Volts Amps Watts Ohms Calculator

Volts · Amps · Watts · Ohms

Ohm's law, power and Tefzel voltage drop in one tool · 12Volt Motorsports
STEP 1

Enter any two values

Type the two you know and the other two are calculated (green). Volts and amps, volts and watts, amps and ohms, whatever the job gives you.

Running voltage, 13.8 V typical
Circuit current
Power
Resistance
STEP 2

Voltage drop on the wire run

Uses the amps from step 1. Enter the one way run length and the Tefzel gauge you are planning, and see the drop at the far end. The table below shows every gauge so you can pick the smallest that passes.

Supply to load. Return path is added automatically.
M22759/32 tinned copper
3% is the usual motorsport limit, 5% for non critical loads.
Ohm's law: V = I × R, P = V × I. Voltage drop uses round trip length and nominal tinned copper resistance for M22759/32 stranded conductors at 20 degrees C; hot engine bays raise copper resistance roughly 4 percent per 10 degrees. Check the ampacity of the gauge separately against the Tefzel ampacity chart, a wire can pass the drop test and still be too small for the current. Questions? Call 0458 102 134 or email 12voltmotorsports@gmail.com

How to Use the Volts, Amps, Watts and Ohms Calculator

Every circuit in a race car comes down to four numbers: voltage, current, power and resistance. Know two and Ohm's law gives you the rest. Enter the pair you have, a 13.8 volt supply and a 10 amp fuel pump for example, and the calculator fills in the power and resistance. Then step 2 does the part most calculators skip: it works out how much voltage that current loses along your actual wire run in each Tefzel gauge, and tells you the smallest size that keeps the drop inside your limit.

Frequently Asked Questions

What voltage drop is acceptable in motorsport wiring?

Three percent is the usual target for anything that matters, injectors, pumps, coils, ECU power. Five percent is tolerable on non critical loads like lighting. Beyond that, pumps slow down, coils weaken and sensors start reading wrong.

Why does the calculator double my wire length?

Current flows out to the load and back through the ground path, so the copper it travels is twice the one way run. Voltage drop is calculated on the round trip. If your ground is a short chassis strap, the real figure is a little better than shown.

Is passing the voltage drop test enough to choose a wire size?

No. Drop is about length, ampacity is about heat. A short run of 22 AWG can pass the drop test at 10 amps and still overheat in a bundled harness. Check the smallest passing gauge against the ampacity chart and take the larger of the two answers.

Why 13.8 volts and not 12?

A running car sits at charging voltage, typically 13.8 to 14.4 volts. Use 12 volts if you are sizing for engine cranking or a dead alternator scenario, the drop percentage gets worse as supply voltage falls.

Does heat change the result?

Yes. Copper resistance rises about four percent for every 10 degrees C. A wire in a 90 degree engine bay has roughly 28 percent more resistance than the 20 degree figures used here, so leave margin on hot runs.

Building the bundle too? Run the wire count through the concentric twist calculator for layer layout and DR25 size, and see all the wiring resources.

Find our guides useful?