Contactor Latching (Start–Stop) Control Circuit: 13-14 NO / 21-22 NC Auxiliary Contacts and Coil Selection
Step-by-step contactor latching circuit with start–stop push buttons: A1-A2 coil, 13-14 NO holding contact, 21-22 NC interlock, 95-96 overload contact and coil voltage choice.
Contactor latching (a holding or seal-in circuit) keeps the coil energized through the contactor's own `13-14` normally open (NO) auxiliary contact after the momentary start button is released. Built with IEC 60947-4-1 contactors and IEC 60947-5-1 control devices, it is the basis of motor control and stops the motor restarting by itself when supply returns after an outage.
> Related Category: Browse models by current and coil voltage in Contactors, Auxiliary Contacts and Control Push Buttons.
1. What the terminal numbers mean
| Terminal | Meaning |
|---|---|
| A1 – A2 | Coil |
| 1-2, 3-4, 5-6 (L1-T1, L2-T2, L3-T3) | Main power poles |
| 13-14 | Auxiliary contact in position 1, normally open (NO) |
| 21-22 | Auxiliary contact in position 2, normally closed (NC) |
| 33-34, 43-44 | NO contacts in positions 3 and 4 |
| 95-96 | Overload relay trip contact (NC) |
| 97-98 | Overload relay signal contact (NO) |
Rule: in a two-digit number the first digit is the position, the second the function. `1-2` is NC and `3-4` is NO, so `13-14` is the NO in position 1 and `21-22` the NC in position 2. Time-delayed contacts use `5-6` (NC) and `7-8` (NO).
2. Start–stop latching circuit step by step
The control circuit is a series chain:
Line (L) → control fuse → overload 95-96 → Stop button (NC) → Start button (NO) → coil A1; coil A2 → neutral (N)
For latching, the contactor's 13-14 contact goes in parallel with the start button:
- After the fuse, the control line enters overload terminal `95` and leaves `96` for the NC contact of the stop button.
- The stop output feeds both the NO contact of the start button and contactor terminal `13`.
- The start button output and contactor terminal `14` are joined and taken to coil terminal `A1`.
- Coil terminal `A2` goes to neutral (or the other side of the control transformer).
Operation: pressing start pulls the coil in, `13-14` closes and current keeps flowing through it after the button is released. Pressing stop or an overload trip on `95-96` breaks the chain, the coil drops out and `13-14` opens; the circuit waits for a new start.
- Several control stations: stop buttons in series, start buttons in parallel.
- Feed a run lamp from a spare NO contact (e.g. `43-44`) and a fault lamp from overload contact `97-98`.
- Use spring-return push buttons; a maintained button defeats the latching logic.
3. The 21-22 NC contact: electrical interlock
In reversing and star-delta circuits two contactors closing together is a phase-to-phase short. To prevent it, the 21-22 NC contact of the other contactor is wired in series with each coil: while K1 is closed, K2 cannot be energized. Reversing duty adds a mechanical interlock as well. If the contactor lacks contacts, add front or side auxiliary contact blocks; to multiply control signals only, choose an auxiliary contactor.
4. Coil voltage selection
| Coil voltage | Use | Note |
|---|---|---|
| 24 V DC | Automation panels with PLC outputs | Fit a freewheeling diode or varistor suppressor |
| 24 V AC | Machine panels with a control transformer | Wet locations and touch risk |
| 110 V AC | Export machinery, transformer-fed control | — |
| 230 V AC | Line-to-neutral control; most common | Needs a neutral |
| 400 V AC | Three-phase panels without neutral | Control taken between two phases |
- A coil pulls in reliably between 85% and 110% of rated voltage; voltage drop on long control runs must not go below that.
- An AC coil draws 6–10 times its holding VA at pull-in (e.g. 70 VA inrush, 7 VA sealed); size the control transformer for it.
- Coil voltage must match the control voltage: 400 V on a 230 V coil burns it out, 230 V on a 400 V coil makes the contactor chatter and weld its contacts.
- For main contact ratings see Contactor AC-1 vs AC-3 Utilization Categories.
5. Button colours and safety (IEC 60204-1)
- Start: green or white, NO contact. Stop: red or black, NC contact, so a broken wire stops the machine.
- Emergency stop: red mushroom head on a yellow background, latching, with positive-opening NC contacts; use an emergency stop button in series at the head of the chain.
- Protect the control circuit with its own fuse (typically 2–6 A).
Frequently asked questions
Is the holding contact wired in series or parallel with the start button?
In parallel. The 13-14 NO contact bridges the start button, so coil current continues through it when the button is released.
Why is the stop button normally closed (NC)?
It sits in series with the coil and must open the circuit when pressed. NC wiring also fails safe: a broken wire or loose terminal stops the machine.
What is the difference between 13-14 and 21-22 contacts?
13-14 is a normally open (NO) contact used for latching and signalling. 21-22 is a normally closed (NC) contact used for electrical interlocking between two contactors.
The contactor pulls in on start but drops out on release. Why?
The holding path is not working: 13-14 is not in parallel with the start button, an NC contact was used by mistake, or the auxiliary contact is faulty.
AC or DC contactor coil?
Use 24 V DC in panels with a PLC and 24 V supply, and 230 V AC in classic line-to-neutral control. A DC coil is silent and less sensitive to voltage dips.