Like many other terms and phrases in the design side of our industry, the phrase “riser diagram” will mean different things to different people. The phrase “one line diagram” shares this fate. It is often necessary to ask someone to clarify what they mean, when then use one of those phrases.
To me, the phrase “riser diagram” is a drawing that shows a building’s principle set of power sources (utility and local generators), switchgear, switchboards, transfer switches, transformers, distribution panels, and branch circuit panels. It differs from my notion of a “one line diagram” in two ways. One is that it does not show all circuit breakers. The other is that it conveys not only the connections (i.e., panel B gets power from panel A, and in turn provides power to panel C), but also gives relative locations.
Imagine a four story building. Take a sheet of paper and draw four horizontal lines (five, if you have equipment on the roof). Label them 1, 2, 3, and 4 (and R, if needed). Every major component on the first floor is depicted with a box (or other symbol) above line “1.” This may include the main service switchboard, a transformer or two, a distribution panel, and a few branch circuit panels. Each is shown as a box, and you connect them with lines. You may even include some indication of the type and size of feeders between boxes. I usually only show breakers on the main board and any distribution panels.
Suppose you have a 480/277V distribution panel, a 480-120/208V transformer, and a few 120/208V branch circuit panels on each floor. You would then put boxes on each “floor” of the diagram, depicting each component, and you would show the feeders that connect the boxes. I generally don’t see any branch circuit loads on a riser diagram. But I would show large loads, such as condensing units, as individual symbols on the riser diagram.
Charles E. Beck, P.E., Seattle
Comments based on 2008 NEC unless otherwise noted.