Cable gauge Battery to switch

Stu Jackson · Aug 6, 2019

The size of the cables depend on the LOAD, not the boat size. It also depends on how you charge your batteries, for example, I have a combined inverter charger and need 2/0 HEAVY cables to and between the batteries. My LOAD on the DC side is meager, though, so the old OEM #4 FROM the bank TO the DC panel has been working fine for 32 years!

You really need to get into how it all is wired and supposed to work, because with what you've shared, no one can answer your question as written.
Good luck.

This may help you get started:
Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html

seadaddler · Aug 7, 2019

1/0 is pretty standard from battery to 1/2 both switch and main breaker panel.
Nick

JimInPB · Aug 7, 2019

ABYC actually has quite a bit to say about wire size - https://www.paneltronics.com/atimo_s/news/E11Excerpts.pdf

If you want the short story, you need to at least know amps & length of wire, then use these charts to look up wire size - https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

Remember that a load 10' away from a battery counts as a 20' run in those charts, because you have 10' going from the negative side of the source to the load & another 10' from the load back to the positive side of the source. That makes a total of 20' of wire in the current loop.

If you use those charts & find that you need a wire size that is way too big to possibly be right, then it's probably correct. Yes, you really do need #8 wire for a 5 amp device that is 35' away from the source, if you want to only have a 3% voltage drop. A 100 amp load 25' away needs 4/0 for the same 3% drop. For a 10% drop, you can downsize to #2

sailme88 · Aug 8, 2019

— I just think oversized is safer in most instances like this. (Sometimes not true)[/QUOTE]

People being people like to save $$$$. To that end they may buy a less expensive 8 ga wire when heavier is called for. Just like friction loss in a hose is reduced by larger diameter hoses, so too is the resistance in cables/wires reduced when heavier cables/wires are employed. This does not mean to run 1/0 to the mast head to power an anchor light....

JimInPB · Aug 8, 2019

Twille said:
I had looked at that cart - my max amp draw would top out at 80 - in a rare high time. The house battery “loop” is 16 ft total so the 1/0 is off the end of the 3% chart — I just think oversized is safer in most instances like this. (Sometimes not true)

A little bigger wire is not a bad thing. Sounds like you are all set.

LeslieTroyer · Aug 8, 2019

I beg to differ on how measuring the line run is being measured here. You need to measure from source to load ignoring any switches or binding posts that come in between. My guess is the starter is the furthest thing, measure from the battery to the switch, then switch to starter then multiply by two.

Yes your getting 3% loss over the segment, but you then need to add the segments to get overall loss. By the logic presented above I could use 6 gauge in 1’ segments for the 16’ with 7 terminal strips every foot and all would be good. - nope

JimInPB · Aug 8, 2019

LeslieTroyer said:
I beg to differ on how measuring the line run is being measured here. You need to measure from source to load ignoring any switches or binding posts that come in between. My guess is the starter is the furthest thing, measure from the battery to the switch, then switch to starter then multiply by two.

Yes your getting 3% loss over the segment, but you then need to add the segments to get overall loss. By the logic presented above I could use 6 gauge in 1’ segments for the 16’ with 7 terminal strips every foot and all would be good. - nope

I just pulled up a layout diagram for a H38. I don't see the nav station (standard battery location) being particularly close to the engine compartment, so if the battery in the standard location is being used to start the motor, your are probably correct. The maximum loop length is probably longer than was stated. If he doesn't have a separate starter battery on that boat, that's a very good catch on your part. I had assumed that his stated loop length was correct. Assumptions can sometimes be dangerous.

quadrille38 · Aug 8, 2019

JimInPB said:
I just pulled up a layout diagram for a H38. I don't see the nav station (standard battery location) being particularly close to the engine compartment, so if the battery in the standard location is being used to start the motor, your are probably correct. The maximum loop length is probably longer than was stated. If he doesn't have a separate starter battery on that boat, that's a very good catch on your part. I had assumed that his stated loop length was correct. Assumptions can sometimes be dangerous.

On my 2008 38 there are 2 4 D batteries connected as a bank. One is located under the port side settee and the other is located under the floor in front of the nav station.
Also under the floor in that same compartment is a starting battery.

When I get to the boat I can check to see what the OEM cable is.

Stu Jackson · Aug 8, 2019

Be careful if you simply multiply by two, because it depends on how your wires are physically arranged. In some of my circuits, multiplying by two would be wrong, because one end of the circuit may be longer, way longer, the other. For example, my AO + is a LOT longer than the (-) because of the negative bus bar I put in the engine compartment.
Measure once, cut twice???

Otherwise, good advice here.