Figuring out AWG cable size for Dual Battery & wiring?

[hostage]

So I made the mistake of not checking the AWG of my current battery and my boat is a ~1.5h round trip away. I thought this would be fairly easy to figure out online, though I was wrong; Google has failed me! I bought 120A Add a Battery kit w/ the isolation relay and 3 way switch.

My boat has a Volvo Penta 5.7 (5.7GiC-300-J) and according to the Volvo Penta Parts, it has a 3884950 Alternator, which looks like it might be rated for 75A. I think I saw a 60A or 65A breaker located behind the current battery kill switch, though don't know where it goes to. Looking at the wiring diagram in the manual I would like to validate that I am reading it correctly:

AFT GROUND BUSS
4 BLK (TO FWD GRND BUSS) - 5/16 RING
10 BLK (ENGINE NEG) - 5/16 RING

MAINPOWER
4 RED - 1/4 RING

So it looks like I will need 4AWG for the +Voltage. I am guessing about 20-25 feet so I can wiring the battery isolation relay as well as the main battery switch. In the "Add A Battery" diagram for the it shows both batteries going back to the ground bus. Should I wire batteries' ground in series or should I do a seperate ground to the bus for the 2nd battery?

For the ring connectors, I do have some tinned copper battery lugs, a hammer crimper, and some shring wrap, so I plan to do custom lengths from one bulk peice. Plan to add 100A Circuit Breakers on either side of the relay. I will also need to add a 16AWG w/ inline fuse for the ground on the isolation relay.

Is there anything I should consider or think about, while doing this?

 

[Scott Danforth]

My battery cables are 3' of 00 between the battery and the switch and 4' of 00 between the switch and the motor. The house wiring is 4gauge for the audio equipment and 6 gauge for lights and fans

Remember if you plan on starting your motor from the second battery, you will have too much voltage drop with a 25' run of 4 gauge. I would suggest 0000 or 350MCM

 

[hostage]

My battery cables are 3' of 00 between the battery and the switch and 4' of 00 between the switch and the motor. The house wiring is 4gauge for the audio equipment and 6 gauge for lights and fans

Remember if you plan on starting your motor from the second battery, you will have too much voltage drop with a 25' run of 4 gauge. I would suggest 0000 or 350MCM

So I did a bit more research and managed to find this burried in the 343 page manuel:

Color | Gauge | Function
Red | 00 | Battery Cable to Engine
Black | 16 to 4 | All Grounds
...
Red | 2/0 | Main DC Panel Feed
Red | 2 | Positive Feed, Starter, Battery
Red | 4 | Positive Feed
Red | 6 | Positive Feed, Alt. Charge
Red | 8 | Positive Feed, Alt. Charge
...
So it looks like my boat uses 00 for the Battery Cable. Granted seeing 16 to 4 AWG for ground is a little bit confusing as the ground should use 00 as well. Also the switching between 00 and 2/0 nomenclature is a bit concering as well.

The 25' is not for one run, I am planning on buying a 25' spool, then cutting it to length as I will need about a 10-12' run going from switch to 2nd battery and only a couple feet between the relay and the two batteries.

 

[hostage]

My battery cables are 3' of 00 between the battery and the switch and 4' of 00 between the switch and the motor. The house wiring is 4gauge for the audio equipment and 6 gauge for lights and fans

Remember if you plan on starting your motor from the second battery, you will have too much voltage drop with a 25' run of 4 gauge. I would suggest 0000 or 350MCM

Wow! The 00 ain't cheap, it looks like about $9/ft! I think I will get a string to measure out exactly what I need and buy a 1ft longer than that! I will use 4AWG for the relay along with 2x100A Breakers as it will be easier to work with and cheaper. I also might just wait till next weekend to double check AWG rating on the cables.

The only thing I need to figure out is if I should just get a short 00 AWG cable to bridge the two batteries or should run a seperate cable to the bus bar?

 

[flashback]

I suppose that would depend on the location of the 2nd battery. I put my 2nd battery opposite the 1st battery in the stern so it was a shorter run to the motor than tying to the negative post of the first.. but no reason you can't.

 

[hostage]

I suppose that would depend on the location of the 2nd battery. I put my 2nd battery opposite the 1st battery in the stern so it was a shorter run to the motor than tying to the negative post of the first.. but no reason you can't.

The battery switch is to the right of the engine, while the battery is mounted well to the left. While I do have a have a lot of room on either side of the engine, it is nice to have a place to put swim toys. I could also put the battery in a box, though it is nice to have batteries on one side and no worries on the other.

 

[flashback]

Again, it's your choice, I was going for weight balance, It may not affect you, your boat's bigger..LOL ..

 

[dingbat]

Wow! The 00 ain't cheap, it looks like about $9/ft! I think I will get a string to measure out exactly what I need and buy a 1ft longer than that! I will use 4AWG for the relay along with 2x100A Breakers as it will be easier to work with and cheaper. I also might just wait till next weekend to double check AWG rating on the cables.

The only thing I need to figure out is if I should just get a short 00 AWG cable to bridge the two batteries or should run a seperate cable to the bus bar?

I suspect if you sat down and did the actual wire sizing calculations based on actual current, duty cycle (%) and insulation (thermal) rating you could down size the battery cables considerably.

 

[Scott Danforth]

Wow! The 00 ain't cheap, it looks like about $9/ft! I think I will get a string to measure out exactly what I need and buy a 1ft longer than that! I will use 4AWG for the relay along with 2x100A Breakers as it will be easier to work with and cheaper. I also might just wait till next weekend to double check AWG rating on the cables.

The only thing I need to figure out is if I should just get a short 00 AWG cable to bridge the two batteries or should run a seperate cable to the bus bar?

I pay $7/ft for 00, and $22/ft for 500mcm

 

[froggy1150]

 

[Scott Danforth]

Small Block pulls 200-300amps (inrush is about 400 amps)
Big Block pulls 300-400 amps (inrush about 600 amps)
6-10 liter diesel pulls 500-700 amps at 12 volt (inrush about 900-1000 amps)
10-15 liter diesel pulls 1000 amps at 12 volt (inrush about 1300-1500 amps)

Fuel solenoids alone pull in up to 75 amps during the pull and only an amp or two for holding.

Since the OP is talking about an SBC, you can sort of extrapolate the chart above.

 

[hostage]

Small Block pulls 200-300amps (inrush is about 400 amps)
Big Block pulls 300-400 amps (inrush about 600 amps)
6-10 liter diesel pulls 500-700 amps at 12 volt (inrush about 900-1000 amps)
10-15 liter diesel pulls 1000 amps at 12 volt (inrush about 1300-1500 amps)

Fuel solenoids alone pull in up to 75 amps during the pull and only an amp or two for holding.

Since the OP is talking about an SBC, you can sort of extrapolate the chart above.

To be safe, I am just going to wait to check the boat out, when I get to it on Saturday.

Call me young, though I still find it funny that a 5.7L is considered a "Small Block". This is the first V8 I have ever owned. Everything I have owned in the past (automobiles and boats) have all had V6s. (3L, 2x3.5L, & 4.3L). In a month the only thing that will use gas at my house will be the boat and my lawn equipment. Everyhing else will be electric.

 

[Scott Danforth]

its still a small block vs a big block.

good luck with the electric

 

[hostage]

I got to the boat this weekend and to my suprise, the battery cable is only 2AWG.

 

[jhebert]

For advice on choosing conductor size in wiring of power distribution circuits, a simple method of calculation of needed wire gauge based on the distance (FEET) and the amount of current (AMPERES) has been worked out for specific voltages and permitted amounts of voltage drop. This method is the AMPERE-FEET method. An explanation and tables are given at

AMPERE-FEET
A Method of Selecting Conductor Size

continuousWave: Whaler: Reference: Ampere-Feet: A simple method for choosing wire conductor size for power distribution.

A method to quickly calculate either the maximum length or the maximum current for distribution of 13.2-Volt DC power using standard AWG conductor sizes.

continuouswave.com