How this calculator works
Each resistor converts its own voltage-current product to heat. Series components carry the same current, while parallel components have the same voltage. A source-current mode first finds network voltage from equivalent resistance. The rating multiplier supplies a design target, not a manufacturer-approved part selection: actual ratings depend on ambient temperature, mounting, airflow and pulse duration.
Formula
Req,series = sum(Ri); Req,parallel = 1/sum(1/Ri); Pi = Vi Ii = Ii²Ri; target rating = multiplier × Pi.
Worked example
A 10 V source across 2 and 3 ohms in series drives 2 A. Powers are 8 W and 12 W; a multiplier of 2 gives rating targets of 16 W and 24 W.
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Frequently asked questions
Should I size by total power?
No. Each component needs its own adequate rating; the table separates individual powers.
Is a factor of two always enough?
No. Check the manufacturer's temperature derating and pulse-load information for the actual conditions.
Can I enter a short circuit?
This model requires positive finite resistances. An ideal short can imply unbounded current with a voltage source.