Circuit Breaker Box Diagram

Circuit Breaker Box Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable circuit breaker box diagram: download the diagram as SVG or open it and print to paper or PDF.

A circuit breaker box diagram maps the internal layout of a residential or light-commercial electrical panel, showing how each circuit breaker connects to the busbars, neutral bar, and ground bar to protect and distribute power throughout a building.

A circuit breaker box — also called a load centre (North America), consumer unit (UK/IEC), or distribution board — is the central point where incoming electrical supply is distributed to individual circuits through protective devices. A circuit breaker box diagram shows the physical arrangement of these protective devices, their ratings, the busbar arrangement, and the wiring connections from the incoming supply through to each circuit.

In a standard North American residential panel, two 120 V hot legs (Line 1 and Line 2) and a neutral arrive from the utility via a service entrance cable or mast. Inside the panel these feed a pair of vertical copper busbars. Each single-pole breaker snaps onto one busbar leg (120 V circuits: lighting, outlets). A double-pole breaker straddles both busbars (240 V circuits: electric range, dryer, air conditioner, water heater). A neutral bar and a separate grounding bar (or a combined neutral/ground bar in the main panel) terminate the neutral and ground conductors from each circuit.

In IEC-standard consumer units (230 V / 400 V systems used in most of the world outside North America), the incoming supply feeds a main switch or combined RCD/main switch. From there, outgoing ways feed individual miniature circuit breakers (MCBs) protecting individual circuits. RCDs (residual current devices — analogous to GFCI breakers) may be grouped in banks to protect multiple circuits, or individual RCBO units may protect each circuit independently.

The diagram must show every breaker position numbered, the rating (amperes) and type (standard, AFCI, GFCI/RCD) of each breaker, the circuit it protects, and the supply voltage. In North America, the NEC (NFPA 70) requires a circuit directory to be mounted inside the panel door. In the UK, BS 7671 requires a Schedule of Circuit Details.

How to wire circuit breaker box diagram

Identify the incoming supply details Determine the supply voltage (120/240 V split-phase in North America; 230 V or 400/230 V three-phase in IEC regions), the service amperage (e.g. 200 A), and the number of phases. This governs the panel type and busbar rating.
Assign each circuit breaker position a circuit and load List all circuits: lighting, general purpose outlets, kitchen small appliance circuits, large appliance (dryer, range, HVAC), and dedicated circuits. Assign each to a breaker position and determine the required ampere rating from the NEC or applicable code's load calculation method.
Determine breaker types required Identify which positions require standard, AFCI, GFCI, or combined AFCI/GFCI breakers per the applicable edition of NEC or local regulations. In IEC installations, determine which circuits need individual RCBO protection vs. grouped RCD coverage.
Draw the panel layout diagram Create a two-column numbered layout (odd/even for North American, sequential for IEC) showing each position, breaker pole count, rating, and the circuit name. Indicate the main breaker at the top. Show the neutral and ground bar positions and the incoming service conductors.
Annotate conductor sizes and types For each circuit, record the wire gauge (AWG in North America: 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A; or mm² in metric regions: 1.5 mm² for 10 A, 2.5 mm² for 20 A, etc.) and the insulation type. Record the feeder cable size from the meter to the main breaker.
Label each circuit on the circuit directory Produce a circuit directory card listing every breaker position number and the circuit it protects. Be specific: not 'bedroom' but 'Master bedroom lights and outlets — upper floor east'. Fit the card in the panel door sleeve and keep a copy with the building records.
Have the installation inspected A licensed electrician must install and certify the panel. In most jurisdictions a building permit and inspection by the authority having jurisdiction (AHJ) are required before the supply is energised. Do not energise the panel before inspection is passed.

Specifications

Standard residential service (North America)	120/240 V, 60 Hz, single-phase, 3-wire; 100–200 A service
Standard residential supply (IEC regions)	230 V / 400 V, 50 Hz; TN-S or TN-C-S (PME) earthing system
Minimum RCD/GFCI trip threshold (personal protection)	30 mA (IEC 61008); 5 mA (GFCI for personnel protection, UL 943)
AFCI requirement (NEC 2020)	210.12: all 120 V, 15 A and 20 A circuits in dwelling unit habitable spaces
Typical wire gauge: 15 A circuit	14 AWG copper (North America); 1.5 mm² (IEC)
Typical wire gauge: 20 A circuit	12 AWG copper (North America); 2.5 mm² (IEC)
Panel short circuit current rating (SCCR)	10 kAIC typical residential; 22–65 kAIC for commercial/industrial
Maximum number of circuits (typical residential panel)	20–42 circuit positions (varies by panel model and local code)

Safety warnings

All electrical panel installation, modification, and repair work must be performed by a licensed electrician with appropriate permits, and inspected by the authority having jurisdiction (AHJ). This applies in all regions including NEC/NFPA 70 (USA), BS 7671 (UK/IEC), AS/NZS 3000 (Oceania), and IEC 60364. This diagram is for educational and reference purposes only.
NEVER remove the dead-front cover of an energised panel without proper personal protective equipment (PPE) including arc flash-rated gloves and face shield, and without ensuring only authorised personnel are present. The busbars behind the dead-front cover remain live even when the main breaker is open.
In a subpanel, NEVER bond the neutral bar to the enclosure or grounding bar. Neutral-ground bonding belongs only at the main panel (or the first means of disconnect). Incorrect bonding creates shock hazards and causes GFCIs/RCDs to trip nuisance-free.
Always verify the supply is dead with a calibrated voltage tester before working on any wiring inside the panel. Do not rely on a breaker or switch position alone — verify with a tester at every terminal.
Aluminium wiring requires aluminium-rated terminals and anti-oxidant compound. Connecting aluminium conductors to copper-only terminals causes galvanic corrosion, loose connections, and fire risk. Check all terminal ratings before use.

Tools needed

Calibrated CAT III/IV voltage tester (non-contact and contact)
Digital multimeter (voltage, continuity, resistance)
Insulated screwdrivers (flat and Phillips) rated to IEC 60900
Torque screwdriver or wrench (to manufacturer terminal specifications)
Wire stripper and crimping tool
Knockout punch set (for new panel installation)
Cable management fish tape or pull rope
Circuit directory card and permanent marker

Common mistakes

Bonding neutral to ground in a subpanel, energising metallic enclosures and causing nuisance GFCI/RCD trips.
Overloading a circuit by doubling up on breaker positions (tandem/twin breakers used in positions not rated for them), exceeding the panel's maximum circuit count.
Using undersized wire for the breaker rating — for example, 14 AWG wire on a 20 A breaker — creating a fire hazard from sustained overload.
Failing to label circuits on the directory card, or labelling them vaguely, leading to incorrect isolation and energised circuit contact during maintenance.
Leaving open knockouts in the panel enclosure, which creates an entry point for vermin, moisture, and insects — all of which can cause faults and arcing.
Ignoring breaker compatibility requirements: not all breakers physically fit all panel brands, and non-listed breakers in a panel enclosure void the installation's safety listing and potentially the building insurance.

Troubleshooting

Circuit breaker trips repeatedly without apparent overload: Cause: Nuisance trip due to inrush current from motor loads, ground fault on the circuit, or a failing breaker with reduced trip threshold Fix: Check for ground faults with an insulation resistance tester on the circuit conductors. If no fault is found, measure actual current draw with a clamp meter. If current is within rating and there is no fault, the breaker may be worn and should be replaced.
No voltage on a circuit even though the breaker is in the ON position: Cause: Loose breaker connection to the busbar, loose terminal at the breaker, or a tripped breaker that did not fully reset Fix: Ensure the breaker is fully reset (press to TRIP then to ON). Check the voltage at the breaker output terminal with the dead-front cover on and using a non-contact tester through the wire entry. If no voltage, switch off the main breaker (with all safety protocols observed) and reseat the branch breaker on the busbar.
GFCI or RCD breaker trips when no ground fault seems present: Cause: Long cable runs with high natural capacitive leakage to earth, neutral-to-ground contact somewhere on the circuit, or accumulated leakage from multiple appliances on one RCD Fix: Measure the actual earth leakage current on the circuit with a leakage current clamp meter. Check that no neutral conductors are touching an earth/ground conductor or metal enclosure. If natural cable capacitance is the cause, split the load across two RCD-protected circuits.

Frequently asked questions

What is the difference between a main breaker panel and a subpanel?

A main panel receives power directly from the utility service entrance and contains the main breaker that disconnects the entire installation. A subpanel (sub-distribution board) is fed from the main panel by a feeder circuit and has its own breakers for a specific area or building section. In a subpanel, the neutral bar and ground bar must be kept separate — they are bonded only in the main panel.

Why must the neutral and ground be separate in a subpanel?

Bonding neutral to ground at a subpanel creates parallel neutral paths. Normal load current would then flow back through the grounding conductor rather than only through the neutral conductor. This energises metallic enclosures and conduits, creating a shock hazard and causing nuisance RCD/GFCI trips. The neutral-to-ground bond belongs only at the main panel or at the first means of disconnect.

How do I read the circuit numbering in a North American panel?

Circuits are numbered sequentially down each side: odd numbers (1, 3, 5…) on the left column, even numbers (2, 4, 6…) on the right. Position 1 and position 2 are on opposite busbar legs. A double-pole breaker occupies two adjacent positions — one on each leg — and its slots are typically labelled with both position numbers (e.g. 7/9 or 7-8 depending on layout).

What is an AFCI breaker and when is it required?

An Arc Fault Circuit Interrupter (AFCI) breaker detects the electrical signature of arc faults — dangerous sparking caused by damaged or loose wiring — and trips before a fire can start. NEC 2020 requires AFCI protection on nearly all 120 V, 15 A and 20 A circuits in dwelling units, including bedrooms, living rooms, hallways, kitchens, and laundry areas.

What size main breaker do I need for a residential panel?

Main breaker size depends on the service amperage provided by the utility and the calculated load of the building (per NEC Article 220 or equivalent). Typical residential services are 100 A, 150 A, or 200 A in North America. 200 A is the current standard for new residential construction. Only a licensed electrician performing a load calculation should determine the required service size.