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Wire size and voltage drop calculator

Voltage drop in a DC or single-phase run, and the size to use.

Try the circuit in the labBuild it on the simulator's bench and measure it.

How it is worked out

Current flows out and back, so the copper length is twice the run: R = ρ × 2L ÷ A with ρ = 0.0175 Ω·mm²/m for copper, and the drop is ΔV = I × R.

Example: 10 A over 5 m of 2.5 mm² at 12 V: R = 0.0175 × 10 ÷ 2.5 = 0.07 Ω, a 0.7 V (5.8 %) drop. 4 mm² brings it to 3.6 %, 6 mm² to 2.4 %.

Low-voltage DC needs much thicker wire than mains for the same power. Voltage drop is one check; the wire must also be rated for the current (ampacity) and protected by a fuse or breaker sized to it.

Philippine wire sizes: mm² to AWG, and the breaker for each

Wire in the Philippines is sold by cross-section in mm² (the Japanese series), not AWG. The sizes, their AWG equivalents and the Philippine Electrical Code ampacity for copper THHN/THWN at 75 °C:

SizeAWGRating (75 °C)Usual breakerUsed for
2.0 mm²1420 A15 A (the small-conductor rule caps it)lighting circuits
3.5 mm²1225 A20 Aoutlets; aircon up to 1.5 HP
5.5 mm²1035 A30 Awater heater to 5 kW; aircon 2–2.5 HP
8.0 mm²850 A40–50 Asub-panel feeders, big heaters
14 mm²665 A60 Aservice entrance of a small house
22 mm²490 A80 Aservice entrance, 100 A main
30 mm²2115 A100 Aservice entrance, inverter battery leads
38 mm²1130 A125 Alarger service

PDX (the flat two-core cable for houses) carries the NM ratings, lower than THHN in conduit: 2.0 mm² 15 A, 3.5 mm² 20 A, 5.5 mm² 30 A. The breaker protects the cable, so the cable's rating must be at least the breaker's; a continuous load such as a water heater or aircon should run at no more than 80 % of the breaker. See the house wiring simulator for circuits you can overload and watch.

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