What Is CAD in Architecture and Engineering?

A CAD drafter draws a 30 foot corridor as 30 feet. Scale enters only when that geometry is shown through a viewport on a sheet, at 1/8 inch equals 1 foot for a floor plan or 1 1/2 inch equals 1 foot for a detail. That separation is why one drawing of a floor can feed the overall plan, an enlarged plan, and the reflected ceiling plan without being redrawn.

Text and dimensions are the exception, because they must read at a fixed size on paper. A note that plots at 3/32 inch on a 1/8 inch scale plan is 9 inches tall in model space. Annotative styles in AutoCAD handle that conversion, and getting the conversion wrong is the usual reason a sheet plots with tiny or giant text.

A line in CAD knows its coordinates, color, line type, and layer, and nothing else. Whether the line is a partition, a duct, or a property line is communicated by the layer name, such as A-WALL or M-HVAC-SUPP, and by the line type and weight that layer plots with.

Because meaning rides on a naming convention, naming discipline matters. A wall drawn on layer 0, or a wall to be demolished left on the new-work layer, plots wrong and misleads every consultant who freezes and thaws layers in that background. Firms write CAD standards largely to keep the convention consistent across people and projects.

A block is a named group of geometry inserted many times, such as a duplex receptacle symbol, a door, or a water closet. Redefine the block and every insertion updates. Blocks can also carry attributes, so a light fixture block can hold its type tag and circuit number as text that a schedule can read.

External references let one DWG display another. The architectural floor plan is attached to the electrical drawing as an xref, so when the architect moves a wall, the electrical engineer sees the move on the next reload instead of drafting over a stale copy.

A typical CAD project splits work into model files by floor and discipline, plus sheet files that reference them and hold the title block. A 40 sheet MEP set can depend on dozens of DWGs. Missing xrefs, broken paths, and an outdated background can make a set wrong without any single line being drawn wrong.

Sheet numbers usually follow the Uniform Drawing System in the National CAD Standard, where E-101 is an electrical plan and M-501 is a mechanical details sheet. The number tells a reader where to look before anyone opens a file.

Many architects model in BIM and export DWG backgrounds for consultants who still draft in CAD, and many renovation, tenant fit-out, and MEP projects never leave CAD at all. Facilities teams frequently maintain their floor plans in DWG for decades after a building opens.

The practical skill on a mixed project is managing exports: knowing which layers a Revit export produced, which to freeze, and when an exported background has fallen behind the model it came from.