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Calculator Answers

Voltage Drop Calculator

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

A voltage drop calculator is an electrical engineering tool that computes the reduction in electrical potential along a circuit's length. The calculator uses Ohm's law and wire resistivity values to determine voltage loss based on wire material (copper or aluminum), wire gauge size, circuit current load, circuit length, and system phase (single or three-phase). It outputs the voltage drop in volts and percentage, indicating compliance with electrical standards. Electricians and engineers use this tool to select appropriate wire sizes and prevent equipment failure.

Enter voltage, current, one-way wire length, circuit type, and conductor information. The calculator returns the voltage drop, percent drop, voltage at the load, and the conductor properties used.

Quick Answer

Calculate the voltage drop of an electrical circuit. Enter the voltage, current, wire gauge, length, and phase to check compliance.

V

e.g. 120

A

e.g. 15

ft

Distance from source to load; the return path is added automatically. · e.g. 100

Circuit type
Conductor material
Electrical estimate only. Real wiring decisions involve code compliance (NEC tables, ampacity derating, conduit fill), temperature, AC reactance, harmonic distortion, and load behavior. Always consult a licensed electrician and the current electrical code.
Voltage drop

Voltage drop

5.79 V

4.82% drop · 114.21 V at load

Resistance per 1000 ft1.93 Ω
Total conductor length200 ft
Voltage drop5.79 V
Percent drop4.825%
Voltage at load114.21 V

DC and single-phase AC: VD = I × R_per_ft × 2 × one_way_length. Three-phase AC: VD ≈ √3 × I × R_per_ft × one_way_length. NEC recommends keeping branch-circuit drop below 3% and total feeder + branch drop below 5%.

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Examples

120V · 15A · 100 ft · 12 AWG copper · 1-phase

Drop ≈ 5.79 V (4.83%)

240V · 20A · 75 ft · 10 AWG copper · 1-phase

Drop ≈ 3.63 V (1.51%)

480V · 50A · 200 ft · 4 AWG copper · 3-phase

Drop ≈ 5.33 V (1.11%)

How it works

Voltage drop is current times resistance, with resistance computed from the conductor table for your wire material and gauge. The circuit type sets the conductor-length multiplier.

DC / single-phase · VD = I × R_per_ft × 2 × one_way_length

Three-phase · VD ≈ √3 × I × R_per_ft × one_way_length

Percent drop · VD / source_voltage × 100

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Code disclaimer. Educational only. Not NEC certified. Does not handle ampacity derating, AC reactance, temperature correction, harmonic distortion, or conduit-fill rules. Always consult a licensed electrician for permitted installations.

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Frequently asked questions

Voltage drop is the loss of electrical potential as current travels through a conductor's resistance. The longer the run and the higher the current, the more voltage is dropped along the wire, and the less voltage reaches the load.

Voltage drop = current × total resistance. Total resistance equals the resistance per 1000 ft (from a published table or the spec sheet) divided by 1000, times the total conductor length (the round trip for DC and single-phase circuits; one-way times √3 for three-phase line-to-line drop).

Current flows out to the load and back to the source, so the total conductor length the current travels is twice the one-way distance. Three-phase circuits have a more efficient geometry; the line-to-line drop uses one-way length times √3.

The NEC recommends (in informational notes) keeping branch-circuit voltage drop below 3% and total feeder plus branch drop below 5%. These are guidelines, not code requirements in most installations. Sensitive electronics, motors, and long runs benefit from staying well below the limit.

Approximate stranded-wire DC resistance values at 75°C, sourced from common electrical reference tables similar to NEC Chapter 9 Table 8. Real-world resistance depends on temperature, conductor temperature rating, and whether the wire is solid or stranded. For exact values, use the manufacturer's spec sheet or the current edition of the NEC.

No. This is an educational estimation tool. NEC compliance requires the official tables, derating for ambient temperature and number of current-carrying conductors, AC reactance for larger conductors, and approval by a qualified electrical inspector. Always consult a licensed electrician for permitted work.