What size wire for a 20 amp circuit 100 feet away?
Short answer: Use 10 AWG copper. At 100 feet with a 16 A load at 120 V, 12 AWG drops about 5.2% (to 114 V); 10 AWG drops about 3.2% and 8 AWG about 2.0%. The breaker stays at 20 A.
Voltage drop by wire size and distance
Copper, 120 V, a 16 A load (80% of a 20 A circuit). Each cell shows the drop and the voltage left at the far end:
| Wire | 50 ft | 75 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 12 AWG | 2.6% · 117 V | 3.9% · 115 V | 5.1% · 114 V | 7.7% · 111 V |
| 10 AWG | 1.6% · 118 V | 2.4% · 117 V | 3.2% · 116 V | 4.8% · 114 V |
| 8 AWG | 1.0% · 119 V | 1.5% · 118 V | 2.0% · 118 V | 3.1% · 116 V |
| 6 AWG | 0.7% · 119 V | 1.0% · 119 V | 1.3% · 118 V | 2.0% · 118 V |
An informational note in NEC 210.19 suggests keeping a branch circuit at 3% or less, and 5% for the feeder and branch circuit together. It is a recommendation, not a rule an inspector enforces, but it is a good one: a 1/2 hp motor on a compressor or table saw at 114 V runs hotter and stalls more easily.
When 12 AWG is still fine at 100 feet
Voltage drop scales with current. If the far end only runs lights and a few chargers (say 5 A), 12 AWG at 100 feet drops 1.6%. Upsize when the run will feed tools, a space heater, a window AC or a fridge in the shed.
Ground wire and the breaker
- The breaker stays 20 A: a larger wire doesn't let you raise it, because the receptacles are rated 20 A.
- NEC 250.122(B): when you upsize the hot wires for voltage drop, the ground must grow in proportion. 10/2 NM-B or UF-B cable already includes a 10 AWG ground.
- 10 AWG is stiff on standard 15/20 A receptacle terminals. Many installers run 10 AWG to a first box and pigtail 12 AWG to the device.
Safety. Wire buried to a detached shed or garage needs UF-B or wire in conduit at the depth NEC Table 300.5 requires, GFCI protection, and usually a permit.