This isn't usually an issue for case fans (they often aren't regulated), but is a serious issue for CPU fans (they are almost always speed regulated.Īccording to the manual (p. When you use a Y-cable to connect two fans to one connector, you end up messing up that tachometer signal, and run the risk of either overheating because the fans are running too slow, or being really noisy because they're running faster than needed. The firmware (or the OS, or the hardware sensor chip, depends on the platform) reads back the fan speed over one of the wires (even on the 3-pin fans), and uses that to regulate the fan speed (using PWM with the 4-pin fans, or by just scaling voltage with 3-pin ones). Trying to run more than one fan from a connector runs the risk of either having both run slow (because they aren't getting enough power), or drawing too much power and causing electrical issues (including being a fire hazzard). Many fan connectors are designed to provide a fixed amount of power, and on most motherboards, this is just enough to power one fan with a reasonable safety margin. You're going to be safer and get better behavior from the system if you put one fan on each connector instead of using a Y-cable.
