Copper sustained its largest one-day decline in nearly three weeks on Thursday as losses piled up late alongside an accelerated downturn in equities and on a bearish technical break that could keep prices under pressure in the near term. In New York, the key December COMEX contract fell 10.20 cents or nearly 3 percent to settle at $3.3825 per lb, near the bottom end of its $3.3625 to $3.4920 range.