It showed the resistance per thousand feet for #4 copper: 0.248 ohms.
Several factors influence the selection of cable thickness, including:
Voltage drop: 2.8% Maximum capacity: 70 Amps Upgrade to 2 AWG for 1% drop. electrical cable thickness calculator
The rain hadn’t started yet, but Marco could feel it in his knees. He was crouched inside the crawlspace of the old Cannery Row building, a flashlight between his teeth, staring at a junction box that looked like a rat king of old wires.
Where:
He needed thicker cable. But how thick? Too thin, fire. Too thick, he’d waste a fortune on copper he didn’t need and couldn't fit through the conduit.
Above him, Elena was waiting. She ran the new coffee roastery. "Marco! Can we flip the breaker yet?" It showed the resistance per thousand feet for #4 copper: 0
Marco held up his phone, the calculator still glowing on the screen. "Because this thing doesn't guess. It uses Kirchhoff’s laws. It knows the resistance of copper at 75°C. It knows that if I go smaller, I lose 12 volts and the roaster thinks it's dying."
The tabulated ampacity ($I_{tab}$) provided by manufacturers must be derated based on environmental factors using correction factors: He was crouched inside the crawlspace of the
Methodology for the Calculation of Minimum Conductor Cross-Sectional Area in Electrical Wiring Systems
Several online calculators are available, including: