Understanding Service Entrance-Rated Transfer Switches

According to the National Electrical Code, all buildings must have a means to disconnect electrical power service from the utility at the site where it enters the building. For buildings that have a standby power system, this can take the form of a service entrance-rated transfer switch. In this post, you’ll learn what a service entrance-rated transfer switch is, where it is commonly used, how it can be used, and different ways it can be configured.

What is a Service Entrance-Rated Transfer Switch?

A service entrance-rated transfer switch is a type of generator transfer switch. It’s the first piece of equipment between the electrical utility and your home or business building. These transfer switches allow you to manually disconnect your building from utility power and to safely switch to another power source if utility power becomes unavailable. These switches generally involve some method of over-current protection that protects the building’s electrical service via a fuse or a circuit breaker. Service entrance-rated transfer switches should be code compliant and have UL-891 and UL-1008 labels.

Where are Service Entrance-Rated Transfer Switches Commonly Used?

Service entrance-rated transfer switches can be used on any building that uses a generator or other secondary power source. This can include homes, but these switches are also used in commercial buildings, heavy industries, or critical infrastructure such as hospitals, sanitation plants, etc. 

Service entrance-rated transfer switches can act as the “brains” of a building’s electrical system. They are the switching mechanisms between two sources of power. Using a service entrance-rated transfer switch allows you to disconnect utility power from your home or business, whether it’s necessary to conduct maintenance or in an emergency. In the event of an over-current, the service entrance rated transfer switch can be used to protect the wires that are feeding into the main electrical panel.

Part of the service entrance-rated transfer switch acts as a method for switching between utility and generator power. When utility power is lost, the switch sends a signal to the generator to start. When the generator reaches the proper frequency, it transfers over to generator power. When utility power comes back online, the service-rated transfer switch senses it, then waits for the utility power to stabilize. Once this delay passes, the transfer switch sends a signal to the generator to power down as utility service is restored to the building.

How Can Service Entrance-Rated Transfer Switches Be Configured?

Service entrance-rated transfer switch installation can be done in four different ways as illustrated by the infographic below.

In the first two options, the disconnect is in a separate box from the automatic transfer switch. The disconnect device, located ahead of the automatic transfer switch, acts as the service entrance. In Option 1, the disconnect device uses a fuse for overcurrent protection, and in Option 2, the disconnect is a service entrance-rated circuit breaker.

In the second two options, the automatic transfer switch itself is service entrance rated and includes an integral circuit breaker for overcurrent protection, so the entire system is located in one box. Option 3 uses a circuit breaker as the disconnect device, while Option 4 uses a contractor.

The most common installation configuration among our clients is Option 4. The advantage is in the system’s simplified design: You don’t have to mount separate pieces of equipment or run wires between them because everything is handled in one enclosure.

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