With alternating current, magnetic fields build up inside the cable and skin resistance comes into play, so cable ampacity doesn't scale linearly with cross-section. In addition, cooling is related to the surface area of the cable, and larger cables have less surface area than two smaller ones to dissipate heat. Ergo, you can safely (and legally) carry more current through 2x 150MCM than through 1x 300MCM, and 2x 1/0 may be sufficient depending on what you intend.
If you're talking DC, reference Chapter 9, Table 8. If you're talking 60Hz AC, reference a combination of 310.60 with the derating rules of 310.1(B)(2)(a)... and the myraid of other codes that may apply.
Edited by SteveFehr (11/07/07 04:33 AM)