12v 'Ring' or 'Radial' circuits??

[HoFFdOg]

Hi guys

I have a question regards a 12v DC power socket circuit im going to install in the truck in addition to the 240v ring main power circuit. Ive worked out the probable loads on this DC circuit & i need 6mm cable. However im unsure as to whether i need to loop this circuit back to the fuse panel (as you would in a domestic ‘ring’ main circuit to the consumer unit) or wire it as a radial circuit so that the circuit ends at the very last power socket & does not return back to the fuse panel (similiar to how a lighting circuit ends at the last ceiling rose).


This would save me a shedload of money effectively halfing the amount of cable i need. However am i more likely to get problems with voltage drop when running multiple appliances from various socket points on the circuit? We’re talking nothing big here, a laptop & external hard-drive, mobile phone chargers, a DAB radio, Ipod; the whole circuit running all this is about 6-10A.


If there are no obvious advantages of doing a 12v ‘ring’ circuit that you guys can see than i would much rather go for the radial circuit. However, im also not willing to cut corners if its detrimental to the efficiancy of the circuit.


This makes me wonder whether i’d even need to bring back my 240v socket circuit back to the consumer unit. Would there be any need for me to do the circuit as a ‘ring’ main or would a radial suffice?


Also using 6mm thick cable am i going to get problems terminating that thickness cable into cigarette lighter sockets, DIN sockets or any other 12v DC socket i get my hands on????? If so any tips to get around that issue?

Thanks for any feedback

Dan.

[Sharkey]

First of all "radial", although I've never heard a simple branch circuit called that. If the current draw of the combined devices is more than a single conductor can handle, then separate the circuits and wire with multiple conductors, each a home run back to the supply.

Using 6mm is insane overkill. That wire (equivalent to AWG #3) is good for 85 amperes minimum. While it's always good to minimise voltage drop, unless you are talking about a hundred feet of run on the length, you could easily get by with 3mm or 4mm if you wanted to stay on the safe side.

More important to me would be that you provide a separate copper conductor of the same size as the supply for the branch circuit to return to the battery negative instead of using the vehicle's metal frame as the return path. Steel isn't that great of a conductor, good, corrosion-free connections are more difficult, and the possibility of electrolysis eating away the metal is a definite possibility.

For connect individual devices (plugs, connectors, switches, etc), I'd recommend tapping off the supply wire with some smaller gauge wire (1.5mm or 2mm) and connecting that through an individual fuse to protect the device from the total amount of current available in the branch circuit. Automotive stores have nice pigtail fuse holders for both cartridge and blade-type fuses that could make connecting to smaller terminals easier.

If I was doing it, , I'd leave the main conductor in one continuous piece and strip off rings of insulation to expose the conductor inside. Then I'd solder the pigtail to the larger wire and use heat shrink tubing to insulate the connection. Unless you want to have big terminal boards and/or dedicated pressure connectors to organise, insulate and support the main wiring, leaving it intact is the best bet.

BTW, "cigarette lighter" plugs are the work of the devil. Get rid of as many of them as you can (I know, a lot of consumer electronics for autos have them at the ends of the cables).

[deltasonics]

Hello again HoFFdOg, as regards the "ring" or "radial" circuit.
I am not sure what those terms mean.
I will describe the circuit in the terms I am used to and you can ask any questions you might have for clarification.

If I'm correct 6mm wire is closest to 10 AWG. Apparently the 6mm dimension includes the insulation. I'll check on this and get back to you.

I'm assuming this is 6mm two-core flex.
Which I'm assuming is equivalent to 10/2 stranded.
Two, stranded insulated conductors in a single jacket.

The cable runs for each circuit will be one piece of 6mm two-core flex of the appropriate length to reach from fuse panel to outlet. This single length will run from fuse panel to outlet.
At the fuse panel you will need a buss for ganging the negative wire terminals. While the positive leads will of course be connected to the fuse for each circuit.

At the outlet end terminate with #10 eyelet type crimp connectors. A two gang barrier strip with #10 screws allows a neat and secure means of connecting your outlets (DC power sockets?) to the supply cable. This will also allow for a branch line to another outlet if desired.
Each circuit requires only two conductors to supply more than sufficient voltage to the loads you describe using 6mm two core flex.

You should have at least a battery switch or master fuse as close to the 12VDC power source as possible.
I hope this makes some sense.
Good luck.

Bill

[Sharkey]

Now we have a discrepancy to resolve.

using 6mm thick cable

Which implies cable that is 6mm in diameter.

Using the chart on the American wire gauge page on Wikipeida, we see that 6mm diameter wire is a bit larger then #3 AWG

6mm wire is closest to 10 AWG

The chart does show that wire with a 6 sq/mm area is approximately equivalent to #10.

Kind of important to know which measurement we are using.

[deltasonics]

Very good point Sharkey.

Here is a link http://www.reuk.co.uk/AWG-to-Square-mm- ... verter.htm
A site I found which may or may not help.

It does make reference to square mm wire gauging.
(As compared to say "sq. inch".)

Is it any wonder the Mars probe crash landed a few years back?


deltasonics said

Apparently the 6mm dimension includes the insulation. I'll check on this and get back to you.

Apparently NOT! Please disregard this statement. Thanks.

Bill

[Sharkey]

??? Why? Was it wired by Lucas?

[TMAX]

AKA Prince of Darkness..

TMAX

[deltasonics]

Wiping the tears from my eyes...

I found a piece of AWG 10/2 in a jacket, stranded 600 volt wire and its outside dimension (the jacket) measures ~11.5 mm wide by 7 mm thick

One of the actual 10 ga. copper stranded conductors measures ~ 3.2 mm D.

The O.D. of one conductor with 600 V insulation is ~ 5 mm.

How do these dimensions compare to 6 mm 2-core flex?

[HoFFdOg]

Hey guys, first thanks for getting back to me on this with your advice. Just to clear up briefly the wire sizing issue. The cable i intend to use for for my DC socket circuits is 6mm2 (cross sectional area) NOT diameter.

Various online cable calculators....err, calculated that i would need an ideal size cable of 6mm2 to carry the combined maxium load, with minimum voltage drop along the length of each branch circuit cable run. Im having two runs, one 16m long and the other 10m long. Each circuit will have approx 2-3 double 12v sockets dotted along the cable run.

Sharkey & deltasonics I will respond to the rest of your other posts & advice later in the week as this is not my computer im currently using, but thankyou both for the sound advice ive had chance to read so far.

Dan

[AccordGuy]

By my calculations a 10A load on a 16m run is going to drop 0.9V (out and return combined) with 6mmsq cables.

But you said that you'll have a couple of spurs (one 16m and one 10m) with sockets along the way so I'm guessing that the load will be split at least 50/50 on each branch.

In that case 5A on a 16m run is only 0.46V (out & return combined) with 6mmsq. You could get away with 4mmsq, as Sharkey said. That would drop 0.69V at 5A. Depends on how fussy your loads are but if they are that sensitive then automotive power is going to upset them anyway as it ranges from 11.8V to 14.5V depending on battery state and whether the engine alternator is running.