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But volts are not power, watts are. Watts are voltage multiplied by the amps. How many watts does the lightbar require? Does it have a spec sheet with it (or online)?
It would be a hell of a 20" light bar to pull more amperage than the 30 amps available on the trailer plug, especially if it is a LED light bar. Yes, I understand the difference and relation between wattage, voltage, and amperage. Now, if you start talking about Laplace's equation or Fourier transforms, I'll need to grab my old textbooks.
As long as your trailer has a battery inline the truck will see the voltage and supply power. These newer trucks only supply power if it sees a voltage ( battery ) in the circuit.