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How to Choose the Right Wire Size (AWG) for Any Circuit
Written for installers: how to go from a load in amps to a wire size you can defend, covering the 125% rule, temperature ratings, derating and voltage drop, with two worked examples.
Why wire size matters
Too-thin wire is the quiet cause of a lot of field problems. It can overheat inside a wall, trip a breaker for no obvious reason, or starve a device of voltage so it works in the morning and fails at night. Choosing the size is not hard, but it is a sequence of small checks, and skipping any one of them is how mistakes get in. The Wire Size Calculator runs the whole sequence below, so use this guide to understand each step and the calculator to get the number.
Ampacity in plain words
Ampacity is the most current a wire can carry continuously without its insulation getting too hot. A thicker wire has less resistance, so it makes less heat for the same current and can carry more. It also depends on the metal: copper carries more than aluminum of the same size. In AWG, remember that a smaller number means a thicker wire, so 10 AWG is thicker than 14 AWG.
In the United States the reference is NEC Table 310.16. It gives one value per size for each insulation rating (60°C, 75°C and 90°C), assuming a 30°C ambient and no more than three current-carrying conductors in the raceway. Everything else in this guide is an adjustment to that starting value.
The six-step method
1. Find the real load
Use the maximum running current from the nameplate or datasheet. If you only know watts, divide by the supply voltage. A camera with IR lights, a motor starting, or a heater all draw more than their idle figure, and the wire has to handle the worst case.
2. Add 25% for continuous loads
If the load runs for three hours or more, multiply the current by 1.25. A 16 A continuous load is treated as 20 A when choosing the wire and the breaker. Lighting, space heaters, EV chargers and many pumps fall into this group.
3. Choose the temperature column
A wire is only as good as the weakest part it connects to. If the breaker lugs are rated 75°C, you size the wire against the 75°C column even if the insulation is rated 90°C. When in doubt, use 75°C.
4. Derate for heat and crowding
A wire in a hot attic or a conduit full of other loaded wires cannot lose heat as easily, so its allowable current drops. Multiply the table value by the ambient correction and by the bundling factor.
| Situation | Typical effect |
|---|---|
| Ambient 40°C with 75°C wire | Table value × 0.88 |
| 4 to 6 current-carrying conductors | Table value × 0.80 |
| 7 to 9 current-carrying conductors | Table value × 0.70 |
| 10 to 20 current-carrying conductors | Table value × 0.50 |
5. Respect the small-wire limit
The code limits protection on the smallest copper sizes to 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG. That is why 12 AWG, not 14 AWG, is the answer for a 20 A circuit even though the 75°C table value for 14 AWG looks high enough.
6. Check voltage drop
Even a wire that passes on heat can be too long for its size. Voltage drop is current times the loop resistance of the run, and resistance grows with length. A common planning target is about 3% on a branch circuit, but the equipment datasheet decides. If the drop is too high, go up one or two sizes until it passes.
Worked example: a long run in a hot space
A 20 A continuous load, 40°C ambient, six current-carrying conductors in one conduit, 150 ft at 120 V, copper at 75°C, 3% allowed drop.
- Required current: 20 A × 1.25 = 25 A.
- Derating: 0.88 (heat) × 0.80 (six conductors) = 0.70.
- 8 AWG is 50 A at 75°C, so 50 × 0.70 = 35 A, which is above 25 A. Heat passes.
- At 150 ft, 8 AWG drops more than 3%, so move to 6 AWG, which comes out near 2%.
The answer is 6 AWG. Without the voltage drop step, an installer would have stopped at 8 AWG and left the load short of voltage at the far end.
Mistakes to avoid
- Sizing by breaker size alone and never looking at the actual load and conditions.
- Using the 90°C column to size a circuit that ends on 75°C terminals.
- Forgetting that neutral and grounding conductors are not always counted the same way in the bundling rule.
- Using the round-trip length instead of the one-way length in a voltage drop formula.
- Mixing up AWG and mm²: they are separate systems and should not be converted by eye.
Where this guide stops
This is a planning guide, not a code book. Motors, transformers, long continuous runs, high temperatures and special locations have their own rules, and the edition of the code adopted in your area decides what is allowed. Use the result as a check on your design, then confirm it against the rules that apply to the job. For low-voltage camera runs, see why cameras fail after dark and the Voltage Drop Calculator.