Open the simulator

Wired it wrong? Find out why.

Build a circuit, switch it, break it. WireWhy calculates the current in every wire, so you see why the GFCI trips, the breaker holds, or the light stays dark.

Open the simulator or start the course →

No account needed · Computers, tablets, and phones · 120/240 V, 60 Hz · NEC words, AWG, and feet

Bathroom light · demo panel
Service 120/240 V L1NG GFCI 15 A CLOSED Switch Neutral bar Light 1 A Service 120/240 V L1NG GFCI 15 A CLOSED Switch Neutral bar Light 1 A
Through the GFCI

L 0.00 A · N 0.00 A · ΔI 0.00 A

The GFCI trips at a 5 mA difference.

What happened

Switch open. Nothing flows.

The light's neutral is on the GFCI breaker's load neutral.

Readings and the finding are the simulator's results for this board. Open this board in the simulator →

The circuit is solved, not just drawn.

A wiring diagram shows what is connected. WireWhy works out what that wiring does: every time you add a wire, flip a switch, or put in a fault, the whole circuit is calculated again.

  1. Your circuit

    Devices, terminals, wires, and the state of every switch and breaker.

    Service 120/240 V GFCI 15 A, 5 mA Switch closed
  2. Solving

    Steady-state AC, by modified nodal analysis with complex phasors.

    V at every terminal I in every wire
  3. Protection

    The fastest protective device that should trip, trips. Then the circuit is solved again, until nothing changes.

    ΔI 1.00 A ≥ 5 mA GFCI trips after 20 ms
  4. What you see

    Currents in the wires, the trip, and a finding in plain words, with the current's path marked wire by wire.

    Finding: critical wires 1 → 5
Water heater on the demo board
load
39 A
cable
14 AWG
breaker
F2 B32
instant trip
no: that needs 5 × 32 A = 160 A
cable
overheats until its insulation fails
found by
the installation tester, insulation resistance
Metal light fixture, ground left off
fault
hot to the fixture's metal
ground terminal
120 V to ground
protection
nothing trips
with PE wired
the breaker or the GFCI clears it

Four things to try in the first five minutes

Follow the current that trips the GFCI

Open the demo panel and switch the bathroom light on. The GFCI trips, and the wires that carry the stray current are numbered from the service.

GFCI tripped after 20 ms at 1.00 A cause: the light's neutral bypasses the GFCI

A traced GFCI trip: numbered wires show the current that bypasses the GFCI breaker's neutral.

Put a fault on a device

A ground fault, a short circuit, or an overload, on any device. Leave the ground off a metal fixture and it shows the voltage you would touch.

Metal fixture: ground terminal at 120 V nothing trips

A ground fault on a metal light fixture with its ground left off: the fixture shows 120 V in red, and the finding explains why touching it is dangerous.

Measure it before you are told the answer

Clip the multimeter to any two terminals, or take the tester for continuity and insulation, with every reading in a results table.

Light hot to neutral: 120 V Ground continuity, panel to light: 0.01 Ω

The multimeter clipped to a light's screws, reading about 120 V, with the reading recorded in the results table against 120 V ± 10 %.

Find the hidden fault

One fault is hidden in a working circuit: a failed light, a loose screw, a ground fault. Switch, measure, name the device or wire at fault, and get a score for the answer, the reading that proves it, and lockout before dead tests.

Hall light dark: hot to neutral 120 V E1, failed device: 85 / 100

A fault board answered: 120 V across the hall light that stays dark is recorded, the light is named as failed, and the answer scores 85 out of 100.

What WireWhy models

Low-voltage installations as an apprentice meets them: panels, lighting, receptacles, motor control, and the tests that prove them.

Supplies120/240 V 60 Hz split-phase · 120/208 V three-phase · 230/400 V 50 Hz and others for comparison
Groundingthe service's neutral bonded to ground at the main panel · an open neutral · touch voltage on exposed metal
Protectionbreakers 15–60 A, one and two poles · GFCI breakers and receptacles (5 mA) · AFCI breakers · fuses · multiwire branch circuits on a 2-pole breaker (NEC 210.4)
Boardpanelboard with a main breaker, 12 to 42 spaces on two legs · neutral and ground bars · surge protectors
WireNM-B in AWG, rated to NEC Table 310.16 · 14 AWG on 15 A, 12 AWG on 20 A (240.4(D)) · overheating when the breaker does not cover the wire
Switchingsingle-pole, 3-way, 4-way, in one, two, or three gangs · push-buttons
Controlcontactors that hold themselves in · interlocked reversing starters · star-delta starters · overload relays · motor protection breakers · phase monitoring relays · time relays · impulse relays · staircase timers · time on the canvas: watch it run, or skip to the next event
Loadslights · water heaters · NEMA receptacles · GFCI receptacles · motors, single- and three-phase, with a terminal box for star or delta · indicator lights
Faultsground fault · short circuit · overload · open or poor contact · failed device · a neutral bypassing a GFCI · a neutral touching ground · a shared neutral on the same leg · an open neutral · ungrounded metal
Instrumentsmultimeter V, Ω · voltage tester with lockout and verify dead · receptacle tester · installation tester: continuity, insulation resistance, loop impedance, GFCI trip time and current, phase rotation · a results table and a test report
Drawingswiring diagram · single-line diagram · board layout · terminal plan · breaker labels

What it is not

  • Not SPICE: no transistors and no transients, only installation wiring at 60 Hz.
  • Not a certified design package: the drawings are for training and planning.
  • Not a substitute for testing a real installation, or for a qualified electrician.
  • No arcs and no surges are solved: an AFCI breaker trips as a GFCI breaker would, and a surge from the service reaches the surge protector by a rule.

Which rules it follows

  • The NEC's main rules for dwellings: GFCI protection, small-conductor sizing (240.4(D)), multiwire branch circuits (210.4); the simulation itself follows physics, not a code book.
  • Local amendments and other code editions are not modeled; check with your inspector.
Everything WireWhy does →

Who uses it

Learning

A course from the first light to motor control, a lab for every device, fault boards and endless fault practice, and US exercises: GFCIs, 3-way and 4-way switches, a 240 V water heater, multiwire branch circuits.

Start the course →

Teaching

Show a shared neutral tripping a GFCI, or an open neutral putting 240 V across two 120 V loads, as often as the class needs. Build exercises that mark themselves, and let a class follow your canvas with a short code.

Working electricians

Check what a customer's odd wiring really does before it goes into a wall: what trips, and what stays hot.

Why I built it

Circuit simulators either teach electronics at component level, or let you draw wiring without saying what the installation does. I wanted one where you could wire a bathroom light wrong, switch it on, and see why the RCD trips.

Mateusz Hinca, WireWhy

Questions

Is it free?

Yes. The canvas, the simulation, exercises, sharing, and picture export are free to use.

Do I need an account?

No. Open the canvas and start. Your circuit is kept in your browser until you share it.

Does it work on a phone or tablet?

Yes. Devices, wires, and the simulation work by touch; a larger screen is more comfortable for big circuits.

Can I use it with my class?

Yes. Share an exercise as a link, or start a follow-along session: the class joins with a short code and sees your canvas live.

Build a circuit. Break it. See what happens.

Open the simulator