Looking for a simple drawing program for equipment and wiring diagrams?

Have you tried the Sketchup free? Think that may cover you.

Welcome to Fluther !

Does sketchup do 2D diagrams or only 3D?

Yes, sketchup will do a 2-D diagram. Just don’t give any of your objects height off the plane.
And if you do make 3-d objects it is not difficult to put the perspective straight over the item and appear to flatten it.

Back before I branched into Web Development, when I was in electronics packaging design and automated process equipment design for the electronics manufacturing industry, it was all Autocad. If you’re delivering professional design services, Autocad from Autodesk is still the premier product for 2 and 3D design.