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I found a new place for my expanded battery bank. It is midship under the floorboards. The only catch is that it is a 15 foot cable run to the battery selector switch. The switch is about 4 feet from the alternator and two feet from the charger. Is this too far away to locate my house batteries?

It is a 300 Ah bank.
 

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No, it's not too far, provided....

1. that you don't skimp on the cabling -- type and size; and
2. that you provide a proper-sized CPD (circuit protection device...fuse or breaker) near the batteries.

I assume you're talking about a house battery bank, not a starting bank. And, I further assume you're talking about a Pearson 365...not a huge boat.

Assuming modest/typical loads for a boat this size, I'd use nothing smaller than AWG 1/0 cabling. And, I'd install an ANL fuseblock and fuse near the batteries, sized for the maximum load expected.

Keep all connections clean and tight...use a proper crimper and heat shrink tubing.

Bill
 

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If anything shorts the batteries the fuse would stop the boat from burning to the waterline. Also is required by ABYC regs.
 

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Over current protection is required to be within 7 inches of the source of power. If it is physically impractical to put the fuse or circuit breaker within seven inches of the source of the power it can be up to 40 inches (101.6 cm) away, if the wire is contained for it's entire length between the source of power and the circuit breaker, in a sheath or an enclosure.

The exception is the cable from the battery to the starter motor. This is not required to be fused and you probably couldn't find a fuse big enough anyway.

The whole point is to protect the wire. By putting the fuse or circuit breaker close to the source of power you protect the entire run of the cable except for the 7 inches or 40 inches from the battery to the protection.
 

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well if space is a total concern you could use these fuses, they fit right on the battery post. but you need this part too.

the important question is how your boat is wired. if your starter is wired right to your start battery then you can get away with a smaller fuse and wire. if your starter is wired to your battery selector switch then you need a bigger fuse and wire just in case you accidentally try to start when you have selected your house bank.

if the starter is direct to a start battery then size your fuse and wire to 150 to 200 % of your max load. ie if you turn everything on your boat on and it only draws 50 amps, then size your house fuse around 100 amps. you also need to look at your charger and alt, make sure it wont blow the fuse either
 

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I like using Line Losses Calculator, so in your case you have 15 feet * 2 which is roughly 10 meters, at 5% loss (14.5VDC and 40 amps (charging)) it gives 2.45mm diameter which is roughly between 0 and 00.

If this battery is charged along with others in the house bank, you want to minimize the voltage drop and resistance as much as possible so it gets fully charged with the others.
 

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Thanks Bill.

Why the fuse blocker close to the batteries? Can you explain what that is for me?
Sorry...was out of town.

I agree with the above responses...good stuff.

BTW, re: voltage drop, the 1/0 I recommended would give a maximum drop of about 3% at 100A draw, which should be sufficient for your house bank. Usually, house draws are far less than this.

Bill
 

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