Long cable runs and thin wire let voltage sag before it reaches the load.
About this calculator
Long cable runs and thin wire let voltage sag before it reaches the load. Motors hum, lights dim, and breakers may trip. This calculator estimates drop in volts and percent from load current, one-way cable length, resistance per metre, supply voltage, and whether the circuit is single or three phase.
Indian household wiring often targets no more than 3% drop on branch circuits. Example: 16 A through 25 m of 2.5 mm² copper (about 0.00741 Ω/m) on 230 V single phase gives roughly 5.9 V drop — about 2.6%, which passes the guideline.
Raise current or length and drop climbs fast. If percent exceeds 3%, the tool flags a warning — consider shorter routing, thicker cable, or a higher supply voltage feed.
Use the [[ohms-law]] calculator for V–I–R checks, the [[electricity]] calculator for monthly bill estimates, and the [[resistor-color-code]] calculator when reading component values on a bench.
Common uses
- Sizing wire for a garage sub-panel 40 metres from the main DB
- Checking drop before installing a 3 HP single-phase borewell pump
- Three-phase workshop feed to a CNC machine at 415 V
- Verifying garden lighting runs on 1.5 mm² cable
- Teaching electrical students the difference between single and three-phase drop factors
Frequently asked questions
What voltage drop is acceptable in home wiring?
Many Indian and IEC guides use 3% maximum on final branch circuits and 5% total from the meter. Above 3% this calculator shows a warning. Sensitive electronics may need an even tighter limit.
Why multiply by 2 for single phase?
Current travels out on the live conductor and back on neutral, so resistance acts over twice the one-way length. Three-phase balanced loads use √3 × I × R × L because only one line is counted per phase pair.
What resistance should I use for 2.5 mm² copper?
About 0.00741 Ω/m at 20°C is a common value for 2.5 mm² copper PVC cable. Thicker 4 mm² is near 0.00461 Ω/m. Check the cable datasheet for your exact product.
How do I find voltage at the load?
Subtract the calculated drop from supply voltage. At 230 V with 5.9 V drop, the load sees about 224 V. The result card labels this as voltage at load.
Does temperature change resistance?
Yes — copper resistance rises as the cable heats. This tool uses the resistance value you enter. For long runs near rating, add a safety margin or use manufacturer tables at operating temperature.