Anyone know the specifics of automotive wiring?

Camman · May 15, 2005

I was doing some thinking... my limited(read very very very very very limited) engineering knowledge started this... would 2 eight gauge stranded automotive wire have the same amount of strands as one 4 gauge? Or would they actually be able to carry more current/foot? I would get into my theory of why I asked this question, but I might make myself look more of a fool than I already do.
I have plenty of 8 gueage wire, I cant find a place locally that sells bulk stranded(this is the key) wire bigger than 2 gauge... I would not like to use welding cable as it is way to large for my comfort. Thoughts, places my dumbass could look.. any useful info please chime in

h8louzn · May 15, 2005

first off what are you lookin for?
Only thing bigger than 2 guage is 0.
4 guage is smaller than 2 guage.
Now I see you live in vegas correct?
If so go find any stereo shop, as there are like 200 of them in vegas.
All of them should have 4 guage in red and black.
Hope that helps

jrichker · May 15, 2005

See http://www.belden.com/Master Catalog PDF/index.htm for complete info on automotive wire.

You will need the Adobe Acrobat viewer which is also a free download – http://www.adobe.com/products/acrobat/readstep2.html

Michael Yount · May 15, 2005

Regardless of the theory, it would be easy enough to check with a digital volt meter. Simply measure the resistance of a same-length piece of the 4 gauge compared to the double 8 gauge. I'm gonna guess that doubling the 8 gauge results in something with even LESS resistance than a single 4 gauge. I don't think there's a proportional relationship between wire/bundle diameter and gauge size - in other words, 4 gauge isn't twice the diameter, or even twice the cross sectional area of 8 gauge.

Michael Yount · May 15, 2005

BTW - if you're looking at battery relocation, I'd want something even bigger than a single 4 gauge. For what it's worth, I came up with a very economical solution by buying WalMart's 'best' 2 gauge 24' jumper cables. I removed the clamps, and joined the two cables together by soldering them into the battery lug. So my hot 12V+ from trunk-mounted battery to the engine compartment is a double 2 gauge run. 4 years and still working just fine.

Camman · May 15, 2005

Thanks for the help, yes I am wondering because I am going to relocate the battery in the 89. In the 65 I used 2 2 guage wires for the starter, and 2 8 gauge for accesories. It worked flawlessly, however I was hoping to run maybe 6 8 guage wires total for this one, as the smaller diameter wire would be easier to run through the cabin. I have a wierd/not average interior plan for this one so the smaller wire is almost a necessity. Thanks alot for the help

edit: Thanks for the link jrichker.. talk about information overload... now I have to read this whole thing... yay!

srothfuss · May 15, 2005

Wiring Questions

OK, Let me start of by saying that I am a M.E. (Mechanical Engineer) and not an E.E. (Electrical.) So with that being said I must say that I don't think you can take (2) 4-gauge wires and make (1) 2-gauge wire. The reason beign that the same amount of voltage will pass thru (2) smaller wires instead of one bigger wire. Thus causing more heat (read: resistance.) If you build too much resistance, your could have an electrical fire.

Real world example: Power lines. All electrical companies use 1 large bundle of wires to transfer massive amounts of electrical power from station to station. If they don't do it, I wouldn't recomend that you do it either...

silverstripes · May 15, 2005

srothfuss said:
OK, Let me start of by saying that I am a M.E. (Mechanical Engineer) and not an E.E. (Electrical.) So with that being said I must say that I don't think you can take (2) 4-gauge wires and make (1) 2-gauge wire. The reason beign that the same amount of voltage will pass thru (2) smaller wires instead of one bigger wire. Thus causing more heat (read: resistance.) If you build too much resistance, your could have an electrical fire.

Real world example: Power lines. All electrical companies use 1 large bundle of wires to transfer massive amounts of electrical power from station to station. If they don't do it, I wouldn't recomend that you do it either...

AGREED

To add something you can't check the load capability of the wire with a volt /ohm meter. Example you could take a single strand of wire 2/32 in diameter and check the resitance and the meter would read 0 ohms or you could take a single strand 1/4 wire and get the same reading. With little current passing through the wire the resistance will be low but when more current is sent heat with build thus resistance will increase.

Saleen0679 · May 15, 2005

If you are simply replacing the one four gauge cable with two eight gauge cables for the same circuit then you should be alright. The two cables would be connected to the same points on either end and in effect would be the equivalent of one larger cable. Current would divide equally between the two cables. If you think about it, what is stranded cable but many wires along the same path. You would be adding strands just not stuffing it into the same insulation. The two eight gauge cables would also have a higher current capacity than the single four gauge as long as both are working together. One concern would have to be the introduction of more points of failure with the addition of multiple cables and grounding would become increasingly important. If one of your cable connections should happen to fail then the remaining smaller gauge cabling may not be able to handle the load where the larger cable could.

For some cable information check out: http://www.powerstream.com/Wire_Size.htm

Camman · May 16, 2005

okie, I am a m.e. student, but... from what I do know of electricity 2 conductors would be more efficient as one.. unless my thinking is flawed it should have more surface area with 2 8 guage than 1 four gauge... hmmm I think I can do the 2 2 guage from walmart i will give that one a shot. I have alot of 4 guage monstor cable, but to be honest I am not a fan of the monster.

Saleen0679 · May 16, 2005

Camman said:
from what I do know of electricity 2 conductors would be more efficient as one.. unless my thinking is flawed it should have more surface area with 2 8 guage than 1 four gauge...

And you would be correct. As I explained above, all things being equal, the two 8 gauge calbles would probably have a higher current capacity when used together than a single 4 gauge cable used alone.

You would have a little more current carrying capacity with the two 8 gauge than with one four gauge because each cable in the same circuit will carry a percentage of the current. If both paths have equal resistance each path would carry half the current.

For solid wire the equation is simple: Double the diameter = quadruple the cross sectional area. With stranded I'm sure a few more variables come into play.

srothfuss · May 16, 2005

Saleen0679 said:
Current would divide equally between the two cables.

You are correct. I still don't think that you should do it.

Lets go over the basics:

For a system wired in parallel (a pair of 8 gauge wires)

V(total)=V(1)=V(2)
I(total)=I(1)+I(2)
R(total)=[1/R(1)]+[1/R(2)]

For a system wired in series (the single 4 gauge wire)

V(total)=V(1)+V(2)
I(total)=I(1)=I(2)
R(total)=R(1)+R(2)

So in theory you could work these equations to come up with similar answers. We have a few knows and assumptions.
a) They system is 12V(total)
b) Resistance per foot for 4 gauge is 0.000292
c) Resistance per foot for 8 gauge is 0.000739
d) Asumming your need to run an exact 20 feet.

The only thing you have left to determine is the current load of the system.

I(series) = 2054.7945 amps
I(parallel) = 405.953 amps

So the parallel system would offer more resistance and less amperage. So to get the Amps from the parallel system you can alter either the resistance or voltage. We know that changing the voltage is impossible, unless you re-wire your car for a 24 volt system. So we are left with the Resistance as the variable. And from the above equations, we know that R=0.000292 will give us the higher amperage. I still don't recomend that you do it.

baskin · May 16, 2005

That's about the most convoluted answer I've ever seen to a question. The bottom line is that you'll need three 8 gauge wires to replace one 4 gauge wire, it will actually work slightly better with the multiple strands since the higher surface area will allow better heat dissipation.

The battery resistance is typically about .015 ohms, so 20% of that resistance in the wiring should work fine. On a hot day, the cranking current will raise the temperature of the wire, assuming a maximum temperature of 125 degrees Celsius, the resistance will go up by a factor of 1.6 over the chart resistance with that temperature.

So, let’s assume 150A cranking current, the battery voltage will drop to about 10.5 V, for a 10% loss of power to the starter, we can allow a .5V drop or a total resistance of ,0033 ohms. At 125 C, 8 gauge wire is .020 ohm per 20 feet, you’ll need 6 of them for the positive run, 6 for the negative run.

Michael Yount · May 16, 2005

I'd add to Baskin's comments - I think the problem with the calculations is that when you bundle multiple wires together (by soldering the ends) - it's not a "parallel" circuit; it's the functional equivalent of one larger wire bundle.

baskin · May 16, 2005

They'll share current pretty well since, if there's any initial imbalance, the wire carrying the most current will heat up which will raise it's resistance. So, even though they're separate, they all end up running the same temperature and act like a single wire The dual 2 gauge loss is less (.35V), but the wire's got a slightly less surface area so the difference between the two will be less than the apparent difference.

srothfuss · May 16, 2005

So if (2) 8-gauge wires are acting just like (1) 4-gauge wire, wouldn't it come down to surface area of the wire?

If you send voltage across 2 wires then you divide the current....But if the wires terminate on the same terminal, do you add the current back up? Forget about Voltage Drop.... I want to understand why it's a good idea to split wires into smaller bundles. The voltage thru 2 wires from 1 source is equal. So why don't more people do this? (like the power company)

Michael Yount · May 16, 2005

stroth-no one said 2 8 gauge wires act like one 4 gauge wire....I think Baskin determined that 3 (three) 8 gauge wires are close to acting like one 4 gauge wire. And if you look at your resistance values - you can see why.

srothfuss · May 16, 2005

Thanks! I am just really confused by all of this electrical stuff.... I should just keep my mouth shut and stick to all things mechanical.

baskin · May 16, 2005

srothfuss said:
So if (2) 8-gauge wires are acting just like (1) 4-gauge wire, wouldn't it come down to surface area of the wire?

If you send voltage across 2 wires then you divide the current....But if the wires terminate on the same terminal, do you add the current back up? Forget about Voltage Drop.... I want to understand why it's a good idea to split wires into smaller bundles. The voltage thru 2 wires from 1 source is equal. So why don't more people do this? (like the power company)

Surface area represents the circumference x the length which determines cooling, cross sectional area is what determines resistance or, given that cooling isn't considered, current capability. The single benefit is that the circumference per cross sectional area is higher on seperate strands, so the cooling is better

The current will split proportional to the inverse of the resistance of each path. If one wire starts to heat up, its resistance will increase, shifting the current over to the other wires. It's not commonly done because it's more expensive.

crazypete · May 16, 2005

I built a custom battery cable by running 6 or 7 (forgot) 12 guage wires together, stripping the end off and merging them all together into a big stick of copper wire, to which I affixed the battery terminal clamp. There was MORE copper there than on the factory batt cable and it served me well through the years.

Anyone know the specifics of automotive wiring?

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