The Single-Line Diagram and Safety

The single-line diagram to the electrical professional is comparable to what a map is to a person driving across the country. If this was “back in the day,” I’d say that before I go on a journey somewhere in the car I grab my Atlas and appropriate maps. Today, I use my smartphone and my “Maps” app.

A map is essential and provides key information, including road markers; types of roads (highway or two-lane or other); cities, towns, and states; and key locations, such as airports and other important venues. The map to the traveler is essential. Similar could be said about the single-line diagram for the electrical professional. Another similar parallel between the map and the single-line diagram pertains to the accuracy of the information. An outdated map can get you lost and an outdated single-line can do worse. The single-line diagram is your road map if you are an engineer, electrical contractor, electrical inspector, maintenance professional, or similar. Accurate and updated singleline diagrams could save a life.
The single-line diagram is sometimes called the one-line diagram. Regardless of how you refer to this document, it has some unique basic qualities that set it apart from other electrical diagrams. Many electrical diagrams can be quite confusing, like the schematics we find on the inside doors of industrial control panels and similar. The single-line diagram focused on here is that which addresses the overall power distribution system. These diagrams are meant to be simple and easy to read, providing important, accurate information to the electrical professional.
The following items should be shown on the single-line diagram:

1. Power sources including utility, generator, and alternative energy solutions. When working on electrical distribution systems, we must understand from where power is coming and how much power can be expected. This could pertain to how much fault current is available based upon the system configuration or just that a presence of voltage should be expected on a specific terminal. This is not only important to the design engineer or installer, it is important for those who work on the power distribution system well after the first installation.

    • Utility: The information shown is obtained directly from the utility and should include the voltage of the source connection, the available short-circuit current and the X/R ratio. The available short-circuit current can be represented as Short-Circuit kVA/MVA or Short-Circuit Amperes. Note who the utility is as well.

    • Generators: The generator plays an important role for the reliability of the power distribution system. Information should include the voltage rating, VA rating either in kVA or MVA and sometimes could be represented instead as Watts. Sub-transient reactance and X/R ratio should also be shown at a minimum. The nameplates of generators will have all of this information and more, as required by the UL listing of the product.

    • Alternative energy solutions: These solutions will be connected to the system via an inverter. The key parameters of voltage ratings and output current ratings are those that should be included on the single-line diagram.
2. Significant power distribution equipment that contain switching devices as well as overcurrent protective devices should be shown. This includes panelboards, switchboards, transfer switches, safety disconnects, and similar equipment.

    • Panelboards, switchboards and similar equipment: Assigned identifiers, voltage rating, rated equipment ampacity, available short-circuit current, shortcircuit current rating (SCCR).

    • Transfer sw