UL 98 vs. UL 508 disconnect switches

UL 98 or UL 508 disconnect switches? Panel builders and OEMs often make poor choices, resulting in project delays and increased costs. What do you need to know when selecting the right switch for your application?

The primary focus of panel builders is typically to ensure a safe, reliable power source to motors and other industrial equipment. But equally important is ensuring the ability to interrupt that power when necessary to control the process and to protect the power circuit and connected equipment. The need to disconnect power is common to every industrial process, but the appropriate components to do that can be significantly different.

NEC Article 430.109 requires a means of disconnecting motors and motor controllers from the circuit. There are two types of switches that, based solely on functional specifications (voltage, amperage, etc.), might seem to be appropriate to meet this requirement: UL 98 (Enclosed and Dead- Front Switches) and UL 508 (Industrial Control Equipment) compliant switches.

UL 98 switches are primarily used as disconnects for service entrance, feeders, branch circuits, and motor circuits. On the other hand, switches complying with

UL 508, the standard for industrial control equipment, are primarily designed specifically for motor disconnecting. Their typical applications differ, but there are many situations where either switch will fulfill the NEC requirement.

Choosing one switch over the other can provide additional end-user benefits or meet unique application needs. Selecting the optimum switch for a given application requires consideration of several factors.

Applications

The standard specifies that UL 98 components can be used as a branch circuit, feeder, or motor-circuit disconnect. UL 508 switches can be used only to start, stop, regulate, control, or protect electric motors. Surprisingly, some panel builders are unaware of this critical distinction. In one case, a panel builder opted for a UL 508 switch as the disconnect for their main panelboard. But when UL inspected the project, the panel was rejected. That meant a costly redesign and modification of the control panel, delaying project completion by more than a month.

Some panel builders attempt to avoid potential application problems like this by using the more- versatile UL 98 switch for motor-protection functions. But this is typically overkill. Motor- protection functions can be provided by a UL 508 switch at a significantly lower component cost that results in a less-expensive finished panel.

Physical design and switch size

The minimum spacing between uninsulated live parts of different polarities in a UL 98 switch is 2 inches over surface and 1 inch through air versus only a 1/2 inch over the surface and 3/8 inch through air for the UL 508 disconnect switch. The larger spacing of the UL 98 switches makes them a more robust choice, but at a higher cost and a larger footprint that requires more panel space.

Location

There are two simple rules for UL 98 versus UL 508 disconnect switches related to their location.

If the disconnect switch is within sight of the motor, controller, and driven machinery (within 50 feet), then a single UL 98 switch can be used to disconnect both the motor controller and the motor circuit.
If the disconnect switch is within sight of the controller, but not in sight of the motor location and driven machinery, and the distance is greater than 50 feet between the motor controller disconnect switch and the motor, then a seocond disconnect must be installed. In this case, a UL 98 disconnect switch must be used for the motor-controller circuit.

A UL 508 switch is highly recommended for the motor disconnect with one exception: it is not required to have the disconnecting means within sight of the motor if the distance is greater than 50 feet between the controller and the motor, and if the controller-disconnect switch can be locked in the open position. In this case, one UL 98 disconnect switch can be used to disconnect both the motor controller and the motor circuit according to 430.102(B)(1) and (2).

Smart switch choices

Choosing the right disconnect switch for your panel starts with an examination of UL 98 and UL 508 standards. Doing so will help avoid choosing non-compliant components and will prevent project delays and unexpected redesign costs. Simply relying on the more-capable UL 98 switch reduces the likelihood of failing a UL inspection, but doing so means increased component costs and panel sizes.

OEMs and panel builders must understand the specifications and standards to provide the right solution to customers with the lowest possible cost while meeting the project requirements. Doing so makes it possible to create more cost- competitive panels that bring greater value to customers and end users.

See related blog post “Control panel, motor control, selection made easy”.