What is the NEC maximum voltage drop?

NEC 210.19(A) recommends no more than 3% voltage drop on branch circuits and 5% combined drop on feeders and branch circuits.

How do I fix excessive voltage drop?

Use a larger wire gauge, reduce the circuit length, split the load across multiple circuits, or increase the system voltage from 120V to 240V.

What is K in the voltage drop formula?

K is the resistivity constant: 12.9 for copper and 21.2 for aluminum (in units of circular mil-ohms per foot). It represents the resistance per unit cross-section per unit length.

Does voltage drop matter for short runs?

For runs under 30 feet, voltage drop is rarely a concern. It becomes significant for runs over 75-100 feet, especially at full load current.

Voltage Drop Calculator — NEC Formula & 3% Check

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Quick Answer

Voltage drop is calculated as VD = (2 × K × I × L) / CM, where K=12.9 for copper, I=amps, L=one-way length in feet, and CM=circular mils of the wire gauge. NEC recommends keeping voltage drop under 3% on branch circuits — at 3% on 120V, end voltage is 116.4V.

Use the Voltage Drop Calculator below to get instant results.

Voltage Drop Calculator

Calculate the voltage drop across a wire run. NEC recommends keeping voltage drop under 3% for branch circuits.

Wire Gauge (AWG): One-way Wire Length (ft): Load Current (A): System Voltage: Wire Material:

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Cite This Tool

Voltage Drop Calculator. (2026). Electrical Calculators Online. Retrieved from https://electricalcalcpro.com/voltage-drop-calculator/

Voltage Drop Calculator

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