Transfer switches are rated for something called the Withstand Current. If there were a short circuit, this is how many Amps of electricity the switch could handle (“withstand”) without allowing the contacts to be welded shut, which could lead to a fire.
The switch must be protected from any possible load, including a short-circuit load, that could be greater than its Withstand Rating.
The Withstand Rating that is required– depends on where the switch is installed.
Installed at the Service Entrance: The maximum required Rating is needed at the Service Entrance for the building, so that is the Rating your switch must have if the Switch is supplying the whole building. You can get this number from your electricity utility – ask them for the Withstand Rating for the Service Entrance for your building.
Installed to run only part of the building: If the switch is only running part of the building and is connected after the main panel, the required Rating would be less. The actual required Rating can be calculated by an engineer if he has all of the information about wire sizes and other equipment between the Service Entrance and where the switch is connected. If you do not want to hire an engineer to do this calculation, just make sure that the switch has a Withstand Rating equal to the Withstand Rating at the Service Entrance – the number you got from the electric utility -- which would be the maximum for this facility.
The Withstand Rating for the switch itself-- is on the switch Specification Sheet.
For instance, the rating might be 10,000A for a 200A standard switch, and 18,000A for a 400A switch. Or a heavy-duty 400A switch might be rated 35,000A if protected by a breaker, or 200,000A if protected by a fuse.
What if the Withstand Rating of the switch is not high enough?
- Put a breaker or fuse in the circuit before the switch. Then the Withstand Rating depends on that breaker or fuse. A breaker will give less protection and a lower Withstand rating, because often it will not break the circuit until after 1 or 2 cycles. The fuse will give the highest Rating, because it breaks the circuit after only about ¼ cycle.
So -- you can install a fused disconnect before the transfer switch, or if you already have a disconnect, install just a fuse block before the switch. Depending on the Amperage required, the fuses might be Class L or J or T or RK1 or RK5 etc.
- You can buy a heavier-duty switch with the required rating.
- You can buy a switch that is otherwise larger than needed for your application, but has the required Withstand Rating.
So the simplest solution is:
- Call the electric utility and get the Withstand Rating for the Service Entrance of the building.
- If your Switch needs a higher Withstand Rating, install a fuse block or a fused disconnect before the Switch.