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Wire, fuses and breakers
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.
0 of 7 done
What you will build
0 projects, built step by step
7 lessons in 7 chapters, about 5 hours on a simulated breadboard. Free, in your browser, every step checked by the simulator.
The map
Chapter by chapter
Each lesson opens when the one before it is done, and each chapter ends in a project.
Chapter 1
Copper resistance
Every wire is a resistor: the AWG table, the round trip, and hot copper.
- Open236 XP
Copper resistance: every wire is a resistor
A 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.
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Chapter 2
Ampacity
Heat is I squared R, and the insulation sets the limit.
- Locked218 XP
Ampacity: how much current before it cooks
Thin 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.
0 of 6 steps
Chapter 3
Voltage drop
An Uno computes the drop for a gauge, a length and a load.
- Locked249 XP
Voltage drop: the volts a run loses
An 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.
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Chapter 4
Fuse curves
A real fuse blows on a short, a current limit saves it, and inrush fools it.
- Locked249 XP
Fuses: time against current
A 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.
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Chapter 5
Breaker trip types
An inverse-time thermal trip, a hot reset refused, and the magnetic trip.
- Locked218 XP
Breakers: thermal and magnetic trips
An 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.
0 of 6 steps
Chapter 6
Coordination
Branch breakers against a main, and a fused feeder that clears the right fault.
- Locked249 XP
Coordination: the right one opens
A 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.
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Chapter 7
Boss build
Size a 12 V camper circuit: load, fuse, wire, drop and a fault test.
- Locked231 XP
Boss build: size a 12 V camper circuit
Size 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.
0 of 7 steps
Syllabus
Every lesson in the course
All 7 lessons, in order, each with what it covers. Any of them opens in the editor right away.
Copper resistance
- 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.
Ampacity
- 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.
Voltage drop
- 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.
Fuse curves
- 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.
Breaker trip types
- 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.
Coordination
- 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.
Boss build
- 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.
Kits
Build it on your bench too
Every lesson runs in the simulator with nothing to buy. When you want the real thing, these kits have the parts.
Questions
About Wire, fuses and breakers
Is Wire, fuses and breakers free?
Yes. All 7 lessons of Wire, fuses and breakers are free and run in your browser, with nothing to install and no parts to buy before you start.
Do I need any parts?
No. Every circuit is built on a simulated breadboard, and the simulator checks each step as you go. When you want to build it for real, the same parts and wiring work on a real bench.
How long does Wire, fuses and breakers take?
About 5 hours: 7 lessons in 7 chapters, each one a few short steps. Your progress is kept, so you can stop after any lesson and pick up where you left off.
Where do I start?
With "Copper resistance: every wire is a resistor", and each lesson builds on the one before it. New to circuits? Electronics basics is the course that starts from zero.