Series Connection Diagram — Terminals, Wiring & Examples
This is a free printable series connection diagram: download the diagram as SVG or open it and print to paper or PDF.
A series connection diagram illustrates how components are wired sequentially so that output of one feeds directly into input of the next, creating a single current path. Series connections appear in terminal block wiring, battery packs, fuse protection schemes, and sensor chains. This guide shows correct series connection methods with terminal numbering, wire color standards, and verification steps.
Diagram checks and scope
- Components
- 5
- Wired connections
- 5
- Automated check
- Automated pin, route, source, and topology checks passed; this is not a safety certification.
A series connection physically links the output terminal of one device to the input terminal of the next in an unbroken chain. In a terminal block panel, series connections are made by running a wire from terminal output screw to the next device's input screw, then onward to the following device. Unlike a schematic that abstracts the circuit, a series connection diagram shows actual terminal numbers, wire colors, and cable references — the information an installer needs to make correct physical connections. When multiple switches are wired in series, all must be closed before current flows to the load — this is the basis for series interlock circuits in safety systems. A door interlock switch in series with a machine start circuit means the machine cannot start unless the door is closed. Additional interlocks (pressure switches, level switches, temperature switches) add more series elements; the load only receives power when every series element passes current. Series switch connections are wired with a common wire entering the first switch input, the first switch output connecting to the second switch input, continuing through all series switches, and the final switch output driving the load. In building automation, series-connected temperature sensors (RTDs wired in series) sum their resistance for a zone average measurement. In industrial current loops, components like barriers, transmitters, and indicators are wired in series along the 4-20mA loop — each device sits in the loop and the full loop current flows through all of them. Terminal block diagrams for series connections show each terminal with its incoming and outgoing wire number, enabling installation from the diagram alone without requiring circuit knowledge. Ferruled wire ends, consistent color coding (red for positive, blue for negative in European practice), and numbered wire markers on both ends of every conductor are professional practices for series-connected terminal wiring.
How to wire series connection diagram
- Document the series chain List every device in the series connection in order from supply to load. Assign a wire number to each conductor segment. Draw the connection diagram showing terminal numbers at each device.
- Label all wires before installation Apply numbered labels to both ends of every wire before routing. Use the wire numbers from the connection diagram. This ensures every wire is identifiable after installation.
- Install and wire each series element Starting at the supply, connect wire to the first device input terminal. Run the next wire from that device output to the second device input. Continue through all series devices to the load.
- Test continuity of the complete series chain With all devices in their normal closed/passing state, measure continuity from chain start to end. Then test each segment individually to confirm there are no unintended open points.
- Apply power and verify function Energize the circuit. Measure voltage at the load — it should be close to supply voltage with all series elements closed. Open each series element one at a time and verify the load de-energizes each time.
Specifications
| Control voltage (industrial) | 24V DC typical |
|---|---|
| 4-20mA loop supply | 24V DC, 500Ω max loop resistance |
| Series switch wire | 14-16 AWG, color per NFPA 79 |
| Terminal tightening torque | 0.5-1.2 Nm (terminal spec) |
Safety warnings
- In a series interlock safety circuit, test each element independently before relying on the series chain for safety — a failed-closed switch defeats the interlock function.
- When disconnecting any series-connected device in an energized loop, de-energize the entire loop first — opening one series element disconnects all downstream devices simultaneously.
- Never use a wire size smaller than the rating of any device in the series chain — the smallest wire determines the safe current capacity of the entire series path.
Tools needed
- Insulated screwdrivers for terminal block screws
- Ferrule crimper and bootlace ferrules (for stranded wire)
- Wire number markers and label printer
- Digital multimeter for continuity testing
Common mistakes
- Connecting switches in parallel when a series interlock is intended — parallel switches create an OR condition (either one passes current) instead of the required AND condition.
- Omitting wire number labels on both ends of series conductors — unlabeled wires create confusion during troubleshooting when any single series element fails.
- Mixing two different circuit's wires at a shared terminal, creating an unintended series connection between circuits with different functions.
Troubleshooting
- Load does not energize with all series switches closed
- Cause: One switch is not making contact internally, or a wire has an open fault in the series chain Fix: Measure voltage at the output of each series element in sequence. Find the last point with supply voltage — the element immediately after is where the series chain is open.
- Load stays energized even when one series switch is opened
- Cause: A parallel wire path bypasses the opened switch, defeating the series interlock Fix: Trace all wires connecting to both terminals of the opened switch. Look for a bypass connection that should not be there. Remove any parallel path to restore series interlock function.
- Intermittent load dropout in a series 4-20mA loop
- Cause: Loose terminal screw in the series loop causes intermittent open circuit Fix: Inspect every terminal in the loop. Gently tug each wire — a loose one will move. Re-terminate with a proper ferrule and tighten to manufacturer torque spec.
Frequently asked questions
How do I wire two switches in series for a safety interlock?
Run the supply positive to the input of Switch 1. Connect Switch 1 output to Switch 2 input. Connect Switch 2 output to the load. Connect the load return to supply negative. Both switches must be closed simultaneously for the load to energize. Label each wire with unique numbers at both ends for clear documentation.
What is a series 4-20mA loop connection?
A 4-20mA current loop connects devices in series: the power supply drives current through a transmitter, then through a safety barrier, then through a display or PLC input, and back to the supply negative. All devices share the same current — the transmitter controls the loop current to represent a process variable. Each device in the series loop sees the same 4-20mA signal.
Why use terminal blocks for series connections in a panel?
Terminal blocks provide a labeled, accessible connection point for each wire in the series chain. Each terminal shows its number, the incoming wire, and the outgoing wire — making series connections visible and verifiable. Terminal blocks also allow a single series element to be disconnected for testing without disturbing the rest of the wiring.
How many devices can be connected in series in one loop?
The limit is the supply voltage divided by the total voltage drop across all series devices. Each device drops some voltage; the supply must provide enough to drive the full current through all of them. In 24V DC control circuits, typically 3-5 devices can be series-connected before voltage drop reduces reliability. In 4-20mA loops, the limit is loop compliance voltage — typically 12-15V for fieldbus barriers.
How do I test a series connection for correct wiring?
With power off, use a multimeter in continuity mode. Place one probe at the series chain input and the other at the output. With all series elements in their closed/passing state, continuity should be present. Open one element at a time and verify continuity breaks each time. This confirms true series connection rather than a parallel path.
Sources and verification
Review status: Not independently reviewed. Automated topology checks confirm stored terminals and routes, not the correctness of manufacturer pin assignments, ratings, regional codes, or installation decisions. Verify those claims against the current primary documentation before use.
- IEC 61082-1:2014 — Rules for electrotechnical documents — International Electrotechnical Commission. Supports: General diagram, drawing, table, and reference-designation presentation conventions. Checked 2026-07-15.
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