Calculator What does the voltage drop calculator calculate?
It estimates how much supply voltage is lost across conductor resistance when current flows. Enter the load current, one-way circuit length, AWG conductor size, copper or aluminum material, circuit type, and source voltage. The calculator derives resistance from its K constant and circular-mil table. For a typical single-phase two-conductor circuit, it uses the outgoing and return path; other systems use a different phase factor. The result is a planning check, not an electrical design approval.
Match the fields to one circuit condition. The voltage drop calculator expects current in amperes, one-way distance in feet, and supply voltage in volts. Select the actual conductor material, supported AWG size, and single- or three-phase model represented by the run. The voltage drop calculator reports both volts lost and that loss as a percentage of the entered source voltage. It does not report the voltage available at the load, although subtracting the modeled drop from source voltage provides that simple arithmetic comparison.
How does the voltage drop calculator formula work?
This calculator uses voltage drop = phase factor × K × current × one-way distance ÷ circular mils. The phase factor is 2 for DC or single-phase circuits and √3 for three-phase circuits. K is 12.9 for copper and 21.2 for aluminum in this simplified imperial model; circular mils come from the selected AWG size. Percentage drop is voltage drop ÷ supply voltage × 100. Alternating-current circuits can also have reactance and power-factor effects.
The voltage drop calculator is proportional to current, distance, and the selected material constant, and inversely proportional to conductor circular-mil area. Doubling current or one-way length therefore doubles the simplified resistive estimate when all other fields stay fixed. Changing supply voltage does not change the calculated volts lost in this formula, but it changes the percentage because the same loss is divided by a different source voltage. These relationships are useful entry checks, not conductor-selection rules.
Voltage drop calculator worked example
Consider a 120 V single-phase circuit with a 100 ft one-way length, 10 A load, and 12 AWG copper conductor. The table value is 6,530 circular mils and copper K is 12.9, so 2 × 12.9 × 10 × 100 ÷ 6,530 = 3.951 V. The percentage drop is about 3.951 ÷ 120 × 100 = 3.293%. This example omits connections, temperature rise, and AC impedance.
Check the voltage drop calculator by multiplying 120 V by 3.293% in decimal form; rounding aside, the result returns 3.951 V. Entering 200 ft for this example would double-count the return path because the single-phase factor already doubles the stated one-way distance. If the same geometry and current are recalculated with a supported larger conductor, the circular-mil denominator increases and the simplified drop decreases.
What are the limits of the voltage drop calculator?
This estimate cannot select conductor size or establish code compliance. Ampacity, overcurrent protection, insulation rating, termination temperature, bundling, conduit fill, ambient conditions, fault current, equipment ratings, local amendments, and three-phase or nonlinear loads require a complete design method. Electrical work can create shock, fire, and property hazards. Have a licensed or otherwise qualified electrician apply the governing code and inspect the installation.
The voltage drop calculator uses fixed table values and a simplified resistance-constant method. It has no inputs for conductor temperature, connection resistance, reactance, power factor, harmonics, parallel conductors, or voltage variation at the source. It also does not validate ampacity for the entered current. Any one of those omissions can matter in an actual installation, so retain the field values as a preliminary scenario and have the complete circuit evaluated under the governing requirements.
How does the voltage drop calculator compare with related calculators?
Voltage drop addresses delivered voltage and equipment performance. Ampacity addresses how much current a conductor can carry safely under specified conditions. A conductor can meet ampacity requirements yet have an unacceptable voltage drop on a long run, or vice versa. Treat both as separate checks within a properly designed electrical system.
Use the voltage drop calculator only for its stated simplified loss estimate after a qualified design has established the relevant circuit data. A percent-difference calculator could compare two modeled results, but it would not add the electrical factors omitted here. Resistance, ampacity, and fault-current analyses answer distinct questions and cannot be inferred from this percentage alone.
Voltage drop calculator questions
Why does the formula double the circuit length?
In a two-wire single-phase circuit, current travels to the load on one conductor and returns on another, so both lengths contribute resistance.
Does a larger wire reduce voltage drop?
Generally, a larger conductor has lower resistance per unit length and therefore lowers resistive drop. Selection must still follow the applicable electrical code and design conditions.
Within the voltage drop calculator, that effect comes from the larger circular-mil value in the formula’s denominator; it does not certify that the selected size is suitable.