Source: https://mokxi.com/learn/path/wire-fuses-and-breakers Updated: 2026-11-11 All paths # 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. Free 7 lessons 5 h 7 chapters 1,650 XP Electrical ยท Intermediate +150 XP 0 of 7 done Start the course free Copper resistance: every wire is a resistor Lesson plan for teachers 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. Open 236 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. 0 of 6 steps Start What is in it Chapter 2 ## Ampacity Heat is I squared R, and the insulation sets the limit. Locked 218 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. Locked 249 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. 0 of 7 steps Chapter 4 ## Fuse curves A real fuse blows on a short, a current limit saves it, and inrush fools it. Locked 249 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. 0 of 7 steps Chapter 5 ## Breaker trip types An inverse-time thermal trip, a hot reset refused, and the magnetic trip. Locked 218 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. Locked 249 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. 0 of 7 steps Chapter 7 ## Boss build Size a 12 V camper circuit: load, fuse, wire, drop and a fault test. Locked 231 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. Chapter 1 ### Copper resistance - 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. Chapter 2 ### Ampacity - 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. Chapter 3 ### Voltage drop - 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. Chapter 4 ### Fuse curves - 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. Chapter 5 ### Breaker trip types - 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. Chapter 6 ### Coordination - 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. Chapter 7 ### Boss build - 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. 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. Have a kit already? ### Find your kit Pick the starter kit you own and run every lesson in its manual with the same parts on the same pins. See every kit 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.