Apparent power S
Apparent power for the selected mode; in sizing modes it includes the entered margin.
Electricity · Industrial electricity
Transformer kVA, current and load sizing
Relate transformer apparent power, voltage and current in single-phase or three-phase systems, or estimate required kVA from active load, power factor and a sizing margin.
Created v1.0
By Thibaut Grzelak, Mechanical Analysis Engineer
02
Results
Line current I
RMS current corresponding to the apparent power and voltage entered.
Relation and numerical substitution
S = k · U · I ; I = S / (k · U) ; Sreq = P · (1 + m) / cosφ—
U is the RMS circuit voltage; for three-phase calculations it is the line-to-line voltage. Sizing modes use electrical active load power and assume a constant power factor. Margin m increases the calculated apparent-power requirement; no standard transformer rating is selected automatically. Losses, efficiency, harmonics, inrush, impedance, internal voltage drop, temperature rise, protection and code coordination are not checked.
01Formulas and symbols
Formulas used
k
1 (1φ) ; √3 (3φ)Phase factor used in the power relationI
S / (k · U)Current from apparent power and voltageS
k · U · IApparent power from voltage and currentSreq
P · (1 + m) / cosφMinimum apparent power from active load, power factor and margin02Assumptions and limits
Scope of validity
- U is the RMS circuit voltage; for three-phase calculations it is the line-to-line voltage.
- Sizing modes use electrical active load power and assume a constant power factor.
- Margin m increases the calculated apparent-power requirement; no standard transformer rating is selected automatically.
- Losses, efficiency, harmonics, inrush, impedance, internal voltage drop, temperature rise, protection and code coordination are not checked.
03Validation example
Reference numerical case
- 3φ · P = 75 kW · cosφ = 0.90 · m = 20 % · U = 400 V
- Sload = 75 / 0.90 = 83.333 kVA
- Sreq = 83.333 × 1.20 = 100.000 kVA
- I = 100000 / (√3 × 400) ≈ 144.34 A
04References
Technical references
- IEC 60076-1 — Power transformers — Part 1: General.
FAQ
Which voltage should I enter?
For single phase, enter the circuit voltage. For three phase, enter RMS line-to-line voltage.
What does the sizing margin do?
It increases the calculated apparent-power requirement to reserve capacity above the entered active load.
Does the calculator choose a standard transformer rating?
No. It calculates a kVA requirement; selection of a catalogue rating remains a project and manufacturer decision.
Are losses and inrush current included?
No. Transformer losses, inrush, protection and short-circuit duties are outside this calculation.