What gauge batt wire?

[jasoutside]

OK fellas, I thought I had a fairly simple question but I suppose not.

My set up is a 89 Merc 4.3 and batts are now at midships with roughly a 15 foot run (give or take about a foot).

So I have been doing my homework, looking at charts, emailing companies who specialize in batt cable, doing calculations, yada yada....

The answers I'm getting range from 2 to 2/0 and everything in between.

Now I know bigger is better and have no trouble with that idea, but, at what point does the batt cable become unnecessarily big? Bigger cable also means more $$, knowwhattamean?

So anyhow, what sort of recommendations do you guys have for me?

Thanks in advance!

 

[fishrdan]

Re: What gauge batt wire?

Play around with this DC wire calculator and see what it comes up with. When using the calculator, use the total wire length, 15' between engine and batt's = 30' of wire. I ran the numbers I though appropriate and came up with either 2/0 or 3/0 cable. (I'm not sure how many amps your starter draws, so I just guessed at 300 amps.) I don't think you could ever get battery cable too big, unless running and bending fat cables would be an issue.

Oh yeah, remember to add up the total length of the wiring run, up-down, left-right, connections to battery switch. It's to total amount of wire between the engine and batteries that count, positive and negative.

 

[Bondo]

Re: What gauge batt wire?

Now I know bigger is better and have no trouble with that idea, but, at what point does the batt cable become unnecessarily big? Bigger cable also means more $$, knowwhattamean?

Ayuh,.... Somethin' I was taught by a mentor,....

Welding cables can many times be bought Cheap, at auction, or wherever,...
Weldin' cables are finer strands, 'n pass more power, more easily...
'n they're built for the "Work" environment... Tough...
Also very pliable/ bendable/ soft, 'n easy to work with...

Any copper cable is Too Big, when it don't fit the wiring ends....

2/0 sounds like the bigger of better, 'n #2 is probably the minimum...

I think I used #1 weldin' cables I stole on mine,... they ain't all that big...
to long ago to remember that detail...

 

[jasoutside]

Re: What gauge batt wire?

I don't think you could ever get battery cable too big, unless

....unless tears well up in my eyes when I pull my credit card out?:facepalm:

Thanks for running the calculations for me dan

Welding cables can many times be bought Cheap, at auction, or wherever,...

I actually just found a buddy who may be able to score some welding cable at bottom feeder pricing

That 2/0 may not be so painful after all

 

[robert graham]

Re: What gauge batt wire?

Ayuh,.... Somethin' I was taught by a mentor,....

Welding cables can many times be bought Cheap, at auction, or wherever,...
Weldin' cables are finer strands, 'n pass more power, more easily...
'n they're built for the "Work" environment... Tough...
Also very pliable/ bendable/ soft, 'n easy to work with...

Any copper cable is Too Big, when it don't fit the wiring ends....

2/0 sounds like the bigger of better, 'n #2 is probably the minimum...

I think I used #1 weldin' cables I stole on mine,... they ain't all that big...
to long ago to remember that detail...

Now there's a piece of information worth remembering!...hadn't thought of welding cables....good idea!

 

[jasoutside]

Re: What gauge batt wire?

K, I may need to pick a new route for these batt lines. If I need to bump it up to 25' what are we talkin here, 4/0?

 

[bruceb58]

Re: What gauge batt wire?

Welding cable isn't tinned so you may have a corrosion issue.

Check out the prices here of a good ebay seller.
http://shop.genuinedealz.com/Marine Grade Wire/Marine Battery Cable/

Why did you move the batteries in the first place? Way better in the stern where there is less pounding and will last longer. Oh yeah...shorter battery cable lengths
What size cable and length are you using now?

 

[jasoutside]

Re: What gauge batt wire?

Welding cable isn't tinned so you may have a corrosion issue.

Check out the prices here of a good ebay seller.
http://shop.genuinedealz.com/Marine Grade Wire/Marine Battery Cable/

Why did you move the batteries in the first place? Way better in the stern where there is less pounding and will last longer. Oh yeah...shorter battery cable lengths
What size cable and length are you using now?

Yah, I was yakkin with that fella from genuine deals earlier today, good guy.

I was just trying to push some weight forward is all. Plus, putting the batts at the stern I'd need to install them about 1/2 to 2/3 up/on the transom. I'd rather not have to install them up there, but, with a 25' run of batt cable that option is looking more appealing

 

[MH Hawker]

Re: What gauge batt wire?

12 v amp per wire gage

in runs or 25 feet or less
20 ga 2.5 amps
18 ga 5 amps
16 ga 10 amps
14 ga 15 amps
12 ga 20 amps
10 ga 30 amps
8 ga 50 amps
6 ga 75 amps
4 ga 100 amps
2 ga 200 amps

 

[bruceb58]

Re: What gauge batt wire?

12 v amp per wire gage

in runs or 25 feet or less
20 ga 2.5 amps
18 ga 5 amps
16 ga 10 amps
14 ga 15 amps
12 ga 20 amps
10 ga 30 amps
8 ga 50 amps
6 ga 75 amps
4 ga 100 amps
2 ga 200 amps

Depends on what your acceptable voltage drop is. Without that spec, these numbers are pretty meaningless. You including the return path in your length? You including the current inrush when the starter first turns?

This is what i would do.

1) Measure what the total length of cable that is in there currently, including ground cable length.
2) Determine the gauge wire you have now.
3) Put both of these numbers into a voltage drop calculator using 200A as the load.
4) Keeping the 200A load number and the vcalculatedop number calclated previously in the calculator, change the number of total feet to your new length and see what gauge cable it spits out.

This of course assumes you are not adding any more connections for battery switches.

 

[MH Hawker]

Re: What gauge batt wire?

Thats at 12 volts and in runs of 25 feet or less acorning to NEMA.

 

[bruceb58]

Re: What gauge batt wire?

Thats at 12 volts and in runs of 25 feet or less acorning to NEMA.

So what is an acceptable voltage drop % according to NEMA?

You do realize NEMA is a guideline assuming many different parameters including voltage loss, insulation material, housing for the cable...none that really apply to a boating environment right?

 

[fishrdan]

Re: What gauge batt wire?

I agree with Bruce, play with the calculator (more-less wire, more-less current, more less allowable voltage drop) to see what affect it has on the recommended cable size. 1' less wire could drop to a lower size of cable...

Thats at 12 volts and in runs of 25 feet or less acorning to NEMA.

Run those numbers through the DC wire calculator I posted up above with a variable entered for allowable voltage drop, I used 5%. For 50' of wiring and 200 amps it says 2ga can handle the current, but when the 5% of allowable voltage drop is figured in, it bumps the cable to 4/0ga. (Add in that the cable is going to be in an engine room and it bumps cable to 1ga and 4/0ga.)

I wasn't exactly sure how much current a starter draws so I googled it (wow was I off), around 175 amps for a V6, 200-225 amps for a V8. Running those numbers (50' wire) through the calculator shows 3ga cable (2ga for engine room) is capable of handling that current, but it recommends 3/0ga cable for a 5% voltage loss.

 

[jasoutside]

Re: What gauge batt wire?

Thanks for helping me out guys, appreciated!

I actually decided this morning to keep the batts at stern side, so my run is all of a few feet now.

 

[bruceb58]

Re: What gauge batt wire?

Thanks for helping me out guys, appreciated!

I actually decided this morning to keep the batts at stern side, so my run is all of a few feet now.

I think that is a good choice overall.

 

[jasoutside]

Re: What gauge batt wire?

Yah, 15 feet seemed borderline reasonable. 25 feet (50' round trip) seemed borderline silly. That just seems like a long way to me anyway.

Thanks again fellas!

 

[Bondo]

Re: What gauge batt wire?

I agree with Bruce, play with the calculator (more-less wire, more-less current, more less allowable voltage drop) to see what affect it has on the recommended cable size. 1' less wire could drop to a lower size of cable...



Run those numbers through the DC wire calculator I posted up above with a variable entered for allowable voltage drop, I used 5%. For 50' of wiring and 200 amps it says 2ga can handle the current, but when the 5% of allowable voltage drop is figured in, it bumps the cable to 4/0ga. (Add in that the cable is going to be in an engine room and it bumps cable to 1ga and 4/0ga.)

I wasn't exactly sure how much current a starter draws so I googled it (wow was I off), around 175 amps for a V6, 200-225 amps for a V8. Running those numbers (50' wire) through the calculator shows 3ga cable (2ga for engine room) is capable of handling that current, but it recommends 3/0ga cable for a 5% voltage loss.

Ayuh,.... I may be completely wrong, but I believe the newer Mercruiser "Lawyer" fuses mounted on the starter at the battery wire are only 75 or 90 amp rated.....

'n the run is only 25', not 50'...

 

[fishrdan]

Re: What gauge batt wire?

Ayuh,.... I may be completely wrong, but I believe the newer Mercruiser "Lawyer" fuses mounted on the starter at the battery wire are only 75 or 90 amp rated.....

'n the run is only 25', not 50'...

Lawyer fuse connects between the starter and engine wiring harness, the battery cable connects direct to the starter (bypasses the fuse)

The run is only 25', but the total length of cable (positive and negative) is 50'

 

[fishrdan]

Re: What gauge batt wire?

I actually decided this morning to keep the batts at stern side, so my run is all of a few feet now.

I agree, good decision to keep them close to the engine.

(at 25', I was wondering where you were going to locate the batteries... On the bow pulpit )

 

[cyclops2]

Re: What gauge batt wire?

Very GOOD voltage drop calculator for any AC or DC runs.

Tha calculator has the distance already set up for the.... 1 way distance & back.
www.powerstream.com
Look ok left side.....TECHNICAL RESOURCES
count down 5 sections to.....OTHER POWER ELECTRONICS RESOURCES
count down 7 items to...WIRE GAUGE & CURRENT CHART

Enjoy use it for house wiring also.....Save the page in your ....favorites