In UL 489 One and Two pole breakers are tested on single phase system while a Three pole breaker is tested on a three phase system.

Unless a three pole breaker is specifically marked for use on a single phase system it should not be applied in that application. Fault currents are greater in single phase center-tapped transformers than in three phase transformers, so the demand of testing on a single phase source is greater than a three phase source.

Because three pole breakers are tested on a three phase system, either one, two, or three poles can be used without limits when attached to a three phase system. This is true for both Thermal-Magnetic and Sold State Trip Units. Some three pole breakers are additionally tested for use on single phase systems and are marked with a note stating “use outside poles when use on single phase”. This note is specific to use on a single phase system (120/240v for example), and not a single phase load, which could be on either a single or three phase system.