When 10 AWG beats 12 AWG

Both gauges show up in the 15 A and 20 A bins. The hang tags both say heavy duty. The copper does not. On a 15 A, 120 V load, 12 AWG is about 2.9 V (2.4%) at 50 feet and 5.8 V (4.8%) at 100 feet. 10 AWG on the same load is 1.8 V (1.5%) and 3.6 V (3.0%). That is why a circular saw or table saw that ran fine on a 50-foot 12 starts to bog, and why shop lights dip when the motor comes up, on a hundred-foot run.

The actual difference

10 AWG copper is thicker. Resistance from NEC Chapter 9, Table 8 is 1.21 Ω per 1,000 ft versus 1.93 Ω for 12 AWG (uncoated, 75 °C). Same amps, same feet, the 10 AWG path drops about two-thirds the voltage of the 12 AWG path. You pay for that in weight and sticker price. You do not pay for a magic “extra heavy duty” coating.

Full rows for 10 A and 13 A live on the master chart. The 100-foot column is the usual fork — see 100-foot gauge.

When 12 AWG is enough

• The run is 25–50 feet on a 15 A tool. That is the default shop buy.
• The load is well under 15 A (lights, chargers, a small drill) even if the cord is long.
• You carry the cord every day and 10 AWG would be a boat anchor for the same job.
• You are not treating the reel as furniture. A temporary reach is a different product than a cord that lives on the floor.

If the hardware-store fork is still 12 vs 14, stay on that page: 12 vs 14. Debating 10 vs 12 means 14 AWG is already out.

When 10 AWG is the right buy

• A 100-foot, 15 A motor — circular saw, contractor table saw, compressor. At 100 feet, 12 AWG on 15 A is already near 5%. 10 AWG is the comfortable pick.
• Lights on the same circuit dip when the motor starts on a 12 AWG run. Nameplate understates inrush. The cord is already the weak link before the cut begins.
• The cord is semi-permanent shop wiring: a table saw parked in the aisle, a compressor that never moves, a reel you do not coil back up. Size it like a feeder, or put a receptacle closer.

Weight, heat, and a cord that lives down

A 100-foot 10 AWG reel is heavy. That is the trade. If you cannot carry it, the honest fix is a receptacle closer to the work — not a thinner cord that runs the motor hot. Leaving any gauge tightly coiled under a 15 A load is a heat problem; unspool what you are using.

Outdoor or open-door shop work still needs a weather-rated jacket in the gauge you already chose. SJTW is the jacket, not thicker copper. A 12 AWG SJTW 100-foot “outdoor” reel is still a 12 AWG resistor.