One-line diagrams, riser diagrams and cut sheets

A one-line diagram draws a building's electrical system as a single line running from the utility's supply to the equipment that uses the power. One line stands for the whole circuit, even when the circuit carries three conductors, and each symbol on it is a device such as a meter, a switch or a panel. A riser diagram does the same for the vertical path in a multistory building, showing how power rises from the service entrance to panels on each floor. A single-story building has little vertical path to draw.

The project's electrical engineer or electrical contractor typically draws it, and the applicant submits it. Central Hudson lists a one-line of the electrical system, from metering to interconnection, among the application documents. National Grid says the diagram helps it understand the layout and configuration of the customer's system so the service is installed correctly and meets safety standards. It also publishes a specification of what the diagram must contain (conductor and conduit sizes, the size of the main switch, fuse or breaker, and the metering arrangement) and states that leaving items out causes delays.

Take the invented Dunmore Road Distribution, with its 1,200-amp, 480-volt, three-phase service, and read its one-line from left to right. The line at the far left is the utility's supply, labeled with the service voltage and phase. It runs to a metering cabinet. At this size the meter typically reads through current transformers in that cabinet, which pass along a small, proportional signal in place of the full current. The line continues to a main breaker labeled 1,200 amps, then to the main switchboard. Four lines leave the switchboard. One goes to the lighting panel, one to the rooftop HVAC units, one to the dock and conveyor panel, and one to the charger room, each labeled with its rating.

The labeled ratings let a reviewer compare the drawing against the load form. A main breaker of 1,200 amps on the diagram, a service of 1,200 amps on the form and a supply voltage of 480 on both are the figures a reviewer reads against each other.

Cut sheets

A cut sheet is the manufacturer's specification page for a piece of equipment, also called a catalog cut. It lists the model, ratings, dimensions and the standards the product meets. A utility does not need the whole catalog. It needs the one page identifying the exact main switch, switchgear section or metering cabinet the applicant plans to install.

National Grid's one-line diagram specification marks some items with an asterisk, namely the main switch with its fuse or breaker, and the metering equipment, and states that those items need catalog cut sheets. Its connections checklist says job owners may ask for cut sheets to confirm that the service equipment meets the required specifications, and sets the line for such requests at services of 200 amps and three-phase or larger. A reviewer typically compares the diagram with the sheet, so a one-line showing a 1,200-amp main breaker is read against a sheet for a 1,200-amp device.

For Dunmore Road, that means a cut sheet for the main breaker and one for the metering cabinet, attached with the one-line.