Does anyone have a table that shows the amp limits and permissible lengths for various wire sizes? What is std practice for sizing fuses? Is the fuse sized by wire limits or demand? Does a soldered connection add to resistance?

Fuses exist to protect the wiring.

Google for an ampacity table for wiring amperage capacity.

Remember that 914 wires are metric sizes, look at the later year schematics for their sizes.

First hit on Google for "wire gauge current rating":

http://www.powerstream.com/Wire_Size.htm

IIRC, the wires in our cars are smaller than that standard calls for.

Fuses are there pretty much to protect the wiring. If something shorts, you don't want the wire acting as the fuse; it can catch fire if it does that. So the fuse blows and stops the current going through.

Fuses are generally chosen for the maximum amount of current you expect to go through the circuit, plus some extra on top of that. (Often a lot of extra, in fact.)

--DD

Kent,
That is a good wire table Dave posted. As for fuses, they are somewhat different depending on the manufacturer. ATC type fuses like I used in my fuse blocks are as follows; 125% of the rated amps continuously, 140% for .5 sec, 300% for 0.1 sec.
If I remember right, I had listed some of the most common loads for our cars and the fuse amperage necessary, you just have to search for that thread.
For the stock Porsche/Vw fuses I don't know what they will allow and did not test any to see what they will allow before blowing.
I also use "offroaders.com for some good info.
Soldering should lessen the resistance of a crimped connection , and keep it lower for a longer period of time. Soldered connections are however somewhat affected by vibration and could cause cracks in the soldered connection if the connections are unsupported, ie: hanging loose and allowed to move around.
Tom

Thx to the Elec Guru's once again. Ampacity, huh? New word for me. I think I'm going to regret this question but what is the difference (or which one should I use) between "maximum amps for chassis wiring" vs. "maximum amps for power transmission"? TIA, Kent

I don't agree with that chart. I used to run a car stereo shop and I've been an electrician for over 25 years.

For an AC circuit, a 14ga wire is good for 15amps. DC actually requires a larger diameter wire for the same amperage. In this case a 12ga wire would be the minimum wire to use for 15amps DC.

This is based on theory and real world experience. This is also based on performance. DC requires larger wire to reduce heat caused by resistance. DC current travels the entire length ow the wire. AC current changes direction 60 times a second.

You can run higher amperage on smaller wires but the added current will cause heat and perhaps a fire or melted insulation.

Automobile manufacturers use the smallest possible wire that will work for the given application. They do not account for age or corrosion. In our cars, the wiring harnesses have reached and passed their shelf life. Cars are not designed to last 40 years. Porsche expected you to replace this car after about 4-5 years. Most original 914 owners did so. All factory warranties are expired so there is no more liability.

Using a larger wire for a circuit is a kin to adding more horse power to your engine. You car can still do 60 mph but the added hp makes it easier to get there. Larger wire has less resistance to current flow and equals better performance. This is especially true when talking about grounds.

Most grounds are undersized by design; Poor design. When our cars were new and had NO rust, the entire chassis was a good ground plane. This allowed for smaller ground wires to be installed. Now our cars are rusty corroded steel held together by spot welds. The ground potential from the headlight to the battery is no longer what it once was. Add in the age and corrosion within the wires and you have very poor performance in your electrical system. Mos t corrosion cannot bee seen without cutting into the wire. If you have ever stripped an old wire and the copper is black or green, this is corrosion. It actually uses up voltage to pass the current past the corrosion. This is a voltage drop. VOLTAGE DROPS CAN ONLY BE MEASURED UNDER LOAD. Many electricians make the mistake of measuring voltage drop on an open circuit. This will show the same or close to the source voltage. Example: Measuring the voltage at the starter cable. It shows 12+ volts. Now measure while the starter is cranking. It will show closer to 8 volts. This is voltage drop.

What can be done?
Upgrade the size of the grounds cable and add additional grounding from the battery to the chassis and from the chassis to the engine block.

Up-size the charge wire from the alternator to the regulator and from the regulator to the battery.

These two modifications will allow your car to charge better and reduce voltage drops. You should also clean and wire brush all the ground connections to the chassis.

Any additional wiring added to you car should be a 16ga or larger to reduce heat and promote better current flow. If you wire up an amplifier, use a 10ga or larger. Up-size you starter power wire to a 2ga or larger to help with starting.

My $.02

The big difference between using a smaller wire for power in an automotive circuit is the voltage drop in the circuit. The longer the run, the greater the voltage drop for a particular size wire. Most wiring tables use a 2% voltage drop in their formula for determining minimum wire size, some use a 5% voltage drop.. A step up in size will improve the performance of the circuit, = less voltage drop. Ever wonder where that voltage goes? It is dissipated as heat along the length of the wire. Put too small a wire in and the wire's ability to dissipate the heat will result in a fire.
Also something that should be considered in selecting wiring is where the wire will be used. If in the engine bay, you should upgrade to a wire that handles greater heat and exposure to oils and gas, such as GXL coded wire. Good to 257 degrees and is resistant to oils and gas.
Tom