General > General Technical Chat
If the electrical energy is outside the wires, how is insulation protecting us?
TimFox:
"You're the expert, but a lotta guys..."
typoknig:
So, rhetorical question, is "current" a measure of energy? Since the answer is "yes", how is it that a 10 AWG solid core wire will carry more amperage than a 10 AWG stranded wire, yet the stranded wire has more surface area?
TimFox:
No, "current" is not a measure of energy.
In normal electrical units, the "watt-second" is energy, usually referred to in physics as the "Joule".
TimFox:
"I don’t like all of the theoretical particles & virtual particles demanded by Quantum stuff."
Another in a series of arguments by several posters that they find something to be "icky" and therefore refuse to deal with it.
T3sl4co1l:
Current is a measure of energy in the sense that it's energy per flux (volt-second). You wouldn't say that a displacement, or a force, is a measure of energy, any more than one is contingent on the other: their product (given a few others conditions) being energy. Or velocity being more analogous to current, the product with potential being power, hence needing time to get energy from voltage and current.
So for example, the say 6AWG or so wires used for overhead power lines -- these might be good for 50A in in-wall electrical wiring, which at 120/240V is merely 6/12kVA capacity. Up on the poles, with free air and ceramic insulators, it takes more current for the same temperature rise, or they may be allowed to run hotter besides, and so distribution lines might run 100A. Carrying 4800V or more, you get capacity upwards of half an MVA quite easily.
Meanwhile, even though the current and thus voltage drop might be higher, the fact that the system voltage is 20+ times higher, means the voltage drop is 10+ times smaller in relative terms, thus the transmission efficiency is extremely high, suitable for distances of tens of miles.
This says nothing of stranded; for mains purposes, there's no difference, the wire is jacketed and the jacket is only slightly larger for stranded, so its temp rise is hardly different. At low frequencies (DC and mains, except for the largest conductors), current flows in the entire cross-section, which is equivalent (they're both called "6AWG" for a reason).
There is some truth to the assertion at higher frequencies, however. Skin effect forces current to flow on the surface of the conductor. A stranded 6AWG wire might have say 10-20% less AC resistance at, oh, a few kHz, compared to solid. It's not much, but it is something. Even better if the strands are fine and individually insulated (litz), in which case the cable might have AC resistance not much higher than DCR, even at a few 100 kHz (where the solid wire is naught but a thin tube, electrically speaking).
(The largest transmission lines use conductors the size of your forearm, typically using aluminum for lighter weight, built around a steel core for strength. The build is stranded for flexibility, which again also increases the skin depth somewhat; but again, this is a small difference, and when quite high currents are required (several thousand amperes), multiple conductors are used in parallel, spaced out a bit so that their magnetic fields don't affect each other quite so much (proximity effect, basically skin effect but for strands a bit further apart from each other). The increased effective diameter of the bundle also reduces electric field and thus corona discharge -- these are very high voltages indeed, pushing a megavolt. Such lines can transmit the power of whole power stations (gigawatts), or indeed some entire countries.)
Tim
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