Wire Size Calculator
Estimate a practical wire gauge (AWG) based on amps, conductor material, and wiring method (NM-B cable vs conduit).
Your Wire Size Estimate
- Enter amperage - Use the expected load current in amps.
- Pick material - Copper and aluminum have different ampacity values.
- Pick the wiring method - NM-B cable (Romex) sizes from the 60°C column; THHN in conduit uses 75°C - the NEC 334.80 rule most tools skip.
- Review result - The calculator returns a smallest gauge that meets or exceeds the entered amps in the selected table.
- Confirm locally - Installation conditions can change allowable ampacity. Use this as a planning estimate.
For a 20 amp circuit you need 12 AWG copper; 30 amps takes 10 AWG; a 50 amp circuit is 8 AWG in conduit or 6 AWG for NM-B cable; and a 100 amp feeder uses 3 AWG copper or 1 AWG aluminum. The catch most calculators miss: NEC 240.4(D) caps the breaker on 14, 12, and 10 AWG copper at 15, 20, and 30 amps no matter what the ampacity table shows, and NM-B cable (Romex) must be sized from the 60 degree column per 334.80. Enter your load amps above and pick the wiring method for a size that respects both rules.
Bigger Load, Bigger Wire
How Do I Choose Wire Size?
Wire size is commonly planned around three things: load current (amps), conductor material (copper vs aluminum), and a reference ampacity table. This calculator returns a smallest gauge that meets or exceeds your input in the selected column as a starting point.
Real installs can change what's acceptable due to temperature, bundling, insulation type, terminations, and equipment limits. Treat this as a planning estimate and verify against your local code requirements and manufacturer specs before purchase or installation.
Example: a 50A load may point you toward a mid-range gauge in copper, but a long run can still require upsizing to reduce voltage drop. If distance is part of your project, run the same scenario through the Voltage Drop Calculator to sanity-check the result.
Wire Size vs Amp‑to‑Wire Size: Which Calculator Should You Use?
These two tools are close cousins, but they answer slightly different questions. Use the one that matches the decision you are making.
Use the Wire Size Calculator when you already know circuit amps (or breaker size) and want a practical gauge recommendation with copper vs aluminum and your wiring method (NM-B cable vs THHN in conduit).
Use the Amp‑to‑Wire Size Calculator when you are starting from an appliance/load amp draw and want a fast “what gauge is this roughly?” conversion before you refine the design.
For long runs or voltage‑sensitive loads, pair either result with the Voltage Drop Calculator.
The Two Rules Most Wire-Size Tools Miss
The first is NEC 240.4(D), the small-conductor rule. The ampacity table lists 14 AWG copper at 20 amps in the 75 degree column, but 240.4(D) still caps its breaker at 15 amps - and 12 AWG at 20, 10 AWG at 30 - regardless of the table. That is why a 20 amp circuit uses 12 AWG, never 14, even though the raw table makes 14 look sufficient. This calculator applies those caps, so it will not hand you a wire the code would reject.
The second is NEC 334.80: NM-B cable (the Romex in most home walls) is sized from the 60 degree column even though its insulation is stamped 90. Sizing Romex from the 75 degree column, which many tools do by default, overstates its capacity. Pick NM-B cable above and the calculator drops to 60 degrees automatically; pick THHN in conduit and it uses 75. The difference is real - a 50 amp circuit is 6 AWG in Romex but 8 AWG in conduit.
Copper vs Aluminum: Why the Gauge Changes
Copper and aluminum carry the same current differently. This quick table shows common planning pairings people compare:
| Circuit amps | Often planned (copper) | Often planned (aluminum) | Notes |
|---|---|---|---|
| 20 A | 12 AWG | 10 AWG | Aluminum typically needs a larger gauge. |
| 30 A | 10 AWG | 8 AWG | Bigger gauge helps keep heating down. |
| 50 A | 6 AWG | 4 AWG | Often compared for larger loads. |
Planning reference only. Always verify for your installation conditions and local code.
Quick Reference: Common Circuits
These are typical pairings people look up (planning reference only):
| Circuit | Typical Breaker | Often Planned Wire (Cu) |
|---|---|---|
| Lighting circuit | 15 A | 14 AWG |
| General outlets | 20 A | 12 AWG |
| Small appliance circuit | 20 A | 12 AWG |
| Dryer / range branch | 30–50 A | 10–6 AWG |
| Subpanel feeder (small) | 60–100 A | 6–3 AWG |
Your project may require a different choice depending on run length, voltage drop targets, and installation conditions.
How This Wire Size Result Is Chosen
Under the hood, this is a planning selection from common ampacity tables (you choose copper/aluminum and a wiring method, which sets the temperature column).
Material effect = Aluminum typically needs a larger gauge than copper for the same amps
Next steps = If the run is long, verify voltage drop separately
This is sizing guidance for estimates and comparisons. Installation method, bundling, insulation rating, and terminations can change the required wire.
Code Notes & Sources
- Conductor ampacities here follow NEC Table 310.16 for copper and aluminum at the 60, 75, and 90 degree columns. Which column applies depends on the wiring method and equipment terminal ratings, not the wire alone.NFPA 70 (NEC) Table 310.16 - Conductor Ampacity
- The small-conductor rule caps overcurrent protection at 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper, whatever the ampacity table shows. This calculator enforces those limits.NFPA 70 (NEC) 240.4(D) - Small Conductors
- NM-B cable is sized from the 60 degree C column even though its conductors carry 90 degree insulation, which is why the wiring-method selector changes the answer.NFPA 70 (NEC) 334.80 - NM Cable Ampacity
Next Steps
Wire size is one input in the circuit. Check the rest:
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