Welding cable ampacity, also known as amperage capacity, current capacity, or amp ratings, refers to the maximum amount of electrical current that a cable can safely conduct.
Amperage, also known as current, is a measure of the number of charged particles (coulombs) moving past a fixed point in one second. Another way to think of it is that amperage is the measure of the volume of charged particles moved per second relative to some fixed point, or the volumetric flow rate through a conductor.
Different welding cables running on the same voltage (e.g. 600V) will have different amp ratings depending on several factors including cable length, gauge, insulation temperature rating, and what kind of machine it is connected to.
The three main factors to consider for any electrical system are voltage, amperage, and resistance. Just like how friction is the resistive force of motion between any two objects (unless an object is moving through a vacuum), the force that resists the flow of electric charges passing through an object is called electrical resistance.
This means that you will have more energy losses in welding cables with higher ohm ratings when transferring electric power, since energy is wasted to overcome resistances in the cable. Furthermore, like what happens with friction, the energy used to overcome electrical resistance becomes heat. Essentially, this means that the more amps you conduct through your welding cable the hotter the wires will get.
The higher the ohm ratings of the welding cable the less amps you will be able to safely run without overheating the cable. Welding cable ampacity also depends on the temperature ratings of the insulation material. The copper wire itself can handle the high temperatures generated by higher amperages, but the insulation protecting the copper wires will melt long before the wires themselves sustain any damage. See welding leads durability for temperature ratings of EPDM and Neoprene rubber, commonly used for welding cables.
These are all factors that directly affect how much electrical resistance you will have in your circuit. Welding cable is made of stranded copper wires and are typically round in shape. What affects your welding cable ampacity are cable gauge, which rates the cross-sectional area in stranded wires, and length.
Higher temperatures increase electrical resistance. The copper wire itself will be much hotter than the ambient temperature: wire charts are often listed with 105° C conductor and 30° C ambient temperature ratings. However, ambient temperature affects the ability of the cable to dissipate heat into the surrounding environment.
Like the reasoning for ambient temperature, multiple cables lying too close to or on top of each other dissipate less heat than those with more space around them.
Overloading your welding cable, also called thermal overloading, means running more current through the cable than the rated welding cable ampacity allows. This causes overheating and can deteriorate or melt the insulation, leaving exposed wires that can get damaged or cause electrocution.
Unless you have a superconductor, you cannot have amperage without voltage. Voltage, also known as the electromotive force or potential drop, is the force that causes charged particles to flow, or "draws" out the current. Like distance, voltage is always measured between two points. Thus, to measure voltage at a single point there must be a common reference point, which is called "ground" by convention and always labelled as having 0V. In reality, any point in a circuit can be called "ground," but for simplicity's sake most people choose ground to be the negative side of a circuit.
In terms of welding cable ampacity (and power distribution in general), higher voltages allow for thinner, lighter, and cheaper cables. This is because the amount of energy carried per charge (coulomb) in high voltage currents is greater. For example, if we think of height as voltage (measured from the floor and "ground" respectively) and mass as the current, then a 1 lb rock falling from 10 meters high will carry more energy than a 1 lb rock falling from 5 meters. To get the same amount of energy from 5 meters as from 10 meters you would need twice the mass, and this analogy is so helpful that the voltage difference between two points is often called the voltage drop. Thus, higher voltages allow for lower currents, because each charge carries more energy, that in turn allow for longer and thinner cables.
In fact, as the voltage increases the amperage MUST decrease to avoid damaging your cables and your machine. See the equation below to see the relationship between how much energy is used in a given period of time (power), voltage, and amperage.
Welding cables are typically rated for 600 volts and used for power supply applications not exceeding 600 volts.
Since most power distribution systems deliver power at fixed voltages, we can calculate amperages based on the given voltage and how much power (watts) we need to use to power a machine using the equation P = V x I where
P = power (watts = joules/sec)
V = voltage (V = joules/coulomb)
I = amperage (I = coulomb/sec)
As you can see, using P = V x I, the units for coulombs in voltage and amperage cancel leaving the units for power, which is joules/sec. Thus, if the amount of power used stays constant, doubling the voltage will halve the amperage and vice versa. (For AC circuits the equation is slightly more complicated. P = V x I x PF where PF stands for the power factor, which is always between 0 and 1.)
NOTE: The equation for resistance is R = V / I and power is P = V x I. By increasing voltage and reducing the current, the resistance increases. In order to keep the power constant higher voltages actually require cables with more resistance. This means that higher voltages allow for thinner and longer cables since thicker and shorter cables have less resistance. (This is also why high voltages are used for power transmission.)
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