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Old 05-15-2019, 07:33 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, 09:06 AM   #2
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No it will not
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Old 05-15-2019, 09:06 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, 09:16 AM   #4
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That makes sense, but,
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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, 09:25 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, 10:30 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, 12:09 PM   #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, 12:16 PM   #8
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Thanks for all the replies. Looks like I'll leave this as it is.
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Old 05-23-2019, 02:26 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 05-23-2019, 03:03 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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Old 05-23-2019, 04:56 PM   #11
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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.



I would think it does make a difference over time if different loads are created in some types of weather conditions, from hot to cold to dampness. Hard life for insulation, electronic components etc.
I would put my money on the mfg recommendations for warranty purposes, unless money and being out of servvice when you need it is not a problem.


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Old 05-23-2019, 05:49 PM   #12
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Quote:
Originally Posted by BlackJohn View Post
I would think it does make a difference over time if different loads are created in some types of weather conditions, from hot to cold to dampness. Hard life for insulation, electronic components etc.
I would put my money on the mfg recommendations for warranty purposes, unless money and being out of servvice when you need it is not a problem.


John
Lots of thinking and speculation, much ado about nothing. Isn't it basically the same as a light that has hot running from one end to the other, but the ground is only 2" long? What was the mfg recommendations , if any?
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Old 05-23-2019, 06:40 PM   #13
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Lots of thinking and speculation, much ado about nothing. Isn't it basically the same as a light that has hot running from one end to the other, but the ground is only 2" long? What was the mfg recommendations , if any?
From a Samlex manual, but this applies to pretty much any inverter, because the principles involved aren't product-specific:

"Do not keep the battery wires far apart. Keep them taped together to reduce their inductance. reduced inductance of the battery wires helps to reduce induced voltages. This reduces ripple in the battery wires and improves performance and efficiency. for details, refer to Limiting Electro-Magnetic Interference" at Section 1.3.4"

It's not resistance that's the issue, it's inductance. When a single conducting wire changes current magnitude (which is happening constantly under load), it creates a magnetic field which temporarily opposes the current flow (the induced voltage they speak of... counter emf). This creates the ripple they speak of. When you run the cables side by side, the magnetic field from one wire actually helps push current along in the other (and vice versa), so the negative effects of inductance in a single conductor are effectively counteracted.

With one long cable and one short, it's impossible to run them side by side. So you have this inductance issue. On top of that, by looping the long cable, you're actually building a coil... an inductor by definition... which instead of addressing the problem is instead significantly magnifying it.

Aside from inverter performance related issues, the ripple can also create RF interference, which could impact other devices nearby.

Again, I can't speak to the magnitude of the impact, or if it would be noticeable at all. You could very well be right... might not make one bit of difference. But the effect itself is real, and if it wasn't potentially problematic, I can't see any reason for (every manufacturer's documentation I've read) to suggest the very same practice, for the very same reasons.


If nothing else, you're reducing your voltage drop, and likely reducing the possibility of the longer cable vibrating/rubbing against an abrasive surface.
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Old 05-24-2019, 01:59 AM   #14
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Originally Posted by TheHubbardBus View Post
From a Samlex manual, but this applies to pretty much any inverter, because the principles involved aren't product-specific:

"Do not keep the battery wires far apart. Keep them taped together to reduce their inductance. reduced inductance of the battery wires helps to reduce induced voltages. This reduces ripple in the battery wires and improves performance and efficiency. for details, refer to Limiting Electro-Magnetic Interference" at Section 1.3.4"

It's not resistance that's the issue, it's inductance. When a single conducting wire changes current magnitude (which is happening constantly under load), it creates a magnetic field which temporarily opposes the current flow (the induced voltage they speak of... counter emf). This creates the ripple they speak of. When you run the cables side by side, the magnetic field from one wire actually helps push current along in the other (and vice versa), so the negative effects of inductance in a single conductor are effectively counteracted.

With one long cable and one short, it's impossible to run them side by side. So you have this inductance issue. On top of that, by looping the long cable, you're actually building a coil... an inductor by definition... which instead of addressing the problem is instead significantly magnifying it.

Aside from inverter performance related issues, the ripple can also create RF interference, which could impact other devices nearby.

Again, I can't speak to the magnitude of the impact, or if it would be noticeable at all. You could very well be right... might not make one bit of difference. But the effect itself is real, and if it wasn't potentially problematic, I can't see any reason for (every manufacturer's documentation I've read) to suggest the very same practice, for the very same reasons.


If nothing else, you're reducing your voltage drop, and likely reducing the possibility of the longer cable vibrating/rubbing against an abrasive surface.
Well said!
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Old 05-24-2019, 07:26 AM   #15
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Interesting. Learn something new every day.

I think you can lessen the inductance coil effect by looping in a figure 8 pattern vs a simple loop.

Also, the way my OEM bus electrical system is made most certainly ignores this. Positive and Negative heavy cables are of different lengths and run nowhere close to each other.

Of course the whole body, frame and motor are grounded to negative so that may make some difference. True with the inverter wiring as well that your entire system is surrounded by negative ground in the form of the bus body. It seems like that could impact the induction field significantly.
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Old 06-01-2019, 04:52 PM   #16
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I follow the engineers recommendations in the manuals. From what I’ve read inverter and battery bank cable layout, routing, proximity, sizing, and length is important for both inductance and resistance is concerned. The manufactures manuals I read years ago said to keep the positive and negative cables the same length and twist the inverter cables together. More recently I’ve read to simply tape or zip tie them together and same length.
Making all the cables between batteries exactly the same length and even using high amperage bus bars to help equalize the cable distribution is recommended. Yachts often do this. The RV world is plagued by short lived and imbalanced batteries due to inadequate cabling.
On my cargo trailer system I wish I had been more careful doing the cables. AGM batteries cannot have a equalization charge like a flooded battery. I added a Victron battery equalizer and it brought them back into balance. I’m hoping this will add a few years of life to my battery bank.
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