Why are 3% and 5% the voltage drop limits?

Wire has resistance, so some voltage is lost as heat before reaching the device. CEC Rule 8-102 makes the limits enforceable: maximum 3% on a branch circuit or feeder and 5% combined from the service to the farthest load. Low-voltage systems like 12 V solar often need an even tighter design target.

Why don't voltage drop calculators agree with each other?

Almost always one of two things: some tools want round-trip wire length where others want one-way distance (VoltDrop always uses one-way and doubles it internally), and some use slightly different resistance values for the wire. VoltDrop shows its full calculation so you can check its numbers against any other tool.

⚡ Voltage Drop Calculator

How much voltage does your wire lose? Enter amps, distance, and wire size — get a clear green/yellow/red verdict with the math shown.

Power type

DC: batteries, solar, vehicles, LED strips.

Voltage at the source

volts

Current (load)

amps

The amps your device or circuit actually draws. If a label says watts, amps = watts ÷ volts.

Wire material

Wire size

AWG = American Wire Gauge. Smaller number = thicker wire. It's printed on the wire jacket.

One-way distance

feet

Measure one way — from the power source to the load. Electricity makes a round trip, and we double it for you. Don't double it yourself.

Why 3% and 5%?

Wire has resistance, so some voltage is lost as heat before it reaches your device. Lose too much and motors run hot, LEDs dim or flicker, chargers underperform, and breakers can trip for no obvious reason.

CEC Rule 8-102 sets a mandatory maximum: keep the voltage drop on a branch circuit or feeder at or under 3%, and the combined drop from the service to the farthest load at or under 5%. These are enforceable limits in Canada, not optional performance guidelines. Low-voltage systems like 12 V solar often need an even tighter design target because 3% of 12 V hurts much more than 3% of 240 V.

Why don't other calculators agree with each other?

Almost always one of two things: some tools want round-trip length where others want one-way (we always use one-way and double it for you), and some use slightly different resistance values for the wire. Open "How we calculated this" above and you can check our numbers against anything.