NEC Wire Size & Ampacity Calculator

Size conductors per NEC 310.16 with temperature and bundling derates, select the breaker/OCPD (NEC 240), and get a pass/fail code verdict. Voltage drop is reported as advisory.

Sizing a solar array instead? Use the Solar PV Wire Size Calculator (NEC 690.8).

Input Parameters

Maximum continuous current draw

Used only for the voltage-drop estimate

One-way distance (calculator uses round-trip)

Caps the usable ampacity per NEC 110.14(C)

NEC recommends 3% for branch circuits, 5% total

Sizes the breaker and conductor at 125% of load (NEC 210.19 / 210.20)

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Results

Enter parameters and click Calculate

Diameter
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Area
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Resistance
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Run calculation to see ampacity table

Wire Sizing Principles

Ampacity

The maximum current a conductor can carry continuously without exceeding its temperature rating. Values come from NEC Table 310.16.

I_corrected = I_base × f_temp × f_bundle

Temperature Correction

When ambient temperature differs from the 30°C base, ampacity must be adjusted per NEC 310.15(B). Higher temperatures reduce the allowable current.

Bundling Adjustment

When more than 3 current-carrying conductors are bundled together, heat dissipation is reduced. NEC 310.15(C) provides adjustment factors.

Voltage Drop

Resistance causes voltage loss along wire length. NEC recommends ≤3% for branch circuits.

V_drop = I × R × L_roundtrip V_drop% = (V_drop / V_source) × 100

Power Loss

P_loss = I² × R

References

  • NFPA 70: National Electrical Code (NEC), 2020 Edition
  • NEC Table 310.16: Conductor Ampacities
  • NEC 310.15(B): Ambient Temperature Correction
  • NEC 310.15(C): Bundling Adjustment Factors