So since we're talking about the expansion of knowledge here, let's get down to the fundamental physics to start with... (all of this is metric and 240v based...)
We know that any material at room temperature will have a resistance that is governed by it's resistivity, it's cross sectional are, the length and the temperature. R = rho*l/A, where rho is resistivity, l length and A the cross sectional area of the
conductor.
Resistivity is temperature dependant and so R = R0[alpha (T-T0) + 1] where R0 is the resistance at the reference temperature T0 and alpha is the temperature coefficient, (which technically is also temperature dependent...)
The physical
effect of that resistance is the conversion of electrical energy to heat. The ability to dissipate that heat effectively is what underpins the whole process of cable selection.
Let's take for example 2.5mm2 twin and earth TPS cable as is used for wiring
power in walls here.
It's normally either V75 or V90. The TPS designation is Thermo Plastic
Sheath, the type of outer
jacket. A V75 cable is designed for a maximum
conductor temperature of 75 degrees C under normal conditions, under
short circuit it can cope with more for a short period.
So basically you need to make sure that the
conductor temperature will be kept under 75 degrees or bad things start to happen... Airflow is what allows the heat to be dissipated and that's where many of the variables come into
play...
So with that in mind, let's look at some numbers.
In free air and not touching a surface, the cable has a
current capacity of 27A. If it touches the surface it's mounted to that comes down to 26A, that's because there isn't airflow on one side now... Now if I go and place the cable in a
conduit that's in free air, the
current capacity comes down again, to 22A for a
flat cable. This is because in
conduit, the air is trapped and can't move and dissipate heat as effectively... Now if I go and surround the cable partially in thermal
insulation, say with one side touching a surface and the other touching an
insulation batt, the capacity is down to 18A. Surround it completely in thermal
insulation and you're looking at 13A, about half of the
current capacity of the cable in free air... Now if you go and bury that cable direct (assuming the cable is rated for that), you get 40A capacity, 31A if in
conduit. That's because the standards assume soil temperature to be 25 degrees C rather than air temperature of 40 degrees C. Because the soil is cooler, it's better able to dissipate heat and so you have a higher continuous
current rating.
The
conductor material makes a difference too because the material changes the resistivity. For instance, a 16mm2 cable in free air with a copper
conductor has a
current capacity of 85A. The same size aluminium
conductor has only got a capacity of 66A.
The
insulation material also makes a difference. If instead of the V75
insulation, I had X90
insulation, I would get 34A capacity out of a 2.5mm twin and earth, if it were say X-HF-110, I get 41A capacity. 41A is a fair
bit more than 26A and I haven't changed the amount of copper, just the
insulation around it...
Ambient temperature I mentioned in passing when talking about buried cable... Remember that you need to
base the sums on the maximum ambient temperature, which is why the Australian Standards are based on a 40 degree ambient... Let's
throw some numbers around, going back to our V75 cable for this. With an ambient temp of 40, the
rating factor is 1. If that comes down to 20, you have 1.28x the capacity. If that goes up to 50 degrees, the cable's down to 0.82x capacity, at 60 degrees it's 0.60x, at 70 degrees it's a mere 0.37x the capacity.
Now if we consider the bunching of circuits. If we look at cables bunched on a surface or enclosed (say multiple wires in a
conduit), then we get these
derating factors: 1
circuit, 1.00, 2 ccts, 0.80, 3ccts, 0.70, 5ccts, 0.60, 10 ccts, 0.48, 20 or more circuits, 0.38. That means our original 2.5mm cable is down to a capacity of 8.36A if in a group of 20 or more circuits, which is less than the standard 10A capacity of a normal
dimmer and so you would need to use a cable >2.5mm2... Earthing conductors, lightly loaded neutrals (something which one cannot assume for entertainment applications) don't get counted in these sums...
You can uprate a bunched set of cables IF and only if you can show that they can never all be fully loaded simultaneously. I would be very hesitant to do this in a theatrical application...
Be aware that all the numbers mentioned so far are for fixed wiring. The principles are equally applicable to flexible cordage, but the standards set a lower capacity to keep the
conductor temperature to 60 degrees to keep the surface temperature of the cables down... 2.5mm2 flex is rated for 20A max. Derate as needed for bunching (say in
Socapex style cables) and other factors mentioned...
Add to that that sometimes the
voltage drop will be the governing factor in cable selection, not the
current capacity, as we try to keep the
voltage at the
point of usage within 5% of that at the
point of supply.
I haven't and I won't talk about
fault current ratings, it's not needed for most people...
All numbers come from AS3008.1.1:1998, Copyright Standards Australia / Standards New Zealand. The US
NEC will have different numbers and you need to ultimately use the code in force in your jurisdiction...
I hope I've given enough background to allow you to now ask more specific questions...
