A locomotive may have a fifth letter, generally denoting a technical variant, subclass or subtype: a variation in the basic model or series, or a different motor or manufacturer. Under the new system, the fifth letter further refines horsepower in 100-hp increments: A for 100 hp, B for 200 hp, C for 300 hp and so on. A WDP-3A is a 3,100 hp (2,300 kW) locomotive, and a WDM-3F is 3,600 hp (2,700 kW).