Tech Tip

SELECTING SWITCHES per NEC and IEC

Article 430 of the US National Electric Code includes two methods for properly sizing disconnect switches:

1. 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.

2. 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.

Selecting Switches per IEC Utilization Categories

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