PROTECTION · CALCULATOR

Maximum wire length (short-circuit)

Free maximum wire length (short-circuit): get maximum resistive loop length from voltage, required fault current for disconnection and external loop impedance.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Voltage

Required fault current for disconnection

External loop impedance

Conductor area

kcmil

Resistivity at operating temperature

Ω·cmil/ft

Set area from AWG (geometric equivalent)

Resistivity starting point:Override with project temperature / manufacturer data.

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

Lmax = (U0 / Ia − Ze) × A / (2ρ)

Simplified equal line/return areas and resistive loop. Enter line-to-ground voltage. Excludes reactance and voltage correction.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Voltage: 120 V · Required fault current for disconnection: 160 A · External loop impedance: 0.2 Ω · Conductor area: 6.53 kcmil · Resistivity at operating temperature: 12.9 Ω·cmil/ft

Maximum resistive loop length: 139.205 ft

This example uses the stated inputs; results for the entered values appear above.

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