Correct feeder cable for distro

It is a long, long time ago that I learned electrical theory, so the laws of physics might have changed since then, but I was taught that ideal inductors were reactive components and thus did not dissipate energy. Real life inductors dissipate energy mainly because they also have a resistive component. Also, as someone previously mentioned, general purpose main cables have two close coupled conductors where the current is in opposite directions, cancelling out any induction effects however you wrap them.

Any cable will have a resistance, albeit very low. The resistance is usually specified in m ohms/metre Current flowing through this resistance will dissipate energy in the form of heat in Joules/sec/metre (W/m). Cables with a larger cross-sectional area have a lower resistance for a given length and, assuming the same current, will therefore dissipate less energy so not heat up as much.

Coiling the cable concentrates a longer length into a smaller area, so more Watts, and inhibits its ability to dissipate the heat. This results in the cable getting hotter, possibly to the point of melting the insulation.
 
The two cores tend to cancel the magnetic properties, but the heating effect should not be underestimated. The cable resistance is a key factor, but so of course is current. Coiling cables adds inductance components. Cables have resistance. They need cooling. If the temperature rises, this changes the electrical properties and it starts to run away. A domestic extension reel highly loaded near to its capacity can turn it into a solid lump of plastic, unable to be unwound. Very common. I’d not want to do this with high capacity cable. Figure 8 and loose laying with space between turns reduces the inductive component so the air can enable cooling. The thermal results just need managing.
 
Fred Campbell said:
The inductance is irrelevant. It is only the resistive component that dissipates power.
Click to expand...

True, we have Famous Dead Guys who made Laws about such things. The question about coiling, figure-8, over-under, or spiral is based on induction being the path for circuit completion and what type of cable management has less inductive paths.
 
Then induction heating with a coil should not work if the resistance is the same as a piece of non coiled conductor. is inductance the only property changing?
 
An induction heater isn't going to heat anything that doesn't allow current to flow, because the "induction" in that device is "induced [eddy] current", and not "inductance (the tendency of an electrical conductor to resist the change of current)".

Only the resistive (real) component of the impedance dissipates power (Joule's Law). But the reactive (imaginary) component of the impedance affects the current flow, and the resistive losses go as I^2R. So if you go from a resistor to a device that has that resistor in series with a perfect inductor (no resistive component to the impedance), your power dissipation at DC doesn't change but your power dissipation with AC goes up with frequency.

If we look at ~100' of feeder formed into a perfectly stacked coil 36" in diameter (10 turns, so a 10" tall stack), we get an inductance of 124uH. Reactance is omega*L, so 2*pi*f*L. At 60hz, this is about 50 miliohms. DC resistance of 4/0 copper is about 5 miliohms for that same 100'. So the the impedance of coil has gone up by about an order of magnitude at 60hz vs. DC. However, it is worth noting that the same 100' piece of 4/0 would have an inductance of about 50uH if not coiled, so 20 milliohms instead of 50 at 60hz.

Let's assume a purely resistive load (the case of a constant power load will be a bit worse, but is much more obnoxious to calculate) of 0.5 ohms (28.8kW, about 240A) and 120V into the feeder. This creates a voltage divider of 55 miliohms through the feeder and 500 miliohms through the load for the coiled case, and 20 miliohms through the feeder and 500 miliohms through the load for the straight cable case. Current in the coiled case is 216A, and 230A in the straight cable case. Ohmic losses are 233W for the coiled cable case, and 264W in the straight cable case, a difference of about 13% with the coiled cable actually being lower loss with a resisitive load (again, a constant power load will have different results, likely higher for the coiled cable case, but similar degree of difference).

So not a huge difference, and a difference that is probably less than the variability in contact resistance of all the connectors in the system (your terminal screws are all torqued to the manufacturer's specifications, and there's no oxidation on any of your connectors, right?). Of course, this all assumes that there's nothing ferromagnetic nearby to dissipate power via eddy currents, so coiling around steel stage legs will give you different results.
 
I have to admit that I’m now hopelessly confused by the science and the real world? So all the stuff about cables going through panel holes, coiling vs non coiling all boils down to using too thin cabling?

i suppose that as you really get it, what do you do with your cables and we will follow.
 
If you have ferromagnetic material near the cable, you will get current flow into that material due the the changing magnetic field. That current circulates and can cause heating of the material. This is the "eddy current" phenomenon, and is how induction heating works. This is a cause for concern, and is covered by Article 300.20 of NFPA 70 (which generally requires conductors for a circuit to be grouped of there to be slots in the metal so that there isn't a closed-loop path for current to circulate).

But the case of the coil functioning as an inductor isn't cause for concern from a heating perspective, and the choice of over-over, over-under, or figure-8 depends on how you choose to manage cable twist, and how much you care about creating an electromagnet.
 
Then induction heating with a coil should not work if the resistance is the same as a piece of non coiled conductor. is inductance the only property changing?
Click to expand...
Induction heating works by using induction to transfer energy from the power source to the object wished to be heated. The system is designed and optimised for that purpose, it is not just a coiled-up mains cable, and while the object gets hot, the primary induction coil stays relatively cool.

Induction can be used to wirelessly charge electronical equipment, such as phones, which it does without setting either the phone or the charger on fire. It is also how transformers work and although designed with a tightly wound coil of wire, they mostly work without burning up. Inductors and induction do not necessarily mean heat.
 
Top Bottom