Wire size from battery to starter for A4?

[rigspelt]

This is the one spec I am stuck on in designing the rewire plan for the boat: size of the cables from the battery to the starter and back again.

1. I understand that the A4 uses less than 200 amps during starting, but what is the maximum amperage to wire for?

2. The boat came with AWG 4 for the power and ground cables, but the tables in the books by Charlie Wing and Nigel Calder suggest to me that AWG is too light for 200 amps.

Nigel wrote (p 164): "Sizing starter motor cables is more complex because the current draw is hard to determine and is intermittent. In practice, the key issue is to keep down the votage drop (see next section) as opposed to meeting the requirements of the ampacity table. This often results in cables that are undersized in terms of ampacity tables, but given the short-term duration of cranking loads, the cable do not become hot enough to create a fire hazard."

Maybe that explains the AWG 4 spec for the A4? Thoughts?

[Kuwait]

Cable size.

#4 AWG is capable of carrying up to 170 amps continuously depending on the type of insulation (rated for 125 degrees C.) and how it is installed. In free air, that is not bundled in with other cables or in a conduit. This information is from the electrical code book. Based on this information, #4 cable should do just fine. Another thing to keep in mind is to always make sure your connections to the battery and other terminals are clean and tight. Dirty or loose connections create heat under load.

[High Hopes]

The Atomic 4 has a small starter. Unless you have exceptionally long cables, #4 is more than enough.

There are at least three design points to be considered when spec'ing out cable sizes.

(1) The first is the maximum current rating. This is the familiar “#14 wire for a 15 amp circuits” code for your home. A cable can be de-rated when in hotter environments, like engine compartments. A cable can also be de-rated to achieve a higher safety factor for different systems, such as marine systems, medical systems, and man-rated space systems.

(2) The second design point is voltage drop.

(3) The third is failure mode scenarios.

Between the first and second items, voltage drop is the overriding item with 12 volt systems. This is because 12 volt systems use more current than 110 volt system. Higher currents mean higher voltage drops (Ohm’s Law). There are tables available on the internet which show how much voltage drop a given circuit will have given the wire gauge, the current load, and the length of the wire.

How much voltage drop can you tolerate for your application? I suspect the A4 will start with 8 volts across the starter terminals. Voltage loss tables will show a drop of about a volt for a typical A4 installation. Given an 8 volt engine start minimum, a 1 volt cabling voltage drop with #4 wire, and a 13.5 volt battery, there is big 4.5 volt margin for cranking the engine. This suggests that a smaller cable gauge could be used.

Now, on to point (3). We still have to deal with the possibility of short circuits. Usually, the high current of a short will quickly fry the contact point of the short and open the circuit. This means that the wire will not have time to heat up, but one never knows what can happen.

For an unrelenting hard short, the energy will be expended inside the battery, in the wire, and in the item causing the short. Fuses can help this situation but there are only so many things you can do and you can’t protect everything. There is a lot of energy in a battery and there is always some situation where it can be unloaded in the wrong direction.

A #4 wire will handle about 1000 amps or so for a long enough time to drain a battery. It won’t be pretty. The wire will get very hot. As it heats, its resistance will increases, which means the current will drop. If you use too big a wire, then all the energy gets expended inside the battery – probably not a good idea for a device containing acid. You probably would prefer to have the wire melt (*which is basically what a fuse does).

Personally, I prefer modestly sized batteries on my 28 footer to limit the magnitude of failures. A melting #4 wire is about as much as I want to deal with at sea.

More than you ever wanted to hear, I’m sure. Use #4 wire.

Steve

[rigspelt]

Steve and others: all that confirms my reading. Thanks for the reality check. AWG #4 it is.