The Control Panel GA Drawing: How to Read One, and How to Produce One That Gets Built Right
The short answer
The general arrangement drawing is the one that decides whether the panel physically works. It shows the enclosure, the mounting plate layout to scale, the door layout, the gland plate, the dimensions, and the free space around every component. Everything else, schematics included, describes what is connected; the GA describes what fits, what cools, and what an engineer can reach at two in the morning.
What it must show
The enclosure itself: manufacturer, part number, external dimensions, IP rating, material and finish, and whether the door is left or right hinged. Hinge side is missed constantly and cannot be changed on site.
The mounting plate, to scale, with every component in position. Not a schematic, a plan. If the drawing is not to scale it is not a GA.
The door layout, with pushbuttons, lamps, HMI, meters and their cut-out dimensions and heights.
The gland plate, with entry positions, sizes and whether entries are top or bottom. Top entry into an IP55 enclosure outdoors is a decision you will regret.
Dimensions and clearances, including the space needed to swing a breaker handle, withdraw a drive, and open the door fully against the wall it will be fixed to.
Laying it out
Heat rises, so put heat at the top and heat-sensitive things below it. Drives, transformers and power supplies generate; PLC CPUs, communication modules and relays suffer. Then calculate the rise rather than guessing: sum the component dissipation in watts, compare against the enclosure's dissipation at your ambient, and size the fan, filter or cooler from that. An Indian plant floor at 45 degrees ambient in May is not the 25 degrees the catalogue assumed.
Segregate power from signal. Keep incoming and motor power on one side, control and analogue on the other, and cross at right angles when you must cross. A VFD output cable run alongside a 4-20 mA pair will put noise on the signal, and it will be intermittent, and nobody will find it for a month. See 4-20 mA current loop troubleshooting.
Terminals at the bottom, in one strip, in order. Field cables come from below and terminate immediately. Terminals scattered around the plate to suit the schematic produce a wiring loom that cannot be traced.
Leave 25 to 30 per cent spare on the plate and the terminal strip. Every panel is modified. A panel with no spare space gets modified anyway, badly.
Wiring ducts sized for the fill they will actually carry, not for the fill on day one, and never above about 60 per cent.
The clearances people forget
- Bending radius for the largest incoming cable, which for a 240 sq mm armoured cable is substantial and decides the height of the gland plate above the floor.
- Door swing against the adjacent structure, and whether a person can stand where the door opens to.
- Withdrawal depth for a drive or a plug-in relay base.
- Arc-flash and working-space clearance in front of the panel to your site standard.
- Handle clearance on a rotary isolator when the door is closed and when it is open.
Checks before it goes to the shop
- Add the component depths to the deepest item and compare with the enclosure depth, including anything door-mounted that closes into the same space.
- Sum the dissipation and check the thermal calculation at the real site ambient, not 25 degrees.
- Confirm the gland plate has room for every cable in the schedule, at its real overall diameter.
- Check the IP rating survives every hole you have put in the enclosure.
- Walk the maintenance sequence in your head: isolate, open, test, replace, close.
Where to learn it
The control panel design and building course covers layout, thermal calculation and construction, and industrial motor control covers what is going inside. For producing the drawing set itself, see EPLAN Electric P8 or AutoCAD Electrical, and the comparison of the three packages.
Frequently asked questions
Is a GA needed for a small panel? Yes. Small panels are where components get placed by whoever is building it, and where the door fouls the isolator.
Who signs the thermal calculation? The designer. It is a design output, not a manufacturing detail, and enclosure vendors provide the dissipation figures to do it with.
What scale? Whatever fits the sheet and is stated, commonly 1:5 or 1:10 for the plate. An unstated scale is the defect.
