Alternator Cable Fuse and Long Pump Branch
On a 12 V boat, a 100 A alternator charges the house bank through a 2 AWG cable with a 14 ft one-way run. The cable is protected at the battery by a 300 A fuse. The cable runs in a 50 °C engine room bundled with two other cables. The builder's table gives 2 AWG 130 A at 30 °C free air, with a 0.82 temperature correction and a 0.70 bundling correction. A separate 10 AWG branch feeds a 25 A deck-wash pump. That branch is protected by a 30 A breaker at the main panel, and the one-way run is 30 ft. The 10 AWG rating is 60 A at 30 °C, corrected by 0.70 for bundling. Use K = 10.7.
1. Evaluate the 2 AWG charge cable and the 300 A fuse. What size cable and fuse would you select?
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Corrected ampacity = 130 x 0.82 x 0.70 = 74.6 A. The alternator can produce 100 A, so the 2 AWG cable is undersized for the source, and the 300 A fuse is far above the cable ampacity. The fuse at the battery is correctly located to protect against battery fault current, but it must be sized to the cable: a 300 A fuse will not open before the 2 AWG overheats. 1/0 gives 170 x 0.574 = 97.6 A, still marginal for 100 A; 2/0 gives 195 x 0.574 = 111.9 A. Use 2/0 with a 125 A fuse, or a fuse sized per the chosen cable and alternator, and verify voltage drop: 2 x 10.7 x 100 x 14 / 133,100 = 0.23 V. Also check the alternator's own output protection requirements.
2. Evaluate the 10 AWG deck-wash branch for both overcurrent protection and voltage drop. What minimum size would you recommend for a 10% drop target, and for a 3% target?
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Ampacity: 60 x 0.70 = 42 A, so the 30 A breaker protects the 10 AWG conductor. Protection is not the problem; voltage drop is. At 25 A: 2 x 10.7 x 25 x 30 / 10,380 = 16,050 / 10,380 = 1.55 V, or 12.9% of 12 V. That exceeds a 10% motor-circuit guideline and is worse at startup. 8 AWG (16,510 cm) gives 0.97 V (8.1%); 6 AWG (26,240 cm) gives 0.61 V (5.1%); 4 AWG (41,740 cm) gives 0.38 V (3.2%). For a 10% target, use 8 AWG with a 30 A breaker; for a 3% target, use 4 AWG. Confirm the pump's starting current and the breaker's motor rating.
3. The owner wants to move the 300 A fuse from the battery end to the alternator end because it is closer to the alternator. Evaluate.
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No. The fuse must be at the battery end of the charge cable because the battery is the high-fault-current source. Moving it to the alternator end would leave the 14 ft cable unprotected if it chafes and shorts to ground; the battery would feed the fault through the cable, and the alternator's 100 A output would not limit it. The alternator end may need its own protection for the alternator's output, but that does not replace battery-end protection. Keep the fuse at the battery, size it to the cable, and protect the cable from chafe.