In this article8 sections
A wire numbering convention is the rule that gives every conductor in a control panel or an instrument loop one identifier, marked on a sleeve or ferrule at each end and shown on the drawing the wiring was built from. It fixes what the number is built from (a sequence, the sheet and line where the wire starts, the potential it carries, the terminal it lands on, the device it serves, or an I/O address), when the number changes, and which documents carry it.
IEC 62491 provides rules and guidelines for labelling the cables and conductors used in industrial installations, equipment and products, to keep a clear relation between the technical documentation and the actual equipment. The scheme a project numbers its wires by is written into the owner's or the panel builder's specification, and it is tested twice: when a technician traces a fault from a label back to the drawing, and when a revision moves wires that are already built.
In short
- One identifier per conductor, taken from the drawing, and the same at both ends of the wire.
- The scheme is a project choice: sequential, sheet and line, equipotential, terminal location, device and terminal, or I/O address. Many projects use two, one inside panels and one for field wiring.
- The convention must state when a number changes: at every device, at every termination, or never.
- Instrument loops add cable and pair numbers, and often cross ferrules that name the far end.
- Fix the numbers once the drawing is issued for construction. Renumbering a built panel means relabelling its wires.
What the standards require, and what they leave open
| Standard | What it covers | What it leaves to the project |
|---|---|---|
| IEC 62491 | Rules and guidelines for labelling cables and cores in industrial installations, equipment and products: coloured cables and designated cores, identification labelling, connection labelling and signal labelling | The physical design and material of labels, and manufacturers' own cable marking |
| IEC 60445 | Identification and marking of equipment terminals and of the terminations of certain designated conductors, and the general rules for conductor colours and alphanumeric notations | Numbering of ordinary control conductors |
| IEC 61666 | General principles for identifying the terminals of objects within a system, built on the IEC 81346 reference designations | The marking of terminal designations on products |
| ISA-5.4 | The minimum content of an instrument loop diagram | How the project numbers its wires |
Two of these qualify the others. IEC 60445 is a basic safety publication, written for the committees that draft product and installation standards, and its requirements apply where another publication refers to them. ISA-5.4 sets what a loop diagram must show, and the loop diagram is where one loop's terminals, cables and wire identifiers appear end to end, but the numbers on it come from the project's own convention.
What the standards share is the link between the label and the documentation. A wire whose label cannot be found on any drawing is unidentified, however neat the sleeve.
The common numbering schemes
| Scheme | The number is built from | Example | Holds up when | Breaks when |
|---|---|---|---|---|
| Sequential | The next free number in a register | 1047 | Numbers are fixed at issue and the register is kept | Nothing in the number leads back to the drawing |
| Sheet and line | The sheet and the line (rung) where the wire starts, with a suffix for a second wire on the same line | 0314, 0314A | Ladder schematics with numbered lines | A revision moves rungs or sheets |
| Equipotential | One number per connected potential, changing only through a device | 120 on every conductor of one node | Fault-finding by continuity; terminal blocks leave it alone | The convention never says what counts as a device |
| Terminal location | The panel, the terminal strip and the terminal | CP1-X2-14 | Panels built from termination schedules | A wire with two different ends needs one end chosen |
| Device and terminal | The device tag and its terminal or polarity | FT101+ and FT101- | Two-wire instrument loops | The device is retagged |
| I/O address | The controller rack, slot and channel, punctuation dropped | 010214 | I/O wiring with a frozen assignment | Channels are reassigned; non-I/O wiring has no address |
| Supply blocks | A number range per supply voltage | 100s for 120 V AC, 200 to 499 for 24 V DC | Reading the supply from the label | A range runs out |
Sequential numbers are the simplest to issue and the least informative to read. A technician needs the register or the drawing index to find a wire, so the scheme suits small panels and projects where the drawing set is always at hand.
Sheet and line numbers carry the drawing location. Electrical drawing tools build them from the line reference where a wire network starts: a wire that begins on line 100 of sheet 02 can be printed as 100 or as 02-100, and a suffix keeps two networks that start on the same line apart. The weakness is the revision that inserts rungs or splits a sheet, because the wire no longer starts where its number says.
Equipotential numbering gives one number to everything connected at one potential. The number stays the same through terminal blocks and changes where a contact, a coil, a fuse or another device separates two potentials, so a technician with a meter knows that every conductor marked 120 should read the same. Drawing tools apply the same idea when they number a whole net or potential with one designation. The convention has to name the devices that change the number, or two drafters will disagree about a link terminal or a disconnect.
Terminal location numbers name where a wire lands, by panel, strip and terminal. One practitioner arrangement gives each panel a location, each strip a number and each terminal a number, and matches the cable to the strip and the wire to the terminal. A wire between two different terminals has two candidate names, so the convention picks one end, usually the end in the panel.
Device and terminal numbers are common in instrument loops, where a two-wire transmitter has a positive and a return conductor. FT101+ and FT101- tell the technician which loop and which leg, and they read the same in the field, the junction box and the marshalling panel.
I/O address numbers turn the controller address into the wire number. They are quick to check against the I/O list, but only I/O wiring has them, and every one moves when channels are reassigned.
Supply blocks reserve number ranges by voltage, so a label tells the technician what the conductor carries before a meter touches it.
Most projects mix schemes, with sheet and line or equipotential numbers inside a panel, device and terminal numbers on field wiring, and the I/O address recorded beside the wire number at the card. The convention has to say which scheme applies where.
Identification labels and connection labels
A conductor end can carry two kinds of text, and IEC 62491 keeps them apart.
| Label | What it names | Same at both ends | Changes when |
|---|---|---|---|
| Identification | The conductor or core itself | Yes | The wire is renumbered |
| Connection, local end | The terminal this end lands on | No | This end is moved |
| Connection, remote end | The terminal the other end lands on | No | The other end is moved |
| Connection, both ends | Both terminals | Yes, as a pair | Either end is moved |
| Signal | The signal the conductor carries | Yes | The signal is reassigned |
An identification label needs the drawing to tell where the wire goes. A connection label tells it on the wire, at the cost of a longer label that goes out of date when either end is moved.
Instrument wiring uses the remote-end form as cross ferruling. Each ferrule carries the far end of its conductor (installation area, panel or junction box, terminal and polarity), so a technician at a junction box can see where every core goes without opening the loop diagram. It is usual between the field and the junction box and between the junction box and the marshalling panel, where the wire runs are longest and the drawings furthest away.
Numbering an instrument loop end to end
Take FT-101, a two-wire flow transmitter wired to an analogue input on PLC-1 through junction box JB-101 and marshalling in control panel CP-1.
| Segment | Cable or core | From | To | Labels |
|---|---|---|---|---|
| Field run | A single-pair field cable | FT-101 terminals + and - | JB-101 terminals 5 and 6 | FT101+ and FT101-, or cross ferrules naming the far terminal |
| Home run | Pair 3 of a multipair cable | JB-101 terminals 5 and 6 | CP-1 strip X2, terminals 13 and 14 | The same wire numbers, with the cable tag and pair number in the cable schedule |
| System side | Panel wiring or a system cable | CP-1 strip X2, terminals 13 and 14 | The analogue input channel on PLC-1 | The same wire numbers, with the I/O address recorded beside them |
Three documents carry this run. The cable schedule holds the two cables, their types, cores and ends. The termination schedule holds every terminal with its wire number, cable and core. The loop diagram shows the run for this one loop, and the I/O list holds the channel and the signal class.
The numbering only works if the four agree. A pair swapped at the junction box during commissioning shows on the ferrules first, and it has to travel back to the termination schedule and the loop diagram, or the next technician traces a wire the drawings say is somewhere else. The route from the I/O list to the terminals is covered in from the I/O list to wiring and termination, and the boxes the run passes through in marshalling cabinets and junction boxes.
Where wire numbers are recorded
| Document | What it shows about a wire | Source of truth for |
|---|---|---|
| Schematic (elementary) diagram | The number of each conductor between device terminals | The number and the circuit it belongs to |
| Panel wiring diagram | The physical terminals and where each wire lands | Landing points inside the panel |
| Termination schedule | Strip, terminal, wire number, cable and core, far end | Ferrule printing and field checks |
| Cable schedule | Cable tag, type, cores, from and to | Field cables and their cores |
| Loop diagram | One loop from instrument terminals to the I/O channel, with junction box and marshalling terminals | Instrument loops |
| I/O list | Tag, I/O address and signal class | The channel a wire ends at |
Ferrules are printed from the approved drawings or the termination schedule built from them, never from what is found in the panel. When the two disagree, the drawing is corrected first and the labels follow, so the record and the hardware stay one.
Revisions, field changes and renumbering
Electrical drawing tools renumber wires automatically when a drawing is retagged, which is useful while a design is moving and costly once ferrules are printed. The same tools let a number be marked as fixed so that a later retag skips it, and they use fixed numbers for preassigned wiring such as instrumentation that does not follow the default format.
A convention that holds through revisions follows four rules:
- Fix numbers at issue for construction. Before that, renumbering costs a regeneration; after it, every changed number is a relabelled wire. The drawing status codes mark that boundary.
- Add without disturbing. A new wire takes the next free number in its range, or a suffix on its line in a sheet-and-line scheme, so its neighbours keep theirs.
- Retire, never reuse. The number of a deleted wire stays out of use, because old redlines, test records and loop folders still cite it.
- Fix the width. One project ended up with duplicate wire numbers when leading zeros were dropped from node and slot numbers, and two different addresses printed the same. A fixed width and a stated character set prevent it.
Field changes come back as redlines. The as-built drawing records the wire numbers as built, and the ferrules have to match it, which is the same discipline the document numbering convention applies to the drawings themselves.
Writing the convention
The convention belongs in the project's electrical or instrumentation design basis, or in the owner's specification that the panel builder works to. It should cover:
- the scope: panel, project or facility, and which scheme applies where
- what each number is built from, with its width, separators and permitted characters
- when the number changes: at each device, at each termination, or never
- identification labels, connection labels or both, and on which wires
- protective earth, neutral and cable shields, with the conductor colours the governing standard requires
- cable tags and core or pair numbering for field cables
- the point at which numbers are fixed, the suffix rule and the retirement of deleted numbers
- the sleeve size and the document the ferrules are printed from
- the documents that carry the numbers, and which one wins when they disagree
The schemes all work. What fails is a convention that leaves one of these questions to whoever draws the next sheet.