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scosch 11-07-2007 12:08 AM

Sizing battery cables
 
What is the general rule of thumb for sizing battery cables? Does one use the amperage draw of the starter (which I cant find either for my Westerbeke 21) and then use the standard 3% voltage drop tables for cable length or does one need to consider the in-rush amperage which Calder suggests could be hundreds of amps for a brief period? Calder suggests fusing the cable at 150% of cable ampacity but what size (and therefore what ampacity) cable does one use other than bigger is better.

sailingdog 11-07-2007 09:55 AM

scosh-

Use the amperage draw of the starter or the output of the alternator—whichever is higher—as a starting point to size your battery cables. You should also consider what output you have on a battery charger or charger/inverter if your boat has one. For instance, say your have an 85 Amp alternator, an 100 Amp battery charger and the starter is rated at 50 amps, you'd probably want to size it for the 100 Amp battery charger.

However, you might want to oversize them a bit, since it would allow you to increase the alternator size at a later date without having to re-wire. Oversizing battery cables is generally not a bad idea in general. :D

Idiens 11-08-2007 02:38 PM

This came from Sterling Power products...

WHAT CABLE TO USE IN sq mm
A charger or inverter up to cable run distance
Current 0-1.5 mtr 1.5 - 4 mtr
0-25 amps 6 mm sq 10 mm sq
25-45 amps 16 mm sq 25 mm sq
45-85 amps 25 mm sq 35 mm sq
85-125 amps 35 mm sq 50 mm sq
125- 180 amps 50 mm sq 70 mm sq
180-330 amps 70 mm sq 90 mm sq

There doesn't seem to be a way to format that table, sorry.

Starters can draw 200-300 amps for a short period.

sailingdog 11-08-2007 02:41 PM

Yes, but that is the in-rush amperage, which is only needed to get the motor turning... and it only lasts a fraction of a second. You really have to size for the longer, constant amperage loads.

Quote:

Originally Posted by Idiens (Post 219672)
This came from Sterling Power products...

WHAT CABLE TO USE IN sq mm
A charger or inverter up to cable run distance
Current 0-1.5 mtr 1.5 - 4 mtr
0-25 amps 6 mm sq 10 mm sq
25-45 amps 16 mm sq 25 mm sq
45-85 amps 25 mm sq 35 mm sq
85-125 amps 35 mm sq 50 mm sq
125- 180 amps 50 mm sq 70 mm sq
180-330 amps 70 mm sq 90 mm sq

There doesn't seem to be a way to format that table, sorry.

Starters can draw 200-300 amps for a short period.


Idiens 11-08-2007 02:46 PM

I think the question was about the fuse size. If you put a 50 amp fuse on a starter motor cable, expect it to blow.

sailingdog 11-08-2007 02:53 PM

Not if it is a slow-blow fuse. If the starter motor is rated for 60 Amps... and you put a 60 amp slow-blow fuse on the line, you should be okay, since the in-rush current duration is so short, it won't generally cause the fuse to blow.
Quote:

Originally Posted by Idiens (Post 219679)
I think the question was about the fuse size. If you put a 50 amp fuse on a starter motor cable, expect it to blow.


Idiens 11-08-2007 02:58 PM

I doubt that, as the surge into a starter motor is quite long.

Not that I would recommend a fuse on a starter cable, in case it blows just when you need the engine most. A manual isolation switch is better in my view.

HoffaLives 11-08-2007 03:29 PM

Starters generally aren't fused; they are usually connected straight through to the battery through the solenoid. The solenoid is, though.
AWG 2 cable would be my choice, but only to the starter. The rest of the cabling can be AWG 4, unless you are connecting to a hi-draw appliance like a windlass.

scosch 11-08-2007 09:32 PM

thanks but....
 
I think I get it. Ignore the high amperage in rush of the starter and use a slow blow fuse rated at around the starter's constant draw. Maybe even at 150% of the normal draw.

And...what happens when you put the battery switch to combine and your house batteries then could provide a continuous 2-400 amps to the starter (or whatever you have it fused at, but probably higher than the battery fuse) but the cable to the starter only has the ampacity necessary for what you thought would be the normal starter draw. Of course its not a problem under everyday circumstances but with a starter problem you could overheat that starter cable. Seems like the cable should be sized for that potential, however unlikely. I suppose just the cable from the start battery to the battery switch could be smaller. Or maybe having a fuse at the start battery and another on the starter side of the battery switch would do it too.

I think the current recommendation is to fuse all circuits, even those to the starter. Blue Sea has just come out with a terminal fuse that makes it a snap to add. Its not on the BS web site but its in their 08 catalog or you can visit Jack Rabbit for a view.

HoffaLives 11-09-2007 01:35 AM

The chances of the starter overheating a 2AWG cable is next to nil IMO. The components inside the starter are far smaller gauge (maybe 10?) and in the unusual event of a direct intternal short, it would almost certainly open inside long before it overheated a multi-strand cable.

From what I understand most cases of overheating/fires come from poor, hi resistance connections. Putting in a heavy fuse doubles the number of connections. Not a big deal, but those connections are more likely to cause problems in the future than the starter, and as a general wiring philosophy it's better to keep all connections as simple and direct as possible. How many people go around tightening all their cable connections as regular maintenance?


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