Is there way to figure out amp carrying capacity of a circuit?

[tortillasoup]

For example, it's pretty well established that a 14ga wire has a 15a carrying capacity for 120vac before it becomes a problem for that circuit. That you should use 4ga battery cables for jumping your car or that you should not use a 14ga wire for a 20amp 120vac circuit because of voltage sag and overheating concerns...

So here is the question. In the case of an electrical ground like that found in a car, you take a multimeter and read the ohms value and it reads 0.00. That looks good, right? But how does one know if that 0 ohms reading is being conducted through the equivalent of a 30ga wire or an 8ga wire? How does one know the load carrying capacity of that electrical ground? Sure you could measure the diameter of the wire and all but what if you can't? What if you know the wire should be sufficient but you can't tell if the wire is compromised or damaged somewhere along the line? What then? In short, what is a quick way to determine the load carrying capacity of a connection without damaging anything?


I found the following pages to be useful and relevant but I'm still at a loss as to how I can answer my question.
http://www.powerstream.com/Wire_Size.htm
http://www.rowand.net/Shop/Tech/WireCapacityChart.htm

 

[Modelworks]

You cannot use an ohms measurement on a cable to determine its capacity. Meters are going to load down the cable with just a few volts at very low ma , not enough to determine the capacity.

The way to determine a wires capacity without damaging it is to place it under load. Connect the wire to a voltage source and on the other end you place a resistor between the wire and ground, 10K resistor is pretty safe for most common DC voltages and wires.
At the power source measure the voltage then measure the voltage at the resistor. The difference in the voltage is the voltage drop caused by the wiring. Generally the large the wire the less that difference in voltage will be.
Loose connections, corroded cables, will have higher resistance and cause a greater voltage drop.

If you wanted to determine if your wire is sufficient for the load you would lower the resistance value, picking the value based on ohms law, if the cable does not drop below what you consider usable at the resistor end and the current does not heat the cable then you know that the wire is sufficient.
Careful using too low a resistor on a small cable, you can melt the wire. The resistor also needs to be able to handle the power you are placing across it. Use wire wound resistors for something like a car. They look like square blocks.

example:
I=V/R

13.8VDC (car) / 10 ohm resistor = 1.38 amps through the wire
13.8VDC * 1.38A = 19 watts, would need a 20watt resistor to test a wire.

 

[William Gaatjes]

Once you have determined the resistance of the wire, you can calculate it to verify your test setup and measurements :

The resistivity if the wire is made of copper is typical 1.68 * 10^-8 Ohm m. But this may be for pure copper. Copper used for standard electric wire might have impurities. These impurities increase the resistivity.
That is probably the reason why this number is more common :

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html

A table for standard wire sizes :

http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/wirega.html#c1

 

[Mark R]

The current capacity of a wire depends on many factors:
The resistivity of the conductor, it's length, diameter, shape, insulation thickness and material, mounting method, type of overcurrent protection, ambient temperature, etc.

The current capacity of a 14 AWG cable varies greatly depending on if it is copper or aluminium, whether it is PVC insulated, silicone insulated, teflon insulated or glass insulated, whether it is packed with multiple other cables or free in air or mounted to metal or mounted to an insulating surface.

The current capacity also depends on the type of short-circuit protection: the continuous current rating of a cable must be reduced if a high-short circuit current is possible; a faster fuse or circuit breaker may allow the continuous current rating to be increased, by reducing the sudden burst of heating during short-circuit usage (similarly, if the current source is inherently current limited, allowance does not have to be made for short circuit current heating).

At the currents you are talking about, measuring resistance is not a useful test, as it cannot identify damage (there are too many variables). Current capacity is ensured by using good quality workmanship, ensuring connections are made using high quality connectors and calibrated tools, and protected from corrosion, and regular inspection for damage.

 

[Gibson486]

Nfpa30

 

[Jeff7]

A handy utility: Saturn PCB Design Toolkit

Enter in the copper thickness (by weight), the allowable temperature rise you'd be willing to live with, the trace width, and a few other parameters, and it'll tell you the capacity of the trace.