What cable/wire for electrics

[lockwood]

This weekend I plan to re-wire my grp yacht. I have bought all the switches and connectors but am unsure about cable/wire.

What diametre is adequate and do I need a special wire, or will B&Q 5mm wire be okay?

 

[Anonymous]

No, don't use domestic wire because it is bare copper and will corrode causing high resistance joints and broken wires. The wire should be tinned (i.e. look silvery). Second, don't use solid conductor wire, you need to use standed, otherwise the wire will break due to the vibration and movement of a boat. Merlin sell suitable wire - I'm sure others do as well.

 

[ShipsWoofy]

Re: 12v bible

Right click THIS LINK and select save target as.....


This should give you all the advise you need.

 

[Robin]

Don't start until you know what the current carrying capacity of each circuit is and understand the need to minimise voltage drop. 12v DC systems need heavier cabling than the size per amperage quoted for use on 220vAC would suggest, ie a 5 amp cable for 220v would be much too small to avoid serious voltage drop at 5 amps with 12v DC.

 

[lockwood]

Everything is just 12v from one single battery. Its only a 22ft Jaguar so I have just a Log, depth, running lights and a cigarette adaptor for my GPS.

 

[Robin]

OK but the running lights will be 10w each or 10/25w assuming a bicolour or tricolour and depending on the model and bulb fitted. If you do not wire these with adequately sized wire there will be significant voltage drop and the lights will be dimmer than intended. The size required depends on the length of run (up and back total, so 20 feet of cable is a 40 foot run) together with an 'acceptable' voltage drop. Also how about charging or is the battery removed and charged ashore - outboard motor or inboard?

 

[lockwood]

The battery is charged with the outboard. I have had the boat a year and the battery has never failed (so far) - although I may get a small solar pad to give the battery a trickle charge as I now only use the boat on weekends.

 

[Evadne]

Merlin sell Lifetime wire, which is probably better but much more expensive than tinned copper wire from CPC or Farnell or Maplins. The spec. is here and the downloadable spec. sheet (adobe acrobat format) includes a handy calculator for voltage drop vs cable size for 12V and 24V systems.

 

[jimbouy]

Robin,

Do you know how AWG ratind relate to other ways of giving the spec of cables?

For example tinned equipment wire described as 32/0.2 is what in AWG.


Jimbouy.

 

[Robin]

Jim

Sorry my fuddled brain can really only work one system at a time and I use metric coss sectional area for 12v electrics! There is a basic conversion table from AWG to Sq mm area in Nigel Calder's book (Boatowners Mechanical and Electrical Manual). If it means 0.2sq mm then that is 24AWG?

Robin

 

[alex_rogers]

Have a look at -

http://www.vehicle-wiring-products.co.uk/

They have an online catalogue and have a big range of cable (rated by typical current draw) and terminals. I've ordered from them in the past and have always received items extremely quickly.

 

[pvb]

Online conversion table...

There's a useful table here on the Index Marine website, giving a conversion from conductors to sq mm size (eg 28x0.3 = 2 sq mm), plus a table giving the sq mm equivalents of AWG sizes.

 

[jimbouy]

Robin,

After a trawl on google I found this.
http://www.enlightenment.co.uk/frames/ref/conv-csa.htm

So in my example it's 18AWG.

usefull to know as tinned equipment wire is easy and cheaper to get hold of.

Regards to all.

Jimbouy

 

[Graham_Wright]

I wish the mast manufacturers would take note of this. My mast arrived with Woolies three core stranded running up a 42ft. mast. I'll be lucky to get a glimmer out of the nav lights!

 

[MarkV]

I think your point is valid but your theory is wrong.

Assuming that the temprature of the wire is constant and therefore the resistance of the wire is also constant, pushing 5 Amps through it will result in a fixed voltage drop regardless of the voltage of the system - Ohms law V=IR where V is the voltage drop, I is the current and R is the restance of the wire. The problem with low voltage systems is that when the voltage is lower you need more current to get the same power - P=IV where P is power (Watts). So a 60W bulb running on 240v will draw a current of 0.25A, change this to a 60w bulb on a 12V system and you have a 5A current draw, 20 times the current will drop 20 times the voltage for the same wire, this will result in 400 times the power loss through the wire as heat. You therefore need to have heaver wire to keep the voltage drop under control given the higher currents involved.

Failure to provide heavy enough wire will turn the circuit into an electric fire element, closely followed by an all out boat fire!

 

[wooslehunter]

That's why power cables are rated for current - assuming the insulation can take the voltage of course. Hence you can use a cable intended for mains at 12v provided you don't exceed the current rating.

In any event, the cables should be protected by a breaker appropriate to the cable current rating. If the load is appreciably less as it can often be, I'd prefer to down rate the breaker to the load.

Here's an example. Use a 1sqmm cable for a long run up the mast for a 25W tri-colour, and there should be no problem. Say you have around 35m of cable there and back, the voltage drop would be minimal at a little over 1v resulting in no noticable change in brightness. The current's only around 2A. Hence a 1sqmm cable is OK for the rating.

The http://www.vehicle-wiring-products.co.uk/ web site shows two kinds of cables: standard and thinwall. The thinwall has a far higher current rating: 16.5A vs 8.75A for bothe types of 1sqmm cables. The resistance is the same for both since it's dependant on the cross sectional area and not the number of conductors. Hence voltage drop and power dissipation is also the same.

In the event of a partial short at the lamp fitting, a system of standard cable and a 6A breaker could dissipate up to 21W in the cable. This would cause some cable heating but if the short got worse, the breaker would blow before any damage was done. If thinwall cable was used and then protected by a 15A breaker, it could withstand far more of a short and dissipate anything up to 135W before the breaker opened. It would get lots hotter. While this would not necessarily damage the cable it could cause damage to something else close by like an older PVC cable in the bundle.

Sounds complicated but use a correctly rated cable and protect with a breaker that's rated for the load and not necessarily to the maximum rating of the cable.

 

[dickh]

Craig - all that has been said is correct - use tinned cable - obtainable from RS or Farnell otr your freindly Industrial Electrical distributor - look in yellow pages. Are you a member of the Jaguar Owners Association? Look at
http://myweb.tiscali.co.uk/westgatefamily/default.htm and please join - lots of knowledge about jaguars from the members.

 

[Robin]

Now I am muddled because I think I thought that was what I said, sort of! /forums/images/graemlins/ooo.gif

The important thing is for him to use the correct size for the job. I don't know what B&Q 5mm stuff is but I suspect if that is the diameter overall the wire (twin or 3 core he doesn't say) would be perhaps 0.75 sq mm only. This would be fine for short runs to low power instruments but for a long run to say a tricolour will give a dimmer light and reduced range. Also, parts of his circuit may well be carrying several loads as I doubt that each item would be wired back directly to the battery. Finally there is also the need to plan for the future, just because today there is only a low power need from the cigar lighter socket to run a GPS doesn't mean tomorrow someone might not want to power say a 12v vacuum cleaner from it. It will not cost significantly more to wire up with decent sized stuff so why not err if anything on the safe side.

I would personally not want to accept a 10% voltage drop as someone else calculated at the tricolour. With his very basic set up the battery is not likely to be fully charged so the voltage may start at no better than say 12.4/ 12.5v, a voltage drop of 10% would drop this to only 11.2/11.25v and that assumes no other losses from corroded connections etc. It is not going to set fire to his mast but the bulb will surely be dimmer than ideal?

 

[Anonymous]

[ QUOTE ]
I would personally not want to accept a 10% voltage drop as someone else calculated at the tricolour.

[/ QUOTE ]I totally agree - we want to be seen and the light output from a tungsten bulb falls off quickly with voltage.

 

[gorb]

In all this technical talk about wire guages and volt drops, can I just make one STRONG recommendation? This is that all terminations are done onto crimp connectors and that a proper ratchet crimper is used (available from the likes of Screwfix for around £20 I think). If one of these silly little plier-like tools is used it will cause nothing but grief! Like marriage, crimp in haste and repent at leisure!