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Old 15-07-2018, 03:31   #1
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Sizing Main Battery Cables

I am having trouble figuring out how to size the main battery cables.
I am using the Blue Seas Circuit Wizard


Single Bank (2)T105 225ah
Internal Length to positive bus and neg shunt=5'
External Length from pos bus to Sw= 2'
How do I complete the circuit for sizing the main cables?
What length do I use?
What load do I use? 225a or estimate the actual load?
Temp=105f not in engine room
Length=7'?
Current= 225a?
Volts=12
Circuit=Main
CCA=450? Trojan T105 have no rating.

Two Banks Joined (4)T105 450ah
Temp= 125f because cables pass through engine room.
Internal Length to wire both banks together across the engine.
Internal Length 18' from Pos Bus to Neg Shunt.
How do I complete the circuit for sizing the main cables?
What length do I use?
What load do I use? 450a or estimate the actual load?
Temp=105f not in engine room
Length=18'?
Current= 450a?
Volts=12
Circuit=Main
CCA=450? Trojan T105 have no rating.
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Old 15-07-2018, 04:08   #2
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Re: Sizing Main Battery Cables

Double the length for the whole circuit. An 18 foot run is a 36 foot circuit.
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Old 15-07-2018, 04:23   #3
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Re: Sizing Main Battery Cables

The total load is not the 450a, its the load in amps of all the circuits turned on at the same time. I just add up the total of all the items that could possibly be run at the same time. I also set the voltage drop for 3%

Length is the measured length from the battery bank to the distribution panel (and return for some calculators)

CCA is not used for these calculations, you'd only need that for a starter circuit.

There are (ironically) much better calculators available. I use on on my ipad for this.
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Old 15-07-2018, 04:32   #4
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Re: Sizing Main Battery Cables

Yes, I already have included the neg cables, but when sizing the large battery cables it is not clear to me how to do it.

What load do I use? I could use the actual Battery Ah, but that is not amps. Do I just estimate all of the loads from the Main DC Panel and the smaller direct Battery Panel that has bilge pumps etc. and use that amperage?

Does the internal storage capacity (225ah) of the Battery which is an indication of potential output get used at all in figuring out cable size?

The other problem I have is how to complete the circuit for the larger cable sizes.
I have already done all the other loads including the full circuit length from + battery post to - battery post. I have those cable sizes determined. Also for power sources such as the charger and alternator, that's done.

Now I am having a conceptual problem with how I am supposed to size the battery wires (both internally between the positive busbar and the negative shunt (including all wires & batteries in between).

Also I don't know how to size the larger wires, for example from the positive busbar up to the 1Both2 switch and it's negative counterpart, whatever that is.
What is the length that I should be using? The loads have already been done and the wires are smaller.


Also since Trojan T105 have no CCA rating, I don't know what to use in Blue Seas Circuit Wizard.
If I Join the two banks port and starboard into (4)T105 and 450ah with a much longer cabling going through the engine room, how does that change the cable size, what are the lengths I should be using to size the cables?
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Old 15-07-2018, 05:09   #5
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Re: Sizing Main Battery Cables

Quote:
Originally Posted by rgleason View Post
Yes, I already have included the neg cables, but when sizing the large battery cables it is not clear to me how to do it.

What load do I use? I could use the actual Battery Ah, but that is not amps. Do I just estimate all of the loads from the Main DC Panel and the smaller direct Battery Panel that has bilge pumps etc. and use that amperage?

Does the internal storage capacity (225ah) of the Battery which is an indication of potential output get used at all in figuring out cable size?

The other problem I have is how to complete the circuit for the larger cable sizes.
I have already done all the other loads including the full circuit length from + battery post to - battery post. I have those cable sizes determined. Also for power sources such as the charger and alternator, that's done.

Now I am having a conceptual problem with how I am supposed to size the battery wires (both internally between the positive busbar and the negative shunt (including all wires & batteries in between).

Also I don't know how to size the larger wires, for example from the positive busbar up to the 1Both2 switch and it's negative counterpart, whatever that is.
What is the length that I should be using? The loads have already been done and the wires are smaller.


Also since Trojan T105 have no CCA rating, I don't know what to use in Blue Seas Circuit Wizard.
If I Join the two banks port and starboard into (4)T105 and 450ah with a much longer cabling going through the engine room, how does that change the cable size, what are the lengths I should be using to size the cables?

Ignore the CCA rating as deep sycle batteries don't have one anyway, it won;t affect the pages abiity to complete the calculation ( leave it blank) the distance is the distance rom the panel to the battery bank. for the load, again, use the total amps of all the circuits on the main panet (assum they are all on at the same time) Ignore the 1-2-both switch ( you shouldn't have one anyway, that 1970's wiring methodology) Measure the distance from the battery positive post of your bank, through the main batt switch to the panel to get your distance.


because your banks are split, to keep it simple, just measure from the farthest set to the main panel and use that as your awg size.

as an example I bought a roll of 0/2 AWG wire to run from my main house bank to my windlass in the bow. I wired up all the battery interconnects and the lead to the main battery switch with that same wire. Its overkill but I had it and there is virtually no voltage drop that my meter can detect. From my main battery switch I ran 4awg cable to the dist panel ( a distance of 6 inches! ) again overkill but I had it on hand and it was way more than the calculator gave me.

to make their calculator work all you need is 3 values. voltage (12), total amps and circuit length ( length from the panel to the battery positive doubled to include the return)
If your bank is split just use the longest distance
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Old 15-07-2018, 06:52   #6
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Re: Sizing Main Battery Cables

Pcmm, this is very helpful. I'll follow that advice and will post a diagram on small boat electrical for sail thread.

Since it is a small boat, the positive busbar with all those fuses will have to mount just above the batteries in port cockpit locker and it looks pretty exposed. Is there a way to reduce number of fuses and meet abyc? Current setup has none! Don't they create heat and voltage drops?

New setup currently shows fuses on batteries (250a), on pos busbar for charger, alternator, solar, combiner, etc.
Are all of these needed?

Should I just remove the 1both3 switch.
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Old 15-07-2018, 07:57   #7
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Re: Sizing Main Battery Cables

Pcmm, what calc values should I use for the internal battery cables, ie those wiring up the batteries between the main + post and the main - post.

There are two options, use port side only for house 225ah or combine port and starboard for 450ah.

My estimated loads are cruising/anchor 80ah/24hr and at sea 129ah/24hr.

I will have 280 to 300 watts of solar.

I am torn between the two choices for house battery size. The port/starboard wiring is not optimal and goes through the engine room. 225ah is a bit low I think, but for cruising I think it will work. It is when at sea that the bigger bank would be useful and that is only an occasional use. Perhaps I should just go with the 225ah house, and wait for battery technology to improve the ah, acceptance and bulk capacity later.
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Old 15-07-2018, 10:12   #8
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Re: Sizing Main Battery Cables

On battery cables, the bigger the better. It will keep voltage drop to a minimum. Why not 1/0 or 2/0?
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Old 15-07-2018, 10:14   #9
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Re: Sizing Main Battery Cables

Quote:
Originally Posted by Terra Nova View Post
On battery cables, the bigger the better. It will keep voltage drop to a minimum. Why not 1/0 or 2/0?
Agreed, just gets hard on the shorter runs to make the 2/0 cable bend sometimes!

I used 2/0 ( wrote it backwards, lol) for all my heavy loads. just bought a 50ft roll and used it up for my windlass, starter, battery inter connects and leads to my main batt switch. easy and allowed me to purchase the connectors in bulk for $ savings.
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Old 15-07-2018, 10:36   #10
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Re: Sizing Main Battery Cables

Quote:
Originally Posted by rgleason View Post
I am having trouble figuring out how to size the main battery cables.
I am using the Blue Seas Circuit Wizard


Single Bank (2)T105 225ah
Internal Length to positive bus and neg shunt=5'
External Length from pos bus to Sw= 2'
How do I complete the circuit for sizing the main cables?
What length do I use?
What load do I use? 225a or estimate the actual load?
Temp=105f not in engine room
Length=7'?
Current= 225a?
Volts=12
Circuit=Main
CCA=450? Trojan T105 have no rating.

Two Banks Joined (4)T105 450ah
Temp= 125f because cables pass through engine room.
Internal Length to wire both banks together across the engine.
Internal Length 18' from Pos Bus to Neg Shunt.
How do I complete the circuit for sizing the main cables?
What length do I use?
What load do I use? 450a or estimate the actual load?
Temp=105f not in engine room
Length=18'?
Current= 450a?
Volts=12
Circuit=Main
CCA=450? Trojan T105 have no rating.
Refer to the appropriate Marine electrical standards.
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Old 15-07-2018, 10:51   #11
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Re: Sizing Main Battery Cables

Quote:
Originally Posted by Terra Nova View Post
On battery cables, the bigger the better. It will keep voltage drop to a minimum. Why not 1/0 or 2/0?
Don't use them because they are expensive, unnecessary, hard to route and hard to add terminals. You would only need 2/0 for engine starting loads.

As was said earlier, add up the maximum load through to panel and select the correct size wire. On a 32 foot boat I would be surprised if the wire size was over #8.
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Old 15-07-2018, 12:52   #12
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Re: Sizing Main Battery Cables

Well, if I were to have Single Sideband Radio, which I do not, I would use at most 40 amps in loads, the blue sea calculator using 125f and 3% drop max. with 16' indicates that I need just AWG 6 to to the + and - posts! So why should I use AWG 2/0 for the main runs?
The internal battery wiring seems to be totally dependent on the expected loads and not battery size or capacity, yet Mainsail wants at least 250a or 300a fuses on the batteries. So I don't really understand this aspect
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Old 15-07-2018, 13:25   #13
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Re: Sizing Main Battery Cables

Terra Nova I guess you're right about the sizing for the main run, but I already have AWG 1 for all the main wiring from + post to - post. And also AWG 1 for all the internal battery wiring.

Is this going to be acceptable for ABYC standards?
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Old 15-07-2018, 14:16   #14
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Re: Sizing Main Battery Cables

Quote:
Originally Posted by rgleason View Post
Well, if I were to have Single Sideband Radio, which I do not, I would use at most 40 amps in loads, the blue sea calculator using 125f and 3% drop max. with 16' indicates that I need just AWG 6 to to the + and - posts! So why should I use AWG 2/0 for the main runs?
The internal battery wiring seems to be totally dependent on the expected loads and not battery size or capacity, yet Mainsail wants at least 250a or 300a fuses on the batteries. So I don't really understand this aspect
Recommended best practice is to wire a SSB directly to the battery, not through a panel. So if you were to add one it would not be a factor.
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Old 15-07-2018, 19:42   #15
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Re: Sizing Main Battery Cables

Thanks so I could get away with less than 6 but 1 should be fine.
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