Connecting a little wire to the middle of a big wire.

[WindyWindyWindy]

So I've got a big fat negative cable (50mm2) running from the stern to the bow for the windlass.
It would be useful to use this as the ground for some other bits and pieces because it's already there and it's nice and big.
The ends of the big wire are quite a way down the boat, but the middle passes quite close to where I need it.

What's the best way to connect a small wire to this big wire?
Is there a way to do it without cutting the big wire to insert terminals?

 

[andsarkit]

You can carefully strip the insulation off and wrap the second cable around it and cover with insulating tape.
This will :
1) Cause ground noise on the second wire when the windlass draws 100A and causes a 0.4V rise in the ground voltage.
2) Allow moisture to enter the big cable and cause corrosion, increased resistance and local heating.
3) Fail just when you need it most.
Or you could run another ground cable and do it properly. Fault finding is much easier with separate circuits that don't interact.

 

[noelex]

You need to add a bus bar, or a terminal post, which means cutting into the wire.

If you do this make sure it is fused adequately. The fuse installed for the 50mm cable will lnot provide protection for a smaller wire.

 

[Refueler]

Cut a narrow band of insulation away at the required position on big wire. Strip sufficient length of insulation for the thin wire.

Using a Cable Clamp - designed specifically for this job - you can clamp the thin wire to the big.

Or you can wrap the thin wire round say 2x .... twist end onto standing part and carefully tighten.

Then paint all with Liquid Insulation ....

 

[WindyWindyWindy]

You can carefully strip the insulation off and wrap the second cable around it and cover with insulating tape.
This will :
1) Cause ground noise on the second wire when the windlass draws 100A and causes a 0.4V rise in the ground voltage.
2) Allow moisture to enter the big cable and cause corrosion, increased resistance and local heating.
3) Fail just when you need it most.
Or you could run another ground cable and do it properly. Fault finding is much easier with separate circuits that don't interact.

I do like the philosophy of this, but in practice you have a remote bus bar at the switch panel, so in a way it's just adding one even more remote.

Also, in defence of the concept, it's all things related to the windlass installation. Like the control box itself, radio remote receiver, I did even contemplate a light for the chain locker.

If you ran 20m of cable for each of these you'd quickly start thinking that it doesn't look right.

The bow nav lights could share the neg as well.

 

[WindyWindyWindy]

Using a Cable Clamp - designed specifically for this job - you can clamp the thin wire to the big.

Got a pic? "Cable clamp" covers a multitude if things...

 

[AngusMcDoon]

Just twisting a cable round is asking for failure however well you seal it. You wouldn't accept that kind of connection long term anywhere else. If you're going to do it you need a clamp that you can screw down tightly. You can get earth rod clamps of varying sizes for clamping round the big cable. They're meant for rods but the kind that clamps from both sides rather than having a screw end bearing on the cable will work. Then you could crimp on a ring terminal on the smaller wire, attach it under the nut, and screw it all down tight, You could use a simple L bracket to attach it to something to stop it moving around, and wrap the whole lot in self amalgamating tape.

These are just examples. You'd have to get sizes to fit...

7TCA083220R0002 CR205 | WJ Furse Copper AlloyEarth Clamp Nominal Rod dia. 9.5mm | RS
160296 | TE Connectivity, PLASTI-GRIP Insulated Ring Terminal, M8 Stud Size, 3mm² to 6mm² Wire Size, Yellow | RS

It's probably cheaper and less faff just to run a separate earth cable though.

 

[andsarkit]

I do like the philosophy of this, but in practice you have a remote bus bar at the switch panel, so in a way it's just adding one even more remote.

Also, in defence of the concept, it's all things related to the windlass installation. Like the control box itself, radio remote receiver, I did even contemplate a light for the chain locker.

If you ran 20m of cable for each of these you'd quickly start thinking that it doesn't look right.

The bow nav lights could share the neg as well.

I assume you will have to run the +12v power cable to each device so you could use a Multicore cable for all the live and grounds so you only need one run. That would need a junction box at a suitable location to take off the individual feeds. Also individual fuses would mean that a failure of one device would not take out the others.

 

[PaulRainbow]

So I've got a big fat negative cable (50mm2) running from the stern to the bow for the windlass.
It would be useful to use this as the ground for some other bits and pieces because it's already there and it's nice and big.
The ends of the big wire are quite a way down the boat, but the middle passes quite close to where I need it.

What's the best way to connect a small wire to this big wire?
Is there a way to do it without cutting the big wire to insert terminals?

It isn't a ground, its a negative cable.

Using it for "other bits and pieces" is generally not a good idea. Making a proper job of it will cost more than running a new cable. Every time you use the windlass the "bits and pieces" will almost certainly suffer an unacceptable voltage drop. This might just be some lights dimming or it might be some electronics resetting, obviously depends what you connect to it.

Best solution would be to run a new cable. You can add up all of the loads you want to connect and use a single negative cable (allow a bit for possible future expansion), install a busbar or terminal post and connect everything to this. There is no need to fuse the negative cables.

If the positives need to be switched at the supply end you will obviously need to run separate cables and these will need to be individually fused at the supply end.

If the positives (or even just some of them) do not need to be switched at the supply end you can run another single cable, rated for the combined loads, to a fuse panel close to the equipment, this cable needs a fuse at the supply end, rated for the combined loads.

 

[Elessar]

100A Busbar With 2 Studs & 5 Screw Terminals

 

[Elessar]

100A Busbar With 2 Studs & 5 Screw Terminals

And use proper shrink wrap crimps.

then it’s a forever job.

 

[LadyInBed]

You need to add a bus bar, or a terminal post, which means cutting into the wire.

If you do this make sure it is fused adequately. The fuse installed for the 50mm cable will lnot provide protection for a smaller wire.

I don't normally fuse the Neg return wire!

 

[LadyInBed]

Is there a way to do it without cutting the big wire to insert terminals?

If you don't want to run a new wire or cut the existing wire, you could drill or preferably skewer into the thick cable and screw in a brass wood screw.
Attach your thin wire with a brass washer, eye terminal and locking washer before screwing in the wood screw.
Seal with hot glue gun and self amalgamating tape.

 

[Elessar]

If you don't want to run a new wire or cut the existing wire, you could drill or preferably skewer into the thick cable and screw in a brass wood screw.
Attach your thin wire with a brass washer, eye terminal and locking washer before screwing in the wood screw.
Seal with hot glue gun and self amalgamating tape.

bodgedy bodge bodge

that's why used boats are so unreliable!

see#10 for something that will last!

 

[LadyInBed]

bodge bodge bodge

Yes, yes, yes!
But just linking to a busbar doesn't solve the OP's question of connecting the busbar to the large diameter neg return cable if, as he has said, he doesn't want to run a separate return wire back to source.

 

[Daydream believer]

100A Busbar With 2 Studs & 5 Screw Terminals

How does that connect to the existing cable?

 

[PaulRainbow]

If you don't want to run a new wire or cut the existing wire, you could drill or preferably skewer into the thick cable and screw in a brass wood screw.
Attach your thin wire with a brass washer, eye terminal and locking washer before screwing in the wood screw.
Seal with hot glue gun and self amalgamating tape.

I hope that no-one who reads this follows this suggestion.

 

[PaulRainbow]

bodgedy bodge bodge

that's why used boats are so unreliable!

see#10 for something that will last!

Or #9 for the correct way of doing it

We don't know what windlass the OP has, but it might not be a good idea to fit a 100a busbar part way along a length of 345a cable.

 

[Refueler]

I hope that no-one who reads this follows this suggestion.

I must admit - I am not in agreement with DRILLING through .... this can cause breakage of wire strands, but skewering to 'spread and separate' strands without breaking any .... I can accept if no other route.

 

[KompetentKrew]

Just tape or heatshrink a second cable to the big fatty, run them both through together at once.

I think 8.5mm² cable should only be about 3mm or 4mm in diameter, and is rated for 63A. 12vPlanet have some 6.0mm² tinned cable that's rated to 50A.

Maybe I use thicker cable than necessary, probably 1.5mm (21A), 2mm (25A) or 3mm (33A) would be adequate, and closer to 2mm diameter.