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Alternator Cable Size Guide

When upgrading to a high-output marine alternator, don’t forget about the all important cable connecting the alternator to your battery bank. Use our Alternator Cable Size Guide to help determine the correct cable size for connecting your high-output alternator.

Most marine engines come with a 6 AWG or 8 AWG alternator positive cable that connects between the alternator and the battery post on the starter. These OEM cables are not sufficient to carry the current supplied by high-output marine alternators.

Connection Strategy

While there are many approaches to battery charging strategy, we recommend connecting your high output alternator directly to the house (or largest) battery bank. The house battery bank can handle the highest charging current due to its size, so the alternator output is utilized most efficiently by going direct to the house bank. We recommend automatic charging relays for charging engine starting and auxiliary battery banks. If the starting and auxiliary batteries use a different chemistry from the house bank, we recommend Balmar’s Digital Duo Charge so the auxiliary batteries do not get overcharged. We do not recommend battery isolators.

You may be surprised at the size cable that is needed to properly handle the current supplied by a high-output marine alternator. Balmar offers alternators at up to 310 amp ratings, for which a single 4/0 (0000) AWG cable is not enough!

Alternator Grounding

Some alternators are case-ground type, which means the DC negative charging current (electrical engineers please don’t comment!) flows between case of the alternator to the engine block through the mounting arrangement. Make sure there is no paint or corrosion where the alternator touches the engine block. The engine’s ground (DC negative) cable needs to be at least as large as the alternator cable.

Other alternators are isolated-ground type, which means there is a DC negative post on the alternator for connection to the boat’s negative bus bar. The alternator’s DC negative cable needs to be at least as large as the positive cable.

Cable Ratings

According to the American Boat and Yacht Council (ABYC) current-carrying conductors must be rated to carry the current load that passes through. Things that affect the current rating of a conductor are the temperature of the hottest space the cable runs through, and how many conductors are bundled together. Our Alternator Cable Size Guide is based on the ABYC formulas. We use the ABYC derating factor for engine spaces, since we have not seen a Balmar alternator operating away from an engine! Our recommendations are based on three (3) or fewer current-carrying conductors in a bundle, and Marine-Grade cable with insulation rated at 105 degrees Celsius. If you have more cables bundled, please consult a qualified marine electrician.

Please note that our Alternator Cable Size Guide recommends the Minimum Cable Size for your alternator. You may want to use even larger (or more) cables if your installation requires long cables, as voltage loss within a cable is proportional to its length.

Minimum Alternator Cable Sizes

Alternator Output (Amps)Minimum Cable Size (AWG)
1700 (1/0)
210000 (3/0)
2500000 (4/0)
3100 (1/0) Doubled Up
Based in 105C rated cable with no more than 3 conductors bundled.

Circuit Protection

Don’t forget protection! All DC positive conductors need to be protected by an appropriately sized fuse or circuit breaker as close to the power source as possible. We really like Battery Terminal Fuses for this purpose, as they reduce cable connections, and you can’t get any closer to the source! The fuse or circuit breaker rating should be at or below the rating of the cable (as installed), although ABYC guidelines permit going up one fuse or breaker rating step if the lower rating step is not quite enough. Keep in mind that high-output marine alternators (and all alternators for that matter) are current-limited, which means they do not have the ability to produce current that exceeds their rating. Because of this, you do not need circuit protection at the alternator as long as the cable is properly sized.

Cable Support

As you may have already guessed, the cables for a high-output marine alternator are quite large and heavy. ABYC recommends that all cables be supported at least every 18″ along their length (by cable ties or conduit). This is very important near the alternator, as the stud that the cable connects to cannot support the weight of a long, heavy cable. Best practice is to support the alternator cable by tying it to the engine and leading the cable down to the boat structure near the engine mounts. If you must suspend the cable between the alternator stud and the boat structure, please leave enough cable to allow for the engine to vibrate.

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