Series and Parallel Circuits

Two ways to connect loads

In a series circuit, current passes through every load in turn. There is only one path, so whatever flows through the first load flows through all of them.

In a parallel circuit, each load sits on its own branch connected across the same two points. The current divides between the branches, and each branch works on its own.

Series circuits

• The same current flows through every load.
• The voltages across the loads add up to the supply voltage.
• Total resistance is the sum of the individual resistances: R total = R1 + R2 + R3.

Each lamp gets only part of the supply voltage, so lamps rated for 120 V glow dimly when wired in series. A larger resistance takes a larger share of the voltage, because the same current flows through every load.

Parallel circuits

• Every branch gets the full supply voltage.
• The branch currents add up to the total current drawn from the supply.
• Total resistance is lower than the smallest branch: 1 ÷ R total = 1 ÷ R1 + 1 ÷ R2 + 1 ÷ R3.

Every lamp now gets the full 120 V and glows at full brightness, and the circuit draws nine times the current of the series version.

Compare them side by side

Both circuits below use the same supply and the same three lamps. Change a lamp's resistance, open a branch, burn out or short a lamp, and compare the readings. Tap any part of either drawing to see what it does.

Why buildings are wired in parallel

Receptacles and lights in a building are connected in parallel. Each device gets the full circuit voltage and can be switched on and off without affecting the others. When one lamp fails, the rest of the room stays lit.

Series connections still show up on every job. A switch is always in series with the load it controls, and a fuse or breaker is in series with everything on its circuit. Some decorative light strings are wired in series too, which is why one loose or missing bulb can darken a whole section.

Opens and shorts

Most troubleshooting comes down to two kinds of fault.

• Open circuit. A break in the path, such as a burned-out filament, a broken conductor or a loose termination. In a series circuit, one open stops current everywhere. In a parallel circuit, an open inside one branch stops only that branch. An open in the shared conductors stops everything past it, which is how one loose connection can leave every receptacle downstream without power.
• Short circuit. A path with almost no resistance, such as damaged insulation where conductors touch. Current rises far above normal until the breaker or fuse opens. A short across one load in a series circuit leaves the other loads carrying more current, so they glow brighter.

In a series circuit with a single open, the full supply voltage appears across the break, while the working loads read close to zero volts because no current flows through them. That reading points straight at the fault. In a parallel circuit, a burned-out lamp still has the full supply voltage across its terminals, and its branch simply draws no current.

Overloads and the breaker

Each load added in parallel adds current. Lower the lamp resistances in the parallel circuit until the total passes 15 A and the breaker trips.

Real breakers respond on a time curve. A small overload can run for many minutes, or even hours, before the breaker trips, while a short circuit trips it almost instantly. The simulator trips the moment the current passes the rating so the cause is easy to see.

Check your understanding

Where this knowledge gets used

• Service openings: Finding opens and shorts is the core of service work.
• Commercial openings: Commercial construction runs on parallel branch circuits and careful load math.
• The journeyman path: What journeymen do and what it takes to get there.

Related lessons

• Ohm's Law: Voltage, Current and Resistance: Change the voltage and resistance on a working circuit and watch the current respond. Then use V = I × R the way it gets used in the field.
• How Three-Way Switches Work: Flip two switches and watch one light. Follow the travelers, switch between a schematic and a hallway view, and track down a wiring mistake.