Circuits, Breakers & Panels
Every wire in a building traces back to a panel. This lesson explains how breakers work, why GFCIs and AFCIs exist, and how to read a residential service panel without dying.
The lesson
Service entrance
Power enters from the utility through a meter, then a main breaker (typically 100A, 200A, or 400A). The main breaker is the ONLY thing between you and the utility transformer — turning it off de-energizes the panel except the meter side.
Hot, neutral, ground
Two ungrounded (hot) conductors enter the panel at 240V between them, 120V each to neutral. Neutral is bonded to ground ONLY at the service panel. Subpanels keep neutral and ground separated. Mixing them downstream creates dangerous current loops.
Breakers vs fuses
Breakers trip and reset. Fuses blow and must be replaced. Both protect WIRES, not equipment. A 20A breaker is sized to protect a 12AWG wire (15A breaker = 14AWG). Never upsize a breaker without replacing the wire.
GFCI — ground fault protection
A Ground Fault Circuit Interrupter compares current going OUT vs coming back. A 5mA imbalance trips in milliseconds — saves lives in wet/grounded spaces (kitchens, bathrooms, outdoors, garages). Required by code in those locations.
AFCI — arc fault protection
An Arc Fault Circuit Interrupter detects the high-frequency signature of dangerous arcing (loose wire, stapled-through romex). Required in most living spaces of newer construction. Different protection mode than a GFCI — they solve different problems.
Tool list
- Insulated screwdrivers (1000V)
- Voltage tester (Wiggy or solenoid type for confirming dead)
- Lockout/tagout kit
- Headlamp (panel rooms are dark)
- AFCI / GFCI tester
- NEC code book or app
Safety — Read or get hurt
- !!Never work in a live panel without proper PPE — arc flash gear required for hot work over 240V.
- !!Lock and tag the main breaker before opening the panel cover.
- !!Neutral wires under load carry the full circuit current — never disconnect a neutral on a live circuit.
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