Wire Size Calculator
Pick a wire gauge for your circuit based on current, conductor material, insulation rating and distance.
How to use the Wire Size Calculator
- Enter the load current.
- Tick Continuous load (runs 3+ hours) – size at 125% if it applies.
- Choose the conductor.
- Choose the temperature rating.
- Enter the circuit voltage.
- Choose the system.
- Enter the one-way length and pick its unit – Optional – used to check voltage drop.
- Enter the maximum voltage drop.
- Press Calculate. The result updates automatically as you type.
About the Wire Size Calculator
Undersized wire overheats and wastes power, while oversized wire costs more and is harder to pull. The right size depends on the current, conductor material, the temperature rating of the insulation and terminals, and the length of the run. For circuits of 100 A or less, many terminals are rated only 60 °C, so the 60 °C column is the safe choice for NM-B (Romex) and small breakers unless the equipment is marked for 75 °C.
Bundling more than three current-carrying conductors, running cable through hot attics or rooftops, or using it in high ambient temperatures all require derating, which this tool does not apply. Results are a starting point for planning; electrical work should follow the current code and be checked by a qualified electrician or inspector.
Formula
The calculator picks the smallest conductor whose allowable ampacity meets the required current, then checks voltage drop:
Required ampacity = Load current × 1.25 (continuous loads)
VD % = k × L × I × R ÷ 1,000 ÷ V × 100, with k = 2 (single-phase/DC) or √3 (three-phase)
Ampacities come from NEC Table 310.16 for not more than three current-carrying conductors at 30 °C ambient. The small-conductor rule caps copper at 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG (aluminum 15 A for 12 AWG, 25 A for 10 AWG).
Example calculation
Using the default values:
- Load current = 40 A
- Conductor = Copper
- Temperature rating = 75 °C (e.g. THWN, XHHW, most terminals)
- Circuit voltage = 240 V
- System = DC or single-phase AC
- One-way length = 50 ft
- Maximum voltage drop = 3%
the calculator returns:
- Recommended wire size: 8 AWG copper
- Ampacity of that size: 50 A
- Voltage drop at that size: 1.3%
- Sized by: Ampacity
Frequently asked questions
What size wire do I need for 30 amps?
Copper 10 AWG is the usual choice for a 30 A circuit. In aluminum, 8 AWG is typically used.
What size wire for a 50 amp circuit?
With 75 °C terminations, 8 AWG copper (50 A) qualifies; with 60 °C NM-B cable you need 6 AWG copper. Long runs may need a larger size for voltage drop.
What is the 125% rule?
Loads expected to run for three hours or more, such as EV chargers and water heaters, must be supplied by conductors and breakers rated at least 125% of the load current.
Can I use the 90 °C column?
Only as the starting point for derating. The final ampacity is normally limited by the 60 °C or 75 °C rating of the terminals on breakers and devices.
Last updated: · Category: Electrical calculators