WORKED FIELD EXAMPLE · HYPOTHETICAL
Why a continuous load changes the conductor comparison
This hypothetical example uses the English page’s existing NEC 2023 reference model: copper THHN/THWN-2, raceway, no more than three current-carrying conductors and ambient temperature no higher than 30°C. It is not a local-code approval.
1. Enter the conditions
Use the example/reset button: single-phase 240 V, noncontinuous load 0 kW, continuous load 7.2 kW, power factor 1, one-way length 100 ft, verified 60°C terminal basis, OCPD 40 A and voltage-drop target 3%.
2. Interpret the result
Actual load current is 30 A. The model’s continuous-load sizing current is 37.5 A. 10 AWG is limited to 30 A in this model and fails that sizing condition. 8 AWG has 40 A capacity at the selected terminal basis, with approximate voltage drop 4.688 V or 1.953%.
3. Check before applying
8 AWG is only a candidate within the stated model. Confirm terminal markings, actual wiring method, corrections and adopted local rules. Motor branch circuits and other excluded applications require separate calculations; do not apply this example to them.
Which US code is used?
This is a limited NEC 2023 reference model, not a claim to use the newest edition or the edition adopted at your site. States and local jurisdictions may adopt different editions and amendments. Confirm requirements with the authority having jurisdiction (AHJ) and a qualified electrical professional.
Power, continuous loads and protection
Use electrical active input kW, not motor shaft kW or kVA. Single-phase I = 1000P/(V × PF); balanced three-phase I = 1000P/(√3 × V × PF). Continuous loads expected for three hours or more are included at 125%; noncontinuous loads at 100%. This tool does not apply exceptions for 100%-rated assemblies. Load current, rather than the 125% design current, is used for the drop estimate.
Insulation and terminal limits are different
The 90°C insulation column is shown for reference only. It does not override lower equipment termination limits. The default 60°C reference covers 14 through 2 AWG in this dataset. Higher sizes are available only with verified 75°C terminations, from 1 AWG through 500 kcmil; 400 kcmil is not included. The dataset omits unlisted sizes rather than interpolating them.
The entered overcurrent protective-device (OCPD) rating must not be below the load-sizing current or exceed the selected ampacity. General small-copper-conductor limits are applied: 14 AWG 15 A, 12 AWG 20 A and 10 AWG 30 A. No next-standard-size-up exception is used. A larger wire alone cannot fix an undersized OCPD.
Voltage-drop estimate
For single phase, ΔV ≈ 2 × 12.9 × I × L / CM; balanced three phase uses √3 instead of 2. L is one-way feet and CM is circular-mil area. This DC-resistance approximation uses a copper K-factor of 12.9 at a 75°C reference and omits AC reactance, conductor arrangement and detailed power-factor impedance effects. It is a screening estimate, especially for long or large AC feeders; use manufacturer impedance data for final calculations.
The 3% default is a user-selected segment design target, not a universal enforceable NEC limit. Check total feeder-plus-branch drop, equipment requirements and applicable special rules.
Worked example: continuous 240 V load
At 7.2 kW electrical input, single-phase 240 V and PF 1, actual current is 30 A. Continuous sizing current is 37.5 A. With a 40 A OCPD, 60°C terminations and 100 ft one way, 10 AWG is excluded by capacity and its 30 A protection cap. The first modeled candidate is 8 AWG, with 40 A ampacity and an estimated 4.688 V drop (1.953%). This is not a final installation approval.
What is not checked?
Temperature/grouping correction, raceway fill, neutral counting under harmonics, faults and interruption ratings, grounding/bonding, motor starting, service/dwelling exceptions, conductor minimums for special uses and local amendments are outside this model. Consult an electrical professional before purchasing or installing wire.
Reference sources · reviewed 2026-10-17