Class II Transformers
Class II transformers must be used in conjunction with Class II circuits. They have a maximum volt-ampere (VA) of less than 100.
The maximum secondary output cannot exceed 30 alternate current voltage (VAC). The most common combination is 75 VA and 24 VAC.
They are regulated by Underwriters Laboratories (UL) guidelines and can be either inherently or non-inherently limited.
Non-inherently limited Class II transformers rely on a fuse or circuit breaker to trip at a preset voltage or an intrinsic coil to impede the voltage if the transformer is overloaded. Non-inherently limited transformers meet UL 1585 requirements and are more common for 60 VA through 75 VA Class II transformers.
The benefit of these transformers is the fact that the circuit breaker or fuse can be reset if they are overloaded.
Inherently limited Class II transformers do not require a fuse. They have a built-in fail safe that causes them to short out and fail if the transformer is overloaded.
These are common on transformers below 50 VA, however, many inherently limited transformers do not meet UL standards.