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Old 05-15-2019, 06:33 AM   #1
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Short neg lead?

We installed the battery bank and inverter in our bus and only have about 3' of 4/0 cable on the POS side, going from the batteries to a 400 amp fuse to the cut-off switch and then into the inverter. I think this is qualifies as a pretty short run for that lead.

The kit came with about 8' of both red and black 4/0 cable, but instead of cutting the NEG cable we just ran it from the inverter to the far side of the battery bank (5-6') and looped the excess inside the battery box. Will the extra length of the NEG lead impact the efficiency of the system?
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Old 05-15-2019, 08:06 AM   #2
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No it will not
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Old 05-15-2019, 08:06 AM   #3
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Not an electrical engineer here but after many years in the trade, we were taught to keep all feeders the same length. More cable means more resistance if one leg is longer than another. I would match them but you probably won't even see a difference in efficiency. And maybe have enough cable left for a spare down the road should you need it.


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Old 05-15-2019, 08:16 AM   #4
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That makes sense, but,
Quote:
Originally Posted by BlackJohn View Post
Not an electrical engineer here but after many years in the trade, we were taught to keep all feeders the same length. More cable means more resistance if one leg is longer than another. I would match them but you probably won't even see a difference in efficiency. And maybe have enough cable left for a spare down the road should you need it.


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Probably only relevant on long runs &/or really huge loads.
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Old 05-15-2019, 08:25 AM   #5
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Originally Posted by haz.matt.1960 View Post
That makes sense, but,Probably only relevant on long runs &/or really huge loads.



Just common practice so I stick with that aspect after having it drilled into me brain so often. Hard to say in this case but in longer ac runs, the load needs to be balanced on each conductor in both single and three phase power setups.


He'll be fine if he doesn't change anything too in my mind.


Ohh and good morninn down in Tejas!


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Old 05-15-2019, 09:30 AM   #6
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Roger that, I hear ya. There have been numerous best practices and procedures that've been drilled into my brain, too.
On average, the first would break, so it would take two, 3/4" masonry bits to get clear thru my cranium...

Quote:
Originally Posted by BlackJohn View Post
Just common practice so I stick with that aspect after having it drilled into me brain so often. Hard to say in this case but in longer ac runs, the load needs to be balanced on each conductor in both single and three phase power setups.


He'll be fine if he doesn't change anything too in my mind.


Ohh and good morninn down in Tejas!


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Old 05-15-2019, 11:09 AM   #7
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Itís a DC system so the relative lengths of the cables would not matter I think. Both neg and pos together are just a loop. If the loop is longer, you will have more resistance, but with the length of run and cable size you have it seems like it would be quite negligible.
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Old 05-15-2019, 11:16 AM   #8
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Thanks for all the replies. Looks like I'll leave this as it is.
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Old Yesterday, 01:26 PM   #9
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Yes, it matters.

Your battery leads should run side-by-side, as close to each other as possible (think speaker wire), in order to minimize inductive losses. Not only are you not doing this, you're actually creating an inductor in the negative lead by looping it back on itself.

How much does it matter? That I couldn't tell you. But I'd be surprised if your inverter manual doesn't include wording very similar to what I just used. I know Samlex & Magnum Research inverters do. So not only is it sound practice, it's manufacturer-recommended.

If it was me, I'd buy another cable assembly to match & run them side-by-side (assuming you don't have the equipment & skill to properly shorten & terminate your existing cable).
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Old Yesterday, 02:03 PM   #10
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Does it make a difference,probably negligible. Does it matter, in this case with the wire in question being 4/0, it won't matter, leave it alone.
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