Windlass Feeder: Voltage Drop and Derated Ampacity
On a 12 V boat, a windlass at the bow draws 70 A running and about 160 A locked rotor. The one-way cable run from the battery to the windlass is 28 ft. The installed conductor is 2 AWG (66,360 circular mils), protected by a 90 A thermal-magnetic breaker at the battery. The builder's ampacity table gives 2 AWG a free-air rating of 130 A at 30 °C. In the engine room at 50 °C the correction is 0.82, and because the cable is bundled with three other cables the correction is 0.70. Use K = 10.7 for copper. The owner reports that the windlass is sluggish and that the breaker feels warm after repeated lifts, but it has not tripped.
1. Calculate the voltage drop at 70 A and at 160 A, and decide whether the existing 2 AWG is electrically acceptable for this motor circuit. State any assumptions.
Read the worked answer
Use Vdrop = 2KIL/CM. At 70 A: 2 x 10.7 x 70 x 28 / 66,360 = 41,944 / 66,360 = 0.63 V. That is 5.3% of 12.0 V and 5.0% of 12.6 V. At 160 A: 2 x 10.7 x 160 x 28 / 66,360 = 95,872 / 66,360 = 1.44 V, or 12.0% of 12.0 V. With a battery at 12.2 V under load, the motor would see about 10.8 V at locked rotor. The running drop is acceptable for a 10% motor-circuit guideline, but the locked-rotor drop is over it. Because locked rotor is brief, this alone may not explain sluggishness; check connections, brushes, and actual battery voltage. Still, the circuit has little margin, so upsizing is justified.
2. Calculate the corrected ampacity of the 2 AWG and determine whether the 90 A breaker and the 70 A load are acceptable. If not, choose a size from 1 AWG (83,690 cm, 150 A at 30 °C), 1/0 (105,600 cm, 170 A), or 2/0 (133,100 cm, 195 A).
Read the worked answer
Corrected ampacity = 130 x 0.82 x 0.70 = 74.6 A. The 70 A load is 94% of that, with no margin, and the 90 A breaker is above the conductor ampacity. That is not acceptable: the breaker does not protect the cable, and a warm breaker may be carrying near its threshold. 1 AWG: 150 x 0.82 x 0.70 = 86.1 A; an 80 A breaker would protect it and the 70 A load. 1/0: 170 x 0.574 = 97.6 A; a 90 A breaker is acceptable. 2/0: 195 x 0.574 = 111.9 A. Voltage drop for 1/0 is 2 x 10.7 x 70 x 28 / 105,600 = 0.40 V; for 1 AWG it is 0.50 V. I would use 1/0 and a 90 A breaker, or 1 AWG with an 80 A breaker, and verify terminal torque.
3. The owner proposes simply replacing the 90 A breaker with a 150 A breaker because the windlass 'only runs for a minute.' Evaluate.
Read the worked answer
No. The breaker is the overcurrent protection for the conductor, not a motor-duty controller. A 150 A breaker would allow 150 A through a conductor whose corrected ampacity is only 74.6 A, creating a fire risk before the breaker opens. The windlass's intermittent duty does not change the conductor's short-time withstand; a stalled motor or chafed cable can draw high current continuously enough to damage insulation. If the motor needs a higher instantaneous trip for starting, use a motor-rated breaker or a fuse selected for both the conductor and the motor, but the conductor must be upsized first. Also inspect the 90 A breaker's terminals; a warm breaker can be a loose connection rather than an overload.