Instrument Cable Schedule: Template, Example, Format and Complete Guide

What is the instrument cable schedule?

An instrument cable schedule is a detailed document that lists all cables used to connect field instruments, junction boxes, marshalling panels, and control system cabinets such as PLC or DCS panels. It provides complete information about each cable, including its number, type, size, length, routing, and termination details.

This document is essential in industrial plants such as oil and gas, petrochemical, power plants, pharmaceutical, and manufacturing facilities. It helps engineers, technicians, and installation teams ensure proper cable selection, installation, and maintenance.

The cable schedule acts as a master reference throughout the engineering, procurement, construction, commissioning, and maintenance phases of a project.

Detailed Explanation of Instrument Cable Schedule

• An instrument cable schedule is a list or table that provides detailed information about the various types of instrumentation cables used in a facility.
• It typically includes information such as the cable type, size, and location, as well as the signal type, signal range, and communication protocol used by each cable.
• The instrument cable schedule is used to identify and document the cables that are used to connect the various instruments and control systems in the facility.
• It is an important document for managing and maintaining the instrumentation cables, as it helps to ensure that they are properly installed, labeled, and connected to the appropriate devices.
• The instrument cable schedule may also include information about the testing and certification of the cables, as well as any repair or maintenance work that has been performed on them.

What is included in the instrument cable schedule?

The cable schedule shall provide the following information:

• Cable type: This includes the type of cable, such as multi-pair, coaxial, or fiber optic.
• Cable size: This refers to the diameter or thickness of the cable.
• Cable location: This is the physical location of the cable, such as a specific room or area within the facility.
• Cable Number: This will be a unique number assigned for each cable for identification purpose.
• Cable Length: This will include the length of the cable.
• Source and Destination Termination Description: This will cover the information of starting and ending connections of a specific cable.
• Cable Glands Type and Size for each cable: This will cover the cable gland details on both side of the cable.
• Signal type: This is the type of signal carried by the cable, such as electrical, electronic, or pneumatic.
• Signal range: This is the range of values that the signal can take, such as 0-10 VDC or 4-20 mA.
• Communication protocol: This is the method used to transmit data over the cable, such as HART or Modbus.
• Testing and certification: This includes information about any testing or certification that has been performed on the cable, such as UL or CSA approval.
• Repair and maintenance: This includes any repair or maintenance work that has been performed on the cable, such as splicing or replacement.

Remarks
Special notes such as spare cable, future use, or installation instructions.

What are the documents necessary to prepare the instrument cable schedule?

Documents required to prepare the Instrument cable schedule are

• Instrument Index – Tag numbers with a wire should be filtered from the instrument index.
• I/O List – Will cover most tag numbers, as it only displays instrument tag numbers that have I/O and thus require wires.
• Instrument Cable Layout – This will be required to understand the required cable length from source to destination.
• Interconnection Block Diagram – This will be required to identify the source and destination termination points.

The majority of tag numbers will be covered by the I/O list because it only displays instrument tag numbers that have I/O and hence need wires.

Please note that instruments that aren’t loop-powered need an extra power supply.

What is the purpose of an instrument cable schedule?

The purpose of an instrument cable schedule is to provide a clear and comprehensive overview of all the cables used in the system, which can be used for installation, maintenance and troubleshooting purposes.

The cable schedule is used to figure out how much cable to buy when making a material take-off. But the cable lengths shown in this document are only approximate. So, there should be a backup plan for getting materials in case a cable gets cut or there is an unexpected obstruction in the field, a riser, etc.

During the construction phase, the cable schedule will also be used. However, it is not recommended to cut the cable based on the length information on the cable schedule. Most of the time, during installation, the cable is taken off of the drum and cut in the field.

It can also be used to ensure that the correct cables are ordered and installed during the initial construction or modification of the system.

How to make an Instrument cable schedule?

• Determine the scope of the schedule. This includes identifying all the instruments and devices that need to be connected via cable, as well as the types of cables that will be used.
• Collect the necessary information. This includes the details of each cable, such as the type and size, as well as the route and termination details.
• Organize the information. One way to do this is to create a table with columns for each piece of information.
• Fill in the table with the collected information.
• Review the schedule for accuracy and completeness.
• Save the schedule and make it available to relevant personnel. It is a good idea to have both a hard copy and an electronic version of the schedule.
• It may be helpful to use a standardized format for the schedule, such as the one provided in industry standards or company guidelines.

Example Calculation: Instrument Cable Length

Cable length is estimated based on routing distance and installation allowances.

Typical cable length calculation includes:

• Cable tray routing length
• Vertical drop to the instrument
• Vertical rise to the control panel
• Termination allowance
• Contingency allowance (usually 5% to 15%)

Example:

Cable tray routing length = 25 meters
Vertical drop to transmitter = 2 meters
Panel termination allowance = 2 meters

Total length = 29 meters

Add 10% contingency = 2.9 meters

Final cable length = approximately 32 meters

This ensures sufficient cable length during installation.

How to Prepare an Instrument Cable Schedule

Follow these steps to prepare a cable schedule:

• Collect instrument tag numbers from the instrument index.
• Identify cable connections using I/O list and interconnection diagrams.
• Select the correct cable type and size based on signal requirements.
• Determine cable routing using cable tray layout drawings.
• Calculate cable length.
• Assign unique cable numbers.
• Enter all information into an Excel sheet.
• Review and verify accuracy.
• Issue the cable schedule for procurement and construction.
• Using a standardized format helps ensure consistency across projects.

Importance Of Cable Schedule During Construction and Maintenance

• During construction, the cable schedule helps installation teams identify the correct cables and routing paths.
• During commissioning, it helps engineers perform loop checks and signal verification.
• During maintenance, technicians can quickly identify cables without tracing them physically.
• It also helps during plant expansion and modification projects.

Download Instrument Cable Schedule Excel Template

You can create the cable schedule using Excel with columns such as:

Excel template for Instrument cable schedule is available for download at the link below