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Series and Parallel Circuits
How resistances combine.
Series
Rt = R1 + R2 + R3. Same current flows through each; voltages divide.
Parallel
1/Rt = 1/R1 + 1/R2 + … Same voltage across each; currents divide. Insulation resistance readings of circuits in parallel combine this way, which is why whole-installation IR values are lower.
Key regulations
643.3