulation type, and installation conditions. However, ampacity alone does not guarantee acceptable voltage drop — a wire that is properly sized for ampacity may still have excessive voltage drop over long distances. This calculator fills that gap by computing the actual voltage drop for your specific installation parameters and comparing it against NEC recommended maximums.
- Select Wire Material: Copper or Aluminum.
- Select Wire Size: Choose the AWG size (smaller numbers = larger diameter).
- Enter Current: Load current in amperes under normal operating conditions.
- Enter One-Way Distance: Distance from source to load in feet.
- Enter Supply Voltage: System voltage (120V, 240V, 277V, 480V).
- Select Phase: Single-Phase or Three-Phase.
- Click Calculate: The calculator computes voltage drop in volts and as a percentage, and indicates NEC compliance.
Interpreting the Results: The calculator shows both the absolute voltage drop in volts and the percentage drop relative to the supply voltage. If the percentage exceeds 3% for a branch circuit or 5% for a feeder plus branch circuit combination, the calculator indicates non-compliance with NEC recommendations. In that case, try selecting a larger wire size (smaller AWG number) or a different wire material to bring the voltage drop within acceptable limits. Moving up one wire size typically reduces voltage drop by about 20-25%.
Understanding the One-Way Distance Parameter: The distance entered should be the one-way length of the conductor from the source to the load, not the round-trip distance. The calculator automatically multiplies by 2 for single-phase (accounting for both the hot and neutral conductors) and by 1.732 for three-phase (the square root of 3 accounts for the phase-to-phase voltage relationship). For three-phase systems, a 3% drop at 480V allows a 14.4V drop total, and the calculator uses the appropriate formula based on the selected phase configuration.
Selecting the Correct Wire Material and Size: Copper is the standard choice for most residential and commercial wiring due to its excellent conductivity and corrosion resistance. Aluminum is lighter and less expensive but requires larger conductors for the same current capacity and special termination techniques to prevent galvanic corrosion. When substituting aluminum for copper, select two AWG sizes larger for equivalent voltage drop performance. The calculator includes both copper and aluminum resistivity constants at typical operating temperatures.
Single-Phase Circuits
V_d = (2 x K x I x D) / CM
Three-Phase Circuits
V_d = (1.732 x K x I x D) / CM
Where: K = resistivity constant (12.9 for copper, 21.2 for aluminum), I = load current (A), D = one-way distance (ft), CM = circular mil area.
Percentage Voltage Drop
V_d% = (V_d / V_supply) x 100
Wire Size Selection
CM = (2 x K x I x D) / V_d_allowed (single-phase) or CM = (1.732 x K x I x D) / V_d_allowed (three-phase)
NEC Recommended Voltage Drop Limits
| Circuit Type | Maximum Drop | Example (120V) |
|---|---|---|
| Branch circuit | 3% | 3.6V |
| Combined feeder + branch | 5% | 6.0V |
| Feeder only | 2% | 2.4V |
Circular Mil Area by AWG
| AWG | Circular Mils | AWG | Circular Mils |
|---|---|---|---|
| 14 | 4,107 | 4 | 41,740 |
| 12 | 6,530 | 3 | 52,620 |
| 10 | 10,380 | 2 | 66,360 |
| 8 | 16,510 | 1 | 83,690 |
| 6 | 26,240 | 1/0 | 105,600 |
| 4 | 41,740 | 4/0 | 211,600 |
Always Use the Correct K Value: K = 12.9 for copper, 21.2 for aluminum, assuming 75 degrees C operating temperature.
Motor Circuits Need Extra Attention: Motors draw 5-7 times running current during startup. Consider using starting current for voltage drop calculations.
- Why is voltage drop more significant in longer wire runs?
- Resistance is directly proportional to length — doubling the length doubles the voltage drop.
- Can I use aluminum wire instead of copper?
- Aluminum has 1.64x the resistivity of copper. To achieve the same voltage drop, use two AWG sizes larger.
- What is the maximum distance for a 120V circuit?
- A 120V, 15A circuit using 12 AWG copper should not exceed approximately 120 feet for 3% drop.
- How does ambient temperature affect voltage drop?
- Conductor resistance increases with temperature (approx. 0.4% per degree C for copper).
- What is the relationship between voltage drop and power loss?
- Power lost as heat in the wiring is calculated as P_loss = I^2 x R_wire, where R_wire is the total resistance of both conductors. This wasted power not only reduces the voltage available to the load but also generates heat that can further increase conductor resistance in a positive feedback loop. In high-current circuits, power loss in wiring can be substantial — a 100A circuit with 2% voltage drop at 240V dissipates 480W of power as heat in the wires, which must be accounted for in thermal management and energy efficiency calculations.
- How do I size wires for long-distance runs?
- For runs exceeding 100 feet, voltage drop typically becomes the limiting factor before ampacity. Calculate the minimum wire size needed for acceptable voltage drop using the formula provided, then verify that the selected wire has sufficient ampacity for the load. In long runs, increasing the wire gauge by one or two sizes often resolves voltage drop issues while adding minimal cost compared to the overall project budget. For very long runs (500+ feet), consider stepping up to a higher supply voltage or using a step-up transformer at the source and a step-down transformer at the load.
- Based on NEC recommendations for US installations. Other countries may have different limits.
- Assumes uniform load distribution. Multiple taps may differ.
- Does not account for temperature derating.
- AC effects such as skin effect and reactance not considered (relevant above 300 kcmil).
- Voltage drop is transient-sensitive (motor starting currents).
Beyond Basic Voltage Drop Calculations
The voltage drop formulas used in this calculator assume a purely resistive load model, which is adequate for most general-purpose circuits but has limitations for certain applications. Motor circuits, for example, have significant inductive reactance that affects both the magnitude and power factor of the current. The actual voltage drop in a motor circuit depends on both resistance and reactance, and the current draw during starting can be 5-7 times the running current, causing a momentary voltage drop that may trip undervoltage protection devices or cause contactors to drop out.
High-frequency AC circuits and large conductors above 300 kcmil experience additional phenomena that affect voltage drop beyond simple resistive calculations. The skin effect causes AC current to concentrate near the surface of conductors at higher frequencies, effectively increasing the conductor resistance. Proximity effects from adjacent conductors carrying current in the same direction also increase effective resistance. For very large installations, such as data center feeders or industrial power distribution, consulting the IEEE Red Book (Std 141) for more comprehensive voltage drop calculations that include conductor reactance is recommended for accurate results.
- NFPA 70: National Electrical Code, Articles 210.19(A) and 215.2(A)(4)
- IEEE Std 141-1993 (Red Book)
- Copper Development Association — Copper Wire Tables
Last updated: May 12, 2026