Battery Cable Gauge

[j442w30]

Hi Everyone,

I've been working on restoring my boat and I decided to relocate the battery to the middle of the boat for better weight distribution. I put a larger engine on last year and with the added weight, gas tank and battery in the rear it made the boat porpoise badly. The engine is a 40 horse Mercury Outboard and the new cables will be about 8ft in length. Is 4 gauge adequate? Thanks for the advice.

 

[Scaaty]

Re: Battery Cable Gauge

8 guage is fine..ya could crank a jet engine on 4​

 

[Kevin W]

Re: Battery Cable Gauge

IMO the bigger the better.
8 ft cable
2ga @ 100amp will drop .26V so 13V will be 12.74V
4ga @ 100amp will drop .41V so 13 V will be 12.59V
8ga @ 100amp will drop 1.03V so 13V will be 11.97V

Starters range in current so assuming its a 40hp it may not get to 100A.

On the cold rainy day that the battery is a little low the 4ga may start it when smaller may not.

 

[Texasmark]

Re: Battery Cable Gauge

Wire sizing is usually accomplished using continuous current ratings, location of wire, ambient temp, and insulation type (how hot can it run).

Resistance of the wire causes the wire to heat up and hence drop voltage as Kevin mentioned....I don't have my books in front of me so I don't know if his numbers are at 25C or at max operating temp.

Regardless, using wire for a starter circuit in a boat is very very intermittent duty. As a result the wire can be downsized significantly from what would be required normally.

Your only caviot in that installation is that you only have a 12v power source so dropping a volt or two can matter whereas at housepower voltages it doesn't.

Since wire weighs and boats are usually rigged out for minimum weight, then you want the smallest wire practical.

But loosing 1 volt to wire resistance during starting, considering that you have all your terminals bright, clean, and shiny, with a fully charged battery in good condition, is no biggie.

Agree that your 40 will probably never see 100A. My manual for my 90 hp says that it could take up to 150 amperes to start the engine, but that's the rating. I doubt my engine ever draws that much. Why? Because the engine is usually very easy to start....except when cold on first start.....but it it's cold, so is the wire so the voltage drop will be lower also......I mean 150 amperes will start a V8 auto engine.

So I would opt for the 8 AWG.

Mark

 

[j442w30]

Re: Battery Cable Gauge

8 gauge would be fine even if they are going to be 8 or so ft long? That would be awesome considering the prices of marine grade wire.

 

[Scaaty]

Re: Battery Cable Gauge

8 gauge would be fine even if they are going to be 8 or so ft long? That would be awesome considering the prices of marine grade wire.

I have simply used at one time plain old quaility 8 gauge twisted copper 12 foot JUMPER CABLES for wire, just cut off the clips, fused on ring shank connectors. Cost $20. A Marine store wants 5 times that for the same thing

 

[Rancherlee]

Re: Battery Cable Gauge

8 gauge is plenty, A ford 302 only sucks 110-120amps while starting. MY Johnson 70hp 4 stroke has only 8 gauge from the factory.

 

[Windykid]

Re: Battery Cable Gauge

Auto parts stores cary 8' cables ready made and fairly cheap.

 

[j442w30]

Re: Battery Cable Gauge

Thanks for the advice guys. I'll check out my local car parts store for a ready made battery cable, lucky for me it's right next door to West Marine.

 

[Scaaty]

Re: Battery Cable Gauge

Thanks for the advice guys. I'll check out my local car parts store for a ready made battery cable, lucky for me it's right next door to West Marine.

Man stay AWAY from West Marine (or after ya buy it go in there and show them what ya got and what ya paid)...they have 4 foot of cable in tiny little packages, and anywhere from $35 to $50...just ONE CABLE! How the hell they stay in business is beyond me!