Electrical Theory

Series and Parallel Circuits

Why one dead bulb used to kill a whole string of holiday lights, and why the outlets in your house are wired the other way.

By IES team, Innovative Electrical Services (IES), Auburn, CA · Reviewed November 2026 · 3 min read

Key takeaways
  • Series: one path. The same current flows through everything and the resistances add up.
  • Parallel: several paths. Every branch gets the full voltage and the currents add up.
  • Home receptacles and lights are wired in parallel, so each one works on its own.
  • Adding loads in parallel lowers total resistance and raises the current the breaker sees.

There are only two basic ways to connect parts of a circuit: one after another (series) or side by side (parallel). Real buildings use both, and knowing the difference explains a lot of everyday behavior.

VSeriesR total = R1 + R2 + R3same current everywhereVParallel1/R total = 1/R1 + 1/R2 + 1/R3same voltage across each branch
Series: one path, so the same current flows through every part and the resistances add. Parallel: each branch gets the full voltage and the branch currents add.

Series circuits

In a series circuit there is a single path. Current leaves the source, passes through each part in turn, and returns. Because there is one path:

  • The same current flows through every part.
  • The resistances add: R total = R1 + R2 + R3.
  • The source voltage is shared between the parts. Each one “drops” part of it, and the drops add up to the source voltage.
  • If any part opens (a burned-out bulb, a broken wire), everything stops.

Older holiday light strings worked this way, which is why one bad bulb could darken the whole string. Switches, fuses and breakers are deliberately placed in series with the load, so opening them stops the current.

Parallel circuits

In a parallel circuit each part has its own path between the same two points:

  • Every branch gets the full voltage.
  • The branch currents add up to the total current.
  • Total resistance goes down as you add branches: 1/R total = 1/R1 + 1/R2 + 1/R3. For two equal resistors, the total is half of one.
  • If one branch opens, the others keep working.

The receptacles and lights in your home are connected in parallel. Every lamp sees about 120 V, and unplugging one doesn’t affect the others.

Try it

Series and parallel resistanceType resistor values in ohms, separated by commas.

Ideal resistors only. Learning tool.

What this means for a breaker

Every appliance you plug into a circuit is another parallel branch. Each one lowers the total resistance, so the total current rises, and that total is what flows through the breaker and the wire back to the panel. A space heater, a hair dryer and a vacuum on one bathroom-and-hall circuit can add up past its rating quickly. The fix is usually a dedicated circuit, not a bigger breaker: the breaker protects the wire, so it has to match the wire size.

Series-parallel in real life

Most real circuits mix the two. A switch is in series with the lights it controls, while those lights are in parallel with each other. Solar arrays are another good example: panels are often wired in series strings to raise the voltage, and strings are combined in parallel to add current.

Sources and further reading

  1. OpenStax, University Physics Volume 2, 10.2 Resistors in Series and Parallel

Local rules decide. These articles explain general ideas and point to the code sections we use. They are not legal or engineering advice and don’t replace the code itself. California adopts the NEC with its own amendments, cities and counties can add more, and the local inspector (the AHJ, or authority having jurisdiction) has the final say on any job. Read the code free at NFPA’s free access page.