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Ohm's law calculator

Enter any two of voltage, current, resistance, and power and leave the rest blank. The calculator solves the others with Ohm's law (V = I times R) and the power relationships (P = V times I). It is for a DC or resistive single-phase circuit; for AC motor and reactive loads, power factor and phase apply.

Worked example

A 240 V circuit feeds a 12 Ω resistive heating element. Find the current and power.

  • Voltage240 V
  • Resistance12 Ω
  1. Current: I = V ÷ R = 240 ÷ 12 = 20 A.
  2. Power: P = V × I = 240 × 20 = 4,800 W (4.8 kW).

20 A and 4,800 W. A continuous 20 A load needs a 25 A circuit (80% rule), so size the breaker and wire accordingly.

Change the numbers in the calculator above to run your own.

anvilfield.com/calculators/ohms-law-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.

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Ohm's law FAQ

What voltage drop is acceptable?

Many designs hold branch circuits to 3 percent and feeder plus branch to 5 percent total. Those are NEC informational-note recommendations at 210.19(A) and 215.2(A), not enforceable limits. The project specification, the equipment's listed voltage tolerance, and the adopted code edition control the actual limit.

How much voltage drop is too much on a 200 ft feeder?

On a 200 ft run, drop above the project target, often 3 percent branch or 5 percent total, is too much. At 208 V that is roughly 6.2 V at 3 percent. Recalculate with the routed length and the real continuous current, then upsize the conductor or shorten the route if you exceed it.

Copper or aluminum: which has less voltage drop?

Copper has less voltage drop than aluminum at the same size, because its resistance per 1000 ft is lower. Aluminum can still meet the target by going up one or two sizes. Size the conductor to the calculated drop and the ampacity table, not to the metal alone.

Do I use one-way or round-trip length in the formula?

Use the one-way routed length in both the single-phase and three-phase formulas. The single-phase formula already multiplies by 2 to account for the conductor and the return, and the three-phase formula uses 1.732 for the phase relationship. Feeding round-trip length into either one doubles the error.

Does the NEC require a maximum voltage drop?

The NEC does not mandate a maximum voltage drop for general circuits. The 3 percent and 5 percent figures live in informational notes, which are advisory. Some specific applications and project specifications do impose hard limits, so verify the adopted code edition, local amendments, and the contract documents before treating any number as enforceable.

More in the Voltage drop field guide field guide.