Open the editorEditor
Lesson plan

Wire, fuses and breakers lesson plan

Why wire gets hot and how to stop it: copper resistance, ampacity, voltage drop, fuse curves, thermal and magnetic breakers, coordination, and a 12 V camper circuit you size yourself.

7 chapters, 7 lessons, about 5 hours, no board. Every lesson runs in the browser, with nothing to install.

Start a class from this course
The unit

Objectives and materials

By the end, students can

  • Every wire is a resistor: the AWG table, the round trip, and hot copper.
  • Heat is I squared R, and the insulation sets the limit.
  • An Uno computes the drop for a gauge, a length and a load.
  • A real fuse blows on a short, a current limit saves it, and inrush fools it.
  • An inverse-time thermal trip, a hot reset refused, and the magnetic trip.
  • Branch breakers against a main, and a fused feeder that clears the right fault.
  • Size a 12 V camper circuit: load, fuse, wire, drop and a fault test.

Materials for the whole unit

  • Arduino Uno R3
  • Battery: AA, coin cell, LiPo
  • Bench power supply x 5
  • Character LCD with I2C backpack
  • Current monitor, INA219
  • Current sensor, ACS712 x 4
  • DC motor
  • Filament lamp x 5
  • Full-size breadboard
  • Half breadboard x 4
  • LED x 2
  • Limit switch, lever microswitch
  • Multimeter x 2
  • Potentiometer x 3
  • Pushbutton x 2
  • Relay module
  • Resistor x 10
  • RGB LED x 4
  • Slide switch x 2

Every part is already on the simulator’s bench, so there is nothing to buy or hand out. The counts are the most any one lesson uses, if you build it on real breadboards too.

Chapter 1. About 40 min

Copper resistance

Every wire is a resistor: the AWG table, the round trip, and hot copper.

LessonTimeWhat students do
Copper resistance: every wire is a resistorA 12 V supply feeds a light bar through a run of wire: the AWG table in ohms per 1000 ft, why the run counts twice, how gauge, length and heat change the volts that arrive.40 min
  1. Run it: a short, fat run
  2. Ohms per 1000 feet
  3. The run counts twice
  4. Three sizes halves it
  5. Hot copper resists more
  6. Predict: half the run

Materials

  • Bench power supply
  • Multimeter x 2
  • Resistor x 3

Check for understanding

Each student finishes "Copper resistance: every wire is a resistor" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Predict: half the run".

Chapter 2. About 40 min

Ampacity

Heat is I squared R, and the insulation sets the limit.

LessonTimeWhat students do
Ampacity: how much current before it cooksThin and thick wire carrying the same current: heat is I squared times R, why 1.5 times the current is 2.25 times the heat, what the ampacity tables mean, and a filament as a wire run hot on purpose.40 min
  1. Run it: one current, two wires
  2. Heat in the thin wire
  3. Heat in the thick wire
  4. Predict: 1.5 times the current
  5. A filament is a wire run hot
  6. What ampacity means

Materials

  • Bench power supply x 2
  • Filament lamp
  • Multimeter x 2
  • Resistor x 3

Check for understanding

Each student finishes "Ampacity: how much current before it cooks" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "What ampacity means".

Chapter 3. About 45 min

Voltage drop

An Uno computes the drop for a gauge, a length and a load.

LessonTimeWhat students do
Voltage drop: the volts a run losesAn Uno computes the round-trip drop for a gauge, a length and a load, flags 3 and 5 percent, names the smallest wire that fits, and checks its own math on a scaled bench stand-in.45 min
  1. Run it: a long 12 AWG run
  2. Shorten it to 100 ft
  3. One size thinner
  4. Find the wire that fits
  5. Predict: half the length
  6. The bench check
  7. Do the 12 V math

Materials

  • Arduino Uno R3
  • Character LCD with I2C backpack
  • Current monitor, INA219
  • Half breadboard x 2
  • Potentiometer x 3
  • Pushbutton x 2
  • Resistor x 2

Check for understanding

Each student finishes "Voltage drop: the volts a run loses" on their own, without opening the hints. The simulator checks 7 of its 7 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Do the 12 V math".

Chapter 4. About 45 min

Fuse curves

A real fuse blows on a short, a current limit saves it, and inrush fools it.

LessonTimeWhat students do
Fuses: time against currentA meter's own fuse blows on a dead short, a current-limited supply keeps it alive, and five cold lamps blow a fuse four times their running current: inrush, time-current curves, fast and time-delay fuses.45 min
  1. Run it: a fused meter
  2. Short it
  3. A current limit saves it
  4. Four lamps
  5. Predict: a fifth lamp
  6. Ride through the surge
  7. Reading a fuse curve

Materials

  • Bench power supply x 2
  • Filament lamp x 5
  • Multimeter x 2
  • Resistor
  • Slide switch

Check for understanding

Each student finishes "Fuses: time against current" on their own, without opening the hints. The simulator checks 7 of its 7 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Reading a fuse curve".

Chapter 5. About 40 min

Breaker trip types

An inverse-time thermal trip, a hot reset refused, and the magnetic trip.

LessonTimeWhat students do
Breakers: thermal and magnetic tripsAn Uno runs an inverse-time overload on a motor: a jam trips in seconds, a mild overload takes much longer, and a hot trip refuses to reset. Then the magnetic trip and the B, C and D curves.40 min
  1. Run it and start the motor
  2. Jam it
  3. Too hot to reset
  4. Predict: a mild overload
  5. Cool down, reset
  6. The magnetic trip, and B, C and D

Materials

  • Arduino Uno R3
  • Bench power supply
  • Current sensor, ACS712
  • DC motor
  • Half breadboard
  • LED x 2
  • Limit switch, lever microswitch
  • Pushbutton x 2
  • Relay module
  • Resistor x 3
  • Slide switch

Check for understanding

Each student finishes "Breakers: thermal and magnetic trips" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "The magnetic trip, and B, C and D".

Chapter 6. About 45 min

Coordination

Branch breakers against a main, and a fused feeder that clears the right fault.

LessonTimeWhat students do
Coordination: the right one opensA panel monitor checks four branch circuits against their breakers and a main, then a fused 12 V feeder shows why the device nearest the fault must open first, and what a fault upstream of it does.45 min
  1. Run it: four branches and a main
  2. The 80 percent line
  3. One branch overloads
  4. Predict: every branch at its limit
  5. A branch fault
  6. A feeder fault
  7. Where coordination is the law

Materials

  • Arduino Uno R3
  • Bench power supply x 5
  • Current sensor, ACS712 x 4
  • Filament lamp
  • Full-size breadboard
  • Half breadboard x 4
  • LED
  • Multimeter x 2
  • Resistor x 10
  • RGB LED x 4
  • Slide switch x 2

Check for understanding

Each student finishes "Coordination: the right one opens" on their own, without opening the hints. The simulator checks 7 of its 7 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Where coordination is the law".

Chapter 7. About 45 min

Boss build

Size a 12 V camper circuit: load, fuse, wire, drop and a fault test.

LessonTimeWhat students do
Boss build: size a 12 V camper circuitSize a real 12 V branch from scratch: add up the load, pick the fuse with the 125 percent rule, pick the gauge for 3 percent drop, prove the fuse clears a chafed wire, then resize it for a longer run.45 min
  1. Run it: the first try
  2. Add up the load
  3. Pick the fuse
  4. Pick the wire
  5. Prove the fuse
  6. Make it yours: a longer run
  7. Your sizing sheet

Materials

  • Battery: AA, coin cell, LiPo
  • Filament lamp
  • Multimeter x 2
  • Resistor x 3
  • Slide switch

Check for understanding

Each student finishes "Boss build: size a 12 V camper circuit" on their own, without opening the hints. The simulator checks 7 of its 7 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Your sizing sheet".

Your class

Teach this unit as a class

Start a class from this course and its 7 lessons are assigned in one go, in order. Students join with a six-character code or printed login cards, and each one’s work comes back to you.

The class tools are free for 30 days for one class of up to 20 students, with no card. The lessons themselves are free either way.

Start a class from this courseSee the demo class
More units

Other lesson plans

Electronics basics20 lessons, about 10 h.Embedded on the Arduino Uno16 lessons, about 12 h.Embedded on the ESP32-C313 lessons, about 11 h.Embedded on the Raspberry Pi Pico7 lessons, about 6 h.Build your own fight stick5 lessons, about 4 h.Embedded on the STM32F4116 lessons, about 6 h.C++ on a board13 lessons, about 10 h.Digital logic and memory4 lessons, about 4 h.Interactive devices on the Uno9 lessons, about 8 h.From gates to a computer23 lessons, about 16 h.Build a claw machine8 lessons, about 6 h.Every unitAll the courses as ready-made units, on the teachers page.