Voltage Drop Calculator Online Free
Voltage Drop Calculator
Calculate Voltage Drop
Calculates based on the resistance and reactance data from the National Electrical Code (NEC).
Calculation Results
Voltage drop is the reduction in voltage across a resistive element when current flows. In electrical wiring, excessive voltage drop in supply conductors means devices receive insufficient voltage, reducing their performance or causing failures. This calculator computes voltage drop in wire runs and helps you select the correct wire gauge for your application. Proper wire sizing is both a performance issue and a safety issue — undersized wiring can overheat and cause fires, while oversized wiring wastes money.
Voltage Drop in Wiring
Long wire runs act as resistors. The total resistance depends on wire gauge (AWG or mm²), length, and material (copper or aluminum). The NEC (National Electrical Code) recommends a maximum 3% voltage drop for branch circuits and 5% combined drop from the service panel to the load. Many engineers target 2% for critical loads like motors or sensitive electronics. Remember to account for the round-trip path — current travels from the source through the hot conductor and back through the neutral/ground conductor, so actual wire length in the formula is doubled.
Voltage Drop = I × R_wire R_wire = (ρ × L × 2) / A Where: ρ = resistivity (copper: 1.72 × 10⁻⁸ Ω·m) L = one-way wire length (m) A = conductor cross-section area (m²) × 2 for round-trip path % Drop = Voltage Drop / Source Voltage × 100%
Practical example: 20A load, 50-foot run of 12 AWG copper at 120V: R_wire = 50ft × 2 / (1000ft/1.59Ω) = 0.159Ω. Voltage drop = 20A × 0.159Ω = 3.18V = 2.65% — just under the 3% NEC recommendation.
Wire Gauge Resistance Reference
AWG (American Wire Gauge) uses a counterintuitive scale: larger numbers mean thinner wire. Each 3 AWG steps approximately doubles the resistance. Each 6 AWG steps approximately doubles the cross-sectional area. Common household wiring uses 14 AWG for 15A circuits and 12 AWG for 20A circuits. Large appliances (ranges, HVAC, EV chargers) use 8-4 AWG.
| AWG | Diameter (mm) | Resistance (Ω/1000ft) | Max Ampacity (NEC) | Common Use |
|---|---|---|---|---|
| 14 AWG | 1.63 mm | 2.52 Ω | 15A | Lighting, outlets (15A circuit) |
| 12 AWG | 2.05 mm | 1.59 Ω | 20A | Kitchen, bathroom (20A circuit) |
| 10 AWG | 2.59 mm | 1.00 Ω | 30A | Dryers, water heaters |
| 8 AWG | 3.26 mm | 0.628 Ω | 40A | Electric ranges, large AC units |
| 6 AWG | 4.11 mm | 0.395 Ω | 55A | EV chargers (Level 2), subpanels |
| 4 AWG | 5.19 mm | 0.249 Ω | 70A | Large subpanels, large HVAC |
Selecting Wire Gauge for Long Runs
For long runs (over 50 feet for 120V, or over 100 feet for 240V), you may need to upsize the wire to keep voltage drop within limits — even if the current capacity of a smaller wire would technically suffice. This is especially common for detached garages, outbuildings, outdoor lighting, and EV charging stations.
| Run Length | 15A Load | 20A Load | 30A Load | Notes |
|---|---|---|---|---|
| Up to 50 ft | 14 AWG | 12 AWG | 10 AWG | Standard sizing — code minimum |
| 50-100 ft | 12 AWG | 10 AWG | 8 AWG | Upsize one step for <3% drop |
| 100-150 ft | 10 AWG | 8 AWG | 6 AWG | Long-run sizing recommended |
| 150-200 ft | 8 AWG | 6 AWG | 4 AWG | Upsize two steps from minimum |
Voltage Drop in DC vs AC Systems
In DC systems (solar, battery banks, 12/24/48V systems), voltage drop is a critical design constraint because the supply voltage is lower. A 1V drop on a 12V system is 8.3% — already over the NEC 5% recommendation. In AC systems, transformers can step up voltage before long runs and step down at the load, making long-distance power transmission practical. This is why the grid operates at kilovolts — it minimizes I²R losses in transmission lines.
Power Lost in Wire = I² × R_wire Example: 10A at 12V through 0.5Ω of wire: Voltage Drop = 10 × 0.5 = 5V (41.7% — catastrophic for 12V system) Power Lost = 10² × 0.5 = 50W wasted as heat in wire Solution: double conductor size (halve resistance) OR halve the current (use higher voltage)
In solar and EV systems, always calculate voltage drop at maximum current. A 100A charge controller needs very short, heavy-gauge wire from the battery bank.
Frequently Asked Questions
What causes excessive voltage drop?⌄
Wire too small (high resistance), wire run too long, high current loads, loose or corroded connections, or undersized wire for the temperature (high ambient temperature increases resistance slightly). Symptoms include dimming lights when loads start, motors running hot or failing to start, tripped breakers on otherwise adequate loads, or equipment performing below specification. Corroded connections are a hidden cause — a single corroded splice can add significant resistance and localized heating.
How do I reduce voltage drop?⌄
Use a larger wire gauge (lower AWG number). Reduce wire run length if possible by moving the panel closer to the loads. Ensure all connections are tight, clean, and properly made (no loose wire nuts, properly torqued terminals). For very long runs, consider increasing the supply voltage and stepping it down at the load end — this is why 240V is used for long runs to outbuildings instead of 120V. For DC systems, parallel wire runs cut resistance in half, or use a higher battery voltage (48V instead of 12V for the same wattage).
What is the difference between voltage drop and voltage loss?⌄
Voltage drop is a general term for any reduction in voltage across a component. Useful voltage drop occurs across loads doing work — the motor, heater, or light is doing something valuable with that energy. Unwanted voltage drop (voltage loss) occurs in supply conductors and connections: the energy is wasted as heat in the wire. In Ohm's Law terms, both are V = I × R, but the goal in circuit design is to minimize voltage drop in supply wiring while delivering the full intended voltage to the load.
What voltage drop is acceptable for LED lighting?⌄
LED lights are sensitive to voltage variation. Most LED drivers operate down to about 10% voltage sag before dimming or flickering, but for consistent color and brightness you want less than 5%. For 12V DC LED strip systems, keep voltage drop under 0.5V (about 4%). For 120V AC systems, the 3% NEC guideline (3.6V drop) is conservative enough for most LED luminaires. Low-voltage outdoor landscape lighting running 12V is most susceptible to voltage drop over long distances — a 100-foot run at the end of a 12V landscape circuit may receive only 10V, causing noticeable dimming.
How does wire material (copper vs aluminum) affect voltage drop?⌄
Aluminum has about 61% the conductivity of copper, so at the same AWG, aluminum wire has about 64% more resistance than copper. Aluminum wiring requires a larger gauge for equivalent performance: 2 AWG aluminum is roughly equivalent to 4 AWG copper. Aluminum wiring is common in utility services, large feeders (over 200A), and long-distance distribution because it is lighter and cheaper per foot. However, aluminum expands and contracts more with temperature, requiring special connectors and anti-oxidant compound to prevent loose connections and arcing over time.