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Specifying a disconnect, short-circuit & overload device in one unit

Question: We received a tech call regarding an application to incorporate a manual starter and two contactors. The manual starter required a means of disconnect, short circuit protection and overload protection. The motor size was 20HP with an FLA of 23.4A @460V. The question from the customer was whether the KTA7-32S-29A, with a dial setting of 24-29A, was an appropriate selection for the application. The customer also asked if an overload incorporated to cover the actual full load rating on the motor would be a better choice.

Answer: What was not clear about the application was “incorporate a manual starter and two contactors”. If there are two motors then it takes a short-circuit device and an overload relay for each motor. The original question indicated only one motor. If we assume one motor then one device is required as follows:

• In the field, someone’s first reaction might be to choose a KTA7-25S-25A based on the Current adjustment range of 18-25A, which fits within the motor FLA of 23.4. However, consider the following details:
  1. If the nameplate of a 20HP @ 460 Volt motor reads 23.4 FLA, then the FLA is the overriding selection factor. However, if you selected KTA7-25S-25A, an inspector may give you a hard time over the fact that the label says 15 HP @ 460 VAC, even though the government passed a law requiring the motor manufacturer to lower FLA to premium efficiency levels.
  2. Consider also that NEC 430-32 indicates for a 1.15 service factor motor that an overload relay trip at less than 125%. Consider further that Sprecher + Schuh overload relays are designed to trip at 120%. This means choosing KTA7-32S-29A with a 24…29A range with the dial set at 24A FLA setting represents a trip at 28.8, or 123% of 23.4 actual FLA. Therefore, choosing the KTA7-32S-29A unit with a dial setting at 24 Amps would be within the range.
• Perhaps more important to the application was that the manual starter required a means of “disconnect, short circuit protection and overload protection”. On page F5 of the Sprecher + Schuh Industrial Controls Catalog we indicate that KTA7-32S-29A is NOT a Self-Protected Combination Motor Starter (Type E) and therefore cannot be applied as the short-circuit device under NEC430-52.c.6
  1. Note that a manual motor starter cannot be used as a short-circuit device, but a Self-Protected Combination Motor Starter (Type E) can be applied as a short-circuit device under NEC430-52.c.6. Most of our KTA7s have both approvals but not all. Careful examination of the UL ratings application chart on F5 shows this.
  2. However, the KTA7-45H-32S is approved as a Self-Protected Combination Motor Starter (Type E), has a range of 23…32A, and has a 20 HP @ 460 VAC rating.

Our recommendation, barring the aforementioned need for clarification regarding the number of motors, was to incorporate the KTA7-45H-32A into the application. This means compliance with the HP rating and the FLA setting needed. The KTA7-45H-32A also provides short-circuit protection as a Type E rated device. There would be no need for a separate overload relay for one motor. Problem solved every way you look at it.