Wire Size Calculator
Calculate the correct wire gauge for your electrical circuit using NEC ampacity tables. Enter your load current, system voltage, wire length, and insulation temperature rating to find the recommended wire size with voltage drop analysis.
How Wire Sizing Works
Wire sizing is the process of selecting the correct conductor gauge for an electrical circuit to safely carry the required current without overheating. The National Electrical Code (NEC) provides ampacity tables that specify the maximum current a conductor can carry under specific conditions. NEC Table 310.16 is the most referenced table for electrical work, covering copper and aluminum conductors from 14 AWG to 2000 kcmil.[reference:0]
Wire ampacity depends on four main factors:
- Wire gauge (AWG/kcmil): Larger conductors have higher ampacity.
- Conductor material: Copper has higher ampacity than aluminum for the same size.[reference:1]
- Insulation temperature rating: 60°C, 75°C, or 90°C. Higher ratings allow more ampacity.[reference:2]
- Installation conditions: Ambient temperature and number of conductors affect derating.
Important: Even when using 90°C-rated wire, you must use the ampacity from the column matching the lowest temperature-rated component in the circuit — usually the terminals, which are typically rated 60°C or 75°C.[reference:3]
How the Wire Size Calculator Works
Step 1: Required Ampacity = Load Current × (1.25 for continuous loads)
Step 2: Find the smallest wire size whose ampacity ≥ Required Ampacity from NEC Table 310.16.
Step 3: Calculate voltage drop using: VD = 2 × K × I × L ÷ CM
Step 4: If voltage drop exceeds the limit, increase wire size.
Where: K = 12.9 (copper) or 21.2 (aluminum), I = current, L = one-way length (ft), CM = circular mils
The calculator recommends the smallest wire gauge that meets both ampacity and voltage drop requirements.
Understanding Wire Gauge (AWG)
AWG (American Wire Gauge) is the standard system for measuring wire sizes in North America. Smaller AWG numbers indicate larger diameter wires with higher ampacity:
- 14 AWG: Common for 15-amp lighting circuits
- 12 AWG: Standard for 20-amp general-purpose circuits
- 10 AWG: Used for 30-amp circuits (water heaters, A/C units)
- 8 AWG: For 40-amp circuits (range, ovens)
- 6 AWG: For 50-amp circuits (EV chargers, subpanels)
- 4 AWG and larger: For 70+ amp service and heavy loads
The NEC also provides overcurrent protection limits: 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG copper.[reference:4]
❓ Wire Size Calculator FAQ
What is ampacity?
Ampacity is the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. The word combines "ampere" and "capacity."[reference:5]
What is the difference between 60°C, 75°C, and 90°C wire?
These are insulation temperature ratings. A higher temperature rating allows the wire to carry more current without damage. For example, 10 AWG copper can carry 30A at 60°C, 35A at 75°C, and 40A at 90°C.[reference:6]
Why is the termination temperature rating important?
Even if you use 90°C wire, the ampacity is limited by the lowest temperature-rated component in the circuit. Most terminals are rated 60°C or 75°C, so you must use the ampacity from the corresponding column.[reference:7]
What is the recommended voltage drop limit?
NEC recommends a maximum voltage drop of 3% for branch circuits and 5% for feeders combined. This calculator uses 3% as the default limit.[reference:8]
What is a continuous load?
A continuous load is a load where the maximum current is expected to continue for 3 hours or more. NEC requires continuous loads to be sized at 125% of the load current.[reference:9]
What size wire do I need for a 100-amp service?
For a 100-amp circuit with 75°C-rated terminations, NEC Table 310.16 requires 3 AWG copper (rated for 100A) or 1 AWG aluminum (rated for 100A) conductors.[reference:10]
What is the difference between copper and aluminum wire?
Copper has higher conductivity and ampacity than aluminum for the same gauge. Aluminum is lighter and less expensive but requires larger sizes for the same current capacity.[reference:11]
How does wire length affect wire sizing?
Longer wire runs have higher resistance, which increases voltage drop. For long runs, you may need a larger wire size to keep voltage drop within acceptable limits, even if ampacity requirements are met.
What is the formula for voltage drop?
For a single-phase circuit: VD = 2 × K × I × L ÷ CM, where K = 12.9 (copper) or 21.2 (aluminum), I = current in amps, L = one-way length in feet, and CM = circular mils of the conductor.[reference:12]
How do I calculate the required wire size for a motor?
Motor circuits require sizing at 125% of the motor full-load current per NEC 430.22. Use this calculator with the appropriate load current and continuous load setting.
What is the ambient temperature correction factor?
NEC Table 310.15(B)(1) provides correction factors for ambient temperatures above 86°F (30°C). Higher temperatures reduce a conductor's ampacity. This calculator includes an ambient temperature input for adjustment.[reference:13]
How does the number of conductors affect ampacity?
NEC requires derating when more than 3 current-carrying conductors are in a raceway. The adjustment factors are found in NEC Table 310.15(B)(3)(a). This calculator includes a conductor count input for derating.[reference:14]
What is the difference between AWG and kcmil?
AWG (American Wire Gauge) is used for smaller wires (14 AWG to 4/0 AWG). kcmil (thousand circular mils) is used for larger conductors (250 kcmil and above). Both are standard in NEC Table 310.16.[reference:15]
What is the maximum overcurrent protection for common wire sizes?
Per NEC 240.4(D), the maximum overcurrent protection is: 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG copper, unless otherwise permitted.[reference:16]
Can I use this calculator for DC circuits?
Yes! This calculator supports DC voltages (12V, 24V, 48V) as well as AC voltages. The voltage drop formula is the same for DC and single-phase AC circuits.
What is the difference between solid and stranded wire?
Solid wire is a single conductor, while stranded wire is made of multiple smaller strands. Stranded wire is more flexible and easier to install. Both have the same ampacity for the same gauge.[reference:17]
How do I size a neutral conductor?
Generally, the neutral conductor must be sized at least as large as the ungrounded conductors, but there are exceptions for balanced loads. This calculator focuses on the current-carrying conductors.
What is the equipment grounding conductor (EGC) size?
NEC Table 250.122 specifies EGC sizes based on the overcurrent protection device rating. For a 100A circuit, the minimum EGC is 8 AWG copper or 6 AWG aluminum.[reference:18]
Is this calculator code-compliant?
This calculator follows NEC Table 310.16 ampacity values and NEC 210.19(A)(1) continuous load requirements. Always verify with local codes and consult a qualified electrician for your specific installation.[reference:19]