How do you size the main fuse and cable for a 72V lithium golf cart conversion?

Direct answer: Work from the current the controller can draw and the pack can deliver. The cable must carry the highest sustained current without overheating, after derating for temperature and routing. The main fuse must be rated for DC at or above the pack’s maximum charge voltage (87.6V here), able to interrupt the pack’s short-circuit current, and sized above normal peak current but below the cable’s ampacity. Have the design checked by a qualified technician and confirm figures with Redway.

Pack figures. Redway’s 72V 100Ah (S72105P), 72V 160Ah (S72160P) and 72V 200Ah (S72210P) list 76.8V nominal, 87.6V maximum charge, 60V cut-off, continuous current of 100A, 160A and 200A, and a peak of 315A for 30 seconds. Our post on choosing between these three packs covers capacity and fit; this one covers the wiring.

Step 1: design current. Find the controller’s maximum battery current and how long it can sustain it. If the controller can draw 300A for a minute on a climb, a pack rated 100A continuous is the wrong match before any wiring choice.

Step 2: cable. Choose a cable whose rated ampacity, from the cable maker’s tables for its insulation temperature and how it is routed, exceeds the design current, and check voltage drop over the full positive and negative run. Use fine-strand cable suited to vibration, crimped lugs of the right size and a proper crimp tool. A cable that is warm after a hard climb is a warning sign.

Step 3: fuse. Choose a fuse with a DC voltage rating at or above 87.6V and an interrupt rating suitable for a lithium pack’s short-circuit current; ask Redway for the pack’s expected short-circuit current. Its current rating should allow the controller’s normal peaks without nuisance blowing and still protect the cable. Fit it as close to the battery positive as practical.

Step 4: check and record. Torque lugs to the specified values, protect cables from chafe and sharp edges, and record cable size, fuse type and rating. The BMS protects the cells; the fuse protects the wiring. Do not replace a blown fuse with a larger one without finding the cause; see troubleshooting BMS lockout.