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darbon

Volt drop, add another cable?

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Hi all,

 

I have a 6mm cable running the length of the cabin (12M) to water pump. It just isn't quite enough, as complete run is 13.5M including tails either end.

 

Can I run another 2.5mm cable alongside the 6mm to create 8.5mm total? Or does it not work like that.

 

Cheers

 

Dan

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It works exactly like that

 

I'm pretty sure both of our pumps run from a single piece of solid core 2.5mm T&E. I'm not saying that this is a good idea, it's just how things were done when she was built

 

Richard

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Guest

It is just the area of copper that you are concerned with in this instance. In industry it was common to use two smaller cables from transformers in parallel since you could bang extra current through compared to the equivalent single (e.g two 400 sq mm compared to a 1000 sq mm). Volt drop is a different issue though; you will be nowhere near to maximum current in this instance.

Edited by Guest

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It works exactly like that

 

I'm pretty sure both of our pumps run from a single piece of solid core 2.5mm T&E. I'm not saying that this is a good idea, it's just how things

Far from best practice. A pedantic BSS inspector might not be happy.

 

Best practice
We highly recommend that new electrical installations be made
with multi-stranded conductors as single solid-wire cables are
vulnerable to breakage where there is high vibration or
repeated flexing.

 

 

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Far from best practice. A pedantic BSS inspector might not be happy.

 

Yes, I know - and what's my 12V system got to do with a BSS inspector?

 

Richard

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Yes, I know - and what's my 12V system got to do with a BSS inspector?

 

Richard

 

As long as each wire is fused correctly for that wire, it shouldn't make a difference.

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Dont forget to fuse each wire separately for its own max capacity (or less).

 

 

Snap

Edited by Justme

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As long as each wire is fused correctly for that wire, it shouldn't make a difference.

 

I see - Did I mention that Tawny wasn't built recently? Curiously, all this best practice has come into being well after she was built.

 

So, would I set out to wire her throughout with T&E, all run off five fuses? No I wouldn't

 

Does it work - yes it does.

 

Am I going to rip out the lining to sort it out when it works....

 

Richard

 

MORE: What I meant to illustrate is that our pumps (plural) work off a couple of strands of 2.5mm successfully, so why doesn't the OPs work on 6mm?

Edited by RLWP

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Its a solution but my concern would be that one or the other of the wires' connections coming adrift wouldn't be easy to detect (after all, the pump might still work, etc) but there would be a potentially live cable flapping around for weeks/months/years. And....you'd need to do similar to the negative return too.

 

I suspect somewhere along the circuit you have a poor connection, to give a sufficient voltage drop to give issues running the pump. My boat's water pump is wired on cable far smaller, and works.


Just reread original post - the complete run is more like 27m because of the return line etc.

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the complete run is more like 27m because of the return line etc.

 

So now we need some one to work out the volt drop and the OP to measure volts at the start of the run and at the pump.

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As always check for volts drop with a multimeter. If volts are lost along a wire then fit more wire, but as so often volts are lost across connectors then they need cleaning and tightening.

 

When you start with 12v any volts drop is too much!

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Its a solution but my concern would be that one or the other of the wires' connections coming adrift wouldn't be easy to detect (after all, the pump might still work, etc) but there would be a potentially live cable flapping around for weeks/months/years. And....you'd need to do similar to the negative return too.

 

On rethinking, you have a point, not with the live side as the fuses will protect you (as they are the right size for the cable), but on the negative side it won't... if one of the negative cables became removed for some reason all the current will be using the other cable which won't be fused and could overheat. So the fuse (or fuses) should be for the smallest cable that the device could use not as per wire as I said before, which will then protect you again.

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On rethinking, you have a point, not with the live side as the fuses will protect you (as they are the right size for the cable), but on the negative side it won't... if one of the negative cables became removed for some reason all the current will be using the other cable which won't be fused and could overheat. So the fuse (or fuses) should be for the smallest cable that the device could use not as per wire as I said before, which will then protect you again.

 

Actually scrap that, only way to safely do it with more than one cable is too correctly fuse each cable (for the size of the cable), this includes both positive and negative.

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Actually scrap that, only way to safely do it with more than one cable is too correctly fuse each cable (for the size of the cable), this includes both positive and negative.

 

Or fuse for the smaller cable size. The OP is thinking of adding another cable to decrease voltage drop, not increase capacity.

From the original post, he's thinking of adding a 2.5mm cable, which should be good for 20A on its own. The water pump is unlikely to take more than 5A or so, so a 10A fuse would protect both cables. If one cable becomes disconnected, voltage drop will increase: if the disconnected cable shorts to hull, it'll blow the fuse, as it would with the cables fused seperately.

 

Iain

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the complete run is more like 27m because of the return line etc.

 

So now we need some one to work out the volt drop and the OP to measure volts at the start of the run and at the pump.

 

Using 27 meters as the length and 10 amps as the current of the water pump you will have a volt drop of 0.848 volts which is less than 10% of the total voltage

 

My question would be why does the water pump not work correct?

 

Keith

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Two fuse option?..I wouldn't do it (own opinion)

 

If there is a problem, you need to fuse to blow and stop it before there are too many sparks or risk of fire.

Two cables ?

The potential for one fuse to blow, but then a delay and the second just has time to 'kick' a spark before the second one blows ?

That's two chances to catch fire to insulation etc.

 

In my 'opinion'...the times I have come up with a quick fix for something has lead to twice as much work.

 

You are talking about 'running a second cable' so why not just run one single large cable and loose the original ?

Your post reads as if you just need to extend it..but then reads as if its the whole cable ?

 

I don't get it ...is it a cost thing ?

Edited by Bobbybass

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Using 27 meters as the length and 10 amps as the current of the water pump you will have a volt drop of 0.848 volts which is less than 10% of the total voltage

 

My question would be why does the water pump not work correct?

 

Keith

 

 

using your figure of 0.848 all it needs is one 'bad' connection and we are talking near or more than 1 volt.

 

assuming fully charged batteries (12.2v) then the pump is only getting approximately 11.2v, not good I would have thought.

 

I think darbon does need to check his voltages and connections and let us know what he finds.

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using your figure of 0.848 all it needs is one 'bad' connection and we are talking near or more than 1 volt.

 

assuming fully charged batteries (12.2v) then the pump is only getting approximately 11.2v, not good I would have thought.

 

I think darbon does need to check his voltages and connections and let us know what he finds.

 

 

With the current ISO 10133 all DC equipment should work with a 10 % volt drop which would be 11 volts in this example

 

I have to say most boat built today will not have 6 mm cable for the water pump

 

Keith

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Well we used 8mm cable for all the water pumps in our new builds!.....what size cable do you think is used now Keith?

 

Cheers

 

Gareth

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Guest

 

Or fuse for the smaller cable size. The OP is thinking of adding another cable to decrease voltage drop, not increase capacity.

From the original post, he's thinking of adding a 2.5mm cable, which should be good for 20A on its own. The water pump is unlikely to take more than 5A or so, so a 10A fuse would protect both cables. If one cable becomes disconnected, voltage drop will increase: if the disconnected cable shorts to hull, it'll blow the fuse, as it would with the cables fused seperately.

 

Iain

Yes, fuse to the smaller cable. It is not a current issue, but a voltage drop issue.

Well we used 8mm cable for all the water pumps in our new builds!.....what size cable do you think is used now Keith?

 

Cheers

 

Gareth

Cables have got bigger over the years!

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Well we used 8mm cable for all the water pumps in our new builds!.....what size cable do you think is used now Keith?

 

Cheers

 

Gareth

 

 

normally 2.5 or 4 mm if the new owner is lucky

 

Keith

Edited by Keith M

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Or fuse for the smaller cable size. The OP is thinking of adding another cable to decrease voltage drop, not increase capacity.

From the original post, he's thinking of adding a 2.5mm cable, which should be good for 20A on its own. The water pump is unlikely to take more than 5A or so, so a 10A fuse would protect both cables. If one cable becomes disconnected, voltage drop will increase: if the disconnected cable shorts to hull, it'll blow the fuse, as it would with the cables fused seperately.

 

Iain

Well done Ian

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