What is the maximum permitted overcurrent device rating for an autotransformer with a rated input current of less than 9 amperes?

For an autotransformer with a rated input current of less than 9 amperes, the applicable code allows for a maximum permitted overcurrent device rating to be set at 167% of the input current. This rating is established to ensure that the protection device can handle inrush currents and temporary overload conditions without tripping unnecessarily, while still providing adequate safety from sustained overcurrent situations.

Choosing 167% ensures compliance with standards aimed at guaranteeing that the device does not exceed what is necessary for safe operation. This percentage provides a balance between allowing for normal operational conditions and ensuring protection against potential hazardous conditions.

While higher percentages, such as 200%, 250%, or lower percentages like 150%, may seem reasonable, they do not align with the specific requirements set forth for autotransformers in this context. The standards are designed to provide the most effective protective measures while allowing operational flexibility, which is precisely what the 167% rating accomplishes.