Sure, the
tensile strength of solid copper (or steel) wire is fine; it's fatigue from tight/repeated bending that weakens it. Solid-core Cat. 5(e) in the usual 24AWG has individual copper cores of about 0.5mm diameter; the flexible equivalent has 7 strands of about 0.2mm diameter (32AWG). That compares sensibly to common flexible mains cords (among others), which also have individual strands of typically 0.15–0.25mm diameter (or 34AWG through 30AWG) – just larger numbers of them, to make bigger overall conductors.
And I'm not sure what you meant
this statement to be:
…and also 24AWG solid wire has the conductivity of aprox. 25AWG stranded wire.
A solid-core wire has the conductivity of a stranded wire the next size
smaller?
Well, given your stated preference, you most likely meant it the other way around, so I'll respond accordingly (though it's a technically valid reply either way):
When specifying multi-stranded conductors, the "nominal" cross-sectional area is that of all the strands combined –
not that of the geometric circle enclosing them. So for 1mm² wire (for example), a solid core would have a diameter of ≈1.128mm (for most intents and purposes, 1.13mm is plenty precise), whereas a flexible equivalent might be made from (as per IEC standards) 32 strands, each 0.2mm (nominal) in diameter. While the enclosing circle for them
is somewhat larger than 1.13mm, the effective conductor area happens to be well within 1% of the nominal (1.00531mm², to the nearest part per million).
Of course, not all arrangements will be
quite this close – especially not the basic 7-strand sets (or even 3 strands in a triangle in some old cables) – but the principle remains. (In power cables made for fixed installation in structures, the strand size is often calculated from the desired area and a "convenient" number of strands, rather than the other way around; for example, 1.5mm² cable used in Australia for lighting circuits usually has 7 strands, each 0.522mm in diameter – or 522μm, if you want to be ultra-metric. Though manufacturers will sometimes "rationalise" a few near sizes into one to simplify the production processes, providing they remain within the permitted tolerances…)
By conclusion, having about the same cross-sectional area means the resistance is about the same, too (at least to DC, but usually well into the audio range if not higher). Stranded twisted-pair cables
do have somewhat higher losses at RF than their solid-core counterparts, I'll give you that; but the usual plan for Ethernet installations is to keep the patch leads reasonably short (usually within 10m combined, or 5m at each end if they're equal), with the fixed (and concealed) cabling (which can, in most installations, be solid-core without a problem) going the rest of the way (the remaining 90m of the usual "100m" limit). As long as it's within specification (and the equipment at each end is
up to specification), it should work fine; like any digital protocol, it either works correctly or doesn't…
EDIT: I have found that in 1.5mm² 7-strand cables, at least in Australia, the strand size has been rounded off to 0.50mm. This gives a true CSA for those of ≈1.374mm². The remainder of this post holds true.
by Behemot » May 15th, 2017, 9:46 pm 
They may well be CCA too (it's hard to tell by looking at such thin strands, but they aren't too easy to twist together).
