Series vs parallel lighting

Series string

Take any one lamp out — the whole string dies, because they share one path.

Parallel set

Take any one out — the rest stay lit, because each has its own path to the supply.

In series the lamps share one path, so losing any one stops them all; in parallel each lamp has its own path, so the others carry on. Pull a lamp from either set above and watch the difference play out — it is the whole lesson in one gesture.

Trace it yourself

Trace the series string first, then the parallel set, and count how many routes your finger had in each.

Series: finger on the supply's upper terminal. Along the top line into lamp 1. Cross it. There is nowhere else to go — the only conductor out of lamp 1 goes into lamp 2. Cross that. Into lamp 3. Cross that. Down the return to the supply. One route, three lamps, every one of them in the way.
Now take lamp 2 out. Trace again. Finger goes into lamp 1, crosses it, arrives at the gap where lamp 2 was, and stops. Lamps 1 and 3 are perfectly good and perfectly dark: the one path is broken.
Parallel: finger on the hot rail on the left. At the first dot you have a choice — every lamp's own feed leaves the rail at its own dot. Take the top one: through lamp 1, across to the neutral rail, home. Go back to the rail and take the second: through lamp 2, home. And the third. Three routes, each with one lamp in it.
Take lamp 2 out. Its route is broken; the other two routes never went near it. Lamps 1 and 3 stay lit.
One more thing to notice in parallel: every lamp is connected straight across the two rails, so every lamp sees the full supply voltage. In the series string each lamp only got a share. That is why series lamps run dim and parallel lamps run at full brightness.

How each one behaves

A series circuit is a single loop threaded through every lamp in turn. Current has exactly one route, so every lamp carries the same current, and each takes only a share of the supply voltage. Break the loop anywhere — one dead lamp, one loose connection — and the current has nowhere to go, so all of them go out together.

A parallel circuit gives each lamp its own connection across the supply. Every lamp sees the full voltage and runs at full brightness, and the lamps do not depend on each other. Remove one and its path disappears while the others keep theirs. That independence, and the equal voltage, is why building and branch lighting is wired in parallel.

What goes wrong

1. Fixtures accidentally wired in series

Symptom: two new fixtures both glow dim, and when one lamp fails the other goes out too.

It happens when the hot is run through the first fixture's two terminals on its way to the second, instead of the hots being tied together and each fixture given its own neutral. Redraw it: the supply's hot enters fixture 1, leaves from its other terminal, enters fixture 2, and only then reaches the neutral. That is the series string above. Each lamp gets a share of the voltage, so both run dim, and either failing opens the only path. The fix is to give each fixture its own connection to both hot and neutral — pigtails at the box, not a chain through the lamps.

2. Hunting the dead lamp in a string

Symptom: a whole string of small lamps is out, and every lamp you test looks fine.

If the string is series, one open lamp — or one bad socket — kills all of them, and you cannot find it by looking, because the good lamps are dark too. Trace it: the hot reaches everything up to the break and nothing after it. That is also the way to find it with a meter: voltage to ground is present at every socket on the supply side of the break and absent on every socket after it. Walk the string until the reading changes; the break is between the last live socket and the first dead one.

3. "Adding one more lamp in parallel changes nothing"

Symptom: every lamp is at full brightness, and the supply conductor and breaker are working harder than anyone noticed.

Half true. Each lamp in parallel sees the full voltage, so the lamps themselves do not care how many neighbours they have. But look at the two rails: every lamp's current joins the rail and flows back through the same supply conductors. Three lamps, three currents added together in the feed; ten lamps, ten. The lamps are independent; the conductor that feeds them all is not. That total is what the series/parallel drill asks you to add up — and how much a given conductor is allowed to carry is the code book's question, not this page's.

En la obra

Nobody wires lights in series on purpose. You meet series by accident — and the accident has a symptom you can learn to recognise from the doorway. The parallel side is where the everyday habits live.

Common mistakes

An open neutral on a circuit that shares one between two hots. With the neutral gone, the loads on the two legs are no longer each across their own supply — they are in series across the two hots, with the full leg-to-leg voltage across the pair. The bigger load starves, the smaller one is overdriven.
The symptom: some lights bright, some dim, and the pattern changes when something else on the circuit is switched. Anything electronic on the bright side is at risk. That is series behaviour in a building, and it points straight at a neutral joint.
Feeding the next device through a device's screws. Every downstream load's current then passes through one receptacle's terminals — a series joint you did not draw. A loose screw there dims or kills everything past it.
Adding "one more" load without adding up the feed. Parallel loads do not care about each other; the conductor feeding them does. The total is arithmetic; what the conductor may carry is the book's.
Reading a series string as a dead circuit. One open lamp in a string kills all of them. Old decorative strings and some low-voltage runs still do this; check one lamp before you check the supply.

What the inspector looks for

Pigtails at the receptacles: the through-current in a splice, one conductor to each device, so pulling a device does not open the run for everything past it.
On a circuit that shares a neutral between two hots, that the neutral is spliced through — not landed on a device where a loose screw opens it — and that the two hots are on different legs and on the kind of breaker the book calls for.
That the neutral and the hot for each load are from the same circuit, so the return current has the road home it was drawn with.
Joints made up properly: tight, no insulation under the screw, the right connector for the count and size of conductors.
Load on the circuit within what the book allows for it — the inspector does the arithmetic against the book, the drill on this site just makes you quick at the arithmetic.

Check it before you call it done

Breaker off, live-dead-live at the first box and the last.
With power off, continuity of the neutral from the last box on the run back to the panel. An open neutral is the series fault in waiting, and it is silent until there is load on both legs.
Energise. Read hot-to-neutral at the last device with everything on: it should be very close to the supply. A reading that drops noticeably when a load is added points at a poor joint upstream.
If two hots share a neutral: with a load on each leg, read each hot to neutral. Both near the supply voltage is right. One high and one low is the open-neutral symptom, live and dangerous — kill it and find the joint.
Unplug one load and confirm nothing else changes. If a neighbour dims or brightens, something is in series that should not be.

Preguntas frecuentes

Why do old string lights all go out when one bulb fails?

Because they are wired in series — one path through every bulb. Break the path anywhere and current stops everywhere. Remove a lamp from the series string in the diagram and watch all three go dark at once. Modern strings add a shunt in each socket precisely to dodge this.

Why are building lights wired in parallel?

So each fixture sits across the full supply on its own path, independent of the others. One can fail or be switched off without touching the rest, and every fixture gets the same voltage. Pull one lamp in the parallel set and the others stay lit.

Do series lamps share the voltage?

Yes — the supply voltage divides across the lamps in series, so each sees only a share of it and burns dimmer than it would alone. In parallel each lamp gets the whole voltage and runs at full brightness. That difference in what each lamp sees is the heart of it.

Ver también

Single-pole switch wiring — one lamp on its own path.
Ohm's law practice — drill the voltage and current maths behind this.
Duplex receptacle wiring — the pigtail habit that keeps a run in parallel.
All wiring diagrams

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