Tool

Voltage drop calculator

Find out how much voltage a copper cable loses over a run, and the smallest size that keeps the drop within your limit.

Smallest copper size within 3% drop

2.5 sq mm

6.2 V lost over 25 m (2.71%)

Voltage drop for each conductor size
Size	Voltage drop	Drop
1 sq mm	15.22 V	6.62%	Too high
1.5 sq mm	10.38 V	4.51%	Too high
2.5 sq mm	6.23 V	2.71%	Within limit
4 sq mm	3.86 V	1.68%	Within limit
6 sq mm	2.58 V	1.12%	Within limit
10 sq mm	1.49 V	0.65%	Within limit
16 sq mm	0.94 V	0.41%	Within limit
25 sq mm	0.61 V	0.26%	Within limit
35 sq mm	0.43 V	0.19%	Within limit

How it is calculated

Voltage drop = k × length × current × resistance. k is 2 for single phase (the current flows out and back) and √3 for balanced three phase. Resistance is the IS 8130 maximum for class 5 flexible copper at 20 °C, corrected to 70 °C, the operating limit for PVC insulation.

Cable reactance is ignored, which is accurate for the sizes shown. A common design target is 3% or less from the distribution board to the load.

What this tool does not check

It checks voltage drop only. A cable must also carry the current without overheating, which depends on how it is installed, how many cables share the conduit and the ambient temperature. Protective devices must match the cable too.

Always have a licensed electrician size the final installation in line with IS 732 and the National Electrical Code. See also our guide to wire sizes in a home.