Series vs Parallel Circuits
In a series circuit, components are connected end-to-end so current flows through each in turn. In a parallel circuit, components are connected across the same two points, so current splits among them. The choice affects how voltage and current distribute, what happens when one component fails, and where each topology fits in real designs.
Last reviewed on 2026-04-27.
Quick Comparison
| Aspect | Series | Parallel Circuits |
|---|---|---|
| Path | One — current flows through every component | Multiple — current splits among branches |
| Current | Same through every component | Divides among branches; sum equals total |
| Voltage | Divides across components | Same across each branch |
| Total resistance | R_total = R1 + R2 + R3 + … | 1/R_total = 1/R1 + 1/R2 + 1/R3 + … |
| If one component fails open | Whole circuit stops | Other branches continue |
| Common uses | Old Christmas lights, current-limiting networks | Household wiring, modern lights, most electronics |
| Adding components | Increases total resistance | Decreases total resistance |
Key Differences
1. How current flows
In a series circuit, there's only one path. Current that enters the circuit must pass through every component in sequence. The same current flows through each.
In a parallel circuit, there are multiple paths. Current splits at the junction; some flows through each branch; sums recombine downstream. The current through any one branch can be very different from another.
2. How voltage distributes
Series circuits divide voltage. The total voltage across the source equals the sum of voltages across each component. A 12 V supply with two equal resistors in series puts 6 V across each.
Parallel circuits share voltage. The full source voltage appears across each branch. A 12 V supply with two parallel branches has 12 V across each branch.
3. How total resistance changes
Adding components in series increases total resistance. R_total = R1 + R2 + R3 + …
Adding components in parallel decreases total resistance. The reciprocals add: 1/R_total = 1/R1 + 1/R2 + 1/R3 + … More paths means less overall opposition.
4. Failure behaviour
A failure in series stops the whole chain. One bulb burning out in an old-style series Christmas string takes the whole string out.
A failure in parallel only takes out that branch. One bulb burning out in modern household wiring leaves the others lit.
5. Where each is used
Series circuits show up where you need to share voltage among components, run a single current path, or use a known current limit. Examples: voltage dividers, current-limiting resistors for LEDs, daisy-chained sensors.
Parallel wiring is the standard for power distribution: every appliance in your home gets the full mains voltage from a parallel circuit. It's also the right choice anywhere you want components to be independent.
6. Mixed circuits
Most real circuits combine both. A computer's power-supply rails are parallel; the components on each rail include series elements.
Analysis usually proceeds by simplifying step by step: combine series sub-groups, combine parallel sub-groups, repeat until the whole thing reduces to one equivalent.
When to Choose Each
Choose Series if:
- Sharing a fixed total voltage across a chain of components.
- Building a current-limiting resistor network.
- Understanding daisy-chained sensors or single-path systems.
Choose Parallel Circuits if:
- Wiring outlets, lights, and appliances in any modern building.
- Anywhere you want components to operate independently.
- Reducing total resistance in a circuit.
Worked example
In a kitchen, the kettle, fridge, and microwave are all wired in parallel from the household mains. Each gets the full mains voltage. If the kettle is unplugged, the fridge and microwave continue. By contrast, an old set of Christmas lights wired in series shared one circuit — one bulb failure broke the loop and turned the entire string off. Modern Christmas lights wire LEDs in parallel groups specifically to avoid that problem.
Common Mistakes
- "Series circuits are always simpler." Conceptually yes, but real systems almost always benefit from parallel for independence.
- "Parallel circuits cost less." They use slightly more wire but eliminate cascading-failure problems.
- "Total resistance always increases when you add components." True for series, false for parallel — adding a parallel branch decreases total resistance.
- "Voltage stays the same everywhere." Only in parallel — in series, voltage drops across each component.