Selecting Switches per NEC and IEC
Tech Tip
Article 430 of the US National Electric Code includes two methods for properly sizing disconnect switches:
- Single Motor Application
A properly sized disconnect switch for a single motor will:
- Have an ampere rating greater than or equal to 115 percent of the rated motor full load current; or,
- Have a HP rating greater than or equal to the rated motor HP (at applied voltage) if the disconnect switch under consideration is HP rated.
- Combination Load Application
A properly sized disconnect switch for a combination load will be selected by adding all the simultaneous individual loads in the circuit under consideration.Using motor nameplate information, load information, and tables from section 430 of the NEC, determine one equivalent full load current and one equivalent locked rotor current. The equivalent locked rotor current can be used with table 430-151 to determine an equivalent HP rating. Select a disconnect switch:
- Greater than or equal to 115 percent of the equivalent full load current; and,
- Greater than or equal to the equivalent HP rating.
| Nature of Current | Utilization Category | Typical Applications | |
| Frequent Operation | Infrequent Operation | ||
| Alternating Current | AC-20A | AC-20B | - Connecting and disconnecting under 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* | |
| Direct Current | DC-20A | DC-20B | - Connecting and disconnecting under no-load conditions |
| DC-21A | DC-21B | - Switching of resistive loads including moderate overloads | |
| DC-22A | DC-22B | - Switching of mixed resistive and inductive loads, including moderate overloads (e.g., shunt motors) | |
| DC-23A | DC-23B | - Switching of highly inductive loads (e.g., series motors) | |
| Mechanical Endurance | Number of Operations | Number of Operations | |
| 100A
315A
>315A |
10,000
8000
2000 |
2000
1800
400 |
|
* Category AC-23 includes occasional switching of individual motors. The switching of capacitors of tungsten filament lamps shall be subject to agreement between manufacturer and user.
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