Catalogs > Industrial Controls Catalog > Motor Control, IEC > Rotary Disconnect Switches > Bulletin 194R Applications and Fuse Descriptions
Bulletin 194R Applications and Fuse Descriptions
Proper Selection of Disconnect Switches
Applications Within Canada and the United States
General
The requirements for disconnect switches used in motor branch circuits rated 600V and less are defined in Article 430, Part J of the U.S. National Electrical Code (NEC), NFPA70. Canadian Electrical Code (CEC) requirements are very similar in the area of motor branch circuit disconnect requirements. For simplicity, we will treat the NEC and CEC requirements as being the same — and reference specific sections of the U.S. National Electrical Code.
The requirements for properly sizing a disconnect switch are dependent on the type of application. The NEC refers to two types of applications: single motor and combination loads. A combination load consists of an application where two or more motors are used together or where one or more motors are used in combination with other loads, such as resistance heaters.
Single Motor Applications
Section 430-110 Paragraph (a) states that the disconnect switch must have an ampere rating of at least 115% of the full-load current rating of the motor.
Example 1: For a motor with a full-load current of 22 A, the disconnect switch must be rated at least 25.3 A (22 x 1.15).
If the disconnecting means under evaluation is rated in horsepower, the selection of the disconnect switch is even more straightforward; a disconnect switch must have a horsepower rating equal to, or greater than the horsepower rating of the motor at the applicable voltage.
Example 2: For a motor with a 10 Hp rating at 460V AC, the disconnect switch must be rated at least 10 Hp at 460V AC.
If the disconnect switch is rated in horsepower, and UL Listed, UL Component Recognized, or CSA Certified, it will meet the requirements for the 115% full load current rating stipulated by the NEC.
Combination Load Applications
Section 430-110 Paragraph (c) addresses the rating of the disconnecting means for combination loads. This paragraph essentially requires that the loads that “may be simultaneous on a single disconnecting means” be combined to provide equivalent full-load and locked-rotor currents for what is then to be considered as a single motor for the purpose of selecting the appropriate disconnecting means. This means that it is necessary to identify the particular combination of connected loads which can be operating simultaneously and will result in the maximum full-load and locked-rotor current sums.
The individual full-load current values are to be selected from Tables 430-148, 430-149, or 430-150 and the locked-rotor values are to be from Table 430-151.
The equivalent single motor full-load current is the sum of the simultaneously operating motor full-load currents and the rating in amperes of other loads operating at the same time. The equivalent locked-rotor current is the sum of the simultaneously started motors’ locked-rotor currents and the full-load currents of the remaining operating motor and non-motor loads.
The disconnecting means shall have a current rating equal to or greater than 115% of the equivalent single motor full-load current and have a horsepower rating equal to or greater than the horsepower rating determined from the equivalent locked-rotor summation.
Consider the following 460V application:
| Load | Hp | Full-Load Current [A] |
| Motor 1 | 5 | 7.6 (simultaneous) |
| Motor 2 | 10 | 14.0 (not included)⋆ |
| Motor 3 | 15 | 21.0 (simultaneous) |
| Motor 4 | 20 | 27.0 (simultaneous) |
| Other | 7.0 (simultaneous) | |
| Total Equivalent | 62.6 (simultaneous) | |
⋆ Motor 2 is not included in the total since it cannot operate simultaneously with the other motors, therefore, the disconnect switch must be rated at least 72 A (1.15 x 62.6).
Consider now the locked-rotor current analysis for the same application:
| Load | Hp | Full-Load Current [A] |
| Motor 1 | 5 | (7.6FLA) 45.6 (simultaneous) |
| Motor 2 | 10 | 84.0 (not included)⋆ |
| Motor 3 | 15 | 126.0 (simultaneous)‡ |
| Motor 4 | 20 | 162.0 (simultaneous)‡ |
| Other | 7.0 (simultaneous) | |
| Total Equivalent | 302.6 (simultaneous) | |
| ⋆ Note again that Motor 2 cannot operate simultaneously with the other loads. | ||
| ‡ The largest equivalent locked-rotor current occurs when motors 3 and 4 start together while the other loads marked “simultaneous” are already operating. Since Motor 1 is not starting with Motors 3 and 4, its full-load current will be added to the total instead of its locked-rotor current. | ||
Table 430-151, which provides the correlation between locked-rotor currents and Hp ratings, shows that a 40 Hp rating is the equivalent for 302.6 locked-rotor amperes.
Therefore, the disconnect selected for this application must have a current rating of at least 72 A and a Hp rating of at least 40 Hp. In this case a Bulletin 194R rated for 100 A and 60 Hp at 460V would be an appropriate choice. What can be seen from this analysis is that, depending upon the number of motors that can start simultaneously, the actual size of the required disconnect is sometimes determined by the equivalent full load current (72 A) and other times by the equivalent horsepower determined from the locked rotor analysis (40 Hp).
Applications Outside the United States and Canada
General
Disconnect switches designed to IEC Standards and used in applications outside of North America are selected based on the ampere, horsepower, or kilowatt rating of the disconnect switch, under various utilization categories. Utilization categories for disconnect switches are as follows:
| Nature of Current | Utilization Category | Typical Applications | |
| Frequent Operation | Infrequent Operation | ||
| AC | AC-20A⋆ | AC-20B⋆ | Connecting and disconnecting under no load conditions |
| AC-21A | AC-21B | Switching of resistive loads including moderate overloads | |
| AC-22A | AC-22B | Switching of mixed resistive and inductive loads, including moderate overloads | |
| AC-23A | AC-23B | Switching of motor loads or other highly inductive loads | |
| ⋆ The use of these utilization categories is not permitted in the U.S. | |||
For any application, the disconnect switch rating (A. Hp, or kW) must be greater than or equal to the application full-load current or power (Hp or kW), in the appropriate utilization category.
Example 1: For a 380V 50 Hz distribution application (AC-22A), with a 63 A full load current, the disconnect switch must be rated at least 63 A at 380V 50 Hz for use in AC-22A applications.
Example 2: For a 415V 50 Hz motor application (AC-23A), with a 75 kW rating, the disconnect switch must be rated at least 75 kW at 415V 50 Hz for use in AC-23A applications.