Transformer Wire Size Calculator
// SCREEN TRANSFORMER PRIMARY AND SECONDARY CONDUCTOR SIZE FROM KVA, VOLTAGE, RUN LENGTH, AND TERMINAL BASIS BEFORE FINAL CODE REVIEW //
Enter transformer nameplate size in kVA for a first-pass conductor screen.
Distance in feet from the source equipment to the transformer primary terminals.
Distance in feet from the transformer secondary to the downstream equipment.
21.1 mm² | 85.0 A
67.4 mm² | 175 A
This tool screens conductor size only. Final transformer work still needs separate review of overcurrent protection, secondary protection method, taps, grounding, terminals, and local amendments.
| Size | Ampacity | Est. Drop | Status |
|---|---|---|---|
14 AWG 2.1 mm² | 20.0 A | 3.60% | Upsize |
12 AWG 3.3 mm² | 25.0 A | 2.26% | Upsize |
10 AWG 5.3 mm² | 35.0 A | 1.42% | Upsize |
8 AWG 8.4 mm² | 50.0 A | 0.90% | Upsize |
6 AWG 13.3 mm² | 65.0 A | 0.58% | Upsize |
4 AWG 21.1 mm² | 85.0 A | 0.36% | Pass |
| Size | Ampacity | Est. Drop | Status |
|---|---|---|---|
14 AWG 2.1 mm² | 20.0 A | 28.74% | Upsize |
12 AWG 3.3 mm² | 25.0 A | 18.07% | Upsize |
10 AWG 5.3 mm² | 35.0 A | 11.33% | Upsize |
8 AWG 8.4 mm² | 50.0 A | 7.15% | Upsize |
6 AWG 13.3 mm² | 65.0 A | 4.60% | Upsize |
4 AWG 21.1 mm² | 85.0 A | 2.88% | Upsize |
Primary and secondary conductor rules can change with the transformer overcurrent protection arrangement. Use this page as a planning screen, not a final code answer.
The lower-voltage side usually carries much more current, so secondary conductors often need more upsizing than the primary.
Voltage drop shown here is only for the selected primary or secondary run. Review the full feeder or branch-circuit path before release.
Transformer secondary tap rules and protection location are not modeled here.
Neutral, grounding electrode, and equipment grounding conductor sizing require separate checks.
Large dry-type transformers can also need ventilation, lug, and enclosure coordination beyond conductor ampacity.
Check the rest of the feeder or branch-circuit path after the transformer conductors are screened.
Review terminal temperature, ambient correction, and conductor-count derating in more detail.
Verify raceway fill once primary, secondary, and grounding conductors are selected.
Compare the transformer result against a broader feeder sizing workflow.
Inspect conductor resistance directly if you want to audit the drop math on each side.
Estimate reel weights and pull loads for larger transformer feeders.
Use the voltage-drop, ampacity, conduit-fill, cable-size, resistance, and wire-weight tools to finish the rest of the transformer wiring check.