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thecookster

12V wire size

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OK, so going to do some jobs on my van and add things like an offside awning light and low level 12v lighting.

 

I'm competent at doing electrics (before anyone starts) but don't know how to calculate what size wire to use.  If it was 240V I'd know exactly what wire to use for what situation!

 

Is there a formula for 12V electrics to determine the right cable size to use?

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Its no different to 240v, it's the current rating you need to consider. That said at 12v, volts drop has a bigger effect than at 240v.  I would assume you'll be using an LED awning light, I would suggest to go for 1.5mm2 more for mechanical strength than anything else.

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Use Ohms Law for DC circuits, that states that V = I R,

(voltage = current times resistance)

(current = voltage divided by resistance)

(resistance = voltage divided by current)

 

Also DC power (Watts) = V I

(power = voltage times current)

(voltage = power divided by current)

(current = power divided by voltage)

 

eg 1  5W lamp at 12V will require a wire to carry 5/12 = 0.42 Amps

eg 2  15W lamp at 12V will require a wire to carry 15/12 = 1.25 Amps

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And power = volts times  amps.

 

So if device states it is 240 watts then 240 = 12 x current in amps.  Therefore current is 20amps. Cables are rated for current. So low voltage DC cabling can tend to be thicker than household mains.

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Just now, WispMan said:

So low voltage DC cabling can tend to be thicker than household mains.

So far as the conductor is concerned, yes. However 12V requires less insulation than 230V so the cable may be thinner overall despite having a thicker conductor.

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Thanks for all the replies!

 

I'll sort out my cable now as the last step in my project!

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I buy my vehicle wiring bits and bobs from Autospark: http://www.autosparks.co.uk/index.php?content_page=51

 

The thinwall cable is excellent - I find the 16.5 amp size easily copes with everything the mighty Lucas E3H dynamo on the Norton produces......

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A very good table on the Caravan Chronicles website

 

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3 hours ago, WispMan said:

And power = volts times  amps.

 

So if device states it is 240 watts then 240 = 12 x current in amps.  Therefore current is 20amps. Cables are rated for current. So low voltage DC cabling can tend to be thicker than household mains.

It actually doesn't. If you consider what the OP is asking about it's lighting, do you really think he's going to run 240 watts of lighting? In general 1.5 mm2 cable will be fine for most things 12v, exceptions being motor movers, fridges and charging circuits.

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Whatever size cable you use, please ensure it has suitable fuse in circuit, and that the cable is suitable for automotive use.

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34 minutes ago, AJGalaxy2012 said:

It actually doesn't. If you consider what the OP is asking about it's lighting, do you really think he's going to run 240 watts of lighting? In general 1.5 mm2 cable will be fine for most things 12v, exceptions being motor movers, fridges and charging circuits.

My experience is in the marine world, and often the problem is not whether the cable will take the current load (by the way low voltage DC will "let the smoke out of the wires" at the drop of a hat), but weather the voltage delivered to the device at the end of the cable run will be sufficient for the device to run.  I have a device aboard that draws 20A max, and the cable run from the battery is about 4m in each direction, I used 6.5sq.mm cable...

A calculator is available here.

 

Several years ago the boat had a Wallas diesel cooker that required 8A through the glow plugs to start it up.  The original wiring was small section (2.5 sq.mm max) but ran a tortuous 15m route, and often the engine had to be started to ensure the cooker would not shut down due to low delivered voltage.

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2 hours ago, Guzzilazz said:

My experience is in the marine world, and often the problem is not whether the cable will take the current load (by the way low voltage DC will "let the smoke out of the wires" at the drop of a hat), but weather the voltage delivered to the device at the end of the cable run will be sufficient for the device to run.  I have a device aboard that draws 20A max, and the cable run from the battery is about 4m in each direction, I used 6.5sq.mm cable...

A calculator is available here.

 

Several years ago the boat had a Wallas diesel cooker that required 8A through the glow plugs to start it up.  The original wiring was small section (2.5 sq.mm max) but ran a tortuous 15m route, and often the engine had to be started to ensure the cooker would not shut down due to low delivered voltage.

Volts drop is a significant factor on 12v systems for sure, the longer the cable run the thicker it needs to be.

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