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Old 06-12-2011
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Battery cable size??

I will have two house battery boxes - each with 4 Costco 225 AH deep cycle 6v 'golf cart batteries - I plan to wire them in a series/parallel combination to get 12 v.

These will power a DC panel with 16 breakers at a nominal 15 amps/.

What size wire do I need to use to wire the batteries and on to the panel?
Also, if I use Bluesea terminal fuses, what size should I use?

Charles
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Old 06-12-2011
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Wire Sizing Charts
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Old 06-12-2011
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battery cable size

I have 3 trojan 105s in each box rigged as you propose. it might be overkill but I use #2 AWG
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Old 06-12-2011
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How far is the run to the panel from the batteries

Dave
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Old 06-12-2011
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Quote:
Originally Posted by micheck View Post
I will have two house battery boxes - each with 4 Costco 225 AH deep cycle 6v 'golf cart batteries - I plan to wire them in a series/parallel combination to get 12 v.

These will power a DC panel with 16 breakers at a nominal 15 amps/.

What size wire do I need to use to wire the batteries and on to the panel?
Also, if I use Bluesea terminal fuses, what size should I use?

Charles
Are these batteries connected to a 1/2/BOTH/OFF battery switch? Can they or could they start the motor if the switch was set right?

You need to know what your max amp draw that your panel can pull from the batteries is? This is normally figured with all loads drawing for safety factoring. All your circuits won't likely be drawing 15A some may be 3A. You take that number and figure it for a 3% max voltage drop to size your panel feed wire.

Fuses are always sized to protect what ever wire size you are using for the wires max rated ampacity. Once you decide what wire you want to use then you choose the fuse to protect the battery wire.

So max panel load in amps @ a max 3% voltage drop for the wire distance, round trip, and this will be the wire size you want.

For the series parallel connections I won't use anything smaller than 1/0 or 2/0.
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Old 06-13-2011
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Originally Posted by Maine Sail View Post
Fuses are always sized to protect what ever wire size you are using for the wires max rated ampacity. Once you decide what wire you want to use then you choose the fuse to protect the battery wire.
I understand 100% what you are saying but don't know how to figure out the answer. I have #2 AWG running to my buss bar etc, but don't know the math (or electronics?) to figure the size of the fuse.

Thanks

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Old 06-13-2011
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Just to note: If you wire the 6v batteries in 'Series' you will have 12 volts at 225 AH.
If you wire the 6v batteries Parallel, you will have 6v at twice the ampherage.
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Old 06-13-2011
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re: wire sizing charts...

this link will let you download the wire sizing chart as a pdf - much more legible than the chart on the web page!

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Calculating current load, wire size and fuse protection

Rik, you need to determine how much current the device at the end of the wire is pulling out of the wire. The fuse should be a little bigger, the most often next available larger size (e.g. current draw between 15 to 19 amps, therefore use 20 amp fuse). If there is more than one device connected to the circuit, add the loads of all the devices connected to / powered by that circuit (e.g. 7 light bulbs at say 1 amp each = 7 amps, therefore use a 10 amp fuse). Sometimes a device like an electric motor, will draw for a very short time a larger current at start up (inrush) than it does when steadily running; in this case a time delay fuse allows the extra start up current to pass without blowing, but will blow if the higher demand is continuous.

The wire should be sized according to the wiring chart to carry the load current you calculated, with a voltage drop acceptable for the devices powered (with too large a voltage drop lights will just go a little dim, but electronics might not work, the engine might not start or the batteries not charge).

In my thinking, it's better if the fuse capacity is lower than the wire capacity, and the load lower than the fuse capacity (but only a little) so that the cheap fuse is protecting both the wiring and the equipment... my 2˘ worth - hope it helps you!

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Originally Posted by rikhall View Post
I understand 100% what you are saying but don't know how to figure out the answer. I have #2 AWG running to my buss bar etc, but don't know the math (or electronics?) to figure the size of the fuse.

Thanks

Rik
Rik,

Genuinedealz has the charts at the link below. On the right scroll down til you hit your gauge then scroll to the far right columns for your inside and outside engien spaces max ampacity. The link provided by Knotted is not what is used in marine applications and is derived from shore based standards.

Below, at the link, are the ABYC max ampacity ratings for MARINE applications and 105C temp rated wire. These tables were derived from the ABYC E-11 Table VI ampacity ratings. The actual table has ampacity ratings for all sorts of temperature rated jackets but in marine applications 105C wire is what is most used and is what all UL1426 wire is rated at.

The ABYC allows you to go to 150% of max ampacity for fuse sizing, but if you can do it, and stay within the 100% range, your doing much better.

Fusing a bank that is used for starting, that the factory "under wired", is a good example of where you might need to go to 150% of the E-11 Table VI ampacity ratings. Most 70's and 80's production boats are under-wired when it comes to battery cable so the 150% rule may need to be used if you want to start the motor off that bank. Personally I prefer to change out the undersized wire and keep the fuse rating at 100%..

Voltage Drop Calculator and Max Ampacity Charts
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Last edited by Maine Sail; 06-13-2011 at 07:01 PM.
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