Sizing Battery Cable for Engine Battery

[Capn_Tom]

The house side of my system has been completely replaced and I am confident that it is well designed based on the information found in this forum. With the impending arrival of my new Beta 38 engine I need to quickly complete the engine starting side of the system. I am having difficulty finding a good resource for cable sizing for this side of the system.

I will be running the positive cable aproximately 5' from a terminal fuse block on a battery with 550 CCA to a ANL fuse where it connects to an ACR. From there it is approximately 2' to the stud on a Blue Sea 8686 battery management panel. From the panel it is 10' to the starter.

The negative cable travels aprox. 5' to the boats negative bus and then 8' to the engine block.

The feed from the 100 amp alternator travels aprox. 7' to an ANL fuse that is ganged to the the house battery through a Maine Sail style copper bus. The house side of the ACR connects here as well.

I assume I size the alternator wire for the 100 amp capacity @7'.

How do I calculate the correct cable size for the starting circuit. I assume I need the amperage draw for the starter. I am expecting that data today.

Thanks

[Tim]

Basic wire size table

[Maine Sail]

I generally size my battery cables at 100% of the maximum peak draw of the starter motor and for a 2% voltage drop. While this in only a short duration in-rush it seems to make big differences in the way motors start. The ABYC is quite absent on starter motor sizing but in conversations with John Adey we are both in agreement on this sizing, at least on the personal level. I just today Installed 2/0 for a 55 amp windlass and 1/0 for battery and starting circuits for a boat I am re-wiring. Beta should be able to supply you with the max starter draw.

Many builders drastically under-size battery cable and they can because the ABYC is absent on starter motor sizing.. On a Catalina we had it came from the Factory with #4 wire, starter drew 170 peak amps and the run was over 20 feet round trip. After converting it to 1/0 it was like I had installed a brand new starter. There was no comparison. Size your alt wire for a max 2% VD as well unless your regulator can sense at the terminal voltage then you could theoretically use 3%, 5% or 10%.

I use this calculator the most..

http://boatstuff.awardspace.com/awgcalc.html

[Capn_Tom]

I was expecting an answer today on the max starter draw from Beta but as yet I haven't heard back from them. The only information in the owners manual lists the starters output as 1.2 kw. I don't tend to think of starters in terms of output but then the Brits speak a bit differently than me. If this number is meaningful I would interpret it as 100 amps at 12 volts but that seems low. In any event I think I'll go with 00 to be safe which I was hoping to avoid since the largest die on my crimpers is 0.

[Maine Sail]

I was expecting an answer today on the max starter draw from Beta but as yet I haven't heard back from them. The only information in the owners manual lists the starters output as 1.2 kw. I don't tend to think of starters in terms of output but then the Brits speak a bit differently than me. If this number is meaningful I would interpret it as 100 amps at 12 volts but that seems low. In any event I think I'll go with 00 to be safe which I was hoping to avoid since the largest die on my crimpers is 0.

One formula I often use to figure starter cables is cubic inches of displacement X 2. So for a Beta 38 with 1498cc that translates to 91.5 CI. 91.5 X 2 = 183 amps. 183 amps X 16 feet (cable length round trip) @ 2% voltage drop = 1/0 wire. I usually prefer to use my DC inrush capable meter to figure max starter draw but the 2X cubic inches is often close going to 2.2 is often better.

Yanmar for example want to see a max of 0.2 volt drop for every 100 amps this equates to a less than 2% VD for the starter motor. Yanmar's 1.0KW starter has a peak loaded draw of about 250 amps. So peak on a 1.2KW should be close to that.