Wire Numbering in Switchgear and Control Panels
Have you ever opened the door to a Relay and Control Panel and been met with
an absolute ocean of tiny, identical wires? If you had to trace a fault
without a standardized system, it would be like searching for a needle in a
haystack-in the dark.
Thankfully, the electrical industry uses a brilliantly logical
Alphanumeric Wire Numbering System
to bring order to the chaos.
Every wire is assigned a
Letter Prefix (which
tells you the wire's overall function) followed by a
Number (which tells you
exactly where it belongs). Here is a cheat sheet to help you read any control
panel like a book:
Each wire shall have a letter prefix to denote its function:
The Phase Numbering Rule: For A through G, the numbers following the letter tell you the exact phase!
10 - 29: Red Phase 🔴30 - 49: Yellow Phase 🟡
50 - 69: Blue Phase 🔵
70 - 89: Neutral / Residual Circuits ⚪
| Circuit Function Letters | Wire Numbers |
|---|---|
| A – Current transformers for primary protection (excluding overcurrent) |
10 – 29 Red phase 30 – 49 Yellow phase 50 – 69 Blue phase 70 – 89 Residual circuits and neutral current transformers 90 – Earth wires directly connected to the earth bar 91 – 99 Test windings, normally inoperative |
| B – Current transformers for busbar protection | |
| C – Current transformers for overcurrent protection (including combined earth-fault protection) and instruments | |
| D – Current transformers for metering and voltage control | |
| E – Reference voltage for instruments, metering and protection | |
| F – Reference voltage for voltage control | |
| G – Reference voltage for synchronising |
| Circuit Function Letters | Wire Numbers |
|---|---|
| H – A.C. and A.C./D.C. supplies |
1 – 69 Switchgear and generators 70 – 99 Transformers |
| J – D.C. supplies |
| Circuit Function Letters | Wire Numbers |
|---|---|
| K – Closing and tripping control circuits | Any number from 1 upwards |
| L – Alarms and indications initiated by auxiliary switches and relay contacts, excluding those for remote selective control and for General Indication equipment |
| Circuit Function Letters | Wire Numbers |
|---|---|
| M – Auxiliary and control motor devices, e.g. governor motor, rheostat motor, generator AVR control, spring charging motors, transformer cooler motor control, motors for isolator operation |
1 – 19 Switchgear and 20 – 69 Generators 70 – 99 Transformers |
* If, for functions A-G and for functions H, J and M, more numbers are required, add multiples of one hundred (e.g. 10-29 may be extended to 110-129, 210-229, etc)
| Circuit Function Letters | Wire Numbers |
|---|---|
| N – Tap change control, including AVC, tap position and progress indication | Any number from 1 upwards |
| O – An indication that the ferruling is not in accordance with the general scheme and that if it is not altered double ferruling will be required for co-ordination with the remaining equipment in the station | |
| P – D.C. tripping circuits used solely for busbar protection | |
| R – Interlock circuits not covered above | |
| S – D.C. instruments and relays, exciter and field circuits for generators | |
| T – Pilot conductors (including directly associated connections) between panels, independent of the distance between them, for pilot-wire protection, for Inter-tripping or for both |
| Circuit Function Letters | Wire Numbers |
|---|---|
| U – Spare cores and connections to spare contacts | Spare cores shall be numbered from 1 upwards in each cable, and shall be so arranged that they can be readily identified on site with the cable containing them. This shall be achieved by suitable grouping, and unless the location of each group is clear from the diagram, the groups shall be labelled. Alternatively the core number shall be preceded by the cable number |
| Circuit Function Letters | Wire Numbers |
|---|---|
| V – Automatic switching circuits not integral with circuit breaker control schemes, i.e. separately supplied, or isolatable from, the circuit breaker control scheme | Any number from 1 upwards |
| W – Light current control connections (generally used for the light current side of interposing relays for control purposes) | |
| X – Alarms and Indications to and from General Indication and remote selective control equipments | |
| Y – Telephones |
Fluency in this
numbering system separates the novices from the veterans. When a breaker fails
to trip during testing, knowing instantly that you need to trace the "K" wires rather than
wasting time looking at the "L"
wires saves critical downtime. It ensures safety, speeds up maintenance, and
allows engineers across the globe to understand the same panel schematics.
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