Circuits & Wire Size

Notes

• **Two separate questions, and only one of them is code.** Ampacity asks whether the wire survives the current — that is NEC Table 310.16 and it is mandatory. Voltage drop asks whether useful voltage reaches the far end — that is a 3% *recommendation*, not a requirement. At 125 ft the run is short enough that ampacity decides and drop is only 2.3%.
• **NEC 240.4(D) overrides the ampacity table for small wire.** 14 AWG is capped at a 15 A breaker, 12 AWG at 20 A and 10 AWG at 30 A, no matter what 310.16 shows. Reading the ampacity column alone and stopping there is the single most common mistake in this whole subject.
• **Aluminum needs roughly two sizes up on the same load.** It carries less current per gauge and has about 1.6 times the resistance, so both constraints move together. It is cheaper per foot and it is what utilities use for service entrances — but every termination has to be listed for aluminum, and antioxidant compound is not optional.
• **Distance is the variable people leave out.** The same 70 A circuit takes 4 AWG in a panel-room subpanel and 4 AWG out to a detached garage. Measure the actual conductor path — around the joists and up the wall — not the straight line across the yard.
• Temperature column matters too. These figures use the 75°C column, which is what the *terminations* on ordinary breakers and lugs are rated for. A 90°C cable does not let you use the 90°C column unless everything it lands on is also rated 90°C, and on residential gear it almost never is.