Class I vs. Class II

For our definition, a Class II fixture is any low voltage fixture or component that has 18 gauge wire connecting to the power source or tap leads. A Class II transformer can only power this type of fixture.

The National Electrical Code (NEC) classifies any transformer with maximum output of 60 watts or less at 12 volts (and marked as such) as a Class II transformer (5 amps or under). Any transformer with a larger output than 60 watts is defined as a Class I transformer. The only way to use a Class I transformer with Class II fixture is to add an in-line 5 amp fuse (or fused plug) between the transformer and the fixture's tap leads. The fuse protects the 18 gauge wire in case of a short and must not be placed in an inaccessible enclosed area. The rationale behind this theory is as follows:

60 watts/12 volts = 5 amps

Factually, 18 gauge wire is rated for no more than 5 amps. The 5 amp protection fuse is necessary when using a Class II fixture on a Class I system.