Battery Cable Sizing

Stu Jackson · Mar 12, 2012

What's the amp draw on the bow thruster?

Consider calling the house bank #1, might be easier to remember, since #2 would be a usually unused item. We call our house bank #1, 'cuz we have switch that doesn't go all the way around as some do.

Joe2954 · Mar 12, 2012

Which battery would you use for starting? - I'm guessing Switch #1 is currently engine battery and #2 is house.
The voltage drop at 90 amps draw (typical windlass running current is a little less than that) for your situation is about 0.4 volts for your 50 ft run (25 + 25 ft).

However, this would jump to about 1.2 volts for 250 amps of starting current if you were using it as the engine battery. Doesn't sound like much but consider the battery may only put out 10 volts at that load.

Is the windlass battery wired to common for the battery switch or elsewhere?

Ron20324 · Mar 12, 2012

What size wire feeds the bow battery to charge it, and what is it wired to?

Deadhead · Mar 12, 2012

Battery Cables

I believe ABYC has published tables that specify minimum wire & cable sizes based upon the anticipated current load, the length of the run, and a maximum allowable voltage drop of either 2% or 10% across the circuit. I think Nigel Calder's book reproduces them - one for actual wire/cable size (1, 1/0, 2/0, etc.), the other for cross-sectional area.

Calder also gives the equation used to develop the tables. I have used the equation because it allows me to include my own safety factor in the circuit design.

I am going from memory though, I keep the book on the boat.

Once you have the cable properly sized have your local chandlery make up the cable with crimped and heat shrunk lugs. It will be money well spent.

Regards.
DH

Maine Sail · Mar 12, 2012

NYSail said:
The isolated battery is 25 feet away from the switch.....

So the question what size wire do I need? 1/0 tinned marine??

Thanks as always!!!!

If you ever needed that battery for starting purposes you're looking at about an 8.4% volt drop on top of the voltage dip of the battery at 200A. If your starter draws more it gets worse. Most engine manufactures want to see less than a 3% voltage drop to a starter from the cable. 50' is a lot of wire to run a starter load through... Even 4/0 @ 200A only gets you down to about 4.2%...

Bill Roosa · Mar 13, 2012

Well you can always "get it to work" the rub is the cost and pain to install and inconvenience to use.
Since you don't use the windlass and only use the bow thruster (on a 36'???) when the engine is on perhaps you should consider moving the battery and running the cables to the bow vs leaving the battery in the bow and running the cables to other batteries.
This would make checking them easier and you would do it more often if they are all co-located, you could design for a 10% voltage drop instead of a 3% resulting in smaller wires and less hassle running them, you could do some balancing of the boat if needed.

Course if you have the space to move the battery this begs the question
Why not just add another battery in that location and then you have a nice bow thruster/windlass voltage boost when you do need it.

Alternately you could use a smaller motorcycle battery to replace the one in the bow and move the one there now to the rear. I’m thinking that a battery would see a bow thruster as if it was a starter motor and not need to be deep cycle in your configuration.

Bill Roosa · Mar 13, 2012

Well as you said you only use it when the motor is on so the alternator is going to help whatever battery gets the load.
If you know the model of the thruster we can probably look up it's power requirements. I'm thinking a HP or two so somewhere between 720 and 1440 watts. That translates to 60 to 120 amps at 12 volts.
Worst case estimate is 120 amps for 5 seconds would only be 120*5/3600=0.16 AH so you don't NEED a huge battery just one that can sustain that load without self destructing. Since the thruster is similar to a big motor cycle starter in both size and usage I'm thinking it can work.
I'd want to know the actual wattage/amp draw before I committed to that route however.
Then the question of how big a cable do you need to both handle the load not taken by the MC batt and recharge the battery after it gets discharged. 0.25 * battery AH capacity is the max amps you are likely to see for the charging part so Im' thinking you would need a cable to handle less than 25 amps (assume < 100 AH MC battery). The amount of current that will flow in the cable when you hit the thruster button is much harder to figure as that would depend on the alternator voltage, the house bank voltage and size and the MC battery voltage and size. Standard practice is to size the cable to handle the full thruster load just to be on the safe side and to provide a sufficient power in the event the MC batter dies. Since this is “non-critical” you can use the 10% voltage drop charts.

Do you know the make/model/size of the hole the prop fits in?

Ron20324 · Mar 13, 2012

NYSail said:
Guess the question would be what power is needed for the bow thruster and windlass....

The question in post 5 was what size is the cable presently there? You are already charging TO the battery, now you just need to get juice FROM it. Kind of how an outboard with electric start.

You said it is connected to an isolator, so is probably already losing half a volt there..