Basic residential circuits
What you’ll learn
- How receptacle and lighting circuits are laid out, and why everything is in parallel.
- Where the switch goes, and what a switch loop is.
- How 120/240 V split-phase gives both voltages in one house.
- What a GFCI and an AFCI each detect, and why they are different jobs.
A house is a handful of circuit types repeated: receptacle circuits, lighting circuits and a few big 240 V loads, all fed from one panel, with every load in parallel so each gets the full voltage. Add the protective devices — breaker, GFCI, AFCI — and you have the first-year residential picture. Each device watches for a different kind of trouble.
Everything is in parallel
Receptacles and lights on a branch circuit are connected in parallel: each one sits directly across the hot and the neutral. That is why every receptacle gets 120 V whether or not anything is plugged into the others, and why the currents of everything running add up at the breaker.
Worked: what a kitchen circuit is carrying
- On one 120 V receptacle circuit: a kettle (1,500 W) and a toaster (900 W).
- Kettle: 1,500 ÷ 120 = 12.5 A. Toaster: 900 ÷ 120 = 7.5 A.
- Parallel currents add: 12.5 + 7.5 = 20 A. Add a microwave on the same circuit — its plate’s input rating, often well over 1,000 W, adds another 10 A or more — and it is well past what a typical 15 or 20 A circuit is meant for.
- This is why kitchens get several circuits. How many, and at what rating, is set by your code book.
Lighting circuits and the switch
A switch goes in the hot. Off means the hot to the lamp is open, so the socket is disconnected — fine for changing a lamp. But a switch is not isolation: the box can still hold an unswitched hot, so before you work on the fixture itself, turn off the breaker and test. When the power arrives at the light box first, a cable drops to the switch and back up: that is a switch loop (or switch leg), and the conductor returning from the switch to the light is the switched hot. Two switches controlling one light are 3-ways; add 4-ways between them for more locations.
120 and 240 in one house
A typical North American house is fed 120/240 V split-phase: two hots and a neutral from the utility transformer. Each hot is 120 V to the neutral, and the two hots are 240 V to each other. Ordinary receptacles and lights use one hot and the neutral (120 V); a dryer, range, water heater or baseboard heater uses both hots (240 V), with or without a neutral depending on whether the appliance has 120 V parts inside it.
GFCI: protecting people from shock
A ground-fault circuit interrupter compares the current going out on the hot with the current coming back on the neutral. They should be equal. If a few thousandths of an amp are missing, that current is going home some other way — maybe through a wet hand to a grounded tap — and the GFCI trips before it can do serious harm. The ones built to protect people trip at around 5 mA. It does not care about total current (that is the breaker’s job), and it works without a ground conductor, because it only compares hot and neutral.
AFCI: protecting the house from fire
An arc-fault circuit interrupter watches the shape of the current for the signature of arcing: a cord crushed under furniture, a nail through a cable, a loose connection sparking in a box. Those arcs may draw too little current to trip a breaker but plenty to start a fire. The AFCI recognises the pattern and opens the circuit.
Breaker or fuse: too much current (overload or short) — protects the wiring. GFCI: current leaking out of the circuit — protects people. AFCI: arcing — protects against fire. Where GFCI and AFCI are required is a code list that changes between editions and between Canada and the US; always check your current book.
Try one: a 4,500 W water heater at 240 V
I = P ÷ V = 4,500 ÷ 240 = 18.75 A, on both hots. The same element on 120 V would draw a quarter of the power — see Ohm’s law and power.
En la obra
Tips from the job
- Before you start a circuit, know its load: add the nameplates. Watts ÷ volts gives the amps the circuit has to carry.
- Test a GFCI with its test button after you install it, and push reset. Tell the homeowner to test it too — the instructions say how often.
- When an AFCI trips repeatedly, it is usually telling you something real: a nicked cable, a loose connection, a shared neutral. Find it rather than swapping the breaker.
- Keep the neutral for each circuit with its own hot. Mixing neutrals between circuits trips GFCIs and AFCIs and confuses everyone after you.
Common mistakes
- Switching the neutral instead of the hot.
- Landing the feed on a GFCI receptacle’s LOAD terminals instead of LINE — the receptacle may not reset, or may not protect.
- Thinking a GFCI needs a ground to work. It doesn’t — it compares hot and neutral.
- Expecting a GFCI to stop an overload, or an ordinary breaker to stop a shock. Each device watches for one thing.
Check yourself
8 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.
Not started yet.
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Receptacles on the same branch circuit are connected to each other in…
Answer and why
B. In parallel: every receptacle gets the full circuit voltage, and one empty or unplugged receptacle doesn’t stop the rest.
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A kettle (1,500 W) and a toaster (900 W) run together on one 120 V circuit. Total current?
Answer and why
C. Parallel loads add: 1,500 ÷ 120 = 12.5 A and 900 ÷ 120 = 7.5 A, total 20 A. (Or 2,400 W ÷ 120 V = 20 A.) Whether that is too much for the circuit depends on its rating and your code book.
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What does a GFCI watch for?
Answer and why
A. A GFCI compares the current out and the current back. If they differ by a few milliamps, some current is going somewhere else — possibly through a person — and it trips.
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What does an AFCI watch for?
Answer and why
B. An AFCI looks at the shape of the current for the signature of an arc, which is how a lot of wiring fires start. It protects against fire; a GFCI protects people from shock.
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The hot carries 10.000 A out and the neutral brings 9.994 A back. How much is leaking somewhere else?
Answer and why
C. 10.000 − 9.994 = 0.006 A = 6 mA. That is enough imbalance for a people-protection GFCI to trip.
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In a switch loop (switch leg), the switch is placed in…
Answer and why
C. The switch interrupts the hot. Switch the neutral instead and the lamp holder is still live with the light off — a shock waiting for whoever changes the bulb.
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On a North American 120/240 V split-phase service, what voltage is there between the two hot conductors?
Answer and why
D. Each hot is 120 V to the neutral, and they are opposite each other, so between them is 240 V. That is what a dryer, range, water heater or baseboard heater uses.
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Where do you find which locations need GFCI or AFCI protection?
Answer and why
B. Where protection is required is a code rule that grows most code cycles and differs between Canada, the US and local amendments. Look it up in your current book.
Preguntas frecuentes
Can a GFCI protect receptacles downstream of it?
Yes. A GFCI receptacle has LINE terminals (the supply) and LOAD terminals; anything fed from LOAD is protected too. Where one GFCI protects several outlets, the downstream ones are generally labelled as GFCI-protected — the device instructions and your code say how.
What is a combination AFCI/GFCI (dual-function) device?
A breaker or receptacle that does both jobs: it watches for arcing and for current leaking to ground. It saves space where both protections are needed. Which you need where is a code question — check your book.
Ver también
- Single-pole switch wiring — the switch in the hot, traced.
- Switch loop wiring — power at the light, loop to the switch.
- GFCI wiring diagram — line versus load.
- AFCI vs GFCI — the short version.
- 240 V receptacle wiring — the two-hot circuit.
- Grounding and bonding — the lesson before this one.
- Trade math refresher — the next lesson.
- All first year lessons
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