Wire Size Calculator

The Wire Size Calculator sizes a conductor against two independent criteria at once and tells you which one decided the answer. A conductor has to be large enough to carry its current without overheating, and large enough to keep voltage drop acceptable. It reports the smallest size meeting each criterion, the governing size, which criterion governs, and the voltage drop at that size.

On short runs ampacity usually governs. On long runs voltage drop governs, often by several sizes. Knowing which one is binding tells you what to change: a longer run needs a bigger conductor for drop, while a heavier load needs one for heat.

The two sides of this calculation do not have equal standing. The voltage drop side is derived from conductor geometry and is sound. The ampacity side is illustrative only. Real ampacity depends on the insulation temperature rating, the ambient temperature, how many current-carrying conductors share a raceway, and the termination rating, none of which this tool resolves. Always size from the electrical code in force in your jurisdiction.

Purpose and model boundary

This model screens a copper conductor ladder against two independent criteria: resistive voltage drop and an illustrative ampacity table. It reports the first size meeting each criterion and selects the first size satisfying both.

The voltage-drop side is an engineering calculation based on workbook resistivity data. The ampacity side is explicitly illustrative and is not a code table. The governing size is therefore a screening result that must be verified against the code and product data applicable to the installation.

Inputs and units

• System selects DC, single-phase AC, or three-phase AC and the associated path multiplier.
• System voltage is the positive nominal voltage in volts.
• Load current is the positive circuit current in amperes.
• One-way circuit length is the positive run length in feet.
• Maximum allowed drop is entered as a percent and converted by the workbook to a fraction.
• Conductor temperature is degrees Celsius and is used for the linear resistance correction.
• Combined derating factor is the single ampacity multiplier in (0, 1]; required pre-derating ampacity is load current divided by this factor.
• Ampacity table multiplier scales the workbook’s illustrative ampacity column and must be positive. It does not replace selection from an applicable code table.
• Conductor material is currently limited by the delivered contract to copper.

Governing relationships

For candidate conductor j, let CM_j be circular-mil area, rho the workbook copper resistivity constant, alpha its temperature coefficient, T conductor temperature, and k_s the workbook stranding factor. Resistance per 1000 ft is:

R_j = rho x 1000 / CM_j x (1 + alpha x (T - 20)) x k_s

Let m_path be the system path multiplier from the workbook (2 for the out-and-back DC/single-phase path and the workbook’s three-phase multiplier for three-phase AC). The drop fraction for each candidate is:

Drop_j = m_path x I_load x R_j x L_oneway / (1000 x V_system)

The percent input is converted to:

Drop_limit = Maximum allowed drop / 100

The voltage-drop criterion passes when Drop_j <= Drop_limit.

For the ampacity criterion:

Required ampacity = I_load / Combined derating factor

Illustrative ampacity_j = workbook table ampacity_j x Ampacity table multiplier

The ampacity criterion passes when Illustrative ampacity_j >= Required ampacity.

The workbook scans candidates in ascending size order. It separately records the first drop pass and first ampacity pass, then records the first candidate for which both tests pass. The relative positions of the individual first passes determine whether voltage drop, ampacity, or both equally govern.

Calculation sequence

1. Validate current, voltage, length, and the two ampacity factors.
2. Convert the allowed percent drop to a fraction and calculate required ampacity after derating.
3. For every size in the workbook ladder, calculate temperature-corrected resistance and drop.
4. For every size, scale the illustrative ampacity and evaluate both pass flags.
5. Select the first drop-only pass, ampacity-only pass, and combined pass.
6. At the combined size, report drop, illustrative ampacity, resistance, and governing criterion. If no combined pass exists, return none and the workbook CHECK status.

Outputs and interpretation

Governing size is the first candidate that meets both workbook criteria. Smallest size meeting the drop limit and smallest size meeting ampacity expose the independent constraints. Which criterion governs identifies which individual pass occurred farther up the size ladder.

Required ampacity after derating is the load current divided by the combined factor. Drop at the governing size, illustrative ampacity at that size, and resistance at that size provide the numeric evidence. The charts show drop bars against the maximum-drop line and the illustrative ampacity across conductor sizes.

Validation and status logic

The workbook evaluates status in this order; the first matching row is returned.

A maximum allowed drop of zero is permitted by the current contract. It normally prevents any positive-resistance conductor from passing and therefore reaches the no-size CHECK. A negative maximum drop is outside the contract bound and should be rejected by the proxy before workbook calculation.

Assumptions and limitations

The drop calculation is resistive. It does not include conductor reactance or load power factor and therefore is less suitable for long runs, large conductors, or low-power-factor AC loads; the separate Voltage Drop Calculator carries those inputs. Resistance uses a linear temperature correction and workbook constants, not manufacturer-specific cable construction.

The ampacity list is illustrative and becomes even more hypothetical when scaled by the user-entered multiplier. The model combines all derating into one factor rather than separately applying ambient, bundling, insulation, and termination rules. It does not evaluate overcurrent protection, conductor insulation rating, terminal temperature, conduit fill, equipment grounding, neutral/harmonic loading, parallel-conductor rules, installation method, local code amendments, or aluminium conductors.

Restrictions and non-computing states

Current, voltage, length, combined derating factor, and ampacity multiplier must be positive. Combined derating cannot exceed one under the current contract, conductor temperature cannot be below the contract minimum, and conductor material is copper only. The result is limited to the workbook’s candidate ladder; a requirement beyond it returns no governing size rather than extrapolating.

Errors and warnings

The LogicCommons proxy enforces contract types, dropdowns, and declared bounds before calling SpreadsheetWeb. The workbook returns NOT VALID for its positive-domain failures and CHECK when the table has no combined passing size. A transport, proxy, or SpreadsheetWeb service failure is not an electrical result and must not be interpreted as a pass, failure, or recommendation.

No table, chart or text from the National Electrical Code or any other standard is reproduced. Conductor resistance is derived from the American Wire Gauge geometry and published resistivity, not copied. The ampacity values used are illustrative typical figures held in the workbook and are not code tables.

• Wikipedia. American wire gauge, for the defining geometric relation the conductor areas come from. https://en.wikipedia.org/wiki/American_wire_gauge
• Wikipedia. Electrical resistivity and conductivity, for the resistivity values and the linear temperature correction. https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity
• Wikipedia. Ampacity, for what the current-carrying criterion represents and why it depends on conditions this tool does not resolve. https://en.wikipedia.org/wiki/Ampacity
• National Fire Protection Association. NFPA 70, National Electrical Code. Consult the edition adopted in your jurisdiction for conductor ampacity, derating and termination temperature ratings. https://www.nfpa.org/codes-and-standards

NFPA, NEC and National Electrical Code are trademarks of the National Fire Protection Association, which does not endorse this workbook. Always size and verify from the code edition adopted in your jurisdiction.

Additional source notes migrated from Methodology

The drop calculation uses American Wire Gauge geometry, workbook resistivity, and linear temperature correction. The ampacity figures are illustrative rather than reproduced code-table values. See the page’s References section for the AWG, resistivity, ampacity, and NFPA sources and verify the final design against the adopted code edition.