Parallel resistors: why unequal branches draw different currents
Calculate each branch current at a shared voltage and check the total against equivalent resistance.
Method
Parallel branches connect across the same two nodes, so each has the same voltage. Their currents need not be equal. For an ideal resistor, branch current is voltage divided by resistance. Add the branch currents to obtain the source current. Equivalent resistance satisfies 1/R = 1/R1 + 1/R2 + … . For positive resistances, the equivalent value is smaller than the smallest individual resistor.
Worked example
Connect 100 Ω and 200 Ω resistors in parallel across 12 V. The currents are 12 ÷ 100 = 0.12 A and 12 ÷ 200 = 0.06 A. Total current is 0.18 A. Equivalent resistance is 1 ÷ (1/100 + 1/200) = 66.67 Ω approximately, and 12 ÷ 66.67 is approximately 0.18 A. Equal voltage does not mean equal current: the 100 Ω branch draws twice as much. Its ideal power is 12 × 0.12 = 1.44 W; the other branch dissipates 0.72 W.
Checks and limits
Check the equivalent value with Series and parallel resistors, then use Ohm’s law for individual branches. These values assume a steady ideal voltage source and resistors with the stated resistance. Real tolerances, heating, source limits and wiring resistance can alter the result. Adding another parallel branch increases the current required from a fixed-voltage source. A calculated wattage is not a component rating or a wiring design approval; use appropriate rated equipment and qualified guidance for real electrical work.