Make a cable blueprint to avoid rework

Many integrators seem to think that, if construction drawings show the number of connections, a cable and conduit schedule may be introduced later for the completion of an automation construction job. This causes inaccuracies and headaches. A well-thought-out construction plan will drive costs down.

If a design is completed and there is clear direction on control room layouts and distances from substations or disconnects, then it’s easier for construction drawings to be accurate. Accurate construction drawings will allow for known pathways and conduit and tray layouts. This equates to accurate cable lengths. Copper costs change daily; thus, it behooves a contractor to complete design at the end of front-end loading of a project so that contractor’s install costs are less. Exact wall layouts, final ceiling heights, actual routes for corridors, plenum, tray and conduit mean that the electrical contractor does not have to guess. This can equate to at least a 30% reduction based on saving material and time.

Cable costs are not just related to material, but also to labor. More runs equal more labor. Longer routes equal more support fixtures, more labor and more materials. This means that, if an integrator does not pay attention to routing on an industrial line, the installation costs skyrocket. Why? The installer takes the risk that the integrator passed on. The solution is to require that routing be part of the construction drawings and completed before bid.

The other problem is rework. The integrator should have a solid design and know where components are going. Rerouting cable can be two to four times the original installation costs. Save yourself the headache, and specify routing.

Additional costs to wiring installations include not just conduit but cable tray, J-hooks, sleeves for floors and walls, as well as reinforcement of tray holders. Sometimes this requires structural analysis for trays to be installed for the purpose of making sure cables will not sag or pull off the wall in long runs.

Creating a good layout and understanding where power feeds are currently makes a difference in machine location and looking at costs, as opposed to having to create new runs. Other things to consider are piping for water, HVAC duct work and other plumbing. Machines may require other chemicals or gases or drains. Understanding the context of the machine and thinking through the machine resources will make installation easier. An example is that if the HVAC must go on the roof, make sure there is a drain pan and a drainpipe separate from return water and with accessible filters for maintenance. Otherwise, there may be water on top of the new drive that was just installed.

If the process going in requires air flow or cooling, then cable trays need to work with or around ducting. Proper planning on paper reduces rework and change orders. How do you reduce conflicts between duct work, drains and electrical cables? Have a meeting with the trades and have them review your installation drawing before signing off on the proposal. A change order during the concept phase is less costly than one during the construction phase.

Engineers also forget about the amount of a cable that can go in a tray, the size of cable and that higher voltages may cause issues with communications or cause noise for data circuits. This means deciding on tray dividers or separate paths. Reuse of trays may not be convenient in brownfield installations because if you break a circuit while pulling cable, then there is uncalled-for downtime.

The other considerations for wiring are the type of cable, the cable housing and the protection required for the application. Distances also play a role. This means that an automation system that has a distributed network or uses cameras must be planned out due to the network cables required for the architecture to work.

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A cabling hierarchy alleviates some of the problem. There are typically four levels of cable hierarchy.

Level 1: Level 1 would be power and high-voltage or non-sensitive conductors. This is where main power feeds, variable-frequency drive (VFD) outputs, motor feeds and ac supplies would go.

Level 2: Level 2 involves moderate, sensitive signals with low-voltage control and ac control. Relay outputs, 120-V analog control circuits and solenoid valves would be at Level 2.

Level 3: DC control and field signals involving 24-V inputs and outputs, PLC communications, and actuator signals, or level sensors, or proximity sensors would be at level 3.

Level 4: Highly sensitive data and analog signals that are susceptible to noise or distance issues or need switching such as fiber, Ethernet and 4-20 mA loops, as well as thermocouple wires would be Level 4.

Unshielded data and power should be separated by 12 inches. Shielded data may be within 6 inches of power circuits. High-voltage, long-distance runs greater than 300 V maintain more than 2 inches and a physical barrier. Metal grounded wireways are essential for data circuits to provide shielding. Using dividers can create distinct compartments. Putting power in bottom or outer trays can help to isolate. Control and data circuits should be in top or inner trays and as far from power sources as possible. It’s also important to not overfill trays.

Other considerations for proper wiring are color coding and labels, so that it’s clear what the voltage sources are and what the cable’s purpose is. DC is typically blue. AC is red. And power circuits use black.

In short, if the right pathway is chosen and proper separation is kept based on cable type, density and location, then cable is more controllable. To do this, the integrator should plan a good layout and come up with routes and connections ahead of time so that construction drawings may be completed accurately. Involving the trades ahead of time will alleviate conflicts and rework. All of these measures will reduce costs on the installation of an automation construction project.