14-Pin Relay Diagram: 4PDT Octal-Style Base, Coil, and Contact Wiring Reference
This is a free printable 14 pin relay diagram: download the diagram as SVG or open it and print to paper or PDF.
A 14-pin relay diagram shows the coil terminals and four independent SPDT contact sets of a 4PDT plug-in relay mounted in a 14-pin octal-style or square-pin relay base.
The 14-pin relay is a plug-in, industrial-grade switching relay with four sets of SPDT (single-pole double-throw) contacts, giving it the designation 4PDT (four-pole double-throw). This configuration provides the designer four independent switching circuits controlled by a single coil — useful for interlocking, sequencing, and logic functions in control panels, industrial machines, and building automation systems.
The physical format is typically a square-bodied relay that plugs into a matching 14-pin relay socket (also called an octal base, though the term is more precisely applied to 8-pin octal types). The 14-pin square format uses two rows of seven pins on a standard pitch, allowing the relay to be unplugged and replaced without disturbing any wiring — a major advantage over hard-wired relays in maintenance-intensive environments.
Pin numbering varies between manufacturers, but the IEC 61810 and widely adopted standards define the following typical assignment for a 4PDT relay: Pins 1 and 14 are the coil terminals (A1 and A2 respectively in IEC nomenclature). The four switching contact groups occupy the remaining twelve pins: Group 1 (COM, NC, NO), Group 2 (COM, NC, NO), Group 3 (COM, NC, NO), Group 4 (COM, NC, NO). In the most common layout, contacts are arranged symmetrically: pin 2 = COM1, pin 3 = NC1, pin 4 = NO1; pin 5 = COM2, pin 6 = NC2, pin 7 = NO2; pin 8 = NO3, pin 9 = NC3, pin 10 = COM3; pin 11 = NO4, pin 12 = NC4, pin 13 = COM4. Always verify the precise pinout from the relay manufacturer's wiring diagram, as minor variations exist between brands.
The relay socket connects directly to DIN rail or panel, and the relay body plugs in from the front. Terminal screws on the socket body connect the wiring, making field installation and relay replacement straightforward. Many relay sockets include a mechanical hold-down clip that secures the relay body against vibration.
Contact ratings for 14-pin relays typically cover 8–16 A per contact at 250 V AC or 30 V DC. Because all four contact poles share one coil, the thermal envelope of the relay limits how many poles can be operated at full rated current simultaneously — consult the simultaneous contact loading chart in the datasheet.
A test button and a mechanical flag indicator are common options on relay socket accessories. The test button manually engages the armature, allowing verification of the contact state without coil power — invaluable during commissioning and fault-finding.
How to wire 14 pin relay diagram
- Obtain and verify the relay wiring diagram Before wiring, locate the relay body's printed wiring diagram (usually on the side or bottom of the relay) or download the datasheet. Confirm pin 1 and pin 14 are the coil terminals (A1 and A2) and identify the COM, NC, and NO assignments for all four contact groups. Do not assume pinout from visual inspection alone.
- Mount the relay socket on DIN rail or panel Clip the relay socket onto a 35 mm DIN rail or screw it to the panel backing plate as applicable. Ensure mechanical clearance for the relay body height above the socket and for the front-access test button if fitted. Torque socket terminal screws to the manufacturer's specification — loose terminations on high-current contacts cause localised heating and eventual failure.
- Wire coil terminals on the socket Connect the positive control voltage to the socket terminal corresponding to relay pin 1 (A1/coil +). Connect the second coil terminal (A2/coil −) to the switching device — PLC output, relay driver, or manual switch — and then to the control circuit common. For DC coils, fit a flyback diode across the coil terminals: cathode to A1, anode to A2.
- Wire load circuits to contact terminals For each required contact group, connect the supply for the load to the COM terminal of that group. Connect the load itself to the NO terminal (active when coil is energised) or to the NC terminal (active when coil is de-energised). Unused contact groups should have their COM terminal left unconnected or connected to a reference — do not leave NO and NC pins floating in high-impedance sensitive circuits.
- Fit load-circuit fuses Fuse each independently loaded contact group with a fuse rated for the load wiring ampacity, fitted in series with the COM supply conductor. Do not rely on the contact rating as overcurrent protection. Label each fuse with the circuit it protects.
- Insert relay body and test manually Insert the relay body firmly into the socket until it clicks into place. Fit the retaining clip if provided. Press the manual test button (if fitted on the socket) and verify that the load circuit activates. Release the button and verify the circuit returns to its de-energised state. This mechanical test does not require coil power and is safe on de-energised load circuits.
- Apply coil voltage and verify operation under load Apply the control voltage to the coil. Confirm the relay indicator LED or flag shows energised. Verify each wired contact group operates its respective load. Monitor relay body temperature over a period representative of normal operation — excessive heat indicates overloading or inadequate ventilation.
Specifications
| Relay Type | 4PDT (four-pole double-throw) |
|---|---|
| Total Pins | 14 (2 coil, 12 contact: 4 × COM, 4 × NC, 4 × NO) |
| Typical Coil Voltages Available | 6 V DC, 12 V DC, 24 V DC, 24 V AC, 48 V AC, 110 V AC, 230 V AC (relay-dependent) |
| Typical Contact Rating (AC) | 8–16 A per pole at 250 V AC (refer to datasheet) |
| Typical Contact Rating (DC) | 8–10 A per pole at 30 V DC (derated; refer to datasheet DC curve) |
| Coil Resistance | Varies with coil voltage: approximately 60 Ω (12 V DC) to 14 kΩ (230 V AC) — refer to datasheet |
| Mechanical Life | 30 million operations minimum (no load) |
| Electrical Life | 100 000 – 300 000 operations at rated load (datasheet dependent) |
Safety warnings
- All wiring of 14-pin relay circuits connected to mains voltages (single or three-phase AC) must be performed by a licensed electrician and must comply with applicable wiring regulations (NEC/NFPA 70, BS 7671, AS/NZS 3000, IEC 60364). Isolate all supplies and verify dead with a calibrated voltage tester before making or modifying any terminations.
- Never exceed the relay contact ratings for voltage or current. The simultaneous loading derating table in the relay datasheet must be consulted — full rated current on all four poles simultaneously may exceed the relay's thermal limits even if each individual pole is within its single-pole rating.
- Do not assume a relay is de-energised because a test button is released or an indicator LED is off. Verify coil voltage is absent with a multimeter and confirm contact state with a continuity tester before touching contact wiring.
- Ensure relay sockets are torqued to the manufacturer's terminal screw specification. Undertorqued terminals on relay sockets carrying motor or heating loads are a known cause of intermittent faults and terminal block fires due to localised resistance heating.
- Replace relays with exact-specification equivalents (coil voltage, contact rating, footprint). Substituting a relay with a higher contact current rating in the same socket is generally safe, but substituting a lower-rated relay into a higher-current circuit is not — contact destruction and fire risk result.
Tools needed
- Digital multimeter (resistance, continuity, AC and DC voltage)
- Insulated VDE-rated screwdrivers (flat blade for terminal screws)
- Wire strippers and ferrule crimper
- Bootlace ferrules (for stranded conductors in relay socket terminals)
- Cable labels
- Logic analyser or simple test lamp (for verifying contact operation during commissioning)
Common mistakes
- Wiring to the relay body pins directly instead of to the socket terminals — the relay body is not wired; all connections terminate on the socket, and the relay body plugs in and can be removed without disturbing wiring.
- Inserting the relay body in the wrong orientation — most 14-pin relays have a polarising key or physical stop that prevents incorrect insertion, but verify the pin 1 marker aligns with the socket pin 1 marker before pressing the relay in.
- Assuming DC and AC contact ratings are interchangeable — a contact rated 10 A / 250 V AC may only be rated for 5 A or less at 30 V DC; always check the DC contact derating in the datasheet.
- Using the NC contact of all four poles without verifying the hold current — in a de-energised relay, all NC contacts are closed, but if the coil is powered and contacts open, inductive load kickback from all four simultaneously switched loads can combine and stress the contact arc suppression capability beyond specification.
- Not labelling which pole (1 through 4) connects to which load — in a panel with multiple 14-pin relays, unlabelled contact wiring is a troubleshooting nightmare when one pole fails and the technician cannot determine which circuit is affected without tracing every wire.
Troubleshooting
- Coil energises (indicator shows) but one or more contact groups fail to switch load
- Cause: Welded contacts from previous overcurrent, loose terminal screw on socket, or broken contact spring Fix: With coil energised, test continuity between COM and NO of the suspect group. If open, contacts are not switching — the relay may have a damaged contact group. If continuity is present but load does not respond, check the load circuit wiring and fuse. Replace the relay body if a contact group is confirmed faulty.
- Relay coil does not energise despite control voltage being applied
- Cause: Open coil winding, control voltage below pick-up threshold, or driver switch not closing circuit Fix: Measure voltage directly across socket coil terminals. If at rated voltage, remove relay body and measure coil resistance directly on relay pins 1 and 14. An open reading confirms coil failure — replace relay. If voltage is below threshold, check control supply and voltage drop across any series switch or wiring.
- Relay contacts causing excessive arcing and burn marks on socket terminals
- Cause: Load current exceeding contact rating, inductive load without snubber, or simultaneous loading exceeding thermal derating Fix: Reduce load current below 80% of contact rating. Add RC snubber across AC inductive contacts (typically 0.1 µF + 100 Ω in series, across each contact pair). For motor loads, consider upgrading to a properly rated contactor. Verify simultaneous loading is within datasheet derating limits.
Frequently asked questions
What does 4PDT mean for a 14-pin relay?
4PDT means four-pole double-throw: the relay has four independent switching contact sets (four poles), each of which can connect a Common terminal to either a Normally Open or a Normally Closed contact (double-throw). All four poles are controlled simultaneously by a single electromagnetic coil. This gives the designer four separately fused or separately loaded switching paths from one coil command.
How do I identify which pins are the coil and which are the contacts on a 14-pin relay?
The most reliable method is to use the relay's wiring diagram, printed on the relay body or in the datasheet. In the absence of documentation, measure resistance between pairs of pins: the coil winding will measure a moderate resistance (typically 100 Ω to several kilohms depending on coil voltage), while contact pin pairs will read either near-zero ohms (NC contacts, coil de-energised) or open circuit (NO contacts). Pins 1 and 14 are coil terminals in the most common 14-pin layout.
Can I load all four poles of a 14-pin relay at their full individual contact current rating simultaneously?
Possibly not. The relay datasheet specifies simultaneous contact loading — the total current across all poles is limited by the relay body's thermal capacity. For example, a relay rated 10 A per pole may only sustain 6 A per pole when all four poles are loaded simultaneously. Always consult the manufacturer's simultaneous loading derating chart, particularly for continuous-duty, high-temperature applications.
What is the advantage of a plug-in relay socket over direct hard-wiring?
A plug-in relay socket allows the relay to be replaced in seconds by pulling the body from the socket, without disturbing any wiring. This dramatically reduces downtime on production lines or HVAC systems. It also allows bench-testing a spare relay before installation, and the socket can be wired and tested before the relay itself is inserted — a safety advantage during panel commissioning.
Is a 14-pin relay the same as an octal relay?
No. An octal relay uses an 8-pin round base with 8 circular contact pins and typically provides 2PDT contacts. A 14-pin relay uses a square-pin base with two rows of seven rectangular pins, providing 4PDT contacts. Both are plug-in formats that use matching sockets and both are common in industrial control, but they are not interchangeable in form or contact count.
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