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Old 18-04-2018, 12:57   #1
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Ok to downsize a 10 cm run of grounding cable?

Hey experts!

I need to reroute my common grounding cable, to install a service shunt (Victron). I bought a length of 35 mm2 AWG2 quality cable, as that is standard according to manufacturer plans. It turned out that on my boat, 50 mm2, AWG0 is used.

The new cable run will be 10 cm long and will see a max load of 60-70 Amps (windlass). According to all gauge calculators this should be OK, but I still would appreciate a sanity check.

Please see attached pics for clarity on my plans. Any critique welcome.



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Old 18-04-2018, 19:23   #2
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Re: Ok to downsize a 10 cm run of grounding cable?

For a 10cm run...that should be no problem. Still...how expensive or inconvenient would it be to discard that and buy the same wire used elsewhere?

Unless there is an issue with terminal or lug sizes not fitting into equipment. When that happens (when the equipment can't take the large cable) it is not unusual to downsize for a very short run (pigtail) like that.

The quality of the crimps is probably more important.
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Old 18-04-2018, 20:12   #3
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Re: Ok to downsize a 10 cm run of grounding cable?

if that max is 70a then you are find with 2awg.

but what I don't understand is why you are putting a shunt in that place in the drawing.

you are going to get strange readings from that spot. bascily current that is flowing between the engine battery and house battery. probably the only thing you'd read is alternator charge current.




looking closer on the lower drawing. you're actually going to be connected in the engine battery and the house battery. to where you wrote engine battery? that is still wrong though. the shunt should go on the wire between the house battery and the engine battery. you monitor one bank at time. house only
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Old 19-04-2018, 00:08   #4
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Re: Ok to downsize a 10 cm run of grounding cable?

Thanks a lot for your input.

Looking at the wiring and diagrams I realized my mistake. As it is right now the negative terminals of all three batteries are connected in series, which I have to reroute (and buy new cabling :-p).

This should be correct, no?


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Old 19-04-2018, 08:58   #5
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Re: Ok to downsize a 10 cm run of grounding cable?

I can't figure anything out from your drawing. Find some sample schematics and emulate their layout.

Treat the House bank (the one you're monitoring) as a single battery in that drawing.

That bank's + and - connections to loads and from chargers are all from the same pair of terminals.

The shunt gets interposed between the negative post of that bank and everything else, nothing bypasses it.

The Starter (engine) batt should not be involved in any of this.
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Old 19-04-2018, 11:24   #6
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Re: Ok to downsize a 10 cm run of grounding cable?

I can't make out from the various diagrams. Just remember everything going through the hot leads is also going through the ground so insure the winch isn't the only thing using that ground.
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Old 19-04-2018, 11:27   #7
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Re: Ok to downsize a 10 cm run of grounding cable?

Workaround:

If you have enough cable, you could rig two wires of 35mm2 alongside each other. That would be more than the 50mm2 you need.

If you have big terminals you could possibly even crimp them together.

If I where in your shoes I'd feel uneasy with 35mm2 instead 50mm2. It's similar to a congestion in a waterhose.
The weakest spot tends to fail in situations like that.
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Old 19-04-2018, 13:17   #8
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Re: Ok to downsize a 10 cm run of grounding cable?

The Victron diagram labeling may be confusing. The Victron diagram label reads "Battery Negative (System Ground)". A more understandable label might be "Return negative" or "Load Negative". You'd want to run a grounding wire from the bottom terminal of the shunt, as depicted in the Victron digram, to the engine block.
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Old 19-04-2018, 13:39   #9
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Re: Ok to downsize a 10 cm run of grounding cable?

On a second glance. It appears you may be confusing earth ground with the current flowing back to the battery, using conventional current flow.JNHO
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Old 19-04-2018, 23:17   #10
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Re: Ok to downsize a 10 cm run of grounding cable?

like I said early. the easiet thing is to cut the cable between the engine battery and house battery and put the shunt in the middle of that cable. if you have a lug tool. no need for cable or to change anything

the better option is to remove the cable from house battery to engine battery on the engine battery side, attach to shunt, buy new cable from shunt to clamp.

don't move the engine battery ground off the clamp like in your later drawing.
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Old 19-04-2018, 23:23   #11
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Re: Ok to downsize a 10 cm run of grounding cable?

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Old 20-04-2018, 04:46   #12
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Re: Ok to downsize a 10 cm run of grounding cable?

Quote:
Originally Posted by earbuilder View Post
Hey experts!

I need to reroute my common grounding cable, to install a service shunt (Victron). I bought a length of 35 mm2 AWG2 quality cable, as that is standard according to manufacturer plans. It turned out that on my boat, 50 mm2, AWG0 is used.

The new cable run will be 10 cm long and will see a max load of 60-70 Amps (windlass). According to all gauge calculators this should be OK, but I still would appreciate a sanity check.

Please see attached pics for clarity on my plans. Any critique welcome.
All cables should have an individual connector, crimped and heat shrink and individually bolted to a bus bar specifically for that purpose of connecting multiple cables...
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Old 20-04-2018, 06:03   #13
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Re: Ok to downsize a 10 cm run of grounding cable?

smac999 option #1 would be my tip as well.
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Old 20-04-2018, 12:02   #14
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Re: Ok to downsize a 10 cm run of grounding cable?

One of us in insufficiently caffeinated. It could be me. From the looks of this picture:
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Old 20-04-2018, 12:04   #15
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Re: Ok to downsize a 10 cm run of grounding cable?

One of us in insufficiently caffeinated. It could be me. From the looks of that picture, the only difference between the old wiring and the new wiring, is the change from dashed line to solid line, and the color. The connection points remain the same even if the lines are drawn in another direction.

Seems like the shunt--which will be useless, since it is not shown ATTACHED to a meter, will be useless. And if the shunt was simply inserted in-line someplace, that's all that would be needed.

This definitely needs more caffeine and new drawings. Complete with the shunt's meter connection, and a note as to exactly what current flow the OP intends to measure with it.
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