Size matters when selecting the right enclosure
Control panel enclosure sizing almost always represents a challenge faced by automation system designers. Electrical room sizes tend to be established last after the floor plan, process areas, mechanical equipment, piping, etc., so they often tend to have too little space allocated.
Another constraint is that the National Electrical Code (NEC) mandates working space requirements around control panels. Two of the most important items to consider when sizing control panel enclosures are panel location and budget. Consider these 12 aspects when selecting the appropriate enclosure size.
1. As always, be sure to investigate and follow specific customer project requirements and specifications when they are provided. Many customers may have installed equipment spares requirements, and future space requirements.
2. What NEMA rating is required for the panel? This is typically defined by the installation area characteristics. If it needs to be NEMA 4X, does it need to be stainless steel or fiberglass? If stainless steel is required, is there any direction regarding 304 or 316? The most common material is 304, but sometimes 316 is required for areas with certain chemical exposure. Not all sizes are available in all materials.
3. How is the panel going to be mounted? Will it be free-standing or wall-mount? If it stands on the floor, are there any mechanical or cleanliness requirements driving the type of legs or stands to use? Does the panel need to be slope top for sanitary reasons? Can it be rigged into place through doorways and paths? After it is installed, are there adequate locations for conduit penetrations and routing? Once these questions are answered, the installer can start looking at specific sizes.
4. If the enclosure is floor-mounted it may be installed on a housekeeping pad (a raised concrete area that lets the enclosure sit above floors which may be washed), so allow for additional 4- to 6-in. above the finished floor. Keep this in mind when selecting the panel height and when door mounted devices are required.
5. If the enclosure is enlarged in the rare case where abundant space is available, do not overlook the additional cost of larger cooling and/or heating devices.
6. Make sure that all components have proper spacing around them per manufacturer’s instructions. This is especially important for heat-generating devices like power supplies and programmable logic controllers (PLCs). Note that most devices should be installed in the "upright" position. Some devices, but not all, can be installed in any orientation.
7. Ensure there is enough space between wire ducts and devices (like terminal blocks) so that the wire can be manipulated, landed, and the wire tag is readable.
8. Try to group like voltages. Specifically, keeping 24Vdc away from ac voltages, even though this typically consumes more space. This is generally not an NEC requirement, since the NEC is satisfied if all the conductors have the proper voltage rating. The reason is to minimize any noise being imposed on the dc wiring. Attempt to make all ac and dc conductor crossings at 90 degrees. However, if ac and dc must be run together, minimize the parallel length and maximize the spacing between the two; installing an EMI barrier (noise shield kit) inside the wire duct may be another option. This will give you the equivalent of 6-in. of air spacing and eliminate the need to run two wire ducts side by side and use up valuable panel space.
9. Estimate the wire count for each voltage type and size wire ducts appropriately. Do not forget to account for field terminations when you count wires. Undersized wire ducts not only won’t meet code, but they will be unworkable.
10. Make sure to take into account the bend radius of wires, cables, tubing, etc., for the panel layout. This is especially important for larger power distribution conductors.
11. Be kind to field installers. When control panels are fabricated in shop conditions, it is far easier to route and terminate conductors. However, once in the field, the situation is physically challenging. Try to provide additional space for field-side connections and service loops.
12. Lastly, keep it simple. You can invest a lot of up front effort to cleverly arrange a control panel interior like you’re playing Tetris. However, this concept is at greater risk of faltering in the fabrication shop or the field. It is usually cheaper to select the larger size enclosure.
This post was written by Joe Weathers. Joe is a senior designer at Maverick Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. Maverick delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization, and more.
Maverick Technologies is a CSIA member as of 7/26/2016.