The short answer
Terminals
The terminal strips are where the panel meets the field, and their design decides how long every commissioning and every troubleshooting session takes. Terminals are grouped by voltage class and function, with power, 120 volt control, 24 volt discrete, analog, and network each on its own strip and each strip separated and labeled; every terminal carries a number that appears on the schematic and in the terminal schedule; instrument loops use fused or disconnect terminals so that a loop can be isolated and tested without lifting wires; shields land on their own terminals or a bar with a defined ground; and a fifth to a quarter of each strip is spare. The terminal schedule, which lists every terminal, its wire numbers, and what is on the other end, is the document that turns a cabinet of wires into something a technician can read.
Key points
- Group strips by voltage class and function, and separate them physically and by label.
- Number terminals to match the schematic and the terminal schedule; the number is the address of the wire.
- Fused or disconnect terminals on instrument loops make testing possible without lifting wires.
- Shields land on dedicated terminals or a bar with one defined ground point.
- Reserve spares on every strip and record them in the schedule.
Grouping
| Strip | Contents | Notes |
|---|---|---|
| Power | Incoming supply, motor leads, and feeders | Sized for the conductors; barriers between phases where required; often power distribution blocks instead |
| 120 volt control | Control transformer secondary distribution, field switches, and 120 volt outputs | Fused terminals for branch distribution; a neutral bar if the circuit is grounded |
| 24 volt discrete | Field inputs and outputs at 24 volts | Two-level or three-level terminals save space; commons on the lower level |
| Analog | Loop positive and negative, and the shield | Fused or disconnect terminals, one per loop, with shield terminals alongside |
| Network and serial | Ethernet and fieldbus | Ethernet usually on patch panels or couplers rather than screw terminals; serial on shielded pairs |
| Intrinsically safe | Barrier field side | Separate, identified, and spaced per the control drawing |
| Ground | Equipment grounds and shield drains | Ground bars bonded to the enclosure; the instrument ground bar with its single bond |
Numbering
Each terminal strip has a designation, typically the letters TB and a number or the voltage class, and each terminal a number within the strip. The schematic shows the strip and the terminal at every field connection, the wire number is on both ends of the wire, and the terminal schedule lists strip, terminal, wire number, device, and field destination. The convention is chosen once for the plant and used on every panel, so that a technician who knows one panel knows them all. Renumbering an existing panel is a project with a wire-by-wire check, and the reason to get it right at the start.
Instrument loops
A 4 to 20 milliamp loop lands on a pair of terminals with the shield beside them. A fused terminal on the positive side protects the loop supply and gives a place to isolate the loop; a disconnect or knife terminal on the negative side gives a place to open the loop for a meter in series or to insert a signal source, without lifting a wire. Test points on the terminal accept meter probes. A panel with plain terminals for its loops is a panel where every loop check means a loosened screw, and every loosened screw is a future intermittent.
Shields and grounds
- Shield drains land on shield terminals or a shield bar, not on a signal terminal or a random ground screw.
- The shield bar is bonded to the instrument ground bar, which is bonded to the enclosure ground at one point; the design drawing shows the point.
- The shield is grounded at one end only, which for panel-to-field cables is the panel end; the field end is insulated and folded back.
- Shield continuity through junction boxes is carried on a terminal, insulated from the box.
Workmanship
- Ferrules
- Stranded conductors get ferrules, crimped with the right tool, one conductor per ferrule unless a twin ferrule is used for a documented purpose.
- One wire per terminal
- Or two where the terminal is rated for it and the drawing shows it; jumpers use the terminal jumper bars, not extra wires.
- Torque
- Screw terminals torqued to the manufacturer value; spring terminals used where vibration or many connections argue for them.
- Labels
- Terminal markers on every terminal, strip designations on the end brackets, and wire labels readable at the terminal.
- Spares
- At least a fifth of each strip spare, labeled as spare in the schedule, and left empty.
Frequently asked questions
- Screw or spring terminals?
- Both are acceptable in a listed panel. Spring terminals are faster, vibration resistant, and need no torque check; screw terminals accept a wider range of conductors and are what many field electricians expect. Pick one per plant and use ferrules with either.
- Where should the field wires land, on the terminal strip or directly on the I/O module?
- On the terminal strip. Direct landing on a module saves terminals and costs every module replacement, every loop test, and every schematic that then has to show module terminals as field terminals.
- How many spares?
- A fifth to a quarter of each strip, plus a spare strip section in the wireway plan. Spares cost little at build and a great deal at retrofit.
- Should I use multi-level terminals?
- For discrete I/O, yes: a three-level terminal carries signal, common, and ground in one width, and the strip is a third the length. Label the levels and show them on the schematic; a three-level strip drawn as single terminals is a trap.
Related topics
- Terminal BlocksThe point where field wiring meets the panel: screw, spring, and push-in terminals, ratings and the UL 1059 groups, fused and disconnect terminals for instrument loops, grounding terminals, multi-level blocks, jumpers and marking, and the layout that makes a loop check or a field change a five-minute job.
- WirewaysPanel wireway design: fill limits and the space terminations need, separating power, control, and signal wiring, routing around heat and drives, sizing rules, the cases where wireway is the wrong choice, and the habits that keep a panel serviceable.
- How to Build Terminal SchedulesProduce the terminal schedule that lets a technician find any wire: define the strips and their numbering, list every terminal with its wire numbers, the device and the field destination, and keep it current through every change.
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- Control SchematicsThe ladder-style drawing of every control circuit in a panel: how a schematic is organized by line number, the conventions for contacts, coils, and cross-references, what belongs on a schematic and what belongs on a wiring diagram, and reading one under pressure.
- Loop ChecksProving every I/O point end to end before a system goes live: what a loop check is and is not, the three-point analog check, discrete and output checks, the two-person method with the sheet that records it, what to do with a point that fails, and why a loop check is not a calibration.
Direct contact
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