Peterbilt 520 Turn Signal Bulb Size

Additional Notes

• The Peterbilt 520 uses the 1157 bulb for the rear turn signal across the 2017-2019 model years.
• The 1157 is an ANSI-standardized bulb type, meaning its physical and electrical specifications are governed by a recognized North American standard applicable in both the United States and Canada.
• The 1157 is a dual-filament bulb, which means it contains two separate filaments within a single envelope, allowing it to serve two distinct lighting functions from one unit.
• The bulb operates on a 12-volt system, consistent with the standard electrical architecture found in most commercial vehicles.
• The two filaments in the 1157 are rated at 21 watts and 5 watts respectively, with the higher-wattage filament typically associated with the active turn signal function and the lower-wattage filament used for a steady running or parking light function.
• The 1157 uses a BAY15d bayonet base, which is an offset dual-contact base. The offset pin configuration is a deliberate design feature that prevents the bulb from being inserted in the wrong orientation, ensuring the correct filament is assigned to the correct circuit.
• Because the BAY15d base uses offset pins, it is not physically interchangeable with single-contact or symmetrical dual-contact bayonet bases, even where the overall bulb size may appear similar.