Here’s the HTML code for the article, ready to copy-paste into WordPress. It covers the wiring diagram, terminal labels, safety steps, and compatibility checks for 120V to 12V transformers.
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Two terminals in, two terminals out: 120V lands on a transformer’s primary side and the 12V load lands on its secondary, as the case label shows.
For the full breakdown, see our best 120V to 12V Transformer guide.
A swapped pair of leads is all it takes to send 120 volts into a 12-volt fixture — and the label on the transformer case is the map that prevents it. Installation sheets spell out how to wire a 120V to 12V transformer the same way every time: supply to the primary, load to the secondary, ground to the case. The letters stamped on the housing are just four manufacturers’ shorthand for those three jobs.
The confusion almost always comes from the markings. H and X, L and N, red and purple, common and 120 volt — different sheets, same split. The table below sorts them out, and the sections after it cover step order and the one compatibility check that matters.
120V To 12V Transformer Wiring: What Each Terminal Label Means
Every unit divides its terminals into a primary side that takes line voltage and a secondary side that sends out low voltage, and no wire ever crosses between them. The letters
