How to choose the right wire size and type

After spending weeks digging and reading, I have finally gotten some information compiled about vehicle wiring.

If I have made a mistake anywhere in here and you are sure, call me on it. This write up is to make things safer, not worse

So here it is:

PART 1


There is a outstanding lack of decent reference material easily available for the backyard mechanic to be able to determine what size and type of wire is suitable for their purpose.


I want to try to simplify things so others can make safe wiring decisions as to what type of wire to use.


First, some lenglthy information on wire, and to try to dispel some myths. I wish I could make it shorter, but you need to know what to get.

IT'S ALL ABOUT TEMPERATURE

What makes a wire hot? All wire has a resistance to the flow of electricity. It takes a moving magnetic field (at the generator) to push a electron from atom to atom. to move that magnetic field takes a outside source of energy. Every time a electron is pushed from one atom to the other, it creates a very small amount of heat. this heat is part of the converted energy of the moving magnetic field. When you take trillions and trillions of electrons moving in a wire, that heat quickly adds up.

That plastic around the copper on our cables really is insulation. but it has two important insulating jobs, very different from each other, but both inextricably linked.

The first job is to insulate the wire from having electrical contact with anything else. This really is a pretty easy job at the low 12 or 24 volts systems we use. even a dry piece of cigarette paper will do the job. So We ask why the thick insulation?

The second job is to insulate the wire from setting anything on fire, while at the same time allowing the heat to escape.

The insulation is to protect the outside environment from that hot wire.


CLASSIFICATIONS:

SAE - Society of Automotive Engineers These are the folks that determine what is the basic standards for wiring (and lots of other things) in automotive use. The SAE types of wire are slightly different than AWG.
AWG - American Wire Gauge. This is what most of us are familiar with from working around the house and electronics. It is also the classification used for marine wire and welding cable

The main differences between the two are in the basic diameter of the copper part of the cable. SAE wire is slightly smaller for the same gauge, but they both are rated to carry the same current. Some SAE wire has less strands (the number of little wires in it) and that can make it much stiffer. On the other hand, when getting to really large sizes like 4/0 (4 ought or 0000), the SAE starst getting strand counts close to AWG, to improve flexibility.

There are other classifications, such as used in the railroad industry, or oilfields, but we don't usually need to deal with them for our vehicles.



INSULATION

After that, The differences mainly come down to the Insulation. This is where most of us get tripped up. There are a plethora of different insulation types, thicknesses, and temperature ratings. It is enough to confuse anyone. To top it all off, when we go down to the local auto supply warehouse chain, the packaging does not tell us if it is good enough for under the car, exposed to the sun, run through engine spaces, or what load it really can carry for a given length.

The reason the insulation is important is primarily due to heat, and then environmental factors such as abrasion and oil resistance.

Every wire carrying current creates some heat. There is a constant amount of resistance to the current flow for every inch of length and every given diameter. If the amount of heat is low, then the insulation can release it to the air without harm. If the heat is high, then the insulation may not be able to release it fast enough and the wire starts getting hotter and hotter. More current (amps), more heat. Eventually, with increasing amperage, the temperature of the wire gets hot enough that the insulation starts to break down, and then finally melts and then burns.

Now that is a situation that we ALL want to avoid.


Each type of insulation has a temperature rating based on the material it is made of.

------Marine wire and cable:
This wire, at least most of it you will find outside of industry, uses PVC soft insulation. Per standards, it is all rated to carry a given load without damage to the insulation at -40 to 105 degrees Celsius (221°F). You will see in a bit that marine wire does have some uses in the automotive field, as there is a SAE classification that has the same insulation in battery and primary wire. Very resistant to chemicals and oil. The main claim to fame for Marine wire is that each of the hundreds of strands in it are tin coated to provide outstanding corrosion resistance. If you have ever changed a terminal on a wire and seen where it is green or gray from corrosion, then you know why the boaters want corrosion resistant wire.


-----Welding Cable:
This wire uses a rubber or synthetic rubber (Thermoplastic Elastomer or TPE) insulation. Most often rated at 600 volts and -50 to 90°C (194°F). Some specific brands are rated to 105°C.

Some types also have double insulation jackets. A thinner inside insulation of one color, and a outside thick layer of another color. This allows you to visually see when the insulation has been abraded away and it is time to get a new cable. This is used a lot in industry for exposed cables. Railway cable for example.

The rubber insulation types should never be used in automotive or marine locations. They are very susceptible to chemical attack from oil, salt, and gasoline. The TPE insulations might be useful, but be very careful as to the specific specifications of the insulation.

When you hear of someone talking about how their welding cable softened under their car, you will know they picked a rubber insulated type.


-----SAE Wire and Cable:
Now this is where it gets more complex. We describe the wires more by the type of insulation rather than the size:

PVC SAE and AWG primary (small gauge) wire is readily available at the local parts stores with PVC insulation.
But what is the insulation type and temperature rating? It is usually not labeled for the cheap wire. Some of these wires can have temperature ratings as low as 67°C. Using these types of wires, you should keep the amperage very low, and use them mostly to light small bulbs and operate relays. (In my personal opinion, The only place these unlabeled wires should be used in a vehicle is in plain sight, with your hand on a fire extinguisher.) Never use them under the hood or where they might be exposed to abrasion.
The exception to this is if they are labeled with a 80 to 105°C rating. 80 degree C used inside the car, 105°C under the hood. The labeling will usually be found printed on the spool and on the wire.


GPT insulation: (General purpose Thermoplastic Insulation) PVC. Rated at 80°C.
Suitable for inside the vehicle use. protect from heat or major bundles. This is the minimum primary wire you should be getting for radios, relays, gauges, dash wiring. roughly equivalent to marine wire, just not tinned and NOT 105 degree C rated


HDT insulation. Same as GPT but much thicker to deal with abrasion and impact.
Good for under the car.



The following are SAE classification Cross Linked insulation wires (PEX or XLPE). Cross linked insulation is Cross-linked polyethylene. It is high on abrasion resistance, and is rated at 125°C (257°F). That temperature rating is for automotive and marine use only. The insulation is a bit stiffer, but it is suitable for any wiring on the vehicle. It is the specified insulation for high amperage cables like starters. Because of the higher temperature rating, Cross-linked insulated wires can carry higher current levels at the same size.

Primary wires
Wires used for distribution and signals from relay boxes, lights, switches, etc. size 22 gauge to 8 gauge.


SXL insulation (primary wire). This is the standard Cross linked insulated wire. Used a lot on Tractor trailers, it has the thickest insulation, making it more abrasion resistant. Great for under the vehicle.

GXL insulation (primary wire). Thin Wall. Thinner than SXL insulation, suitable for all wiring in automobiles. The thinner coating makes it slightly cheaper and more flexible, but still tough for under vehicle use.

TXL insulation (primary wire) Ultra thin wall. Thinner than GXL, but still plenty tough for under the hood and inside the vehicle. Takes up less space in a tight bundle of wires. Used for general circuit wiring by all the auto manufacturers. There is probably a pile of this in your car.



Battery Cables:
Wires for main power distribution from alternator, battery, etc. Sizes 6 to 4/0 (0000)


SGT insulation (Battery cable) PVC. 80°C. (try to use the ones rated for 105°C) This can be used anywhere under the hood of a vehicle, if properly sized and protected. Due to temperature deratings, (I will get to that later) the size of the wire chosen can end up much larger than cross -linked insulated wire for the same amperage

SGR insulation (battery Cable) PVC (EPDM )105°C Good anywhere under the hood and under the car. Thick insulation self extniguishing. Similar to marine insulation.

STX insulation (Battery Cable) Cross-Linked. 125°C. Good anywhere under the hood and under the car. Thinner insulation than SGX. Excellent for all purposes.

SGX insulation (Battery Cable) Cross-Linked. 125°C. Good anywhere under the hood and under the car. Thickest of the Cross-linked insulations. Excellent for all purposes and excels in abrasion and exposure.
In sizes 1/0 and larger to 4/0, this wire also has the added advantage of more strands, making it much more pliable and easier to fit in tight places and bends. For example, 2/0 SGT and STX wire has 133 strands. 2/0 SGX has 1254 strands. Marine 2/0 cable, renown for it's suppleness has 1330 strands.



Whew! That was a lot to absorb! But you can see that knowing which ones are available and their properties will help you make a informed, safe decision.



How to Choose a Wire Insulation:


So, the first thing to ask yourself is where will the wire be used? In other words, what will it be exposed to? Engine compartment heat? Salt and rocks under car? Abrasion?

Pretty much it comes down to this:

If it is primary wire only inside the vehicle in a protected location, you can use PVC. (GTP, HTP)

Primary wire under the hood or inside the car, use TXL.

Primary wire used under the car, Use SXL or GXL .

If it is battery cable used inside a vehicle, use any of them, but oversize the wire to keep the heat below a maximum of 80°C. and fuse the wire at the source!

If it is inside the vehicle and in the engine compartment, use any of the types, but the Cross linked is preferable in nearly any situation. Smaller size, slightly higher cost, better abrasion resistance.


Starters and heavy draws Use SGX. You can use any of them, but SGX is king for abrasion, heat, and oil. Why save a couple of dollars for this?

Under the vehicle: Any of the cross linked will work fine. They are tougher for the job. If you use SGT or marine (PVC), Protect it in a loom or by careful placement.



Next: Ratings and how to determine size.




.....

 

Wow, Bion, thats alot of info there, well done, Gigger..........reps sent

 

Great Post! Good Job! Rep sent, and this should be made a "sticky" in the electrical Forum!

 

I'm in rep jail for Bion but people need to make it rain rep points down on Bion for the info in this thread.

Stewart

 

Thanks, folks.

I hope to get part 2 finished tomorrow.

 

Great post, thanks for the info.

 

Reps sent ,great info,thanks.

 

Part 2

Much as I want to get to determining wire size, first we need to have a grasp of ratings.


RATINGS, SHMATINGS....

We read and are told the temperature ratings of different wire insulations. So what is a rating?

Insulation Ratings:
Every type of insulation will have temperature points where it will become brittle and crack from cold and points where it will melt, char and then burn. After testing, the engineers determine each of those points. They also will test to see the dielectric ability of the wire to see what voltage it takes to break through the insulation.

Armed with all that information on when a insulation will break down, they can determine what the maximum SAFE points will be.

It gets a bit more complicated than this, but those points are essentially the real safe ratings. every rating will have a conservative factor, meaning a point that is well within the safe zone that is the "declared" rating. This provides a safety buffer, and is usually determined through experiments and trial and error.

Below is a fictional line graph I made up to show how it works.




Elecrical Ratings:
Each insulation will have a electrical rating. This is the maximum safe voltage and frequency that it can block. The higher the voltage, the more chance the electrons will jump from the wire to another source of ground. The higher the frequency (in AC circuits) the greater the tendency to jump or dance to the other source of ground. The ratings are somewhat conservative, to make the same kind of safety factor as with heat.


Current Ratings:
Wire thicknesses determine how many electrons, or current (amps) can run through them. It seems some folks think that the bigger the wire, the higher the voltage. Any wire can carry extremely high voltage, but it is size that flows the current.

To use the age old example of water in a pipe, consider voltage as pressure and current as flow volume. You can have 20, 000 psi of water in a 1/4 inch pipe, but to flow large amounts you would do better with a 3 inch pipe.


The higher the current, the more heat, and any wire, given a high enough current flow, will reach the melting temperature of the metal it is made of.

So, to get a higher currrent (amps) for a given voltage, we need to increase the diameter of the wire so it does not heat up enough to melt or even damage the insulation. (fuseable links are an exception to this. they are a special wire designed to melt and fail if too much currrent goes through them. )

Scientists determine the amount of current that a metal can carry based on a standard temperature and pressure (STP) of the surrounding air.

You can find charts on the internet that will tell you the maximum current that wire of various gauge size can carry.

One thing to be aware of. The charts may or may not have a temperature listed. This is important and leads us right into the idea of DERATINGS.


Deratings:
Remember the Idea that "It's All About Temperature"? Well, temperature affects a wire in another way, too.

When a metal gets colder from the outside air, it has a higher resistance to electricity. It makes more heat. This works out pretty good, as the insulation does not have to work as hard to blow off the excess heat to the air.

But, as the ambient temperature around the metal gets hotter, the insulation has to work harder at blowing off the heat to that air.

For example, a wire has a known maximum amount of current that it can flow safely continuously at a air temperature of 20 degrees C (68F). If we increase the air temperature to 100 degrees C (212F), that means the insulation cannot release the heat as fast and heat begins to build. The insulation will eventually melt and fail. So how to turn down the heat in the wire? Reduce the current!

This means that at higher temperatures, wires have to have a different current rating. This derating is usually expressed as a percentage of the standard rating.

To make it even more interesting (!) each type of insulation can release heat differently, so each insulation has a different derating percentage.

You will see deratings listed as single conductors, such as the example above, and then various combinations of multiple conductors in a tight harness. The reason for the combinations is that each wire creates heat. put all those wires together and they keep each other warm, increasing the heat on each wire.

For battery cables and primary wire used in our vehicles, you usually see two ratings. The primary rating, and then the engine space rating. The engine space rating is the DERATED rating due to the higher temperatures in a engine compartment.

These current ratings are available from the manufacturers of the wires. Sometimes you can get them from large suppliers. They show the MAXIMUM current the wire can flow. Try not to use the maximum, but to go somewhat below this, maybe 20% less.

looking around the internet extensively, The absolute best simple general guide charts I have found for temperature and ratings are available at two locations:

This one shows the standard and the engine compartment ratings. At the bottom of the catalog. Download a copy and save it for future reference. The catalog is a very large download.

http://www.colemancable.com/industri...trial-2010.pdf






The above charts from Coleman Cable are available as pictures in my gallery

https://www.ford-trucks.com/user_gal...id=245525&.jpg

https://www.ford-trucks.com/user_gal...id=245526&.jpg


Or to read it MUCH better, as a pdf available at this link:

WIRE TEMPERATURE CHART.pdf - PDF Archive


This one from CustomBatteryCables.com shows the engine compartment ratings:

Technical data



Voltage Drop.
Going back to the idea of water in a pipe, with pressure as the voltage and amount of flow as the current, think of voltage drop this way: The longer and thinner a garden hose, the less and less pressure (and flow) you end up getting at the end of it.

The longer and thinner the wire, the less and less voltage (and current) you get out of it at the end.

Wires also need to also be sized according to a voltage drop amount. For most electronic circuits and lights, 3% is the generally acceptable amount. (.36 volts in a 12 volt system)

However, to save some money and weight, many times a manufacturer sizes a wire to have 10% voltage drop (1.2 volts) for such items as moters, compressors, etc..

Now keep this in mind. In the marine world, where keeping heat on wires down is paramount, they always use the 3% voltage drop. Why?

Again, it its heat and better performance. The more voltage drop means more resistance in the wire and that means more heat. Less voltage drop is less heat.

Less voltage drop at the end of the wire means less heat that the motor makes, too. It means better sound performance in audio power lines, and brighter lights.

I urge you to make sure your wiring has no more than 3% voltage drop. It makes for bigger wires, but better performance. It does not usually affect short runs under 5 feet, but some runs we make can be as long as 20 feet.

To check your voltage drop, use one of the available calculators on the internet.

One I like is Voltage Drop, Power, Battery Calculator.

Another very powerful wire size calculator is from Blue Sea systems

Blue Sea Systems

If you have too much voltage drop, go to a large enough wire to keep it at 3% or less.




Choosing a Wire size:


Sure, you could pick the absolute minimum wire size from the above charts to carry your load. But is that a good idea?

The primary thing to keep in mind is heat. A larger wire will be able to dissapate the heat better in any situation. If it is a wire that carries a load of a seat motor or window motor that is used for just a few seconds no matter what, then getting the minimum size wire would probably be fine.

However, if it is a wire that would be carrying constant loads that would allow heat to build in the wire, like lights, inverters, compresssors, radios, amplifiers, then it is a good thing to choose larger wires to prevent them from getting too hot and possibly damaging things they touch, like other wiring, looms, interiors, etc.

This is especially true if the wires are going to be bundled together or have insulating items like loom, tape covers, or sleeves around them, buried under a carpet, or placed inside a pillar or seat.

For example: you want to carry a 80 amp load to a amplifier. so you choose the minimum wire to carry this, a 125° TXL wire size 8. That wire operating for a period of some minutes or hours time could reach temperatures of over 200° F, especially if it is in covered. Would you want that to be inside the car buried under a rug?

In this case, it may very well be better to choose a wire with lower temperature rating, (which causes it to be a larger wire for the same amperage) just to keep the chance of extreme high temperatures down, and therefor the risk of fire.

One concept that I follow is to size my wires using the 105°C chart, but use 125° insulation

Going up a size never hurts, and brings a extra safety margin for these constant current loads.

Cool wires are safer wires, and always check your voltage drop!




Intermittent Current OverLoads.

In all the discussion above, there is one huge assumption. That the circuit will always be flowing no more than the maximum it is rated for. Lights, relays, wipers, seat motors, etc. are all sized the normal way, to carry the load safely and constantly.

But we all know this is not the way it works in some situations. Manufacturers of winches are notorious for providing wire that is so small that it seems that it could not do the job, but yet it does. The starter wire from the battery is another example of a wire that beggers the imagination, but it works fine.

So what are they trying to do? Save money at our expense? How can this be safe, having a wire on there that can only safely carry 250 amps according to the chart, when the winch or starter can pull 500 to 600 amps?


Again, It is all about Temperature, and, in this case, Time.

As the current flows through the wire, it makes heat. This heat is created and builds up over a period of time. So for a certain period while the over current is flowing, the wire and insulation is operating within the safe zone. The greater the over current, the shorter the length of time before insulation failure.

With certain things like starter motors and winches, they too will make heat from the current flow and the heat will build in them. They also will have a temperature failure point and stop working until they cool down.

So here is where the dance begins. Manufacturers, in order to conserve money, materials (copper) and to keep the wire sizes smaller, size the wire and insulation type so the wire will get hot but will still be in the safe zone at the time the winch or starter fails.

Let's face it, a wire rated and operating at 125° C (257°F) is a really hot wire. You get instant burns from touching it, but the wire is at the top of the rated safe zone. If the wire is sized just right, the winch or starter will already be at a MUCH higher temperature and stalled, failed, and you have hopefully turned them off. I guess you can say that the winch or starter is the fuse to protect the wire!




Now, finally, we can determine what wire and size we would need for a given situation.

Who knew so much engineering time and effort goes into creating a simple (!) wire.

Thanks for reading!!!

Bion Rogers

 

You folks think I should post this in the electrical and wiring forum too?

Can I post it there since it is already posted here?

 

If you ask a moderator, they can make it a sticky in the electrical file

 

Most definitely! This should be posted in the electrical forum where it will help the most people.

Thank you for all the work you've done on this!

 

Bion ,thank you for taking the time to post this information. Bookmarked and reps sent!!!

 

Let's Sticky this!

 

Thanks everyone. I'm really glad that you like the thread. I hope it can open some light for folks (especially the young ones) about the mysteries of auto wiring.

While I was researching, I saw so much stuff written on blogs and forums that was either wrong as heck or just downright dangerous. Quite a few were the blind leading the blind. I remembered when I was young I had no Idea what to do with wiring and just took any advice that was given by anybody. How many times that advice was dead wrong!


As to posting on the electrical forum, I sent Franklin2 a pm asking about it.


Thanks!


Ps. I'm finally done with any revisions. I added a section on voltage drop too. I had forgotten to add that one!

 

I'm sure the mods will put the info in the electrcal file Bion, Well done