The first thing that you need to consider is where the Control Panel is going to be located. Where it is located will help determine what type or "rating" the enclosure will need to be. If it is located on the plant floor will it be in an area that is going to be washed down? Is it going to be outside? Is it in a classified area?
Once you know where the panel will go then you can choose the NEMA rating you will need.
What is a NEMA rating?
NEMA stands for the National Electrical Manufacturers Association
This association created a way to classify enclosures based on the types of environments an enclosure might have to provide protection from. The job or purpose of an enclosure is to keep the elements out.
How Big Should My Enclosure Be?
Once you have figured out what rating your enclosure needs to be, the next thing you need to do is figure out how big it should be. It is good practice to leave enough room for expansion. This is especially true for terminal strips. The general rule of thumb is 20% spare.
At this point you should start putting together a Bill of Materials of what is going to be in the control panel. It is good practice to also keep copies of all the vendor cut sheets for all the components in a file or electronically in a folder on your computer as reference. Most people use Excel to put together a Bill of Material