Hello- What is the yellow and the black leads for on the 120 supply voltage side. Using a 12 vdc output transformer here to feed 12 vdc LED lighting. thank you

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There are four (4) wires on the primary/line voltage side of many Semper Fi transformers.  White, Black and Green are Neutral, Hot and Ground.  The Yellow is an Optional Boost tap that increases the output voltage off the transformer.  When you want a slight boost to compensate for voltage drop, wire your line voltage hot to the Yellow lead instead of the Black wire and cap the black. You can Not use the black and the Yellow wires simultaneously.