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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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Pulley Selection Guide

Alternator Pulley Selection Guide

The standard diameter pulley for Balmar alternators is 2.7″. While all Balmar high-output marine alternators are rated for much higher speeds, the ideal sustained operating range is 4000-4500 RPM.

Divide your crankshaft pulley diameter by the alternator pulley diameter to get the speed ratio. Multiply this by the engine’s cruising RPM to get the alternator RPM.

For Example:

  • Engine crankshaft pulley diameter: 5.0″
  • Alternator pulley diameter: 2.7″
  • Engine cruising RPM: 2400

5.0″ ÷ 2.7″ = 1.85 (Speed Ratio)
1.85 x 2400 RPM = 4440 RPM (Alternator)

If your engine is typically run at low speeds, or if it has a small crankshaft pulley, select a smaller alternator pulley.

CAUTION: Use of an undersized pulley may cause the alternator to exceed its RPM rating when the engine is run at wide-open throttle (WOT). Use the formula above to determine the alternator RPM at max engine speed.

Note: Certain marine engine applications have specific pulley requirements, and other pulleys are available. We have attempted to make the selection as easy as possible for you, and the selections provided will fit most engine applications. However, if you have questions, please contact us.

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Marine Engine Applications

Marine Engine Alternator Applications

Use the table below to lookup your engine manufacturer, and select the proper alternator mounting style.


Engine Type

Balmar Alternator Series


Motorola Style (Note 1)


Off Engine Only, Most Isolated Ground (Note 1)


Hitachi Style or Motorola Style (Note 1)


Motorola Style(Notes 1, 2)


Mounts vary by engine model

Detroit Diesel

Delco Style (Note 1)


Motorola Style (Note 1)


Motorola Style (Note 1)


Motorola Style (Note 1)

Hawk Marine

Motorola Style (Note 1)


Motorola Style (Note 1)


Motorola Style (Note 1)


Motorola Style, some Hitachi Style (Note 1)

John Deere

Motorola Style or J-180 Style (Note 1)

Lehman (Ford)

Hitachi Style or Motorola Style (Notes 1, 3)

Lehman (Sabre)

Hitachi Style, requires tension arm mod.


Varies by model (Notes 1, 4)


No standard (Note 1)


Hitachi Style (With Spacers) (Note 1)


Varies by model Hitachi Style or Delco Style (Note 1)


Hitachi Style (With Spacers) (Note 1)


No standard (Note 1)


Hitachi Style


Motorola Style (Note 1)


Delco Style (Notes 1, 5)

Perkins 4107,4108

Delco Style (Note 1)

Perkins (other)

Motorola Style (Notes 1, 5)

Perkins-Volvo M

Hitachi Style (Isolated Ground)


Motorola Style

Universal (Atomic)

Delco Style (Note 1)

Universal 30, 50

Motorola Style (Note 1)

Vetus Denouden

Hitachi Style (Note 1)

Volvo 2001, 2003

Delco Style (Note 1)


Varies Hitachi Style or Delco Style


Hitachi Style

Yanmar 6LP

Delco Style

  1. Mounting configuration may vary by model and year. Follow directions for determining proper replacement alternator by visual inspection described on following page. Cat 3208 may use 8-Series alternator.
  2. Some Crusader models may require alternator with an offset tensioning arm.
  3. Dual belt models will require special Lehman pulley with 3/8″ spacing between belts.
  4. May require Lugger mounting kit.
  5. Use original alternator pulley.
  6. Use 9mm x 4″ pulley if belt is driven directly off flywheel.
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Alternator Selection

How Do I Choose The Proper Alternator For My Vessel?
Once battery capacity and battery type have been determined, the size of the alternator can be calculated. If the batteries used are standard or deep cycle flooded, the alternator output should be sized at 25 percent of the combined capacity of all house batteries. If your house battery capacity is 400 amp hours, for example, the alternator’s rated output should be 100 amps. If AGM or gel technology is used, the alternator output may need to be increased to 35-50% of the combined battery capacity, as those battery technologies will demand greater output.

Will A High Output Alternator Work With My Engine?
Depending on your application, the size of the engine and drive belt may limit ability to support a high-output alternator. In most cases, an engine with a 3/8″ wide drive belt will support an alternator rated at 70 to 80 amps. A greater load may result in belt dusting and premature belt failure. A single 1/2″ belt will typically support a 100-amp to 110-amp alternator. It may be necessary to consider converting to a dual pulley system if a 120-amp or larger alternator is used. Some engine applications use serpentine (flat) belts to drive the alternator and auxilliary loads. In most cases, a 5-groove or 6-groove serpentine belt will be compatible with alternators rated to 150 amps.

What About Mountings?
Nearly all marine engine manufacturers rely on one of four specific alternator mounting styles:

  • 1″ Single Foot (Motorola style) – Mounting arm at engine pivot point measures 1″ from front to back. (See Image 1 at left)

  • 2″ Single Foot (Delco style) – Mounting arm at engine pivot point measures 2″ from front to back. (See Image 2 at left)

  • 3.15″ I.D. Dual Foot (Hitachi style) – Mounting arm at engine pivot point features two mounting feet, one in front, one in rear. Distance between mounting feet is 3.15″. (See Image 3 at left)

  • 4″ I.D. Dual Foot (J-180 style) – Mounting arm at engine pivot point features two mounting feet, one in front, one in rear. Distance between mounting feet is 4″. (See Image 4 at left)

Our Alternator Application Guide provides a general guideline for various engine applications, matching engine manufacturer and model to the corresponding alternator series and model. Unfortunately, actual mounting styles can vary based on model year, marinizer, and other factors. We strongly recommend comparing your existing alternator to the different mounting configurations shown at left to ensure a proper fit and an easier installation.

How Does My Alternator Size Up?
Making sure your new alternator fits into the same location as your old alternator can make all the difference in avoiding installation irritations. Balmar alternators are broken down into three categories: small case, large case, and extra-large case.

Small case 6-, 7- and 8-Series alternators are usually appropriate to be used in place of standard alternators on most marine gasoline and diesel engines. Some larger engines may support large or extra-large case 94-, 95-, 97- or 98-Series alternators in the factory position, or may be mounted on a custom bracket.

Keep in mind each alternator series is unique in its dimensions. It is always wise to compare the dimensions of your existing alternator to the replacement alternator to ensure a proper fit.