EPS · power steering · column swap

Electric power steering wiring: power, ground, ignition and CAN

An electric power steering (EPS) unit — a motor and its controller on the column or the rack — draws little while driving straight but 50–80 A when you turn the wheel at a standstill. It needs a thick fused feed from the battery, an equally thick short ground, an ignition wire that wakes it up, and usually a CAN pair that tells it the road speed.

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The connections

B+: battery + → 60–80 A fuse within ~30 cm of the battery → thick cable to the EPS power connector. Not through the fuse box or the ignition switch.
GND: from the EPS ground lug to clean body metal next to the column, the same size as the power cable and as short as possible.
IGN: an ignition (15) circuit through a 5–10 A fuse to the wake-up input. It carries almost no current — it only tells the controller to start.
CAN-H / CAN-L: a twisted pair to the car’s CAN bus, where the ABS or engine ECU sends the road speed. Swap units often have a speed or assist-level input instead.

Typical loads on 12 V

Load	Power	Wire length	Current	Fuse	Wire
Parking lights	10 W	4 m	0.83 A	2 A	0.35 mm² (22 AWG)
Headlight bulb H7	55 W	3 m	4.58 A	7.5 A	0.75 mm² (18 AWG)
Pair of headlights	110 W	3 m	9.17 A	15 A	1.5 mm² (16 AWG)
Horn	60 W	2 m	5 A	7.5 A	0.5 mm² (20 AWG)
Wiper motor	120 W	2 m	10 A	15 A	1 mm² (17 AWG)
Radiator fan	250 W	2 m	20.83 A	30 A	4 mm² (12 AWG)
LED light bar	300 W	4 m	25 A	35 A	6 mm² (10 AWG)
Audio amplifier	1000 W	5 m	83.33 A	125 A	35 mm² (2 AWG)

Fuse and cable by EPS current

Peak draw	Fuse at the battery	Power and ground cable (up to 3 m)
40 A (small column EPS)	50 A	10 mm²
60 A (typical column EPS)	80 A	16 mm²
80 A (rack EPS, heavy car)	100 A	25 mm²

Size by the peak current at a standstill (in the unit’s data, or its original fuse), not the average while driving. Voltage matters too: below about 10–11 V most units cut the assist, so a thin cable or a poor ground feels like a heavy wheel.

Ignition, not permanent power, on the wake-up wire

On a permanent feed the controller never sleeps and slowly drains the battery. Use a circuit that is live only with the ignition on — not ACC, which drops out while cranking and resets the unit. With the engine off, steering still works, but all the current comes from the battery.

Road speed and the warning light

The controller gives full assist when parking and less at speed. Without a speed signal (no CAN, a different car’s bus) it either gives full assist all the time or a fixed fallback level and lights the EPS warning. A light with a heavy wheel usually means low voltage, a bad ground, a missing speed signal or a torque sensor that needs calibrating.

Check the power side in the simulator

Run the live demo: an 80 A fuse at the battery, 16 mm² power and ground, and the ignition through a 7.5 A fuse to IGN.
With the ignition off the EPS sleeps. Switch it on: the unit wakes up and, steering at a standstill, draws about 58 A.
Click the power cable: 3 m of 16 mm² loses about 0.2 V. Try 6 mm² to see the overload warning.

Questions

Can I take the EPS feed from the fuse box?

No. A fuse-box circuit is rated for 10–30 A; the EPS needs its own fused cable straight from the battery. Only the thin IGN wire comes from the fuse box.

Why is the steering heavy and the EPS light on?

Check the voltage at the unit while turning the wheel at a standstill: under 11 V points to the cable, the fuse holder or the ground. If the voltage is fine, look at the speed signal and the torque sensor calibration.

Does a column EPS swap need CAN?

Not always. Many swap controllers work standalone with an assist knob or a speed input; the car’s own unit usually needs its CAN messages or it falls back to a fixed assist.

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