What Is the Wire Size Calculator?
The Wire Size Calculator is a free online tool designed for contractors and DIY builders who need quick, accurate calculations in the construction and building space. By entering your system voltage, phase, load amperage, you get instant results including recommended wire, ampacity rating, actual voltage drop. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting recommended wire right can make the difference between success and costly mistakes. In construction and building, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by contractors and DIY builders worldwide, giving you confidence that your numbers are correct. Use it to plan your projects with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your system voltage and need to find the right recommended wire. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
Wire Size Calculator
NEC Copper Wire Ampacity & Sizing Reference
Ampacity ratings (THHN 75°C) and typical max distance at 3% drop, 240V.
| Wire Gauge | Ampacity (Cu) | Ampacity (Al) | Max Ft @ 20A/3% |
|---|---|---|---|
| 14 AWG | 15A | — | 64 ft |
| 12 AWG | 20A | 15A | 101 ft |
| 10 AWG | 30A | 25A | 161 ft |
| 8 AWG | 40A | 30A | 256 ft |
| 6 AWG | 55A | 40A | 407 ft |
| 4 AWG | 70A | 55A | 647 ft |
| 2 AWG | 95A | 75A | 1029 ft |
| 1/0 AWG | 125A | 100A | 1637 ft |
How to Use This Calculator
- Enter Your System Voltage: Start by entering your system voltage — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: phase, load amperage, one-way distance, conductor material, maximum voltage drop. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your recommended wire, ampacity rating, actual voltage drop. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
Wire sizing requires balancing two NEC requirements: the wire must handle the current (ampacity) and keep voltage drop within acceptable limits. The larger of the two determines the final wire size.
The basic rule:
- First, determine the minimum wire gauge from NEC ampacity tables (Table 310.16) based on the load current
- Then calculate the minimum gauge needed for voltage drop using: CM = (2 × K × I × L) / VD_max
- Use the larger of the two wire sizes — ampacity and voltage drop must both be satisfied
For long wire runs, voltage drop typically requires a larger wire than ampacity alone. This calculator checks both and gives you the correct size for your specific installation.
Tips & Considerations
- Double-check your system voltage before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both recommended wire and ampacity rating — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to plan your projects.
- If you're unsure about your maximum voltage drop, start with a conservative estimate and adjust from there.
Frequently Asked Questions
What size wire do I need for a 30 amp circuit?
For a 30-amp circuit, NEC requires a minimum of 10 AWG copper or 8 AWG aluminum wire. However, if the wire run is longer than about 100 feet, you may need to upsize to 8 AWG or 6 AWG copper to keep voltage drop under 3%. Always check both ampacity and voltage drop.
How does distance affect wire size?
Longer wire runs create more voltage drop, which may require a larger wire gauge. A 12 AWG wire might work fine for 20 amps at 50 feet, but at 150 feet you might need 8 AWG to stay within NEC voltage drop limits. Distance is the most common reason for upsizing wire.
What is the difference between copper and aluminum wire sizing?
Aluminum has higher resistance than copper, so it requires larger gauges for the same ampacity. For example, a 100-amp service needs 3 AWG copper but 1 AWG aluminum. Aluminum is cheaper per foot but the larger conduit and connectors can offset savings.
What is the NEC 80% rule for wire sizing?
NEC 210.19(A)(1) requires that conductors for continuous loads (running 3+ hours) be rated for 125% of the load. This effectively means using only 80% of the wire's ampacity. A 20A continuous load requires wire rated for at least 25A.
What conduit size do I need for my wire?
Conduit size depends on the number and size of wires inside. NEC limits fill to 40% of conduit area for 3 or more wires. For a typical 3-conductor run of 10 AWG THHN, 1/2-inch EMT conduit works. This calculator provides conduit recommendations based on your wire size.
Should I use THHN or Romex (NM-B) wire?
THHN is individual conductors pulled through conduit — used in commercial work, outdoor runs, and exposed locations. Romex (NM-B) is a cable with multiple conductors in a sheath — used in residential wall and attic runs. THHN has higher ampacity ratings than NM-B at the same gauge.