ZRD - FAQ Alternator Sizing

High Output (Dual) Alternator Sizing

Review all of the FAQ links, especially the DC charging link, to understand what goes into the requirements stage. When you have a good feel (understanding) of the various factors that are involved, you will probably agree with ZRD's standard solution.

After years of extensive first hand experience, research, and numerous installations, a second alternator is required to ensure that the large house battery bank is kept properly charged. This high output alternator needs to produce in excess of 175 amps of 12 volt DC output. Some of the most significant factors are available charging time, wire sizing, installation, available space, reliability, and cost per amp. With those factors in mind, the ZRD ZCCJE1222 alternator has met and exceeded all of our requirements. There is no compromise in any area with this product. It is as close as perfection gets. It is our most highly recommended and sought after solution.

With the multitude of pulley ratios available between the alternator and power take-off (PTO) pulleys, the following information may vary, but may be used as a guide. The data is from an installation using a 2.7" alternator pulley with a 6" PTO producing a 1.0 to 2.22 engine rpm to alternator rpm. Refer to the output graph with the ratio developed for your specific installation to see how your output scenario may vary from the following.

At normal engine idle, the ZCCJE1222 puts out around 120 amps DC. Before cruising speed is attained for most vessels, 220 amps is possible. With a standard installation, this equates to 120 amps of DC output at 840 rpm and 220 amps at 1500 rpm. As we do not recommend running a power plant engine on the hook (again, review other links), correct normal charging is easily accomplished during location changes or transits while cruising, even at low power settings. This ensures that a strong adequate source of DC power is readily available for all needs. You can personalize this to your situation by looking at the power curve on the Alternators & Regs link and use the standard ratio of 2.6 to 1 to correlate alternator rpm with engine rpm.

A common question we receive from prospective clients is their desire to add a second small alternator (normally found or inherited - the reasons are as varied as the types). Simply, the 99% response is, "do not do it". The normal costs associated with installing a second alternator make the return so little compared to a high output alternator. The only direct savings is the price differences of the two alternators. The difference is normally small. Everything else is the same or even higher for the smaller alternator due to the required specialized manufacturing. This discussion is only dealing with the costs of adding a second small alternator. The previous sections show why the correct solution, regardless of cost, is a high output alternator. Again, our 100% choice is a ZRD ZCCJE1222.

Now one of the most common questions - Why is the First, Start, Engine, Original, Single, Only, or any other name one may call it by, alternator limited to 94 amps? The answer is a combination of simple physics and design. To start with, there is normally only a single sheave V-belt pulley with a 3/8" or 1/2" belt supplied from the OEM engine manufacturer. Regardless of pulley / belt size supplied, any alternator beyond this sizing (94 amps at 12 vdc) will cause undue stress to the many engine components (bearings, water pump, pulleys, belts, ...). These components were designed by their respective engineers to be subjected to a certain side loading (normally rated in HP) and not more than that. Our research has found that this equates to no more than a 94 amp alternator being installed in this location, regardless if it fits or is made to fit. Some peole install oversized alternators in this location and then subject themselves to overwear, belt problems, and premature failures of many parts. If the original manufacturer had designed this area to carry higher loads, they would have supplied a double pulley - not just on the crankshaft, but also on the water pump to esure equal loading. One can add a double or serpentine pulley to the crankshaft pulley, but not the water pump. This also requires special loading and design. This is in fact what ZRD does when we design a Second High Output Alternator solution for our customers. In summary, simply install no more than a 12 vdc - 94 amp alternator in the standard OEM engine location. There are even some OEM engines that restrict this even further, but not many. If your needs dictate, adding a Second High Output Alternator is the solution.