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ASCO Transfer Switch Frame Type Explained

When selecting an ASCO transfer switch, discussions often reference the term frame type. This article explains the term and its relevance to switch ratings.

Background Information

Selecting a transfer switch involves ensuring that its ratings meet or exceed the requirements of the application in which it will be used. For ASCO transfer switches, ampacity ranges are grouped into different frame types. Understanding frame types is useful in discussing transfer switch selection and is referenced when transfer switches are ordered.

ASCO Frame Type Basics

For ASCO transfer switches, models are organized, in part, by frame type. This refers to the specific category of the transfer switch, which includes amperage range, internal cabling and bus, connectors, switching mechanisms, controllers, communication devices, mechanical bracing, accessories, and more.

A specific type of frame can be used to build models with different ampacities. For instance, an ASCO SERIES 300 J-frame transfer switch can be obtained with ampacities ranging from 150 to 600 Amps (A). If an application requires a greater ampacity, then a different frame type must be used.

ASCO Withstand and Closing Ratings Table

The amount of current that ASCO Switches are rated to carry is specified in a table of Withstand and Closing Ratings (WCRs). An abbreviated WCR table for ASCO SERIES 300 transfer switches is shown in Figure 1. When selecting any ASCO transfer switch, users should refer to the latest version of the table for ASCO 7000, ASCO 4000, or 300 series switches, which can be accessed by searching for Engineering Application Information - Withstand and Closing Ratings For Transfer Switch Equipment (Publication 1128) on the ASCO Power Technologies website. For more information regarding WCR ratings, see the ASCO White Paper entitled UL1008 Transfer Switch Withstand and Closing Ratings.
When selecting a transfer switch, someone can find the frame designation in the left-hand column of the table above. The adjacent columns specify the ampacities available for transfer switches manufactured with that frame. Specifiers need to identify an ampacity that meets or exceeds the needs of the application.

Thereafter, the columns at right indicate ratings for the amounts of fault current that a particular model can safely withstand and close on, as verified through UL 1008 testing. For instance, if a 480-Volt (V) application requires an ampacity of 600 Amps and a fault current rating equal to or less than 85kA, then a J-frame model would provide adequate service when equipped with the appropriate specific breaker. However, if the application requires a fault current rating greater than 85kA, a larger frame transfer switch with the appropriate breaker will be required, such as an H-frame transfer switch. For more information on WCRs, specific breakers, and UL 1008 testing, see the aforementioned references.

What If More Than One Frame Type Offers the Same Ampacity?

Table 1 shows that both D and J-frame switches are available in models with ampacities of 200A. Which design should be chosen?

The answer depends first on the fault current ratings needed for the application. The 200A D-frame switch carries ratings as great as 200kA at 240V, 85kA at 480V, and 14kA at 600V when equipped with particular circuit breakers. If the possible fault current at the installation location will exceed the D-frame ratings, then the 200A J-frame model may be required. This unit is rated for fault currents of 200kA at 240V, 200kA at 480V, and 42kA at 600V when equipped with specific circuit breakers.

What is the Trade-off?

Greater ampacities and ratings typically require frames of greater size and weight. Larger and heavier components are needed to manage greater fault currents and additional enclosure volume is needed to prevent the resulting temperatures from increasing to levels that could damage materials in the device. The dimensions of ASCO 300 SERIES transfer switches can be found in the ASCO Digital Binder.

Looking at the dimension information, a SERIES 300, 200A, D-frame switch measures 18 x 31 x 13 inches (457 x 787 x 330 millimeters) and weighs approximately 75 pounds (34 kilograms). The higher-rated 200A J-Frame model presents more than twice the volume, measuring 24 x 56 x 14 inches (610 x 1422 x 356 millimeters), and weighs approximately 270 pounds (123 kilograms). Specifiers must evaluate whether the larger unit will work in the space available for this equipment. Likewise, larger switches incur additional costs.

In some cases, ratings will not be the reason for considering a different frame type. When many power cables enter a switch enclosure, or where large cable diameters require sufficient cable bend space, cable pull boxes can be added. Pull boxes can provide additional space for routing conductors while avoiding the need for a different frame type.

Ordering Transfer Switches

A second instance where frame type is referenced is in ASCO transfer switch ordering codes like the one shown in Figure 2. The frame type is typically the first variable in the 16-digit ordering code for ASCO transfer switches.

The ordering code for ASCO SERIES 300 Transfer Switches is shown below and can be accessed through the ASCO Digital Binder. Ordering codes for ASCO 4000 SERIES and 7000 SERIES switches can be accessed in the Digital Binder here and here, respectively.