How do you wire a timing relay? Most electronic timing relays bring the auxiliary or control voltage into two dedicated supply terminals, commonly marked A1/A2, and present one or two change-over output contacts numbered per IEC 60947-5-1 convention (11-12-14 for an instantaneous contact, 15-16-18 for a timed one). Whether the unit needs only those two supply terminals or a third trigger terminal depends on the function: a plain on-delay reads the supply itself as the start signal, while an off-delay or multifunction timer usually needs the supply held on continuously and a separate contact to start the count. Get the topology wrong on a true off-delay or a star-delta timing relay and the unit either refuses to time out or fires the moment power is applied. This guide covers 2-wire connections, 3-wire connections, instantaneous-versus-timed contact wiring, star-delta interlocking, and the wiring mistakes that show up on the bench as "wrong" timing when the relay itself is fine.
Supply Terminals and Contact Numbering
A1 and A2 carry the control or auxiliary voltage that powers the internal oscillator. Nothing times without it. The output side is a change-over contact — one common, one normally-open, one normally-closed — and on multi-contact or multifunction units, an extra instantaneous contact may share the same terminal block as the delayed one. Mixing the two up is the single most common wiring error on a first-time install: the panel looks correct, the relay pulls in immediately, and the electrician assumes a bad unit rather than a swapped contact.
Some ranges add a second, independent change-over contact (DPDT) for driving two loads from one timing event — a pilot lamp plus a contactor coil, for instance — without paralleling loads on a single contact rated for a few amps.
2-Wire Connection: Supply-Triggered On-Delay
On a basic on-delay (function A), the two supply terminals do double duty as the trigger. Apply voltage to A1/A2, the internal counter starts immediately, and the output changes state once the set time elapses and stays there until supply is removed. No third wire is needed because there is nothing to distinguish from the supply itself — power-on is the start event. This is the simplest circuit in the family and it is why single-function on-delay units are usually the cheapest members of a range.
3-Wire Connection: A Separate Trigger Input
Off-delay, true off-delay, and most multifunction/multi-range timers need to tell "supply present" apart from "trigger active," so they add a third terminal. The supply stays connected to A1/A2 continuously; a separate start contact — a pushbutton, an auxiliary contact, a PLC output — feeds the trigger terminal to begin or end the timed interval, depending on the function selected. Control-signal off-delay works this way: remove the trigger and the timer starts counting down while the supply is still present, then drops the output when the set time expires.
True off-delay is wired differently again. It stores enough energy internally (typically in a capacitor) to complete the timed interval even if the whole unit loses supply mid-cycle — useful where the trigger and the main supply are the same contact and both disappear together, such as a motor contactor's auxiliary feeding both. What we see in the field: panel builders sometimes assume every off-delay timer is "true" off-delay and wire the trigger straight off the same breaker as the supply; on a control-signal variant that just removes both signals at once and the delayed drop-out never happens because there's no stored energy to fall back on. Check the datasheet's function code before assuming which topology you have.
Instantaneous vs Timed Contacts — Wire the Right One
Multifunction and multi-range devices frequently bring out both an instantaneous contact (changes with the trigger, no delay) and a timed contact (changes after the set interval) on the same terminal strip. They look identical on the terminal cover. Landing the load on the instantaneous contact when the delay was the whole point of the specification defeats the purpose silently — the circuit still "works," just not the way the panel was designed to. Always cross-check the terminal legend against the specific function selected on the rotary switch, not against a wiring diagram from a different function on the same range.
Formula: Repeat Accuracy Error on the Wired Timing Interval — Source: IEC 61812-1, Clause 5
Δt = ± (a% × tset)
| Symbol | Description | Unit |
|---|---|---|
| Δt | Maximum expected variation between successive timed cycles | s |
| a | Repeat accuracy rating from the timer's datasheet | % |
| tset | Time value set on the dial or DIP/rotary range | s |
This isn't a wiring number, but it matters when you're troubleshooting a "miswired" complaint: a timer set to 30 s with ±1% repeat accuracy can legitimately land anywhere in a 0.3 s window cycle to cycle. If the observed spread is wider than that, look at the wiring — a loose trigger connection or a trigger pulse shorter than the relay's minimum pulse spec will produce inconsistent timing that has nothing to do with the accuracy rating.
Star-Delta Timer Wiring and Interlocking
A star-delta timer drives two outputs from one timed sequence: close the star contactor, run the motor up to speed, open star, hold a short transition (dead-time) with both contactors open, then close delta. Wiring both outputs to their respective contactor coils isn't enough on its own — the star and delta contactors also need a mechanical or electrical interlock so a welded or slow-dropping contact on one side can't be bridged by the timer closing the other side too soon. See how to wire a star-delta motor starter for the contactor-side interlock wiring; the timer's job here is only to sequence the coil signals with the transition dead-time built in.
Some ranges bring the star and delta outputs out as two independent change-over contacts on one timer body; others use a dedicated star-delta module with the transition time on a separate, finer-resolution dial than the run-up time. Either way, the transition setting is the one dimension on this wiring job that is genuinely safety-critical — set it too short and the two contactors can overlap.
Common Wiring Mistakes That Cause False Timing
Reversed supply and trigger terminals. Swapped instantaneous and timed contacts on a multifunction unit. A control-signal off-delay wired as if it were true off-delay, so the delayed drop-out never happens once both signals vanish together. A trigger pulse shorter than the relay's specified minimum, which some units ignore entirely and others latch on unpredictably. And a load current on the output contact that exceeds its rating — the contact welds shut, and the "timer" appears to stop switching even though the internal timing circuit is working exactly as set. None of these show up as a fault code; they show up as a sequence that doesn't match the dial.
For deeper background on function selection before wiring, see how to select a timing relay for your application and the broader timing relay engineering guide. For multifunction-specific terminal layouts, see multifunction and multi-range timing relays. Stocked ranges from Schneider Zelio Time and ABB CT-D are listed under timing and control relays.
Frequently Asked Questions
What's the difference between 2-wire and 3-wire timing relay wiring?
2-wire uses the supply terminals as the trigger — the timer starts counting the moment it's powered. 3-wire keeps the supply continuous and adds a separate terminal that a start contact or PLC output drives to begin or end the timed interval, which is needed for off-delay and most multifunction devices.
Can I power a timing relay's supply and trigger from different sources?
For a control-signal off-delay or multifunction unit, yes — the trigger circuit is electrically separate from the supply as long as both stay within the relay's rated voltage and the trigger meets the minimum pulse duration. For a 2-wire on-delay there is no separate trigger terminal to power.
Do all off-delay timers need a separate trigger terminal?
No. A control-signal off-delay does, because it needs continuous supply to keep counting after the trigger drops. A true off-delay stores enough energy to complete the interval even if supply and trigger disappear at the same time, though many true off-delay units still bring out a dedicated trigger terminal for wiring flexibility.
What happens if I wire the load to the wrong output contact?
On a unit with both instantaneous and timed contacts, landing the load on the instantaneous one removes the delay from the circuit entirely — the relay still operates correctly internally, but the downstream equipment reacts immediately instead of after the set interval.
How do I interlock a star-delta timer's outputs?
Wire the timer's two outputs to drive the star and delta contactor coils through the contactors' own mechanical or electrical interlock contacts, not directly coil-to-coil. The timer sets the transition dead-time; the interlock is what stops a stuck contact from bridging star and delta.
Why does my timing relay fire instantly instead of after the delay?
Most often a range/multiplier switch left on its lowest setting, an instantaneous contact wired where the timed contact should be, or — on a 3-wire unit — the trigger terminal tied permanently to the supply so the relay sees a start signal the instant it powers up.
Conclusion
Wiring a timing relay comes down to three questions: does the function need a third trigger terminal or does the supply itself act as the trigger, which of the available output contacts is the timed one, and — for star-delta — is the transition output feeding the contactor interlock chain and not just the coil. Get those three right and the timing behavior matches the dial; get one wrong and the relay looks faulty when the wiring is what's actually off. Cross-check the terminal legend against the exact function selected, not against a generic diagram, and the rest of the install follows.