What may happen when changing from a regular to a higher power headlight bulbs?

■ Risk of short circuit
A 51W 12V bulb will draw 4.25 amperes* of current. That current flows through the wiring from the power source to the bulb. The wires should be able to handle the current flowing through it (i.e., wire is rated 5 amps). When you change to 81W 12V, the bulb will draw 6.75 amps of current from the power source. In our example, the 5-amp rated wire won't be able to handle the current flowing through it because it is beyond its capacity. If there is a fuse along the line, the fuse will just blow up. But if there is none, the wires will heat up and may melt the insulators causing a short circuit. For me, an electrical short circuit is the last thing that I would want to happen to my car.

■ Lense deterioration
The headlight lense maybe able to handle only a certain temperature. Changing to a high power bulb will cause a significant increase in temperature (i.e., halogen bulb) that maybe beyond the lense's capacity resulting to faster deterioration.

* power = pressure × current
51W = 12V x 4.25A
81W = 12V x 6.75A

power in watts
potential/pressure in volts
current in amperes

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