How it works

A useful distinction

Percent drop depends on supply voltage

The same 3 V drop is 2.5% of a 120 V supply but 12.5% of 24 V. Compare both volts and percentage when evaluating scenarios; an identical absolute drop has a different relative effect at lower voltage.

Matching the circuit model and conductor length

Voltage drop depends on the circuit type, current, conductor properties, and length convention. Check whether the selected mode expects one-way distance or the full conductor path before entering a value. AC impedance and simple DC resistance are different models. A calculated drop does not by itself verify ampacity, protection, installation conditions, or compliance with an electrical code.

DC two-wire circuit: formula and assumptions

Loop resistance = 2×resistivity×one-way length/area; voltage drop = current×loop resistance.

Uses nominal 20°C resistivity and linear temperature correction. This is a simplified DC estimate, not an NEC ampacity or AC impedance lookup.

Entered resistance or AC impedance: formula and assumptions

Drop = k × current × one-way length × longitudinal coefficient / parallel conductors.

k=2 for DC/single phase and √3 for balanced three phase. With R/X, coefficient = R×PF ± X×sqrt(1−PF²). Entered effective coefficients are used directly; this is a small-drop approximation.

Wire gauge, area & nominal resistance: formula and assumptions

AWG diameter(mm)=0.127×92^((36−gauge)/39). DC loop resistance=2×resistivity×length/area.

Equivalent solid geometry is separate from insulated cable diameter. Nominal silver/gold/copper/aluminum constants are not manufacturer cable tables, NEC impedance values or ampacity ratings.

2017 NEC impedance reference: formula and assumptions

Line-to-line drop (V) = √3 × current(A) × length(ft) / 1000 × [R × PF + X × √(1 − PF²)].

Uses the numerical resistance and reactance relationships from 2017 NEC Chapter 9 Table 9. The lookup assumes 75 °C conductors, 60 Hz, three single UL RHH Class B stranded conductors in a cradled arrangement, up to 600 V. Length is converted to the table’s 1000-foot basis. This dated reference does not establish current local code compliance.

Worked example

Using the DC two-wire circuit example:

Inputs

Source voltage (V): 12
Current (A): 10
One-way wire length (m): 10
Conductor cross-section (mm²): 2.5
Conductor: Copper
Conductor temperature (°C): 20

Method

Loop resistance = 2×resistivity×one-way length/area; voltage drop = current×loop resistance.

Result

Example result

Voltage drop: 1.38 V

Drop percentage: 11.49%

Load voltage: 10.62 V

Loop resistance: 0.14 Ω

These are illustrative inputs. Change the values to explore a different scenario.

What to keep in mind

Cable construction, contact resistance, and conductor grade affect the result. It does not certify electrical safety or code compliance.

Formula references

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions

Should cable length include the out-and-back path?

Enter one-way length. The model applies its two-wire or balanced three-phase circuit factor separately.

What is the difference between R/X and an effective coefficient?

R/X mode combines resistance and reactance with power factor and load direction. An entered effective longitudinal coefficient is used directly.

Does the result determine safe ampacity or code compliance?

No. Nominal geometry and voltage-drop estimates do not replace manufacturer cable data, ampacity assessment or the applicable installation requirements.

Prepared by Finflo. See our methodology and editorial policy.

Voltage Drop Calculator

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