Electrical Circuit Builder

You draw a motor control or power circuit on a numbered sheet, wire it, and press Run. Contactors pull in, motors turn, overload relays trip and a short circuit trips the breaker. A built-in check tests the wiring rules and then operates the circuit by itself. The sheet exports as a PDF with a title block, wire numbers, parts list, wire list, terminal plan and motor circuit table.

It is a drawing and logic-checking tool. The model is idealised: steady-state voltages, simple thermal and timer models. There is no inrush, no voltage drop and no fault-current calculation. It does not replace your design calculations, IEC 60204-1, local wiring rules or the manufacturer’s coordination tables. A qualified person must review any circuit before it is built or energised.

Who it is for: freshers learning how a starter works, and the design team that wants a quick, consistent starter drawing with a parts and wire list.

1. Press the button below, or choose Direct-on-line (DOL) motor starter in Load an example (the page opens with it by default).
2. Press ▶ Run in the pinned controls at the top right. The mode shows Running.
3. Click the START push-button (S2) on the sheet. K1 pulls in and the motor M1 runs forward. Let go of the mouse: K1 stays on. It is sealed in by its own auxiliary contact.
4. Click STOP (S1). K1 drops out and the motor stops.
5. Click ▶ Run again, run the motor, then click the overload relay F2. It trips, opens its NC contact in the control circuit and the motor stops. Click it again to reset.
6. Press ✎ Edit to go back to drawing. Open Check the circuit and press Check circuit.

The check reports 0 errors, 0 warnings. In its automatic run, pressing START briefly makes M1 run forward at 0.5 s and K1 pull in, and it stays on after the button is released (sealed in). Starting and then pressing STOP stops M1 at 3.5 s.

Load this example

Pinned simulation controls (top right). Run, Pause, Step, Reset and Edit (stop and go back to drawing), a speed choice from 0.25× to 4×, a mode label (Editing or Running) and the simulated time. They stay on screen as you scroll.

User manual button. Next to Full screen in the toolbar. It opens this manual in a panel on the right, also in full screen, so you can read it and load its examples while you work. Close or Esc shuts it.

Sheet toolbar. Load an example, Undo, Redo, Rotate, Duplicate, Delete, Clear (empties the sheet), Save .fec file, Open file, Full screen (gives the sheet the whole window), Light sheet (switches the sheet colours for printing and screenshots; press again to go back), zoom −, + and Fit, and Wire numbers Show or Hide.

Sheet tabs. Each tab is one drawing sheet. + Sheet adds one and Rename renames the current one. Wire numbers, parts list and wire list run across all sheets.

Palette (left of the sheet). Symbols grouped as breakers, isolators, rotary cam switches, fuses, contactors, push-buttons and switches, sensors, lamps, loads, motors, drives, PLC points, text notes, custom blocks, sheet links and about 250 IEC 60617 symbols. Expand all and Collapse all open or close every group.

Settings panel. Click a symbol or a wire and its settings appear under the sheet: designation, ratings, contact type, coil, wire colour and cross-section.

Below the sheet, the page continues with Motor circuit selector, Simulate, Check the circuit and Parts, wire list and PDF, each as its own numbered section.

Placing a symbol

Drag a symbol from the palette onto the sheet, or tap it to place it. Click it to select it. R rotates the selected symbol and Delete removes it. Duplicate copies it.

Drawing a wire

Drag from the small circle on one terminal to a circle on another terminal. Select a wire to set its colour, cross-section and cable type. Drag the handle on the middle run of a wire to move the run.

Net labels instead of long wires

Put a net label (L1, L2, L3, N, PE, 24V, 0V) on any terminal. All labels with the same name are connected without a drawn wire, which keeps a busy sheet readable. All earth labels join one protective-earth net.

Designations

Reference designations (Q1, K1, M1…) are assigned automatically and can be overridden in the settings panel. A contact takes the designation of the coil it belongs to. Designations must be unique; the check reports duplicates.

Contacts and coils

Drop a contactor or relay coil, then drop its contacts and choose that coil in the contact’s settings. A coil shows where its contacts are (the cross-reference under it) and each contact shows its coil.

Several sheets

Use + Sheet. A sheet link is a wire that leaves one sheet and arrives on another.

Press Run, then operate the circuit with the mouse:

• Push-buttons are momentary: they hold while the mouse is down.
• Selector switches and the E-stop latch on each click.
• Breakers switch when clicked.
• Overload relays trip when clicked; click again to reset.

Colours while running. Amber lines are a phase, blue the neutral, red and slate the DC positive and negative, green the earth. Grey lines are dead. Motor, contactor, lamp and drive states are listed under the sheet.

Editing the sheet stops the simulation. Use Pause to freeze it and Step to move on one change at a time.

What the simulator decides

• It builds the nets from wires and labels, closes the contacts in their present state and finds which nets are connected to each source.
• Each load is compared with its rating. It runs between 85 and 115 percent of rated voltage; outside that you get an undervoltage or overvoltage flag, and an AC/DC mismatch is flagged.
• A path joining two different sources is a short circuit, and the lowest-rated breaker or fuse in the path trips or blows.
• A three-phase motor runs in the direction set by the phase order, so swapping two phases reverses it.
• Above 105 percent of the relay setting the thermal element heats up and trips, faster at higher current. It then opens NC 95-96 and closes NO 97-98.
• A frequency inverter needs power on L1, L2, L3 and runs when exactly one of its forward and reverse inputs sees 24 V.

The PLC input module reads 24 V DC signals. The output module switches its channels from a few lines of logic in its settings: one rule per line.

• Q0.0 = (I0.0 | Q0.0) & I0.1 is a latch: start I0.0 holds itself on through Q0.0, stop I0.1 breaks it.
• ! is NOT, & is AND, | is OR.
• TON(x, s) is an on-delay and TOF(x, s) an off-delay of s seconds.

Timer relays have on-delay and off-delay modes and a delay in seconds. Time passes at the speed set in the simulation controls. The PLC example in chapter 10 shows a latch, a lamp and a horn after 5 s.

Open 04 Check the circuit and press Check circuit. It runs in two parts.

Rules and ratings

Every terminal connected, every contact has a coil, designations are unique, voltages match, and the breaker and contactor sizes cover each motor current.

Automatic run

It switches on the supply, presses each START briefly and together, operates the selectors, then each stop button, the E-stop and an overload trip, and reports what switched, shorts, overvoltage and supplies that do not match the load.

Messages are marked OK, NOTE, warning or error. Click a line to select the device it refers to on the sheet. A NOTE (for example “KA1 is not used”) is information, not a fault.

A check passing means the circuit behaves as drawn under the idealised model. It does not prove the design is safe.

Section 02 sizes the protection and cable for one motor and then draws a complete, wired starter for you.

1. Enter the motor power (kW) from the nameplate.
2. Choose the supply voltage (230 to 690 V, 3-phase, 50 Hz) and the starting method: Direct-on-line, Forward/reverse, Star-delta or Frequency inverter.
3. Read the table of sizes and the reason for each.
4. Press Place this circuit on a new sheet. A new sheet is added with every device wired and its values set.

How the numbers are made

Current comes from power, voltage and a typical efficiency and power factor for the size. The contactor is the next AC-3 rating at or above that current. The supply breaker is about 2.5× the current. The overload range is chosen so the setting sits inside it. The cable is sized for 125 percent of the current. Replace the calculated current with the nameplate value and check the maker’s coordination (type 1 or 2) tables.

Hand check: 7.5 kW at 415 V, DOL

• Current 13.8 A. From I = P ÷ (√3 × V × η × pf): 7,500 ÷ (1.732 × 415 × 13.8) = 0.76, so the tool assumed η × pf of about 0.76.
• Breaker: 2.5 × 13.8 = 34.5 A, so the next standard size, MCCB 40 A (or gG fuse 32 A).
• Contactor K1: 18 A AC-3, at or above 13.8 A.
• Overload relay: 11 to 16 A, set to 13.8 A.
• Cable: 125 % of 13.8 = 17.3 A, so 2.5 mm² + PE.

Enter 7.5 kW, 415 V, DOL

The lists in section 05 update as you draw.

• Parts list: position, designation, description, quantity and a typical series. The series are examples only; confirm part numbers and ratings with the supplier.
• Motor circuits: motor, kW, volts, full-load current, protection, contactors, cable and connection.
• Wire list: wire number, from, to, cable size, colour, cable type and sheet. Terminal plan: strip, terminal, type and where each side goes.
• Parts list CSV and Wire list CSV download for a spreadsheet.

PDF. Fill Title (printed in the title block), and optionally Prepared by and Drawing number, then press Download PDF. It has every sheet with its frame, column and row grid and title block, then the parts list, motor circuits, wire list, terminal plan, the check results and the simulated behaviour.

Wire numbers. Every electrically separate net gets a number. Wires joined through a terminal, a net label or a junction keep the same number. Colours follow the common IEC convention: black power, blue neutral, green/yellow earth, red AC control, dark blue DC. Cable sizes are for copper PVC in conduit at about 30 °C; derate for grouping and temperature and check voltage drop.

Each example below was run through the check; the results quoted are what the tool reports. After loading, press Run and try them yourself.

A. Direct-on-line starter

Start/stop with a sealed-in contactor. MCCB for short-circuit protection, thermal overload F2 with its NC contact in the control circuit. Result: passes with 0 errors and 0 warnings. START makes M1 run at 0.5 s and it stays on; STOP at 3.5 s stops it. Load this example

B. DOL with E-stop, run lamp and trip lamp

As A, with an emergency stop in series, a green run lamp from a K1 contact and a red trip lamp from the overload relay’s 97-98 contact. Result: passes. START makes M1 run, K1 pull in and H1 light at 0.5 s. Click the overload relay while it runs to see the trip lamp. Load this example

C. Forward / reverse with interlock

Two contactors; K2 swaps phases L1 and L3. Each contactor’s NC contact is in series with the other’s coil, so both cannot pull in together. Result: passes. FORWARD runs M1 forward with K1 and H1; REVERSE runs it in reverse with K2 and H2. Delete an interlock contact and run the check to see what the tool says about the missing protection. Load this example

D. Star-delta with timer

K1 line, K2 star, K3 delta, on-delay timer KT1 of 6 s. The motor needs six terminals. Result: passes. START at 0.5 s: M1 runs in star (reduced voltage) with K1, K2 and KT1. At 6.5 s (0.5 + 6) KT1 times out, K2 drops out, K3 pulls in and M1 runs in delta. In star each winding sees about 58 percent of line voltage and the line current is a third of the delta value. Load this example

E. Frequency inverter, forward / reverse selector

Inverter U1 fed from Q1, motor on its U V W outputs. FWD and REV selector switches connect the drive’s own +24 V to its digital inputs. Result: passes. Operating S2 (FWD) starts U1 forward at 0.5 s; S3 (REV) runs it in reverse. If both inputs are on, the drive does not run: it needs exactly one. Load this example

F. PLC inputs and outputs, 24 V DC

A power supply makes 24 V DC. START and STOP feed input module A1; output module A2 runs the logic in its settings: a latch, a lamp, and a horn after 5 s. Result: passes, with one NOTE that KA1 has no contacts referring to it. START turns Q0.0 and Q0.1 on at 0.5 s, KA1 energises and H1 lights; at 5.5 s Q0.2 comes on and H2 sounds. Open A2’s settings to read the logic. Load this example

G. Sequential start of two motors

START runs motor 1 and the timer; after 5 s motor 2 starts. Either overload relay stops both. Net labels L1, L2, L3 and N distribute the supply. Result: passes. M1 runs at 0.5 s with K1, KT1 and H1; at 5.5 s KT1 times out and K2 pulls in, so M2 runs and H2 lights. Load this example

H. Size a starter and place it

Star-delta for a 15 kW motor at 400 V gives a current of 27.7 A: MCCB 80 A, line contactor K1 32 A, delta contactor K3 18 A (58 percent of 27.7 is 16.1 A), star contactor K2 12 A (33 percent is 9.1 A), overload 22 to 32 A set to 27.7 A, cable 6 mm². Press Place this circuit and a new sheet with the wired starter appears. Enter 15 kW, 400 V, star-delta

• Motor does not run in Run mode. Check there is a source (a breaker or label feeding L1, L2, L3), the breaker is switched on, the overload relay has not tripped (click it to reset), and the contactor coil has voltage.
• A terminal is reported as not connected. Wires must end on the small circle of a terminal. Click the line in the check list to find the part.
• Contact has no coil. Select the contact and choose its coil in the settings.
• Duplicate designations. Two parts with the same tag. Change one in the settings.
• Overvoltage or undervoltage. A 230 V load on 400 V, or a 24 V coil on the mains. Check the supply and the load rating.
• Short circuit. A path joins two different sources, such as L1 to N, or both forward and reverse contactors closed. The lowest breaker in the path trips.
• Drive will not run. It needs power on L1, L2, L3 and exactly one of forward or reverse on.
• Star-delta does not work. It needs a motor with six terminals (U1 V1 W1 and W2 U2 V2).
• Simulation stops when you touch the sheet. That is by design; press Run again.

Save .fec file downloads your drawing; Open file loads it again. Use this for any work you want to keep or share. The page also keeps an automatic copy in the browser it is opened in, but that copy can be lost if the browser data is cleared and it is not shared with other computers.

The example buttons and the Load an example list ask before they replace a drawing. Save first.

Short-circuit current and breaker discrimination, voltage drop, cable derating, motor starting current and torque, harmonics, power factor correction, earth-fault loop impedance, and detailed PLC and drive parameters. Symbols follow IEC 60617 drawing conventions in simplified form. The part series are typical examples only. Safety circuits must be designed and validated to ISO 13849 and IEC 60204-1.

• v13 · 10 Oct 2026
  Added the page background image and this user manual, which also opens from a User manual button next to Full screen with eight worked examples that load onto the sheet. The manual is updated with every change to the tool.
• v1 to v12
  Earlier builds: numbered sheets with frame and cross-references, wire numbers, net labels, sheet links, motor circuit selector with placing a starter, simulation with overload and thermal model, PLC and timer logic, about 250 IEC 60617 symbols, automatic check, and PDF with wire list and terminal plan.