Economic Conductor Sizing (I2R Payback)
Whether upsizing a conductor pays for itself in reduced I^2R heat loss: three-phase loss = 3 x I^2 x R, so the annual saving = (loss at the small size - loss at the large) x run hours x rate, and payback = the added copper cost / that saving. A 100 A feeder from 0.20 to 0.125 ohm over 4,000 hr at $0.12 saves $1,080/yr, paying back an $800 upsize in 0.7 years; at 40 A the same upsize saves only $173/yr, a 4.6-year payback -- upsizing only pays on heavily loaded, long-hour feeders. A screening estimate; the code minimum still governs the conductor.
Formula and source
loss = 3 x I^2 x R / 1000 (kW, per size); annual_saving = (loss_small - loss_big) x hours x rate; payback = upsize_cost / annual_saving.
Economic (loss-based) conductor sizing, standard energy-engineering practice (NEC Informative Annex D / IEEE economic-conductor methods) by name; first-principles I^2R loss.
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