[preservedkillick]

Hi everyone,

I've got a 3 cylinder Vetus diesel engine, and I would like to calculate the electrical load (amps) needed of the starting circuit to determine wire gauge needed. The engine is 979cc or roughly 60 cubic inches.

I've read somewhere that a basic formula is 1amp per cubic inch. Anyone have anything more accurate than that?

Based on my formula, I'm looking at 60 amps?

Preserved.

 

[Maine Sail]

The in-rush can easily exceed 250-500A plus. Continuous average will likely be in the 150-200A range.. Starter motors are very poorly rated and the kW rating is pretty meaningless. These motors are nearly creating a dead short in the system for a very short period of time called the in-rush. For your boat 1/0 is pretty standard for an engine of that size. Even Catalina ships 2/0 on engines of that size these days. Early on they shipped 4GA and many owners chewed through starters due to excessive voltage drop between the batteries and starter......

Here's a Mitsubishi with a 1.2kW starter motor (your Vetus is a Mitsu block). Note that this starter motor exceeded 640A and as seen below averaged 286A for the start duration (loaded starter to unloaded)

The AMPS number is averaged over the start duration:

The short answer is that unless you own the expensive equipment to test your starters in-rush and average, and that equipment is fast enough and designed to do this, go big with the wire..... The difference in cost between 1GA and 1/0 is not worth worrying about..

 

[preservedkillick]

Thanks Maine Sail.

One more question related to this. I'm replacing one section of my starting circuit wiring. I've got a wire running from the battery (20 inches) that goes to a Blue Sea switch, then to the starter motor (~5 feet). I'm replacing the short 20 inch wire. Can I get by with less since that section is so short? I've got some bends to accommodate and a smaller wire size would be easier to install. I was thinking 2awg would be plenty for this. No?

 

[Capt Len]

Why skimp? same size wire and all the terminals are the same .

 

[preservedkillick]

Why skimp? same size wire and all the terminals are the same .

The 2awg is a little thinner & more flexible. With the way by panel & switch is setup, the thinner wire will be much easier to install, in this case. That is..if the wire size is adequate.

 

[mitiempo]

Why skimp? same size wire and all the terminals are the same .

I agree. All the battery/starting wiring I have done is either 1/0 or 2/0 and I have never had a problem with bends. Unless you are tying the wire in knots you shouldn't have a problem.

Engines start faster with larger wire.

 

[MarkSF]

Could I recommend Genuinedealz for custom battery cables? Their prices are so good, you won't mind using 2/0. Just used them for my rewire (new cables and battery switch). They even use the proper 4 point crimper for the terminals. Top quality and they charge less for custom cables than I can buy the raw materials for.

 

[chucklesR]

I recommend you use the time you save on bending the proper gauge wire to install a fire extinguisher next to it.