Start with the current source, then select the conductor
For a single continuous-duty motor covered by the usual U.S. NEC motor branch-circuit rule, calculate the required conductor ampacity from the applicable motor full-load current before choosing an AWG. Schneider Electric’s Load Planning guide describes the 125% requirement under Article 430.22 and the applicable 430.6 full-load-current tables. Select the ampacity column for the actual insulation and marked equipment termination ratings; do not assume a universal 75°C terminal basis. This is a method reference, not evidence that its code edition is adopted at your location.
Worked input example: a 10 hp, 460 V, three-phase motor
Illustrative design worksheet, not an installation schedule. The 2023 NEC Table 430.250 reproduced in Eaton’s 2023 code-change reference, printed page 39, gives 14 A at 460 V for a 10 hp three-phase induction motor with normal torque at usual speeds. Verify that entry and the motor classification in the adopted edition before using it. For the ordinary single continuous-duty case, the starting ampacity requirement is 14 × 1.25 = 17.5 A. A different motor type, duty or drive arrangement can require a different rule.
| Worksheet input | Example value | What it controls |
|---|---|---|
| Table current | 14 A, 2023 reference table | 17.5 A conductor requirement in this example |
| Nameplate current | 13.2 A, synthetic | Separate overload selection; do not substitute it silently for table current |
| One-way length | 30 m, synthetic | Operating and starting voltage-drop review |
| Conductor and terminals | Copper; actual insulation and terminal ratings to be collected | Available ampacity column and allowable adjustments |
| Installation | Actual ambient temperature, current-carrying conductor count and wiring method | Correction and adjustment factors |
If the applicable correction factor were 0.80 and no other adjustment applied, the unadjusted table ampacity would need to be at least 17.5 ÷ 0.80 = 21.875 A. The 0.80 here is an arithmetic teaching input, not a claimed temperature or bundling factor. Retrieve the actual factors for the installation. Check the terminal limit separately; a 90°C insulation marking does not automatically permit 90°C terminal ampacity.
Using the estimating tool
The wire size calculator estimates general conductor ampacity and voltage drop. It does not select the motor-current table or implement every NEC Article 430 exception. For a preliminary comparison with this worksheet, enter 17.5 A as the already adjusted design current and turn the tool's continuous-load option off to avoid multiplying by 125% twice. Enter 460 V, three-phase and 30 m one-way, then supply the actual material, terminal rating, ambient and grouping inputs. A returned size still needs motor-specific protection and installation checks.
Finish the selection worksheet
- Record location, locally adopted code edition, motor type, duty, horsepower, voltage and phase. For U.S. work, verify the applicable rules in NFPA 70; the published 2026 edition is not automatically the local edition.
- Record the required current source for conductors, overloads and short-circuit/ground-fault protection separately.
- Choose a conductor whose allowable ampacity meets the requirement after corrections and terminal restrictions.
- Check running and starting voltage drop, grounding, raceway fill, disconnect, protective devices, equipment listing and manufacturer instructions. Preserve these results with the chosen conductor rather than treating one AWG output as the entire design.
For Japan, use Japanese installation requirements and the exact cable manufacturer's data. NEC AWG tables and this U.S. motor example do not establish a Japanese cable size.