Over my career I cannot think of a single time when I regretted running spare cables or oversizing field junction boxes. However, I can think of too many instances where I ran out of room and/or capacity much sooner than I had expected, and wished I had run MORE spares.
Concept: The incremental purchase cost of a 36 pair cable over a 24 pair cable is practically negligible when compared with the labor cost of running either cable. Install spare wire capacity whenever possible, or at least plan for future expansion when sizing junction boxes, cable trays, and control system I/O.
Details: During system design, always run spare cable, oversize the field junction boxes, and buy extra I/O cards. Ideally, the cost of this spare capacity can be worked into the project cost during the estimating phase so the money will be available when the project is approved. Even if the spare capacity was not included in the original budget, the additional cost of adding the extra capacity is usually low enough that it can be incorporated with no significant impact on the budget. If the project budget is so tight that larger cables or spare capacity cannot be installed, at least PLAN for future additions. Install slightly bigger field junction boxes and cable trays so that more cables can be added later. Oversize conduits, and leave a draw string in them so wires can be pulled in on future projects. Leave blank spaces in the I/O cabinets so I/O racks can be added later, or at least leave space in the room so I/O cabinets can be installed in a future project.
This concept is particularly true when running fiber optic cable. The labor to run a 6 fiber, 12 fiber, or 24 fiber cable is essentially the same and the material cost difference is low. Running a cable with less than 12 fibers is pointless, and if expansion is even slightly possible, run a 24 fiber cable.
Watch-Outs: Always ask about future expansion plans during the design phase of an automation project. By knowing how the ultimate system might appear, you can make minor design changes that will make future growth much less costly and difficult. If the system will double in size, you can lay out the I/O cabinets, power supplies, etc. in such a way that they can be easily upgraded in the future without burdening the current project significantly.
Exceptions: Occasionally an automation project involves a machine or a process that is so mature that future expansion is unlikely. This is not a common occurrence, but if this situation applies, clearly the cost of adding spare capacity would not be justified.
Insight: Project managers hate adding spare capacity after the project has been approved because they consider it “scope creep” and not part of the project. Plants adore spare capacity because it allows the execution of process improvement projects at a much reduced cost. In the long run, the company certainly saves money, but the project manager is not nearly as concerned with the long run as the execution of his or her project. If the spare capacity is included in the original project estimates, then all parties win. The project manager is happy because the project scope has not changed, and the plant and company are happy because they can expand in the future at a much reduced cost.
Rule of Thumb: A common rule of thumb for most systems is to add at least 25 percent spare capacity to the original design. Depending upon the future plans of the plant, a higher percentage might be justified. Adding less than that figure is usually not advisable unless the plant and/or process are mature and unlikely to change.