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Wire Size Calculator
The minimum conductor for a load is the smallest size whose NEC Table 310.16 ampacity meets the load at the insulation rating you are actually allowed to use. Long runs are then often limited by voltage drop rather than by heat, which is why a 100 foot circuit sometimes needs a larger conductor than the ampacity table alone would suggest.
Wire size calculator
Drop over 3%, size up 8 AWG copper is rated 50 A in NEC Table 310.16 at this insulation rating. The 3% figure is the NEC’s informational recommendation for branch circuits, not a requirement.
NEC Table 310.16 ampacity, copper
| AWG | 60 °C | 75 °C | 90 °C | Ω/1000 ft |
|---|---|---|---|---|
| 18 | – | – | 14 | 7.95 |
| 16 | – | – | 18 | 4.99 |
| 14 | 15 | 20 | 25 | 3.14 |
| 12 | 20 | 25 | 30 | 1.98 |
| 10 | 30 | 35 | 40 | 1.24 |
| 8 | 40 | 50 | 55 | 0.778 |
| 6 | 55 | 65 | 75 | 0.491 |
| 4 | 70 | 85 | 95 | 0.308 |
| 3 | 85 | 100 | 115 | 0.245 |
| 2 | 95 | 115 | 130 | 0.194 |
| 1 | 110 | 130 | 145 | 0.154 |
| 1/0 | 125 | 150 | 170 | 0.122 |
| 2/0 | 145 | 175 | 195 | 0.0967 |
| 3/0 | 165 | 200 | 225 | 0.0766 |
| 4/0 | 195 | 230 | 260 | 0.0608 |
This is a starting point, not a final answer. Table 310.16 assumes not more than three current-carrying conductors in a raceway at 30 °C ambient. The installed ampacity must still be corrected for ambient temperature (310.15(B)(1)), derated for conductor count (310.15(C)(1)), and limited by the lowest-rated termination in the circuit (110.14(C)), which is 60 °C on most equipment at 100 A or less. Verify against the code edition your jurisdiction has adopted, and have anything you are unsure about checked by a licensed electrician.
Wire size by circuit amps
The minimum conductor for each common breaker size, with the NEC 240.4(D) small-conductor cap applied. Use the 60 degree C column unless every termination in the circuit is listed for 75 degrees, which NEC 110.14(C) requires you to confirm.
| Circuit | Copper 60 °C | Copper 75 °C | Aluminium 75 °C |
|---|---|---|---|
| 15 A | 14 | 14 | 12 |
| 20 A | 12 | 12 | 12 |
| 30 A | 10 | 10 | 10 |
| 40 A | 8 | 8 | 8 |
| 50 A | 6 | 8 | 6 |
| 60 A | 4 | 6 | 4 |
| 70 A | 4 | 4 | 3 |
| 80 A | 3 | 4 | 2 |
| 100 A | 1 | 3 | 1 |
| 125 A | 1/0 | 1 | 2/0 |
| 150 A | 3/0 | 1/0 | 3/0 |
| 200 A | – | 3/0 | – |
How voltage drop is calculated
Voltage drop over a single-phase run is Vd = 2 × K × I × L / cmil, where K is 12.9 for copper and 21.2 for aluminium, I is the current in amps, L is the one-way run in feet, and cmil is the conductor area in circular mils. The 2 accounts for current travelling out and back. For three-phase, replace the 2 with 1.732.
The NEC does not require a voltage drop limit on ordinary branch circuits. It recommends 3 percent for a branch circuit and 5 percent total including the feeder, in the informational notes to 210.19(A) and 215.2(A). Those notes are advice, not rules, but equipment behaves badly at the end of a long undersized run, and motors in particular draw more current at reduced voltage, which heats the conductor further.
Common questions
What size wire do I need for a 100 amp subpanel?
- For 100 A at the 75 degree C column, 3 AWG copper is listed at 100 A and 1 AWG aluminium at 100 A. Many installers use 2 AWG copper or 1/0 aluminium for margin and for voltage drop on longer feeder runs. Check that the panel and breaker terminations are rated 75 degrees C before using that column.
Does wire size depend on the length of the run?
- Ampacity does not, but the usable size often does. Ampacity is set by heat and is independent of length. Voltage drop rises directly with length, so beyond roughly 100 feet it frequently becomes the deciding factor and pushes you one or two sizes up.