For a 100 A feeder at 300 feet (one-way), you need 2/0 AWG copper or 4/0 AWG aluminum at 240 V and a 3% drop. Feeders are usually budgeted tighter — at 2% (leaving 3% for branch circuits): 3/0 AWG copper / 300 kcmil aluminum.
| Drop budget | Copper | Aluminum |
|---|---|---|
| 2% (feeder best practice) | 3/0 AWG | 300 kcmil |
| 3% (NEC branch note) | 2/0 AWG | 4/0 AWG |
| 5% (NEC total) | 2 AWG | 2/0 AWG |
A 100 A run at this distance is subpanel feeder territory: aluminum (SER, mobile-home feeder, or XHHW in conduit) is the standard economic choice at these sizes with AL-rated, torqued terminations. Where the table says parallel sets, NEC 310.10(G) applies — 1/0 minimum per conductor, matched lengths. Budget 2% here so downstream branch circuits keep 3% of the 5% total; model the whole chain in the multi-segment calculator.
Aluminum, almost always — at 100 A sizes the price gap is large and modern AA-8000 feeder cable is fully standard. The table shows both at every budget.
Feeder practice reserves drop budget for the branches beyond the subpanel — the 2% column is the professional default, and some bids also size for future load growth.