Every signal in a plant travels on a cable that must be bought, routed, pulled, glanded and terminated without confusion. A clean schedule lists each one once, with its route and ends, so design, purchase and site teams work from the same truth.
The cable schedule is the master register that ties field instruments, junction boxes and control cabinets together. A well built instrument cable schedule saves cable, prevents wrong terminations and speeds up loop checks.
What Is an Instrument Cable Schedule?
An instrument cable schedule is a design document, usually a spreadsheet or a database report, that lists every instrument cable in a project with its unique number, type, size, from and to points, route and length. It is the single reference used to order cable, plan trays and terminate both ends correctly.
The schedule covers branch cables from field instruments to junction boxes, multipair main cables from junction boxes to the marshalling cabinet, and the system cables that run inside the control room. It sits next to the instrument index, the JB schedule and the loop drawings.
Merobix describes a cable schedule as the master register of every cable in an installation, where each cable number appears only once. That single rule prevents two crews from pulling the same cable or leaving one out.
10 Essential Columns of a Cable Schedule
| No | Column | What to Enter |
|---|---|---|
| 1 | Cable number | Unique tag such as IC FT1001 |
| 2 | Cable type | Single pair, multipair, triad, IS or non IS |
| 3 | Size and cores | For example 1 pair × 1.5 sq mm |
| 4 | Screen and armour | Individual and overall screen, armoured or not |
| 5 | From | Instrument tag or JB and its terminal |
| 6 | To | JB, marshalling cabinet or system cabinet |
| 7 | Route | Tray or trench sections the cable follows |
| 8 | Length | Estimated length in metres with margin |
| 9 | Gland details | Gland type and size at each end |
| 10 | Remarks and revision | Spares, IS notes, status and revision mark |
Automation Forum adds that many projects also record signal type, range, protocol and test details. Keep extra columns only if someone will maintain them, because an outdated column is worse than no column.
The gland column must match the cable outer diameter and the area classification, which is explained in cable gland types and selection. The route column later feeds the cable tray fill calculation for each tray section.
Automation Forum points out that the lengths in a cable schedule are only approximate. On site the cable is taken off the drum and cut to the real routed length, so the schedule length is mainly for purchase and drum planning.
Cable Types Found in an Instrument Cable Schedule
Runs from one field instrument to a junction box, usually 1 pair with overall screen.
Carries many pairs from a junction box to the control room, often with individual and overall screens.
Three cores for 3 wire RTD signals.
Conductors of matched alloy so no new junction error is created.
Specified impedance cable for fieldbus networks.
The BHEL technical specification for instrumentation cable lists F type cables with individually screened pairs and G type cables with only an overall screen, in sizes from 2 to 16 pairs. The reasons for screening and twisting are covered in shielded cable and twisted pair.
The same BHEL specification calls for 0.5 sq mm conductors on single lengths up to 500 m and 1.5 sq mm above 500 m, with FRLS PVC sheaths. Thermocouple loops always use matched thermocouple extension wire, never ordinary copper pairs.
Cable Numbering Philosophy
A good number tells you the signal family and the end points without opening the drawing. A common style is IC FT1001 for the branch cable of transmitter FT1001 and MC JB101 01 for the first multipair main cable leaving junction box JB101.
Prefixes often separate signal groups, for example A for analog, D for digital, IS for intrinsically safe and T for thermocouple. The same tag then appears on cable markers at both ends, which makes the loop checking procedure much faster.
Freeze the numbering rule in a project philosophy document before the first schedule issue. Renumbering hundreds of cables after ferrules are printed costs time and creates errors.
Segregation and Intrinsic Safety
Intrinsically safe circuits must stay separate from non IS circuits, a key point in explosion proof vs intrinsically safe instruments. MTL notes that IS cables are frequently identified by a bright blue outer sheath, and the schedule should show which cables are IS.
| Signal Group | Typical Cable | Segregation Practice |
|---|---|---|
| IS analog and digital | Blue sheath, screened | Separate JB, tray or partition |
| Non IS analog | Black sheath, screened | Own JB and multipair |
| 24 V DC digital | Screened pair or multipair | Separate from analog where possible |
| Power to instruments | Armoured power cable | Separate tray, keep distance |
| Fieldbus | Specified bus cable | Own trunk, terminators noted |
MTL recommends separate trays for instrument and power cables, with good spacing between them, even where screens or armour make it optional. The IS cables of each zone also follow the hazardous area classification drawing.
Where IS and non IS terminals share an enclosure, IEC 60079 11 asks for a clearance of 50 mm between them, or a suitable partition. The cable schedule remarks column is a good place to flag such shared boxes.
Cable Length Formula
The schedule length is estimated from the tray route plus vertical runs and termination allowance, then raised by a contingency. Automation Forum suggests a typical contingency of 5 to 15 percent to cover field changes.
Order length = L schedule rounded up to the next 5 m
Example:
Route = 120 m, vertical = 8 m, termination = 1.5 m per end, C = 10 percent
Base = 120 + 8 + 3 = 131 m
L schedule = 131 × 1.10 = 144.1 m
Order length = 145 m
Instrument Cable Schedule Length Calculator
Long runs also need a voltage drop and loop resistance check, as shown in instrument loop impedance and 4 to 20 mA accuracy. The calculator above only estimates the cable to buy.
Second Worked Example: Sizing a Multipair Cable
A junction box JB101 collects 10 analog signals, and the project rule asks for 20 percent spare pairs. The required pairs are 10 × 1.2 = 12, so a 12 pair cable fits exactly if it is a standard size in the project.
If only 10 and 16 pair sizes are approved, choose 16 pairs and record the 6 spares in the remarks. Spare pairs should be terminated at both ends, which saves a new cable when a 4 to 20 mA loop is added later.
How to Prepare an Instrument Cable Schedule in 8 Steps
The instrument cable schedule must agree line by line with the JB termination schedule and the loop drawings, so the same cable number appears in all three. The cabinet side is detailed in PLC wiring from field instrument to system cabinet.
Review Checklist Before Issue
- Every cable number is unique and follows the numbering rule.
- Each instrument in the index has a branch cable row.
- From and to terminals match the JB schedule and loop drawings.
- IS cables are flagged and routed apart from non IS cables.
- Spare pairs meet the project percentage.
- Cable type matches the signal, including thermocouple and bus cables.
- Gland type and size fit the cable diameter and area class.
- Revision clouds and status are updated.
Generate the cable schedule, JB schedule and loop drawings from one database in tools like SmartPlant Instrumentation, or from one master sheet. Copying data by hand between documents is the most common cause of mismatched terminals.
During commissioning, every row is ticked off in the cold loop and hot loop testing. Signal noise on long runs, discussed in ground loop causes and prevention, is also easier to trace when screens and earthing points are recorded.
- Accurate cable purchase and drum planning.
- Clear routing and tray fill control.
- Correct terminations at both ends.
- Fast fault tracing during operation.
- Lengths are only estimates until routed.
- Needs strict revision control.
- Errors spread to JB and loop drawings.
- Large projects need a database, not just a spreadsheet.
Who Uses the Cable Schedule
Merobix notes that a cable schedule links to loop diagrams and the SCADA I/O list, so a fault seen on a screen can be traced to a specific cable. For colour rules on the cores, see cable colour codes for field instruments.
BHEL Instrumentation Cable Specification
Cable Schedule and JB Wiring Video
Instrument Cable Schedule FAQ
It is a register that lists every instrument cable with its number, type, size, ends, route and length. Design, purchase and site teams all work from this one controlled document.
It links the instrument index to the JB schedule and the loop drawings of the project. Each cable number must appear only once in the whole register, with no exceptions.
The core columns are cable number, type, size, screen and armour, from, to, route and length. Gland details and remarks with revision status complete the list of ten essential columns.
Some projects add signal type, range, protocol and test results as well. Add extra columns only if someone will keep them updated through design, construction and handover.
Add the tray route, the vertical rises and drops and a termination allowance at each end. Then apply a contingency of about 5 to 15 percent to cover changes found in the field.
The result in the instrument cable schedule is used for purchase and drum planning only. On site, each cable is cut to its real routed length during the pull.
One multipair cable from a junction box replaces many single pair cables running to the control room. It saves tray space, glands, cable markers and pulling time on large plants.
Individually screened pairs keep analog signals clean over long runs near other cables. Always include spare pairs so that future signals can use the same cable without a new pull.
Many projects keep about 20 percent spare pairs in each multipair main cable. The exact figure comes from the project specification or the client engineering standard.
Spares should be terminated at both ends and listed in the instrument cable schedule remarks. Loose spares coiled inside a box are often damaged and cannot be used without rework.
IS cables are flagged in a separate column or by a prefix in the cable number. MTL notes that they often use a bright blue outer sheath for easy identification on site.
They are routed through separate junction boxes and trays from non IS cables wherever possible. The remarks column should flag any shared enclosure that needs a partition or a 50 mm clearance.
Maintenance teams use it to trace a faulty signal back to a particular cable, junction box and terminal. Modification projects use it to find spare pairs and free routes for new instruments.
Keeping it updated after every plant change is therefore vital. An outdated instrument cable schedule can easily mislead a technician during a breakdown at night.
Related Articles
- Marshalling Cabinet DCS Wiring
- Cable Tray Fill Calculation
- Shielded Cable and Twisted Pair
- Cable Gland Types and Selection
- PLC Wiring from Field Instrument to System Cabinet
External References
- Technical Enquiry Specification for Instrumentation Cable, BHEL
- Instrument Cable Schedule Explained with Example, Automation Forum
- Intrinsic Safety, Wikipedia
What We Learn Today
- An instrument cable schedule lists every cable once with its number, type, size, from and to points, route and estimated length for purchase.
- Branch cables run from instruments to junction boxes, while multipair main cables with about 20 percent spare pairs run to the marshalling cabinet.
- Schedule length equals route plus vertical runs plus termination allowance, raised by a 5 to 15 percent contingency and rounded up for ordering.