Voltage Drop Calculator

The Voltage Drop Calculator works out how much voltage is lost along a conductor run, for direct current, single-phase and three-phase circuits. It reports the drop in volts and as a percentage, the voltage remaining at the load, the power lost as heat in the conductors, the conductor resistance at your operating temperature, and whether the run meets the drop limit you set. It also reports the smallest conductor size that would meet that limit.

Conductor resistance is derived from the American Wire Gauge geometry and published resistivity rather than read from a table, so the drop side of the calculation owes nothing to any copyrighted table.

This is an estimating aid, not a code compliance check. It calculates voltage drop only. It does not determine conductor ampacity, overcurrent protection, termination temperature ratings, or anything else required to size a circuit safely. Always size and verify conductors from the electrical code edition adopted in your jurisdiction.

Purpose and model boundary

This model estimates conductor resistance, voltage drop, load voltage, and resistive conductor loss for DC, single-phase AC, and three-phase AC circuits. It also searches the workbook's fixed conductor-size list for the first size that satisfies the entered voltage-drop limit. It is a preliminary estimating tool, not an ampacity calculation, code-compliance determination, or final electrical design.

The spreadsheet is the calculation authority. The page sends the named inputs to the calculation service and displays the workbook's returned results and status; no electrical equation is reimplemented in browser code.

Inputs and units

The selected size is converted to circular mils internally. Current is divided equally among parallel sets; the power-loss result remains the total across all sets and phase conductors.

Governing relationships

For an AWG conductor, diameter in inches follows the geometric series

d = 0.005 × 92^((36 − gauge)/39),

and circular mil area is the square of the diameter expressed in mils. kcmil selections use their stated circular-mil area. The workbook derives resistance rather than reading a code table:

R_20 = ρ × 1000 / CM

R_T = R_20 × [1 + α(T − 20)] × K_strand,

where ρ and α are the workbook's material constants, CM is circular-mil area, and K_strand is 1.00 for solid or 1.02 for stranded construction.

With I_set = I/N, one-way length L in ft, reactance X in ohm/1000 ft, and sin φ = sqrt(max(0, 1 − pf²)), the AC drop is

ΔV = m I_set [R_T pf + X sin φ] L / 1000,

where m = 2 for single phase and m = sqrt(3) for three phase. For DC,

ΔV = 2 I_set R_T L / 1000.

The workbook then calculates drop fraction = ΔV/V, load voltage = V − ΔV, and

power loss = conductor count × N × I_set² × R_T × L / 1000,

using two current-carrying conductors for DC or single phase and three for three phase.

Calculation sequence

1. Validate current, voltage, one-way length, selectors, parallel sets, and drop limit.
2. Convert the selected conductor size to circular mils and derive resistance at 20 °C.
3. Apply the material temperature coefficient and solid/stranded factor.
4. Divide current among parallel sets and calculate voltage drop using the selected circuit branch.
5. Calculate drop percent, load voltage, and total conductor power loss.
6. Evaluate the fixed size list from smallest to largest under the same assumptions and return the first size at or below the entered limit.
7. Return the workbook's status before the numerical results are interpreted.

Outputs and interpretation

The model returns conductor circular mils, resistance at temperature in ohm/1000 ft, current per parallel set in A, voltage drop in V and percent, load voltage in V, and total conductor power loss in W. Meets_The_Allowed_Drop compares the selected size with the entered limit. Recommended_Minimum_Size is the smallest size in the workbook's bounded search list that satisfies that limit, and Drop_At_The_Recommended_Size reports its calculated percent drop.

The recommendation addresses voltage drop only. A larger conductor, additional parallel sets, or a shorter circuit generally lowers drop, but every candidate must still be checked for ampacity, installation method, terminals, overcurrent protection, and the governing code.

Validation and status logic

The workbook evaluates the conditions in this order:

This calculator also enforces the declared selector lists, positive whole-number parallel sets, power factor from 0 through 1, nonnegative reactance and drop limit, and the workbook's published numeric bounds.

Assumptions and limitations

• Parallel sets share current equally and have identical length, material, construction, size, temperature, and reactance.
• The AC branch uses a single displacement power factor and the entered series reactance; harmonic, skin, proximity, mutual-coupling, unbalance, and transient effects are not modeled.
• Conductor temperature is uniform along the run, and the linear resistance correction remains suitable over the entered range.
• The source voltage is the declared DC, single-phase, or three-phase basis used by the workbook.
• The fixed search is discrete and bounded; it is not an economic or installation optimization.

Restrictions and non-computing states

The tool does not calculate ampacity, insulation rating, neutral loading, grounding conductor size, overcurrent protection, conduit fill, short-circuit duty, terminal ratings, derating, starting current, voltage regulation, or code compliance. It does not derive reactance from conductor spacing or raceway geometry. If no fixed-list size satisfies the limit, no out-of-range recommendation should be inferred. A NOT VALID: status supersedes displayed numerical residue.

Errors and warnings

A rejected entry means the request did not satisfy the published input rules. NOT VALID: means the workbook refused the calculation state. CHECK: means the calculation completed but the selected size exceeds the entered voltage-drop criterion. A calculation-service failure is an availability problem, not an electrical conclusion.

The delivered Voltage_Drop_Calculator.xlsx References sheet documents the AWG geometric series, the resistivity-and-temperature resistance model, the AC voltage-drop relationship, and the fact that resistance is derived rather than copied from a code table. The published input rules, protected tests, audit, and reviewer packet verify units, exact status precedence, the parallel-set power-loss calculation, and the bounded conductor search.

The reviewer packet records independent cross-checks against Voltage Drop Calculator .org, Infinity Calculator, Calculator.net, ElectricalToolbox, and Simulations4All. Those comparisons are validation aids rather than code-compliance sources. The workbook reproduces no National Electrical Code table or text and must be checked against the code edition and licensed professional requirements governing the installation.