Ampacity, voltage drop & short-circuit thermal withstand for medium-voltage transformer primary feeders — NEC 450 / IEEE C57 / ICEA basis
Dwg No.MV-001
ScaleNTS
MethodNEC 450 / ICEA P-32-382
Rev—
Reference tool only.Preliminary sizing aid for a medium-voltage transformer primary feeder. Cable ampacity, reactance and short-circuit withstand values used are representative/typical figures — verify against ICEA P-32-382 / P-54-440, the cable manufacturer's data, and NEC Article 450 before finalizing a design. Not a substitute for a qualified electrical engineer's design and a formal short-circuit/coordination study.
1
TransformerRating, primary voltage, impedance
▾
Sets the code reference cited for primary overcurrent protection sizing in the results and report. CSA C22.1 governs by default; switch to NEC only where the project is under U.S. jurisdiction.
2.4 kV
4.16 kV
12.47 kV
13.2 kV
13.8 kV
24.94 kV
34.5 kV
Typical %Z rises with rating — roughly 2–5% for smaller units, 5.5–7%+ for larger distribution-class units. Use the nameplate value once a unit is selected.
Insulation voltage class should be rated for the system voltage (100% or 133% insulation level per the cable manufacturer's recommendation for the grounding configuration in use).
3
Upstream Short-CircuitAvailable fault current at the primary terminals
▾
Default 90°C/250°C matches typical 90°C-rated EPR/XLPE shielded MV cable — confirm the exact short-circuit temperature limit against the cable manufacturer's data for the insulation actually selected.
4
Primary Protection (optional)Clearing time at the fault current above, if known
If you know the primary fuse/breaker's actual clearing time at the fault current entered in Step 3, enter it here to check the cable's short-circuit withstand against it. If left off, the tool instead reports how long the selected cable can withstand that fault current before reaching its thermal damage limit.
Result — Recommended Primary Cable
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Recommended size
Transformer FLA (primary)
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Required ampacity (125% FLA)
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Cable ampacity (this size)
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Voltage drop, this length
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Short-circuit withstand time
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Max primary fuse
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Max primary breaker
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