Series and parallel circuits: why are the sockets in your home wired in parallel?

Anyone who remembers old-style Christmas lights knows how it went: when a single bulb blew, the whole string went out, and you had to test the bulbs one by one to find the faulty one. At home, though, when the kitchen light fails the fridge keeps running, and switching off the kettle plugged into one socket does not turn off the television. The difference between these two experiences comes from one of the most basic ideas in electric circuits: connecting in series and in parallel. In this article we compare the two kinds of connection using Ohm’s law, show the difference in numbers with a worked example, and explain why the sockets in your home are wired in parallel. At the end you will also find important safety notes about household electricity.

The basics first: current, voltage, resistance

To understand an electric circuit we need three quantities:

• Current ($I$): The amount of electric charge passing through a cross-section of a conductor per unit time. Its unit is the ampere (A).
• Voltage ($V$): The electric potential difference between two points; you can think of it as the “push” that moves charges around the circuit. Its unit is the volt (V).
• Resistance ($R$): How much a circuit component opposes the current. Its unit is the ohm ($\Omega$).

The relationship between these three is given by Ohm’s law:

There is also the idea of power: the energy a component uses or converts per unit time. Power is voltage multiplied by current:

The unit of power is the watt (W). How bright a bulb is relates roughly to the power delivered to it.

A common analogy compares electricity to water in pipes: voltage corresponds to water pressure, current to the amount of water flowing through the pipe, and resistance to how narrow the pipe is. Like every analogy it has its limits, but it is very useful for building a first intuition.

Series connection: one path, a shared fate

In a series connection, components are placed one after another along a single path. There is only one path the current can follow.

A series circuit has three basic properties:

1. The current is the same everywhere. Because there is only one path, the same current passes through every point of the circuit.
2. The voltage is shared. The source voltage is divided between the components in proportion to their resistances: $V_{\text{total}} = V_1 + V_2 + \ldots$
3. Resistances add up. The equivalent resistance is the sum of the individual resistances:

The most striking consequence of a series circuit is that if one component breaks the circuit (for example, if a bulb’s filament snaps), the path is cut completely and no current flows through any component. That is exactly how old Christmas lights behaved.

Where is series connection useful?

Series connection is not always a disadvantage. A switch is connected in series with the lamp it controls; when the switch is opened, the path is cut and the lamp goes out. Fuses and circuit breakers are also connected in series with the circuit they protect: when the current reaches a dangerous level, they cut the path and protect everything in the circuit. So series connection is exactly the right choice wherever “all or nothing” behaviour is wanted.

Parallel connection: each component gets its own path

In a parallel connection, components are connected side by side between the same two points. Each component reaches the source through its own branch.

The basic properties of a parallel circuit are:

1. The voltage is the same across every branch. Because all the branches are connected to the same two points, the voltage across each component equals the source voltage.
2. The current is shared. The total current leaving the source divides between the branches: $I_{\text{total}} = I_1 + I_2 + \ldots$
3. The equivalent resistance goes down. The reciprocal of the equivalent resistance is the sum of the reciprocals of the resistances:

Because each new component connected in parallel opens a new path for the current, the equivalent resistance is smaller than even the smallest individual resistance. This means the total current drawn from the source rises as components are added. Keep this point in mind; it matters a great deal for safety at home.

In a parallel circuit, even if one branch breaks, the other branches are unaffected. Each component works independently.

Where do the rules come from? Kirchhoff’s laws

The rules for series and parallel circuits are not separate facts to memorise; they come from two basic conservation principles. These were formulated in the 19th century by the German physicist Gustav Kirchhoff.

• The junction rule (conservation of charge): The total current entering a junction equals the total current leaving it. Charge does not pile up at a point or appear from nowhere. The way current divides between branches in a parallel circuit, with the branch currents adding up to the source current, comes from this rule.
• The loop rule (conservation of energy): Around any closed loop, the sum of the voltage rises and drops is zero. The fact that the component voltages in a series circuit add up to the source voltage comes from this rule.

Anyone who grasps these two principles can work step by step not only through series and parallel connections, but also through complex circuits that mix the two.

Worked example: the same bulbs, two different connections

Suppose we have a 12 V source and two identical bulbs, each with a resistance of 12 $\Omega$. (In real bulbs the resistance changes as they heat up; here we treat it as constant to keep things simple.)

If we connect them in series

The equivalent resistance:

The current through the circuit:

The voltage across each bulb is $V = I \cdot R = 0.5 \cdot 12 = 6$ V. The power of each bulb is $P = V \cdot I = 6 \cdot 0.5 = 3$ W. The total power is 6 W.

If we connect them in parallel

Each bulb is connected directly to 12 V. The current through each bulb:

The total current drawn from the source is 2 A. Since the equivalent resistance satisfies $\frac{1}{R_{\text{eq}}} = \frac{1}{12} + \frac{1}{12}$, we get $R_{\text{eq}} = 6\ \Omega$; and indeed $12 / 6 = 2$ A. The power of each bulb is $P = 12 \cdot 1 = 12$ W, and the total power is 24 W.

What does the result tell us?

In parallel, each bulb receives four times as much power as in series, and shines noticeably brighter. But there is a price: the total current and power drawn from the source have also quadrupled. In a battery-powered circuit, this means the battery runs down faster. In a household circuit, as we will see below, it sets an important limit for the wiring and the fuse.

Why are the sockets in your home wired in parallel?

Now we can answer the main question. The sockets and lights in a home are wired in parallel because:

1. Every appliance must get the same voltage. In Turkey the mains supply is nominally 230 V at 50 Hz, and appliances are designed for this voltage. In series, the voltage would be shared between appliances, so none of them could work properly.
2. Appliances must work independently of each other. In parallel, when you switch off an appliance or a bulb fails, the others are unaffected. In series, switching off one appliance would switch off every appliance on that line.
3. Each appliance draws as much current as it needs. A charger draws little current and a kettle draws a lot; parallel wiring lets each appliance draw the current that matches its power at the same voltage.

The limit of parallel wiring: overload

An important consequence of parallel wiring is that every appliance running on the same line adds to the total current. Let’s look at an example. The current drawn by a 2000 W kettle at 230 V:

If another 2000 W electric heater is run on the same line, the total current rises to about 17.4 A. The circuit breakers commonly used on socket circuits in homes are usually rated at values such as 16 A. In that case the circuit breaker trips and cuts the power to the line. This is not a fault; it is exactly what should happen: when cables are pushed to carry more current than they can handle, they heat up, and that can cause a fire.

For the same reason, plugging several high-power appliances into a single multi-socket extension lead is dangerous. An extension lead actually opens new parallel branches on the same line, and all of their current passes through the same cable and the same wall socket.

Electrical safety: do your experiments with batteries

If you want to try out what we have described in this article, do it only with batteries and low-voltage circuit kits (for example, between 1.5 V and 9 V). A battery holder, a few small bulbs, some wire and a switch are more than enough to see the difference between series and parallel connections.

Mains electricity in your home is a completely different matter:

• Never touch sockets, the inside of sockets or exposed wires, and never push any object into them.
• Do not try to open, dismantle or repair sockets, the fuse box, light fittings or wiring yourself. This work must only be done by authorised, qualified electricians.
• Do not touch electrical appliances or switches with wet hands; water makes it easier for electricity to pass through the body.
• Do not use damaged cables or plugs; if you notice a socket that gets hot, smells or sparks, stop using that line and tell an expert.

In modern installations, as well as circuit breakers there is a residual-current device (RCD). This device cuts the power within a very short time when it detects a small difference between the current going out on the line and the current coming back, that is, when some of the current is flowing to earth by another path, for example through a person’s body. RCDs used to protect life usually have a sensitivity of 30 mA. These devices save lives, but no protection system is a substitute for being careful.

In summary

• In series, the current is the same everywhere, the voltage is shared and the resistances add up; if one component breaks, the whole circuit stops.
• In parallel, the voltage is the same across every branch, the current is shared and the equivalent resistance goes down; each branch works independently.
• Switches and fuses are connected in series with the circuits they control.
• The sockets in your home are wired in parallel so that every appliance gets the same voltage and works independently.
• Running many high-power appliances on the same line increases the total current; that is why circuit breakers exist.
• Do experiments only with batteries; never interfere with mains electricity.

If you are curious to learn more about electricity and circuits, have a look at our physics articles and science topics. In Kuazara’s interactive circuit lessons you can connect bulbs and switches virtually in series or in parallel, without any risk, and understand how the current changes by trying it yourself.