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 We Calculate This
This wire size calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
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.
When Would You Use This Calculator?
This wire size calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
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.