Electrical · Lesson 4 of 13Intermediate9 min read

Circuits and Wiring

From the panel, wires fan out to every room. Each run is a branch circuit, with its own breaker and its own limit. This lesson explains how those circuits are wired, what the wire sizes and colors mean, and why plugging in one more appliance can trip a breaker.

Branch circuits are wired in parallel

A branch circuit is the wiring between the last breaker or fuse and the outlets it serves. A typical circuit feeds several outlets and lights. They’re wired in parallel: each one connects across the same hot and neutral wires, so each gets the full 120 volts, and turning one off doesn’t affect the others.

There’s a catch. In a parallel circuit, the currents add up. Every device draws its own current through its own branch, and all of it flows through the shared wire back to the breaker. The breaker sees the total.

Amps = watts ÷ 120 volts for each load. Add one more appliance and the total passes 20 amps. Wattages are typical label values within the ranges listed by Virginia Cooperative Extension.

I_total = I₁ + I₂ + I₃ + … (each I = watts ÷ 120 volts)

Wire size and ampacity

Wire size in the U.S. is given as a gauge number (AWG). The smaller the number, the thicker the wire. Thicker wire has less resistance, so it heats less for the same current. A wire’s ampacity is the current it can carry continuously without getting too hot.

That’s why every circuit’s breaker is matched to its wire. The rule is that conductors must be protected from overcurrent according to their ability to carry current safely. Typical pairings for copper wire, from University of Missouri Extension, are in the table. The code and the conditions of each installation set the exact limits, so this is a guide to reading a circuit, not to designing one.

Typical copper wire sizes and breakers (University of Missouri Extension); watts are amps × volts
Copper wire	Typical breaker or fuse	Watts at 120 V	Watts at 240 V
14 gauge	15 amps	1,800	3,600
12 gauge	20 amps	2,400	4,800
10 gauge	30 amps	3,600	7,200
8 gauge	40 amps	4,800	9,600

Cables, conduit and colors

Most homes use cable: insulated wires bundled in an outer jacket. Two-conductor cable has a black and a white wire; three-conductor cable adds a red one. Both usually include a bare or green-insulated ground wire too. In some buildings the wires run instead through a raceway such as metal or plastic conduit, an enclosed channel built to hold wires.

The insulation colors tell you each wire’s job:

Black or red (sometimes blue): hot, or “ungrounded,” wires that carry voltage.
White or gray: the neutral, or “grounded conductor,” that carries current back.
Green, green with yellow stripes, or bare: the equipment ground, which carries current only when something goes wrong.

Colors help, but they aren’t proof. Wiring can be old, altered or done wrong, and insulation colors vary by country. Never trust a color to tell you a wire is safe to touch: electricians test (Lesson 9).

Splices and connections are made inside boxes, such as junction boxes and the boxes behind outlets and switches, usually with twist-on wire connectors. Outlets and switches are called “devices”: parts that carry electricity but don’t use it.

Dedicated circuits

Some appliances need a circuit of their own. Electric ranges, clothes dryers and water heaters get 240-volt circuits. Level 2 electric-car chargers usually need a dedicated circuit too; DOE’s Alternative Fuels Data Center says a typical 30-amp home charger requires a dedicated 40-amp circuit under the National Electrical Code. The U.S. Fire Administration says to plug major appliances such as refrigerators, stoves, washers and dryers directly into a wall outlet and never use an extension cord with them.

Why an overloaded circuit trips

Add up the watts of everything running on one circuit and divide by 120 volts. If the total is more than the breaker’s rating, it will trip, quickly for a big overload and slowly for a small one (Lesson 3). For example, a 1,500-watt space heater plus a 1,000-watt microwave is 2,500 watts. Divided by 120 volts, that’s about 21 amps, too much for a 15-amp circuit and even a 20-amp one.

The fix is to move a load to a different circuit, not to fit a bigger breaker. Single plug-in loads have limits too. OSHA’s workplace rules cap one cord-and-plug load at 12 amps on a 15-amp receptacle and 16 amps on a 20-amp receptacle, 80 percent of the rating. Try it in the kitchen below.

Wattages are typical label values within the ranges listed by Virginia Cooperative Extension and DOE; check your own appliances. Time is sped up, and the breaker uses the same trip model as Lesson 3.

Older wiring: aluminum branch circuits

Some homes built between 1965 and the mid-1970s were wired with aluminum branch-circuit wire. CPSC reports that homes built before 1972 and wired with aluminum were 55 times more likely to have one or more connections at outlets reach “fire hazard conditions” than homes wired with copper. The connections can overheat without warning. Signs CPSC lists include face plates on outlets or switches that are hot to the touch, flickering lights, circuits that don’t work, and the smell of burning plastic at outlets or switches. CPSC’s advice is to have a qualified electrician make one of its recommended repairs, and not to try it yourself.

Parts and what they do
Part	What it does	Signs of trouble
Branch circuit	Carries power from one breaker to its outlets and lights.	Trips when too much is plugged in.
Hot wire (black or red)	Carries the voltage from the breaker.	Damaged insulation; scorching at a connection.
Neutral (white or gray)	Carries current back to the panel.	Any problem here needs an electrician.
Equipment ground (green or bare)	Gives fault current a safe path back so the breaker trips.	Two-slot outlets in older homes have none.
Cable or conduit	Holds and protects the wires.	Cut, crushed or cracked jackets.
Box	Encloses splices and devices.	Missing covers; loose devices.
Wire connectors	Join wires inside a box.	Heat, discoloration or a burning smell at an outlet or switch.
Dedicated circuit	Serves one big appliance alone.	Sharing it with other loads; extension cords on big appliances.

StopWhen to call an electrician

Running a new circuit, adding outlets, changing a breaker, or working on aluminum wiring is work for a licensed electrician, usually with a permit and inspection. Never “fix” a breaker that keeps tripping by putting in a larger one: the wire is the weak link, and it would overheat first. Warm or discolored outlets and switch plates, flickering lights, or a burning smell mean stop using that outlet or switch and have it checked.

You canWhat you can safely do

Read appliance labels and add up the amps on each circuit.
Move a heater, kettle or hair dryer to a different circuit when a breaker trips.
Map which outlets each breaker controls, with a helper and a lamp.
Plug major appliances straight into the wall, never into an extension cord.

Check yourself

Tap an answer to check it, or open “Show the answer.”

Outlets on a branch circuit are wired in parallel. What does that mean for current?
Show the answer
The currents of all the running devices add up at the breaker. Each device draws its own current, and the shared wire and breaker carry the total.
Which is thicker: 12-gauge or 14-gauge wire?
Show the answer
12-gauge. Smaller gauge numbers mean thicker wire. 12-gauge is typical for 20-amp circuits, 14-gauge for 15-amp.
What is the green or bare wire for?
Show the answer
It gives fault current a path so the breaker trips. The equipment ground carries current only when something goes wrong, such as a hot wire touching a metal case.
A 1,500-W heater and a 1,000-W microwave run on one 15-amp circuit. What happens?
Show the answer
About 21 amps flows, so the breaker trips. 2,500 W ÷ 120 V ≈ 20.8 amps, more than 15.
A breaker keeps tripping when two appliances run. What’s the safe fix?
Show the answer
Move one appliance to another circuit. The breaker protects the wire. A bigger breaker would let the wire overheat.

Go deeper

University of Missouri Extension G1409: Is Your Wiring System Safe and Energy Efficient? Wire sizes, service size and signs of an inadequate system.
U.S. Consumer Product Safety Commission: Repairing Aluminum Wiring (Publication 516, 2011) CPSC on aluminum wiring and the approved repairs.
U.S. Consumer Product Safety Commission: CPSC Guide to Home Wiring Hazards (Publication 518) Warning signs in home wiring.
Science Lab: Circuit Builder Wire series and parallel circuits and compare the currents.
Being an Adult 101: Reset a Tripped Circuit Breaker The appliance-label math, step by step.

Sources

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