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On many THC15A timers, terminals 1/L and 2/N power the timer; a relay switches the load through COM and NO or NC. For a normally-off load, supply line usually feeds COM (often via a jumper from 1/L), and the load’s switched line connects to the terminal marked NO. Neutral goes to the timer and directly to the load. But THC15A is a family name, not a universal wiring standard: confirm the voltage and printed terminal diagram on your exact unit before connecting anything.
This is mains wiring. Turn off the breaker and verify the circuit is de-energized before working. If you cannot positively identify the terminals, voltage, grounding, and required protection, stop and use a qualified electrician.
Identify your THC15A before wiring
Do not start with a pinout copied from a listing. THC15A units are sold in different voltage and contact configurations, and their terminal numbering and published diagrams can conflict. First:
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstall- Read the rated input voltage on the timer’s front, side, or terminal label. A 110–120 V unit and a 220–240 V unit are not interchangeable.
- Photograph the actual terminal markings and any diagram printed on the device before disconnecting existing wires.
- Find the labels for supply (
L/Nor1/2) and relay contacts (COM,NO,NC). - Check whether the timer is a single-output model or a variant such as
THC15A-2C. - If the printed diagram and an online manual disagree, follow the device’s own labels and supplied instructions. Do not wire by terminal number alone.
GEYA’s THC-15A material describes a 220–240 V AC version, while its manual lists other order-voltage options, including 110–120 V and additional AC/DC ratings. That is why the unit’s nameplate—not the THC15A name by itself—must determine the supply connection. See the manufacturer’s product information and manual.
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What the terminals do
| Marking | Typical function |
|---|---|
L or 1 |
Line/hot supply that powers the timer electronics |
N or 2 |
Neutral supply to power the timer |
COM |
Common relay contact; the switched supply is usually fed here |
NO |
Normally open: connected to COM when the timer commands the relay on |
NC |
Normally closed: connected to COM in the relay’s unenergized state, and opens when the relay changes state |
In a commonly encountered generic layout, terminals 3, 4, and 5 are NC, COM, and NO, respectively. Some listings describe the contacts differently. One example of the common arrangement is shown in this THC15A manual listing; it is not a substitute for checking your own timer.
The timer needs its supply even when the load is scheduled off. Its relay is a switch: it does not necessarily produce a separate output voltage. If the relay is isolated and you want to switch the same line that powers the timer, the supply line must also be routed to COM—often with a short jumper from L/1—provided the unit’s diagram confirms that arrangement.
Normally-off load: the common wiring arrangement
Use NO when the load should ordinarily be off and turn on during a programmed ON period. The following diagram applies only when the actual timer has separate L/N power terminals and an isolated relay with COM and NO:
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Supply line/hot ─────── 1 / L (timer power)
│
└─────────────── jumper to COM
Supply neutral ──────── 2 / N (timer power)
│
└──────────────────────────── load neutral
NO (the terminal actually marked NO) ── load line/hotFor a common 3-NC / 4-COM / 5-NO version, this usually means: incoming line to 1; incoming neutral to 2 and the load neutral; a jumper from 1 to 4/COM; and the load’s switched line to 5/NO. If your unit marks a different terminal as NO, use that marked contact instead of assuming it is 5.
Do not add a jumper merely because another model uses one. Follow the timer’s wiring diagram; some designs may not require or permit the same connection.
120 V and 220/240 V considerations
120 V single-phase example
On a compatible 120 V unit, the usual arrangement is line/hot to L/1, neutral to N/2, line/hot to COM (often via a confirmed jumper), NO to the load’s hot lead, and neutral directly to the load. Protective earth/ground remains continuous to the equipment and enclosure as required; it is not a substitute for neutral and is not switched by this diagram.
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- 28 ON/OFF Programs: Easily set up to 28 daily or weekly switching cycles with a 1-minute minimum interval for lights, pumps, and heaters
- Easy-to-Read Screen: Features a digital LCD display showing real-time clock and program settings, ensuring hassle-free scheduling and monitoring
- Easy Installation: Designed with a standard 35mm DIN rail mounting bracket for quick, secure integration into electrical panels or control boxes
220/240 V single-phase example
The relay principle is the same, but the timer’s input rating and the local circuit arrangement must match. A typical diagram may power the timer from the rated supply, feed one ungrounded conductor to COM, and route NO to the load, with the other supply conductor going directly to the load. Not every 240 V circuit has a neutral: some use two ungrounded conductors. Do not label or connect a conductor as neutral unless it is actually neutral under the installation’s design.
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For any circuit where both ungrounded conductors must be disconnected, a single-pole timer contact may not provide the required isolation. The wiring, disconnect, overcurrent protection, enclosure, conductor sizing, and grounding must meet applicable local requirements. Use an electrician if you are unsure.
Normally-on wiring with NC
Use NC when the intended behavior is for the load to be on while the relay is in its normal, unenergized state, and to be interrupted when the timer changes the relay state. On a verified 3-NC / 4-COM / 5-NO layout, feed line to COM (and to L/1 as required), connect NC/3 to the load’s switched line, and connect neutral directly to the load.
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- AC 220-240V Input: Powered by a standard AC 220-240V input, making it perfectly compatible with household and light industrial control systems
- 16A Switch Capacity: Designed with a robust SPDT relay that safely handles loads up to 16A (240VAC max), perfect for high-power industrial or home appliances
- 28 ON/OFF Programs: Easily set up to 28 daily or weekly switching cycles with a 1-minute minimum interval, ideal for lights, pumps, and heaters
- Easy-to-Read Screen: Features a digital LCD display showing real-time clock and program settings, ensuring hassle-free scheduling and monitoring
- Easy Installation: Designed with a standard 35mm DIN rail mounting bracket for quick, secure integration into electrical panels or control boxes
NC is not automatically a safe “fail-on” setting. Loss of timer power, a failed relay, or incorrect programming can leave equipment in an undesirable state. Do not use this arrangement where an unintended energized load could cause injury, fire, flooding, or equipment damage.
When to use a contactor instead of switching the load directly
A marking such as “15 A” does not establish that the timer can safely switch every 15 A load. Ratings depend on voltage and load category. Motors, pumps, compressors, transformers, refrigeration equipment, and large lighting banks can draw substantial starting or inrush current. One THC15A datasheet, for example, lists 16 A at 250 V AC for resistive loads but 10 A for lagging/inductive loads; your specific model may differ. Check the datasheet for its load-category ratings.
For an appropriately rated contactor arrangement, the timer switches the contactor coil; the contactor’s main contacts switch the load:
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- 28 ON/OFF Programs: Easily set up to 28 daily or weekly switching cycles with a 1-minute minimum interval, ideal for lights, pumps, and heaters
- Easy-to-Read Screen: Features a digital LCD display showing real-time clock and program settings, ensuring hassle-free scheduling and monitoring
- Easy Installation: Designed with a standard 35mm DIN rail mounting bracket for quick, secure integration into electrical panels or control boxes
Control circuit:
Supply line ── timer COM
Timer NO ───── contactor coil A1
Supply return ─────────────── A2
Power circuit:
Supply ── suitable protection/disconnect ── contactor main contacts ── loadThis is a conceptual diagram, not a complete installation design. The contactor coil voltage must match the control circuit, and the contactor’s main contacts and protection must suit the actual load. A contactor is generally preferable for motors, pumps, compressors, loads near the timer’s limit, high-inrush lighting, or frequent switching. GEYA’s product information includes a contactor wiring option; see its THC-15A page.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Install and program the timer
- Turn off the relevant breaker and verify absence of voltage with an appropriate tester. Do not rely on a switch position alone.
- Mount the timer securely on a compatible DIN rail inside a suitable enclosure.
- Confirm the timer’s input voltage and printed terminal diagram once more before wiring.
- Make supply, relay, load, neutral, and protective-earth connections as required by the specific device and local electrical rules. Check that conductors are properly sized and firmly secured.
- Have the completed work inspected before restoring power. Energize only when the enclosure is safe and all exposed conductors are protected.
- If the display or stored settings behave unexpectedly, use RESET only as directed by that model’s manual; it may clear settings.
- Set the current day and time, program the required ON and OFF events, and select
AUTOrather than permanentONorOFF. - Test the relay and load in a controlled way before relying on the schedule.
Common button labels include RESET, a clock key, H+, M+, D+, P, and MANUAL, but names and behavior vary. In GEYA’s manual, the clock is adjusted with the clock key and time/day buttons; P cycles through ON and OFF program slots, and MANUAL selects operating modes. The manual specifies up to 16 ON and 16 OFF settings, while another THC15A datasheet describes 17 of each. Treat capacity as model-specific rather than universal.
Troubleshooting
The display is blank
- After isolating the circuit, check for loose or incorrect connections at the timer supply terminals.
- Confirm that the measured supply matches the timer’s nameplate rating.
- If the correct rated supply is present and the manual calls for initialization, try RESET as directed.
- If the display remains blank despite verified correct power, the timer may be faulty and should be replaced.
The load is always on
- Confirm the load is on NO, not NC, if it should be normally off.
- Check whether the timer is in permanent
ONmode. - Check that the relay contacts were identified from the actual markings, and that COM has not been mistaken for an output.
- If the wiring and settings are correct, overload or inrush may have damaged or welded the relay contacts.
The load never turns on
- Check for permanent
OFFmode and confirmAUTOis selected. - Verify that COM receives the intended line feed and the load is connected to the marked NO contact.
- Check the programmed day, time, ON/OFF sequence, and any separate breaker or fuse for the load.
- Make sure the timer has the correct supply and that the load circuit has power.
Manual operation works, but the schedule does not
Check the current day and clock, program entries, and operating mode. A manual override can hold the output permanently on or off instead of following the schedule. Confirm that program slots were entered in the intended order and not canceled. Some versions use 12-hour time display or day groups, so verify the display conventions in the model’s manual.
The terminal numbers do not match an online diagram
Stop. Do not guess or transfer wires based on numbers from another THC15A. Compare the printed labels on the device with its supplied diagram. If the contacts remain unclear, have a qualified person identify them with the device safely isolated. Published THC15A listings do not all agree on contact numbering.
The timer clicks but the load does not run, or fails after a motor is connected
Check the load circuit, protection, COM feed, and switched contact identification. A click does not prove the relay contacts can handle the load. Motor starting current or other inrush may exceed the timer’s suitable rating; use a correctly selected contactor and protection rather than repeatedly testing an overloaded relay.
Before restoring power: final checks
- The supply voltage matches the timer nameplate.
- The actual terminal labels and diagram—not a generic pinout—were followed.
- The COM feed or jumper is installed only where the device diagram calls for it.
- NO is used for normally-off operation or NC for the intended normally-on behavior.
- Neutral and protective earth are correctly continued; neither is casually substituted for the other.
- The load rating, including inductive or inrush demand, is suitable; a contactor is used where appropriate.
- Conductors are secure, the enclosure is closed, and required protection and disconnects are in place.
- The timer is set to the right clock/day and
AUTO, and both manual and scheduled operation have been verified.
Local electrical codes govern the installation. For hardwired mains circuits, uncertain conductor identification, or any load whose failure could be hazardous, have a qualified electrician select and verify the wiring.
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