CAN bus wires are twisted together to cancel out electromagnetic interference that would otherwise corrupt the signals traveling between sensors, controllers, and other electronic modules in a vehicle. The two wires carry the same signal in opposite polarity, and twisting them ensures that any external noise hits both wires equally, so it cancels itself out at the receiving end.
How Two Wires Cancel Noise
A CAN bus uses two wires, typically called CAN High and CAN Low. These carry complementary signals: high-speed pulses of equal strength but opposite phase. When the receiving device reads the difference between the two wires, any interference that affected both wires identically simply disappears from the calculation. This approach is called differential signaling.
The key word there is “equally.” For this cancellation to work, both wires need to pick up the same amount of noise. If one wire sits closer to an interference source than the other, it absorbs more of that energy, and the math no longer works out cleanly. This is where twisting becomes essential.
What Twisting Actually Does
When two wires run parallel without twisting, one wire is always slightly closer to any nearby source of electromagnetic energy, whether that’s an ignition coil, an electric motor, or a power cable. That proximity imbalance means one wire picks up more induced voltage than the other, creating noise in the signal.
Twisting the wires around each other constantly swaps their positions. Over one half-twist, wire A is closer to the interference source. Over the next half-twist, wire B takes that position. The noise induced in each small segment alternates direction, so when you add it all up along the length of the cable, the unwanted voltages largely cancel each other out. The tighter the twist (more twists per meter), the better this cancellation works, because each segment where one wire is “winning” the noise race gets shorter.
There’s a physics principle behind this. A changing magnetic field induces voltage in any loop of wire, and the amount of voltage depends on the area of that loop. Two parallel wires form a single large loop with a big area, which picks up a lot of interference. Twisting creates many tiny loops that alternate in orientation. Each adjacent pair of loops has its induced voltage pointing in the opposite direction from its neighbor, so they cancel. The net pickup across the full cable length drops dramatically.
Why This Matters Inside a Vehicle
Modern cars are electrically noisy environments. Fuel injectors, alternators, spark plugs, electric power steering motors, and window actuators all generate electromagnetic fields. A typical vehicle might have dozens of electronic control units communicating over the CAN bus, managing everything from engine timing to airbag deployment. Corrupted data on that network could mean a delayed brake signal or an engine misfire.
The twisted pair design gives CAN bus a natural layer of protection without requiring heavy, expensive shielding on every wire run. The twist rate (sometimes called pitch) is specified to balance impedance across the cable and match the frequencies the CAN bus operates at. Standard CAN runs at up to 1 Mbit/s, and the twist rate needs to be tight enough that noise sources operating at those frequencies get properly canceled across each segment.
Twisted Pair vs. Shielded Cable
You can also block interference by wrapping wires in a metallic shield, which physically blocks external electromagnetic fields. Shielded twisted pair (STP) cables combine both approaches and offer less noise than unshielded twisted pair (UTP) alone. But shielding adds weight, cost, and bulk. In an automotive context where hundreds of meters of wiring snake through tight spaces, every gram matters.
For most CAN bus applications, unshielded twisted pair provides enough noise immunity on its own. The differential signaling handles the heavy lifting, and the twist keeps the two wires balanced so that the cancellation works reliably. Some higher-speed or safety-critical applications do add shielding, but the twist remains the foundation of the noise rejection strategy regardless.
What Happens Without the Twist
If you ran CAN High and CAN Low as flat parallel wires, or worse, routed them along separate paths through the vehicle, the system would still use differential signaling. But the noise arriving at each wire would no longer match. The receiver subtracts one signal from the other, and any noise that didn’t cancel would appear as part of the data. At best, this triggers retransmissions and slows the network. At worst, it causes communication errors that affect vehicle systems.
This principle was recognized over 125 years ago, when early telephone engineers discovered that running a direct wire and its return wire at equal distances from nearby electrical circuits prevented inductive disturbance. As one early description put it: if both wires are affected equally, the current generated in one neutralizes and destroys that created in the other. Twisting was the simplest way to guarantee that equal exposure, and the same logic applies to CAN bus wiring today.