Transformers - kVA Ratings vs. Volt and Amps

Alternating voltage and induced electro magnetic field - e.m.f. - in a transformer.

A transformer can transfer electrical energy from one electric circuit to an another through inductively coupled conductors.

When current in the primary₁ circuit changes, a changing magnetic field will induce a changing voltage in the secondary₂circuit .

transformer - primary and secondary coil

For an ideal transformer the induced voltage and current can be expressed as

U₁/ U₂= N₁/ N₂

= I₂/ I₁ (1)

where

U = electrical potential difference (Voltage)

N = number of turns of wire

I = current (Amps)

Example - Secondary Voltage in a Transformer

The secondary voltage in a transformer with 500 primary turns, 3000 secondary turns and 230 V alternating primary current can be calculated by transforming (1) to

U₂= U₁ N₂/ N₁

= (230 V) 3000 / 500

= 1380 V

= 1.38 kV

Single Phase Transformer Full Load Current (ampere)

Single Phase Transformers - kVA Ratings vs. Volt and Amps
Transformer Rating Apparent Power (kVA)	Full Load Current (A)
Transformer Rating Apparent Power (kVA)	120 V	240 V	480 V
0.05	0.42	0.21	0.10
0.075	0.63	0.31	0.16
0.1	0.83	0.42	0.21
0.15	1.25	0.63	0.31
0.25	2.08	1.04	0.52
0.5	4.17	2.08	1.04
0.75	6.25	3.13	1.56
1	8.33	4.17	2.08
1.5	12.5	6.25	3.13
2	16.7	8.33	4.17
3	25	12.5	6.3
5	41.7	20.8	10.4
7.5	62.5	31.3	15.6
10	83.3	41.7	20.8
15	125	62.5	31.3
25	208	104.2	52.1
37.5	313	156.3	78.1
50	417	208.3	104.2
75	625	313	156
100	833	417	208
167	1392	696	348
200	1667	833	417
250	2083	1042	521

Three-Phase Transformer Full Load Current (ampere)

Three Phase Transformers - kVA Ratings vs. Volt and Amps
Transformer Rating Apparent Power (kVA)	Full Load Current (A)
Transformer Rating Apparent Power (kVA)	240 V	480 V
3	7.2	3.6
6	14.4	7.2
9	21.7	10.8
15	36.1	18.0
22	53.0	26.5
30	72.2	36.1
45	108	54.1
75	180	90.2
112.5	271	135
150	361	180
225	541	271
300	722	361
500	1203	601
750	1806	903

Example - Secondary Voltage in a Transformer

Single Phase Transformer Full Load Current (ampere)

Three-Phase Transformer Full Load Current (ampere)

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