DIY Repair Guide

Wiring & Electrical

Circuits, gauges and devices. Drawn out, with the code limits.

Circuits, conductors, devices and the code limits that apply to them. Drawn out, because a wiring question is almost always easier to answer with a diagram than with a paragraph.

These guides cover what fits what, how much a circuit can carry, and where the line sits between homeowner work and licensed work.

The Principle Behind Most Electrical Rules

Nearly every rule in residential wiring exists to keep a conductor from carrying more current than it can shed as heat. Once that clicks, the rules stop being arbitrary.

The breaker protects the wire. It does not protect the appliance. That single sentence explains why 14 American Wire Gauge (AWG) gets a 15-amp breaker and 12 AWG gets 20, why fitting a larger breaker to stop nuisance tripping is dangerous rather than merely non-compliant, and why a longer run sometimes needs thicker cable for the same load.

Load, Not Outlet Count

A persistent myth holds that a circuit may carry ten outlets. For dwellings the National Electrical Code sets no receptacle limit at all. That figure comes from a commercial calculation of 180 volt-amperes per outlet, applied to homes where it does not belong.

What does constrain you is watts. A 20-amp circuit carries 2,400 watts flat out and 1,920 for anything running continuously. Twenty LED lamps is under three amps. Two space heaters is twenty-five and will trip. The count told you nothing.

Where the DIY Line Sits

Rules vary by jurisdiction, and some places require licensed work for anything inside a box. Broadly, though, the risk gradient is consistent.

  • Generally reasonable: replacing a like-for-like receptacle or switch, changing a light fixture, resetting a GFCI, identifying a circuit.
  • Get advice first: adding a circuit, wiring a 3-way, anything on aluminum branch wiring, anything in a bathroom or outdoors.
  • Licensed only: anything inside the panel, service changes, subpanels, and any circuit above 30 amps in most places.

Two Habits Worth Adopting

First, verify dead before you touch it, every time, with a non-contact tester. Panel labels are wrong often enough that treating them as authoritative is how people get hurt.

Second, use the screw terminals rather than the back-stab holes on receptacles and switches. Loop the wire clockwise so tightening pulls it in. Back-stabbed connections work loose over years of heat cycling, and a loose connection arcs, which is the failure behind a great many intermittent outlets and a number of house fires.

GFCI and AFCI: What Each One Watches For

Two protective devices appear throughout these guides, the ground-fault circuit interrupter (GFCI) and the arc-fault circuit interrupter (AFCI), and they are frequently confused because both live in the same places and both trip.

Ground-fault circuit interrupter (GFCI) devices compare the current going out against the current coming back. If they differ by a few milliamps, current is escaping somewhere it should not, potentially through a person, and it cuts in milliseconds. Required in kitchens, bathrooms, garages, basements, laundry areas and outdoors, and it exists to protect people.

Arc-fault circuit interrupter (AFCI) devices listen for the electrical signature of arcing, the sputtering discharge that precedes many electrical fires. Required on most living-area circuits under current code, and it exists to protect the building.

They solve different problems, which is why some locations now require both. A GFCI will not detect an arcing connection inside a wall, and an AFCI will not notice current leaking to earth through someone standing in a wet basement.

Reading What Is Already in Your Walls

Before adding to an existing circuit, work out what you have. Three things tell you nearly everything.

  • The gauge printed on the cable sheath. A marking of 12/2 means 12 AWG, two conductors plus ground. That determines the maximum breaker regardless of what is fitted.
  • The breaker rating. If it disagrees with the wire, the wire is right and the breaker is a problem.
  • Whether there is a ground. Two-slot outlets usually mean an ungrounded circuit, which carries specific rules about what may be fitted and how it must be labelled.

Labelling the Panel Is Worth an Afternoon

Panel labels are wrong in a large share of houses, either because circuits were added and not recorded or because rooms were repurposed. That is a nuisance most of the time and a hazard when someone relies on a label instead of a tester.

Mapping it properly takes one person a couple of hours with a plug-in outlet tester or a lamp. Switch off one breaker at a time and walk the house recording every dead outlet, light and appliance. Write the result on a printed sheet inside the panel door rather than on the tiny factory strip, which never has room for anything useful.

Do it once and every subsequent electrical job in the house gets faster and safer.