5' of single 4/0 AWG or 5' of 2 AWG to hook up batteries

corn18

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I need to move my 3x206Ah 12V SOK batteries to a different place (under the front bed in my 5er). I'll need to run 5' of something from the batteries to my control center. I can run 5' of single 4/0 (400 amps) or 3x5' of 2 AWG. I like the 2 AWG route because it is so much easier to work with, but wither would do. Appreciate your thoughts.

DC wiring 290RL batt cable.jpg
 
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Dzl

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I'm not sure I understand the question or the schematic.
Let me check my understanding:
You are deciding between using 3 x 2AWG or 1 x 4/0 for a 5' (round trip??) run, with a max current of (___A)?

I understand the lefthand side of the diagram, What I don't Understand from the diagram (right hand side) is if you will have three batteries and three 2AWG cables why spend money on (what I believe is) a Victron busbar at all? Or is that more than just a busbar? Since you will have 3 x 2AWG going in and 3 x 2AWG coming out? Is there a purpose for this that I'm overlooking.

One thing I'm not considering yet is the fusing, which is important, but I want to just understand the flow of it and the details/goals first.
 

corn18

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I'm not sure I understand the question or the schematic.
Let me check my understanding:
You are deciding between using 3 x 2AWG or 1 x 4/0 for a 5' (round trip??) run, with a max current of (___A)?

I understand the lefthand side of the diagram, What I don't Understand from the diagram (right hand side) is if you will have three batteries and three 2AWG cables why spend money on (what I believe is) a Victron busbar at all? Or is that more than just a busbar? Since you will have 3 x 2AWG going in and 3 x 2AWG coming out? Is there a purpose for this that I'm overlooking.

One thing I'm not considering yet is the fusing, which is important, but I want to just understand the flow of it and the details/goals first.
I updated the diagram. I messed up the labels.

Those are Lynx distributors with fuses.

Open for discussion on fusing.
 

Hedges

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2 awg might be rated for 130A, but you show 150A fuse.
6 current-carrying conductors - in a bundle? If so, derate current. If single conductors in free air OK to carry more (190A)

Longer run before combining might help balance current. Might, depends on wire resistance and battery IR.
 

Dzl

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I updated the diagram. I messed up the labels.

Those are Lynx distributors with fuses.

Open for discussion on fusing.
Ahh, now I see, I was misunderstanding.

Seems like the issue with the right hand diagram, is that you don't have any overcurrent protection covering the 2AWG wire runs. I see there are fuses in the Lynx distributor, but that can't protect against a short in the 5 foot, 2awg runs, before the Lynx. Unless the SOK batteries have an integrated fuse maybe?

The same issue is true theoretically with the left hand diagram, but its 1' vs 5' of unprotected wire.

Possibly this is over cautious on my part. But from the diagram it seems you are going for cautious/robust overcurrent protection, so maybe we share a similar perspective here. I believe that the marine guidelines (which is largely what forms my own perspective) say main battery protection should have a catastrophic fuse within ~7" of the battery terminals or 6ft if the wire is completely protected by sheathing. This is for a marine application (which I think translates pretty well to mobile, if your situation is something else, these standards may not be relevant)
 

time2roll

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I like a fuse close to the battery so I would be with picture #1. However the 400T seems redundant.
 

corn18

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Can I bus the left diagram at the batteries and put a 400A type T right there after the bus? Just leave out the lynx distributor.
 

time2roll

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Can I bus the left diagram at the batteries and put a 400A type T right there after the bus?
With #2 good for at least 210 amps I think this is fine. I assume the SOK will shut down if much over 210 is drawn from it. This would happen if a bad connection developed and one or two batteries stopped supplying power.
 

corn18

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2 awg might be rated for 130A, but you show 150A fuse.
6 current-carrying conductors - in a bundle? If so, derate current. If single conductors in free air OK to carry more (190A)

Longer run before combining might help balance current. Might, depends on wire resistance and battery IR.
The Blue Sea circuit wizard says I need 4 AWG for 12V/125A/5' in a bundle. So 2 AWG should be plenty.

I used a 150A fuse because 2 AWG is good for 180A according to the wizard.
 

corn18

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This is what it would look like in the whole system:

DC wiring 290RL.jpg
 
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Hedges

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The fuses would protect against overload, but not against short circuit between battery and fuse.

The Blue Sea circuit wizard says I need 4 AWG for 12V/125A/5' in a bundle. So 2 AWG should be plenty.

I used a 150A fuse because 2 AWG is good for 180A according to the wizard.

What temperature wire do you use, and what did Blue Sea assume?

NEC ampacity for 90 degree insulation, single conductors in free air (not bundle) shows 190A for 2 AWG.


130A in bundle of 3 current-carrying conductors.
Derate to 80% (104A) for up to 6 in a bundle.

 

corn18

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The fuses would protect against overload, but not against short circuit between battery and fuse.



What temperature wire do you use, and what did Blue Sea assume?

NEC ampacity for 90 degree insulation, single conductors in free air (not bundle) shows 190A for 2 AWG.


130A in bundle of 3 current-carrying conductors.
Derate to 80% (104A) for up to 6 in a bundle.

NEC ampacity is different than blue sea calculator. I used 90deg 12V 125A 5’. I can never figure out why they are different so I use blue sea as I have an RV and it’s like a boat on wheels.
 

Hedges

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So I don't think it considered a bundle of 6 wires. If yours are bundled that interferes with cooling. If they are spaced apart for air circulation they can handle more current.
 

corn18

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So I don't think it considered a bundle of 6 wires. If yours are bundled that interferes with cooling. If they are spaced apart for air circulation they can handle more current.
I’m going to go with the larger 4/0 for the 5’ run.
 

Dzl

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NEC ampacity is different than blue sea calculator. I used 90deg 12V 125A 5’. I can never figure out why they are different so I use blue sea as I have an RV and it’s like a boat on wheels.
Blue Sea assumes 105*C wire unless you say otherwise, this is the norm for quality marine wire (but not a given). Its up to you to input proper values with the calculator, wire temp rating is one of the user inputs. This probably accounts for the difference of you are seeing. NEC and ABYC (marine ratings) are fairly harmonized from what I remember, once you make sure all the variables are the same, I think there are slight differences, and some larger differences for small wires (10AWG and below) but mostly the same.

Also FYI, you can see screenshots of some of the NEC and ABYC ampacity tables in the albums section of my profile. These are just the ones I found most relevant, including the base tables. But the NEC especially has different tables and modifiers for different conditions.
 
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