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Wire Size & AWG Calculator

Determine the correct wire size and AWG for your electrical project based on NEC Table 310.16 ampacity ratings and rules.

NEC Table 310.16, 310.15(B)(1), 310.15(C)(1)

Design Parameters

#8 AWG50% of capacity used
Quick Start
A
ft
Conductor Material

Per NEC 110.14(C), ampacity is capped at the breaker/lug rating even if the wire itself is rated hotter. Most circuits ≤100A use 60°C or 75°C terminations — check equipment labeling.

Wire Size Results

Recommended Minimum Size#8 TW Copper
Design Load: 20AAllowable Capacity: 40.0A

50.0% of capacity utilized

Derating Breakdown

Base Ampacity (Table 310.16)40 A
× Temp Correction (86°F)× 1.00
× Bundle Derating (3 wires)× 1.00
= Derated Ampacity40.0 A
Capped at 75°C termination (110.14(C))40.0 A
Voltage Drop Check2.62% (3.14V)

Calculations based on NEC 2023 guidelines. Results are for estimation purposes — verify against local code before installation.

Verified against NEC 2023 · Table 310.16, 310.15(B)(1), 310.15(C)(1)

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Frequently Asked Questions

How do I choose the correct wire size for a circuit?↓

Wire size is primarily determined by the circuit's overcurrent protection device (breaker) rating and the load it will carry. You must also consider temperature ratings (60°C, 75°C, 90°C), continuous loads (125% rule), and voltage drop over long distances.

What is the 125% rule for continuous loads?↓

The NEC requires that the wire size and overcurrent device for a continuous load (a load running for 3 hours or more) must be sized at 125% of the continuous load, plus 100% of the non-continuous load.

Can I use a smaller wire if it has a 90°C insulation rating?↓

While 90°C wire (like THHN) has a higher ampacity, you generally must size the wire based on the lowest temperature rating of the connected equipment or terminals (often 75°C or 60°C) per NEC 110.14(C). The 90°C ampacity is mainly used as a starting point for derating.

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