Voltage Drop Calculator

Voltage drop occurs when electrical current flows through a wire — resistance in the wire causes a portion of the voltage to be lost as heat before reaching the load. Both the Indian standard IS 732 (wiring) / IS 3043 (earthing) and the US NEC (National Electrical Code) recommend a maximum of 3% voltage drop for branch circuits. This calculator uses the formula: V_drop = I × R × 2L / 1000, where the factor of 2 accounts for the round-trip wire length. R is in Ω/1000m for mm² (Indian) wire sizes and Ω/1000ft for AWG (US) sizes.

This tool is built for electricians, electrical engineers, solar installers, and DIY homeowners who need to confirm that a cable run is thick enough to deliver clean power to a load. It is especially useful for long runs — a borewell pump 50 m from the meter, an inverter feeding a distant room, an AC or geyser on a dedicated line, or an LED strip in a large hall. You enter the current (amps), the one-way distance, the wire size, and the source voltage; the calculator returns the volts lost, the percentage drop, the voltage that actually reaches the load, and a clear pass / fail against the IS 732 and NEC 3% guideline.

Understanding the Formula

The full equation is V_drop = (2 × L × I × R) ÷ 1000. Here L is the one-way distance to the load, I is the current in amps, and R is the per-unit-length resistance of the conductor. The factor of 2 is essential and the most-forgotten part: current has to flow to the load and back through the return conductor, so the wire it travels through is twice the one-way length. Resistance is published per 1000 units, so the formula divides by 1000 to scale it to your actual run. Once you have the volts dropped, the percentage drop is V_drop ÷ source voltage × 100, and the voltage at the load is simply the source voltage minus the drop. A larger conductor cross-section means lower resistance and therefore a smaller drop.

Worked Example: A 16A Socket Run at 230V

Imagine a 230V single-phase socket circuit carrying 16 A to a point 20 m away, wired in 2.5 mm² copper (resistance ≈ 7.41 Ω/1000m). The round-trip length is 2 × 20 = 40 m. The drop is (16 × 7.41 × 40) ÷ 1000 = 4.74 V. As a percentage that is 4.74 ÷ 230 × 100 ≈ 2.06%, comfortably inside the 3% guideline, and the load sees about 225.3 V. If you doubled the run to 40 m one-way, the drop would roughly double to about 4.1% — now over the limit — and you would step up to 4 mm² cable to bring it back into range. This is exactly the kind of decision the calculator is meant to make instant.

Practical Tips

Aim to keep branch-circuit drop at or below 3%, and total drop (feeder plus branch) below 5%. If you are over the limit, the cheapest fix is usually a larger wire size — moving from 2.5 mm² to 4 mm² cuts resistance by roughly 38%. Where you cannot upsize, shorten the run or move the distribution board closer to the load. For high-draw appliances — geysers, ACs, motors, borewell pumps — start at 4–6 mm² and verify with this tool rather than defaulting to 2.5 mm². Note that this calculator uses copper resistance values; aluminium conductors have roughly 1.6× the resistance, so an equivalent aluminium run drops more voltage.

Common Mistakes to Avoid

The most frequent error is omitting the factor of 2 and calculating drop on the one-way length, which understates the real loss by half. The second is mixing units — using metres of run with an Ω/1000ft (AWG) resistance figure, or vice versa; keep mm² with metres and AWG with feet. Third, people size wire purely for ampacity (whether it can carry the current without overheating) and forget that a long run can pass the ampacity test yet still fail the 3% drop test. Finally, do not ignore drop on low-voltage systems: a 0.36 V drop on a 12V LED line is only 3% in percentage terms, but LEDs are sensitive and that small loss can visibly dim the output.

Indian mm² Wire Resistance (Ω per 1000 m, copper)

Standard IS 732 cable sizes: 1.5 mm² = 12.1 Ω/km, 2.5 mm² = 7.41, 4 mm² = 4.61, 6 mm² = 3.08, 10 mm² = 1.83, 16 mm² = 1.15, 25 mm² = 0.727 Ω/km. Larger cross-section = lower resistance.

AWG Wire Resistance (Ω per 1000 ft, copper)

Larger AWG numbers = thinner wire = higher resistance. AWG 14 = 3.14 Ω/1000ft, AWG 12 = 1.98, AWG 10 = 1.24, AWG 8 = 0.778, AWG 6 = 0.491 Ω/1000ft.