I have seen it both ways. Jim Walls ---------------------------------------- From: "Cor van de Water via EV" <[email protected]> Sent: Friday, April 15, 2016 1:49 PM To: "Electric Vehicle Discussion List" <[email protected]> Subject: Re: [EVDL] Aluminum cables David, I think there is a little confusion between "individual wires" and cable. What I typically see is a steel core (for example 1/4" dia) with 6 alu wires wound around it, each another 1/4" so total cable dia is 3/4" and to the electric field it looks like a wire that is 3/4" dia with the core 1/4" steel and the "skin" of alu is 1/4" thick so this captures almost all of the skin-effect current (6mm versus 8mm) Hope this clarifies,
Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info http://www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: EV [mailto:[email protected]] On Behalf Of EVDL Administrator via EV Sent: Friday, April 15, 2016 12:57 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Aluminum cables On 14 Apr 2016 at 14:31, Jim Walls via EV wrote: > Because of skin effect, the outer portion of a wire is carrying most of > the current, so having a steel core does not affect the current > carrying capability much. The stuff I have seen has individual wires a > little under quarter inch in diameter with the steel core being about > 1/3 of the diameter. So the AL part would be !/4" / 3 == 1/12" thick or 2mm thick. Under Skin Effect Wikipedia (not always reliable) says that at 60Hz skin depth is about 8.5mm. So I would think that here skin effect wouldn't apply much if at all, and the entire conductor would be carrying the current roughly equally. But then I'm not an engineer. As that hobbyist-not-engineer I would also think that the effective DC or LF AC resistance of the wire would be equivalent to two resistors in parallel, with the resistances of the core and the shell proportional to their respective cross-sectional areas and to their materials' respective resistance per unit of length. (But how would this be affected by the fact that they are in contact all along their lengths?) A 6mm dia conductor would have a total cross sectional area of 28.26 mm^2. If the steel core is 2mm dia that is 3.14 mm^2, so the AL area is much larger, 25.12 mm^2. I found the resistance of 2/0 carbon steel wire listed as 0.00212 ohms per meter. That is a 67.43 mm^2 cross section. A steel cross section of 3.14mm^2 would then be 0.0455 ohm/m. The resistance of 2/0 AL wire is 0.00042 ohms/meter, so an AL cross section of 25.12mm would be 0.00113 ohms/meter. Thus, ignoring skin effect and assuming that the conductors being in contact the whole length has no effect, the effective resistance of a 1m length of this 6mm steel core cable should be 1/(1/R1 + 1/R2) == 0.00110 ohms (1.1 m- ohm). For comparison, a copper wire of the same size would be 0.000598 ohms/meter (about 0.6 m-ohm). That's roughly half. BUT, I think I recall reading that an important limitation in long distance power transmission is the tensile strength of the material, and that copper would need to be larger than necessary for the power itself just to support its own weight. It would be more expensive, too. Someone correct me if my memory is off here. > Cables are made from a bunch of these wires. What I was holding was a > bundle about an inch and a half in diameter. So, in a stranded cable, does skin effect apply to the entire twisted bundle, or to each strand individually? If the former, then you're not getting any skin-effect advantage from the AL's conductivity over the steel's, because they're intermixed rather than having an actual steel core. It might be just the resistances in parallel. If the latter, uh, then it would appear that skin effect doesn't help much there either because of what I said above. But I might be totally wrong about this. I really don't understand skin effect very well, sorry. 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