Planning guide / voltage-drop
Introduction
The voltage drop calculator estimates the voltage lost along a conductor for a single-phase or three-phase circuit. It uses entered voltage, load current, conductor material, conductor size, and one-way route length. The result includes volts of estimated drop, the drop as a percentage of entered voltage, and estimated load voltage after subtracting the drop. It is useful for comparing circuit scenarios while planning a route or reviewing a proposed conductor size.
Choose the phase and material to match the circuit being evaluated. Use a current value that belongs to the same load scenario as the entered voltage, and use the planned one-way route length. Select a conductor size from the list so the calculator can use its circular-mil area. If the route length is entered in meters, the calculator converts it to feet before using the formula.
This is a simplified resistive estimate. It uses a stated 75°C conductor constant and listed circular-mil areas; it does not compute conductor impedance, reactance, power factor, or temperature correction. It can help identify a scenario for further review, but it does not establish equipment performance or a code determination. The on-page 3% and 5% values are informational recommendations described by the calculator as not code requirements.
Method / Calculation
How it works
The calculation uses a material constant K, current I, one-way length L, and circular-mil conductor area CM. The single-phase formula includes a factor of two for the outgoing and return path represented by this simplified model. The three-phase expression uses the square root of three. These are separate formulas; select the phase that matches the circuit rather than applying both factors.
K is 12.9 ohm-cmil per foot for copper and 21.2 for aluminum in this calculator. The selected conductor size supplies CM: for example, the listed 12 AWG entry is 6,530 circular mils. The same numeric length must be in feet before substituting it. When the user selects meters, the program multiplies that length by 3.28084 to convert it.
After calculating voltage drop in volts, the tool divides that value by the entered circuit voltage and multiplies by 100 for percent drop. Estimated load voltage is the entered voltage minus the calculated drop. The calculator’s formula trace shows the selected K, CM, and converted length so the substitution can be checked.
Single-phase VD = 2 × K × I × L ÷ CM
Three-phase VD = √3 × K × I × L ÷ CM
Percent drop = VD ÷ entered circuit voltage × 100
Estimated load voltage = entered circuit voltage − VD; meters × 3.28084 = feet
- Select single-phase or three-phase and enter the circuit voltage and current. The current is the scenario input; it is not inferred from load kW or from conductor ampacity.
- Select copper or aluminum. This determines K: 12.9 for copper and 21.2 for aluminum in the calculator’s stated approximation.
- Choose the conductor size to obtain the listed circular-mil area CM. CM is the circular-mil area from the calculator’s table, not an area in square inches or a diameter.
- Enter the one-way route length in feet or meters. If it is in meters, convert to feet by multiplying by 3.28084 before substituting into the equation.
- Multiply the phase factor by K, current, and length, then divide by CM. Keep the single-phase factor 2 or the three-phase factor √3 aligned with the selected phase.
- Divide the voltage-drop result by the entered voltage and multiply by 100. Subtract the drop in volts from entered voltage to produce the estimated load voltage.
The calculator cites NEC (NFPA 70), Chapter 9 conductor properties and Table 8 circular-mil areas, with K values stated as 75°C copper and aluminum approximations. Its assumptions explicitly omit impedance, reactance, power factor, and temperature correction. If those factors matter for the actual circuit, the simplified result is not the complete calculation.
The percentage comparison is a review cue. The page describes 3% for a branch circuit and 5% for feeder plus branch as informational recommendations, not requirements. Use the values and language displayed by the calculator, then verify applicable project criteria and perform the engineering review needed for the actual installation.
Calculation trace / Illustrative inputs
Fully worked example
Illustrative calculation: use the calculator’s starting values of a 120 V single-phase circuit, 16 A current, copper conductor, 12 AWG size, and a 100 ft one-way length. This is a worked scenario, not a measurement from an installed circuit.
- For copper, use K = 12.9 ohm-cmil/ft. The listed area for 12 AWG is CM = 6,530 circular mils, and the entered one-way length is L = 100 ft.
- Single-phase drop = 2 × 12.9 × 16 × 100 ÷ 6,530 = about 6.32 V.
- Percent drop = 6.32 V ÷ 120 V × 100 = about 5.27%. Use the unrounded value for the displayed calculation if available.
- Estimated load voltage = 120 V − 6.32 V = about 113.68 V.
- The percentage is above the calculator’s informational 3% branch-circuit reference. That comparison is a prompt to review assumptions and project criteria, not a finding that the scenario is or is not compliant.
Illustrative result: approximately 6.32 V drop, 5.27% of 120 V, and an estimated 113.68 V at the load.
Estimates for planning only, not a substitute for a licensed engineer's design or manufacturer data. This example uses the tool's 75°C K constant and selected conductor area; it leaves out impedance, reactance, power factor, and temperature correction.
Review / Input checks
Common mistakes
- Entering total conductor route distance instead of one-way length. The calculator’s single-phase formula already includes a factor of two. Enter the one-way route length described by the field and formula, rather than doubling it and then applying the factor a second time.
- Mixing feet and meters. K is expressed per foot and the formula uses length in feet. Select meters only when that is the value being entered; the calculator converts it. Do not convert a value yourself and also label it as meters.
- Using a different phase factor. Single-phase uses 2 in this model; three-phase uses √3. Confirm the circuit configuration before selecting the phase, especially when comparing a line-to-line system with a branch circuit.
- Treating the selected size as a complete conductor design. The tool uses the selected listed circular-mil area in one equation. It does not check ampacity, protection, terminal ratings, installation method, or conductor impedance. A voltage-drop estimate cannot replace those separate checks.
- Calling the informational percentage threshold a code requirement. The calculator labels the 3% branch and 5% combined recommendations as informational, not requirements. Compare with the actual design criteria instead of presenting the screen’s threshold as a compliance result.
Questions / Planning use
Frequently asked questions
Does the length field mean one-way or round-trip distance?
Enter one-way circuit length. The single-phase formula already includes a factor of two. The three-phase formula uses √3 in this calculator. Do not double the length and also apply the displayed phase factor.
What conductor constants and areas does the calculator use?
It uses K = 12.9 for copper and 21.2 for aluminum, described as 75°C approximations, and the circular-mil area attached to the selected conductor size. The listed 12 AWG value is 6,530 cmil. Check the page’s source and assumptions for the full method boundary.
How is metric route length handled?
When the meter option is selected, the calculator multiplies the entered length by 3.28084 to obtain feet, because K is stated per foot. If feet is selected, the entered number is used as feet.
Are 3% and 5% drop limits mandatory?
The calculator describes 3% for branch circuits and 5% for feeder plus branch as informational recommendations, not code requirements. Confirm the criteria that govern your project and do not use this comparison as a compliance determination.
Does the result include conductor impedance and power factor?
No. This simplified calculation uses K, current, length, and circular-mil area. The stated assumptions omit conductor impedance, reactance, power factor, and temperature correction. Use a more complete method when the circuit conditions require those factors.
Continue / Related tools
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