ELECTRONICS · CALCULATOR

Resistor to reduce voltage

Free resistor to reduce voltage: get series resistance and dissipation from input voltage, desired load voltage and current. With the formula shown.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Input voltage

Desired load voltage

Current

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

R = (Vin − Vout) / I

Only valid for a known constant current; not a regulated power supply.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Input voltage: 12 V · Desired load voltage: 5 V · Current: 20 A

Series resistance: 0.35 Ω · Dissipation: 140 W

This example uses the stated inputs; results for the entered values appear above.

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