Transformer Current Reference & Sizing Calculator
Full load current (FLA) lookups, wire gauge recommendations, and fuse sizing for single-phase and three-phase transformers. Use as a starting point for feeder sizing and OCPD selection — always confirm against the nameplate and applicable code.
Why FLA matters
FLA is the baseline for selecting conductors and overcurrent protection. Undersizing based on incorrect FLA is a code violation and fire hazard.
Voltage advantage
Higher voltage = lower current. Distribution at 480V or 600V reduces conductor size and heat versus 120/240V primaries.
Three-phase benefit
Three-phase spreads power across three conductors — lower per-conductor current for the same kVA means smaller wire and less voltage drop.
Nameplate is final
Calculated FLA is a guide only. Actual FLA varies with impedance, efficiency, and temperature class. Always confirm with the nameplate.
All full load amperage (FLA) values, wire gauge suggestions, and fuse/breaker sizing shown on this page are typical calculated figures for general planning purposes only. Values are derived from standard formulas and common NEC/CEC ampacity tables (THWN-2 copper at 75°C) and do not account for individual transformer design variations, impedance ratings, efficiency losses, temperature rise class, winding configurations, conduit fill, ambient temperature derating, harmonic loading, voltage drop, or manufacturer-specific tolerances. Actual FLA and required conductor/protection sizes will differ from calculated values.
Always verify using the transformer nameplate. All conductor sizing, OCPD selection, fuse sizing, and electrical installations must comply with the applicable NEC, Canadian Electrical Code (CEC Rule 26-250), or standards enforced by the Authority Having Jurisdiction (AHJ). This reference does not constitute engineering advice. Always engage a licensed professional for final design, sizing, and installation decisions. The publisher assumes no liability for errors, omissions, or outcomes from use of this information.
Transformer parameters
Enter kVA, phase, and both primary & secondary voltages above to calculate both sides at once.
| Side | Application | FLA | Min conductor | Suggested fuse (TD) | Suggested breaker | NEC ref |
|---|
Wire: NEC Table 310.16 / CEC Table 2 — THWN-2 at 75°C, single conductor in conduit, 30°C ambient. Fuse = standard dual-element time-delay next size up. Breaker = standard inverse-time next size up. Always confirm both sides with nameplate and AHJ.
Why calculate both sides?
Primary and secondary conductors are completely separate runs requiring independent sizing. High primary voltage = low primary current; low secondary voltage = high secondary current. Both must be sized correctly.
Single-phase formula
FLA = (kVA × 1000) ÷ V
Applies to both primary and secondary — use each side's voltage independently.
Three-phase formula
FLA = (kVA × 1000) ÷ (V × ?3)
?3 ? 1.732. Use each side's own voltage. Primary and secondary phase must match.
Primary OCPD (NEC 450.3)
Unsupervised installations: primary OCPD ? 125% of primary FLA. Supervised industrial: ? 250%. If no secondary OCPD, primary must be ? 125%.
Secondary OCPD (NEC 450.3)
With primary protection present: secondary OCPD ? 250% of secondary FLA. Without primary protection: secondary ? 125%. Always size conductors at 125% FLA minimum.
Aluminum conductors
Aluminum requires a larger gauge for the same ampacity. Use AL/CU-rated lugs and anti-oxidant compound at all terminations on both primary and secondary sides.
Formula: FLA = kVA × 1000 ÷ Volts · Doubling voltage cuts current in half. Low-voltage primaries (120V) produce very high primary currents even at modest kVA — always check both sides of the transformer.
Formula: FLA = kVA × 1000 ÷ (Volts × ?3) · Three-phase current is lower by a factor of ?3 versus single-phase at the same voltage. Medium-voltage entries (2,400V+) are for reference — confirm with utility and equipment specifications.
Why FLA matters
FLA is the baseline for conductor sizing, overcurrent protection, and load coordination. It directly drives breaker ratings, fuse sizing, and wire gauge selection.
Nameplate always wins
Calculated FLA is a guide. Actual FLA varies with transformer impedance, efficiency, temperature rise class, and manufacturer design. Always confirm with the nameplate.
Primary vs. secondary FLA
Calculate FLA for both primary and secondary voltages. Both sides need properly sized conductors and protection. Primary I × primary V ? secondary I × secondary V.
Low-voltage primary caution
120V or 240V primaries can produce very high currents at modest kVA. A 25 kVA / 120V transformer draws over 208 A primary — sizing must reflect this on both sides.
Distribution voltage advantage
Operating at 480V or 600V vs 120/240V can dramatically reduce feeder conductor size, cutting material cost and voltage drop on long runs.
NEC 450.3 — OCPD sizing
Supervised industrial: primary OCPD ? 250% of rated primary current. Unsupervised: ? 125%. Secondary OCPD ? 125% or 250% based on primary-side protection present.
Inrush & time-delay protection
Transformers draw 8–12× FLA on energization. Select dual-element time-delay fuses or inverse-time breakers rated for inrush to prevent nuisance tripping on startup.
Temperature rise classes
Common dry-type classes: 80°C, 115°C, 150°C, 220°C rise over 40°C ambient. Higher rise ratings allow a smaller unit but produce more heat — ventilation requirements increase.
Power factor consideration
FLA tables assume unity power factor (PF = 1.0). For loads with lower PF, apparent power (kVA) drawn from the transformer exceeds real power (kW). Always size on kVA, not kW.
Impedance (%Z) impact
Transformer %Z affects available fault current on the secondary. Lower %Z = higher fault current. Use nameplate %Z in fault current calculations for OCPD and switchgear interrupting ratings.
CEC reference (Canada)
Canadian installations reference CEC Rule 26-250 for transformer overcurrent protection. Confirm with the CEC edition adopted by your province and the AHJ.
Aluminum conductor notes
Aluminum wire requires 1–2 gauge sizes larger than copper for the same ampacity. Use aluminum-rated (AL/CU) lugs and apply anti-oxidant compound at all terminations. CEC Rule 12-118 applies.
Conduit fill & derating
More than 3 current-carrying conductors in a conduit requires ampacity derating per NEC 310.15(C) / CEC Table 5C. This may require a larger wire than the base FLA calculation suggests.
Always consult the AHJ
Local amendments to NEC or CEC may impose additional requirements. The Authority Having Jurisdiction has final say. Consult a licensed engineer or inspector when in doubt.
