Login:   Login...      

26/02/16

Permalink 09:29:02 pm, Country: South Australia, Posted by : Brent, 1292 words   English (AU)

How do you choose the right cable rating?

There is discussion all over the Internet about cable rating for trailer / caravan lights brakes etc. Unfortunately most of it is confused, misleading or just plain wrong.

I will attempt to shed some light and clarity to this as I have just been through this exercise, for about two weeks.

Just a quick aside to put my credentials.
I have an Advanced Diploma in Electronic Engineering (Analogue) and an Advanced Diploma in Electronic Engineering (Digital). I work, for the last 10 years, in the Electrical Industry as a line designer for both overhead and underground.

OK, let's just totally ignore the automotive industry cable sizes, they are just plain deceptive. I will come back to this with a couple of tips which will help identify the real ratings.

The correct way to describe cable is through the conductor size in square millimetre. This is the International standard and all conductor of the same size and type will have the exact same rating.

I have used two terms here which are often used interchangeably but should not be mixed up.

Conductor describes the material which conducts the electricity, cable is covered conductor. The covering used for a cable determines it's voltage rating nothing else.

Conductor can be made up of wire, tubing, flat bars, solid billets etc. Conductor doesn't care about the voltage, just the current. Too much current will melt it. A very large voltage with no current has no impact at all.

For the purposes of this post I'll just talk about the makeup of the cable we will use for our wiring harness. The conductor will be made up of multiple strands of wire but I don't care about how many strands or what size they are because the all important current rating is the same for all square millimetre of conductor of the same type regardless of the number of wires.

The only real difference between single strand cable and multi-strand cable of the same rating is how flexible it is. Lots of thin wires are way more flexible than one single thick wire. When you have to pull it through a frame or wall cavity you'll appreciate the multi-stranded cable. The sheath doesn't just protect the conductor from it's surrounds electrically but also physically which is why we use cable, no bare conductor for this type of wiring.

If you want to charge the battery in your caravan or camper while travelling then you will need a specialised circuit and you should be using heavy duty cable via an Anderson plug/socket set. A solar panel on the roof of your caravan or camper will also do this job while travelling. This is not part of this post.

This is just about the lights and electric brakes.

For the lights you really only need 3 sq mm copper conductor. This is good for a continuous current of 18A. Your tail and stop lights are probably rated at about 21W. 21 watts at 12 volts is 21 / 12 = 1.75 amps.

The other thing you need to take into account is voltage drop. The formula for voltage drop is length in metres x current in amps x 0.017/conductor size in sq mm = voltage drop. Juggling this around to get the minimum conductor size and assuming that your total length of cable is 10 metres (from the source in the tow vehicle to the lights on your caravan / camper) we get 10 x 2 x 0.017 / 0.36 = 0.944 square mm. 0.36 is the largest acceptable voltage drop on a 12V line, is used 2 amps so we had a buffer in case your lamps are 24W.

Anyway, at just under 1 square mm that's a pretty easy conductor size to buy. This means that the cheap 7 core trailer cable sold almost anywhere is good enough for these lights as that is usually at leas 3 sq mm copper core..

It isn't any where near good enough for the electric brakes though. The electric brakes typically draw 4 amps per wheel at full braking load. You'll almost never need this amount of current but you should make sure that it is available. Running the calculation again we get 10 x 4 x 0.017/0.36 = 1.88 square mm. This looks easy but you have to remember that this is for one wheel, for a single axle this is 3.76 sq mm and for a tandem axle this is 7.52 sq mm at the connector.

You can't buy these fractional sizes so you need to buy the rounded up size so that's 4 sq mm for a single axle and 8 sq mm for a tandem. You will also need to have the equivalent size for the earth wire in the connector so all that current has some way of returning to the battery. Most professional tow bar installers will use 5 sq mm for the electric brake installation for two main reasons. That's the size which easily fits the flat plug they use and it is good for about 80% of the maximum load that could be required. Mostly you won't use 80% load, you will usually be around 20% load so 5 sq mm is fine. It will also carry the 100% load for a few seconds which is probably all it will ever need in an emergency stop situation.

I'm a bit more of a belt and braces type so my rewire will be using 4 sq mm in parallel with the existing 3 sq mm from the Jaycar trailer cable. The Jaycar cable may be bigger than 3 sq mm but I cannot find out what it is so it's safer to assume a smaller size. By running these cables in parallel it's the same as having a 7 sq mm cable installed. I'll do the same for the earth return cable.

I trust that makes a bit of sense to you. Ask questions if you need more clarification.

Here are a couple of links to help out.
The first is here on this site but reproduced from another site.
http://thetravellersportal.com/index.php/2016/02/26/auto_cable_by_collyn_rivers.

The second is here http://electrical-engineering-portal.com/electrical-thumb-rules-you-must-follow-part-1#thumb-rules the only bit relevant to you is the first section.

There are some things you need to take into consideration when playing with these and other calculations. All results have some caveat or other placed on them. The ability of a cable to carry current is determined not just by conductor size but also the length of circuit, the type of cover, is the cable exposed to the air, is the current momentary (pulse), steady or variable, what voltage is the circuit and many other things.

For a 2.5 sq mm copper cable, the current-carrying capacity can vary from around 15A to around 36A, depending on the factors described above. I would not use 2.5 sq mm for anything bigger than the tail lights on my caravan and nor should you.

In the rules of thumb from the second link using 4 x sq mm of cable is relatively safe as long as you also check for voltage drop. The 6 x sq mm is for copper conductor for a sustainable current in your circuits. It is assumed that the cable is electrical cable not automotive cable.

This is what you can do to get something sensible from the automotive industry cable ratings.
They will often tell you that the cable is rated at, lets say 10A. This is obviously for 12 volt circuits and usually is the maximum rating of the core, just below melting. They are also usually assuming approximately 2 metres of cable being used. Start by halving the current rating, then divide it by 4. That should give you an approximation of the sq mm area of the copper which you can then work with.

However, if the insulation around the copper core seems excessively thick just walk away. Excessively thick is thicker than the diameter of the copper core.

Comments:

No Comments for this post yet...

You must be logged in to leave a comment.

Login .. Register

Places to Stay, Things to do.

This is where you can put in the details of where you went, how you got there, where you stayed, what it cost, what you did, how you liked it and other comments.

Or, if someone else has already been there and made a section, you can add your comments to what is already there.

RULES: If you play nice I will leave it alone. If you get offensive, I will take it down. So, no profanity, no abuse, no spam links, keep it real and useful for others and it'll all be good.

March 2016
Mon Tue Wed Thu Fri Sat Sun
<<  <   >  >>
  1 2 3 4 5 6
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31      

Archives

Syndicate this blog XML

What is RSS?

A couple of Free RSS feed readers.
Internet Explorer
Mozilla

powered by
b2evolution