Wire Size Calculator (Ampacity, 125% Rule and Voltage Drop)
What gauge wire you need for a circuit: copper or aluminum size from the NEC ampacity table at the right temperature column, with the 125% continuous-load rule, the small-conductor breaker limits and an optional voltage-drop check for long runs.
Formula
Design amps = load × 1.25 if continuous. Size = smallest AWG whose ampacity in the 60 °C (or 75 °C) column, capped by 240.4(D) for 14/12/10 AWG, is at least the design amps. Voltage drop % = 2 × K × I × L ÷ cmil ÷ V × 100 (√3 for three phase; K = 12.9 copper, 21.2 aluminum); step up until within the limit.
Worked example
Inputs: Load current = 24 A; Continuous load (3 hours or more)? = Yes (size at 125%); Conductor = Copper; Wire type = THHN/THWN-2 or similar in conduit; Breaker/terminal rating = 60 °C (default for 100 A or less); Circuit voltage = 240 V; Phase = Single phase; One-way length (0 = skip voltage drop) = 100 ft; Maximum voltage drop = 3 %.
| Design amps | 30 |
|---|---|
| Ampacity column c | 60 |
| Size by ampacity awg | 10 |
| Ampacity used a | 30 |
| Voltage drop pct at that size | 2.49 |
| Recommended size awg | 10 |
| Voltage drop pct at recommended | 2.49 |
| Max breaker small conductor rule | 30 A (NEC 240.4(D)) |
Continuous loads (3 h or more) are sized at 125%. Circuits of 100 A or less, or 14–1 AWG, use the 60 °C column unless the equipment is listed for 75 °C (NEC 110.14(C)); NM cable always uses 60 °C. 240.4(D) caps the breaker on 14, 12 and 10 AWG. No ambient-temperature or bundling corrections are applied. Voltage drop uses the same K-factor method as our voltage-drop calculator.
FAQ
What size wire for 20 amps?
12 AWG copper: 20 A in the 60 °C column, and 240.4(D) limits 12 AWG copper to a 20 A breaker. 14 AWG is limited to 15 A.
What size wire for 30, 40 and 50 amps?
Copper at 60 °C: 30 A needs 10 AWG, 40 A needs 8 AWG and 50 A needs 6 AWG (55 A). 8 AWG (50 A) works for 50 A only where the 75 °C column applies, and never for NM cable.
What is the 125% rule?
Loads that run 3 hours or more are continuous; size the conductor and breaker for 125% of them. A 24 A continuous load needs 30 A of ampacity, so 10 AWG copper.
When do I need bigger wire for distance?
When voltage drop gets too high. A 40 A load at 240 V, 150 ft one way: 8 AWG copper drops 3.9%, 6 AWG drops 2.5%. Many designers aim for 3% on a branch circuit.
Can I use the 90 °C column for THHN?
Only as the starting point for temperature and bundling corrections. The final ampacity is limited by the terminal rating, usually 60 °C or 75 °C (NEC 110.14(C)). This calculator does not apply corrections, so check with your inspector or an electrician.
Sources
- Cerrowire Ampacity Charts: allowable ampacity by AWG, copper 60/75/90 °C and aluminum 75/90 °C (based on the 2017 NEC, no temperature correction or adjustment)
- PrepPass, Branch Circuits, Feeders, Conductor Sizing and Overcurrent (2023 NEC references): 125% of continuous loads; 110.14(C)(1) 60 °C for circuits 100 A or less or 14–1 AWG unless listed higher, 75 °C above; 240.4(D) 15/20/30 A for 14/12/10 AWG copper; NM cable at 60 °C (334.80)
- EC&M, Code requirement for sizing the overcurrent protection of conductors (2017 NEC 240.4(D)): 12 AWG aluminum 15 A, 10 AWG aluminum 25 A
- NFPA 70 (NEC) Chapter 9, Table 8: conductor properties (circular mils)
Related
- Wire Size & Voltage Drop Calculator (AWG, Copper/Aluminum)
- Conduit Fill Calculator (NEC Chapter 9, EMT and PVC)
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Last reviewed 2026-10-30.