G’day! I’ve recently put together a couple of basic videos on your 12 volt system. Dual batteries, DC-DC chargers
inverters, solar and isolators
and I wanted to get a little bit technical on something that a lot of people ask about and a lot of people get wrong.
And that is wire size.
Automotive wire is described or
marketed with a different set of rules compared to electronics cable, while auto wire is described based on its overall diameter
Including insulation many installs call for a certain wire gauge.
So if you'd headed to the parts store and picked up a length of six millimeter twin core automotive wire
This is what you'd end up with
Now the interesting thing about this is the overall diameter of the wire including the insulation is about 4mm
The diameter of the wire itself is about 2.4mm
Which gives you an area of about 4.59mm^2
If you wanted 6mm^2 squared cable you'd need to step up to about a nine gauge
Which gives you about 2.9mm diameter?
So you can really easily see the difference here between the "6mm cable" and the 6mm^2 cable
but if your installation calls for six gauge cable,
6 AWG or 6 B&S you need to step up to something this size
that would give you about
4.1mm of conductor diameter and about 13.3mm^2 of area
and finally if you were using actual 6mm diameter, it would be this big
Six millimeter diameter will give you three gauge wire and here's your really easy comparison
This is called "6mm wire"
This has an area of six square millimeters
This is actually six gauge wire and this has a wire diameter of six millimeters
no wonder it can be so confusing
And there's one other thing you need to look out for and that is the thickness of the insulation
Check this out
These wires look about the same thickness, but check out the difference once you strip that insulation
Each of these wires has a certain amp rating that you shouldn't exceed around 40 amps for the "6mm" automotive wire
About 65 amps for the nine gauge
About a hundred amps for the six gauge and about a hundred and fifty amps for the three gauge
As we said before the maximum Amp rating for 6mm
Automotive twin core wiring is 40 amps so you might be tempted to use it for something like a 20 amp DC DC charger
But you have to take into account that's its maximum amp rating at 25 degrees ambient and over a certain length
Double the length and you double the resistance and for every degree above 25 degrees ambient resistance increases as well
So if this wire was at 45°C, it would be an 8% increase in resistance
So it's important to know the amp rating of the wire gauge you're using but it's also important to remember
voltage drop. The thinner or longer the wire you're using the more resistance and the more
Resistance in the wire the more voltage drop that's why it's so important to use the correct gauge of wire and not add any unnecessary length.
I'm going to use my DC power supply to run a pair of powerful 12-volt spotlights
Then I'm going to use two different gauges of wire and demonstrate the massive difference in voltage drop between the two
Now this can be set to exactly 14 volts and it'll stay there
I'll connect up the thin wire to the power supply and then to the driving lights. Then I'll use my
multimeter set to voltage to measure the difference between the start and end of the wire.
So you saw there
There's about 14 volts coming out of the power supply and we're losing around 2 volts through the circuit at the start of the test
By the end of the test we were nearly at 3 volts because this wire was heating up
increasing the resistance and sending more current through.
Then I'll connect my thick wire the same way, and keep in mind these are the same length overall
Again, I connect up the positive wire attach my multimeter and start the test this time the voltage drop is a
tiny 27 millivolts, about a hundred times less than the thin wire.
The Amps was so much higher in the first test because the resistance in the thin wire was drawing much more power and
converting it into heat. In a dual battery system that's just wasted energy that could have been used to charge your second battery
So in that test you saw we were only losing 27 Millivolts (0.027V) along this wire
That means that the amps didn't climb because the resistance wasn't there and the voltage wasn't dropping
You can estimate voltage drop using some pretty simple maths
if you want to check your own installs too.
Times the current of the appliance in amps by the length of wire to the appliance in meters
by 0.017
That's the resistivity of copper and then divide that number by the area of the wire in millimeters squared
So for example a 20 amp DC DC charger over 8 meters of wire
times 0.017 then divided by 4.59 which is the area of the "6mm" automotive wire
gives you 0.59 volts of voltage drop
Then you need to consider the length back to the first battery on your earth return
Double your answer for total voltage drop. That's 1.18 volts
You can adjust the above equation for temperature too if your wire is at 25°C use 0.017
but if it's more like an ambient of 30 degrees use 0.177
for 40 degrees use 0.0184 and for 50 degrees use 0.0191
How hot do you think the underside of your vehicle gets while you're working hard in low range?
At 50 degrees you get 1.33 volts of voltage drop. The hotter it gets the more voltage drop.
So if you think "6mm" automotive wire with its max Amp rating of 40 amps is enough for your 20 amp
DC DC Charger then think again
You should aim for less than 3% voltage drop in your system
Three percent of 13 volts is 0.39 volts
at 50 degrees Celsius
This example is already over 10%. That voltage drop will seriously affect the performance of your charging system.
Finally, if you're planning to run your earth back through your vehicle's chassis and body and you might think you've got plenty of metal and
Area to play with you have to be aware that steel has about ten times higher resistance than copper
You never know quite how well it's all connected and it makes it very difficult to calculate the voltage drop
So if you want to really efficient and effective 12-volt system
Make sure you run a heavy-duty wire all the way back to the earth terminal of your battery
Remember back to part 1 where we talked about your standard alternator that can put out about 14 volts
the obvious problem with voltage drop is that if you're running power from the front of your vehicle all the way to the back and
You're losing one or two volts by the time it reaches the battery in the back. It won't be high enough to charge
The problem is made even worse if you've got a temperature
compensating alternator while it's dropping the voltage for the battery under the bonnet to suit the ambient temperature down to say
13 volts if you're losing one or two volts between the front and the rear of the vehicle
You're already down to 12 or 11 volts way less than you'd need. That's why it's so important to use the right gauge of wire.
The final thing to get into before you start running wiring through your vehicle is safety
Running a positive cable from your battery requires a fuse or circuit breaker as close to the battery as possible
So if you're running a dual battery system and connecting both batteries
You'll need a fuse at both ends.
In an electrical circuit the fuse is designed to blow or melt when a certain current flows through it
The fuse is designed to blow or melt when a certain current flows through it
They're labeled with an amp rating and available anywhere from milliamps to hundreds of amps in size
you need to match the fuse size to the circuit or accessory that it's protecting
If your driving lights need 10 amps to light up and you put a 1 amp fuse in the fuse will blow
before the lights illuminate
on the other hand if you use a fuse that's too large on thin wire the wire will melt before the fuse blows and
That's not good. Here's an extreme example
I'm putting a three hundred amp fuse on this thin wire here and short-circuiting it onto a battery
Short-circuiting this battery means it's able to put out three thousand amps of current
So be careful, whenever you're tightening battery terminals, bridge the terminals and you'll definitely see some sparks
the other thing to be aware of is any jewelry, whether it's rings, watches, bracelets or necklaces because if they arc out it'll hurt
and while you're hooking everything in your dual battery system together take the fuses out. That way nothing is live while you're working
Let's look at a basic dual battery system that I've mocked up without fuses
So we've got our main power lead and a basic isolator connecting the positive terminals
and we've got a separate earth lead plus our starter battery is earthed to the vehicle chassis
Imagine the main positive lead was wearing on a sharp part of your vehicle's chassis or body and eventually it
contacted, without a fuse that current would be free to go through the whole circuit heating up the battery
damaging your 12-volt system and potentially your vehicle
That's why you need a fuse at both positive terminals of both batteries. If there is a short circuit it'll blow and protect your vehicle
that's why it's so important to use the correct wire gauge account for any voltage drop and
don't take any shortcuts as your 12-volt system will suffer
Plus make sure you're protecting yourself, your vehicle and your 12-volt accessories with the correct fuses
Use the right gear and you'll get years of enjoyment out of your 12-volt setup. As always if there's any questions
make sure you throw them in the comments below.
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