Transformers

Transformer formulas, delta-wye connections, full-load currents, and OCPD sizing.

6 reference items · 3 sections

Transformer Formulas Transformer Connections Transformer OCPD Sizing

Transformer Formulas

Turns Ratio

V_p / V_s = N_p / N_s = I_s / I_p

Transformer Turns Ratio

Variable	Description	Unit
V_p	Primary voltage	Volts
V_s	Secondary voltage	Volts
N_p	Primary turns	count
N_s	Secondary turns	count
I_p	Primary current	Amperes
I_s	Secondary current	Amperes

Variations:Find V_s: V_s = V_p × (N_s / N_p)Find I_p: I_p = I_s × (V_s / V_p)

Voltage ratio equals turns ratio. Current ratio is inverse of voltage ratio (power in = power out for ideal transformer).

Transformer Current from kVA

NEC 450.3

I = (kVA × 1000) ÷ V

Single-Phase Transformer Current

Variable	Description	Unit
I	Full-load current	Amperes
kVA	Transformer rating	kVA
V	Voltage (primary or secondary)	Volts

I = (kVA × 1000) ÷ (V × 1.732)

Three-Phase Transformer Current

Variable	Description	Unit
I	Full-load current	Amperes
kVA	Transformer rating	kVA
V	Line voltage	Volts

Quick Reference: Common Transformer Currents

Transformer Full-Load Currents

kVA	120V 1Φ	240V 1Φ	208V 3Φ	480V 3Φ
3	25	12.5	8.3	3.6
5	42	20.8	13.9	6.0
10	83	41.7	27.8	12.0
15	125	62.5	41.6	18.0
25	208	104	69.4	30.1
37.5	313	156	104	45.1
45	375	188	125	54.1
50	417	208	139	60.1
75	625	313	208	90.2
100	833	417	278	120
150	1250	625	416	180
200	1667	833	555	240
300	2500	1250	833	361
500	4167	2083	1388	601

1.Values rounded to nearest whole amp for large values

2.Use these for sizing OCPD and conductors

Transformer Connections

Delta-Wye Connection

Most common three-phase distribution transformer connection.

Delta (Δ) Primary

• Line voltage = Phase voltage • Line current = Phase current × √3 • No neutral on primary side • Provides isolation from primary ground faults

Wye (Y) Secondary

• Line voltage = Phase voltage × √3 • Line current = Phase current • Neutral point available • Common: 208Y/120V, 480Y/277V

Voltage Relationships

Common Delta-Wye Configurations

Primary (Delta)	Secondary (Wye)	Phase-Neutral
480V	208Y/120V	120V
480V	480Y/277V	277V
4160V	480Y/277V	277V
12,470V	208Y/120V	120V

Wye-Wye Connection

Both primary and secondary in wye (star) configuration. • Both sides have neutral available • Requires careful attention to harmonic currents • Must handle zero-sequence currents • Less common than delta-wye • Used when neutral is needed on both primary and secondary

Delta-Delta Connection

Both primary and secondary in delta configuration. • No neutral on either side • Can operate as open-delta if one transformer fails (at ~57.7% capacity) • Good for industrial loads without neutral requirement • Line voltage = Phase voltage on both sides

Transformer OCPD Sizing

Transformer Overcurrent Protection

NEC 450.3 Transformer OCPD Sizing

Maximum OCPD for Transformers >600V

Location	Protection Method	Max % of Rated Current
Primary only	Fuse	300%
Primary only	Circuit breaker	300%
Primary + Secondary	Primary fuse	250%
Primary + Secondary	Secondary fuse	250%

Transformers 600V and Below

Maximum OCPD for Transformers ≤600V (NEC 450.3(B))

Condition	Primary Max	Secondary Max
Primary only protection	125%*	N/A
Pri ≥9A + Sec ≥9A	250%	125%*
Pri ≥9A + Sec <9A	250%	167%
Pri <9A + Sec ≥9A	167%	125%*
Pri <9A + Sec <9A	167%	167%

1.* If 125% does not correspond to standard size, next higher standard size permitted

2.Primary current = kVA × 1000 ÷ V_primary (single-phase)

3.Primary current = kVA × 1000 ÷ (V_primary × 1.732) (three-phase)

4.Where primary OCPD meets requirements, secondary protection is not required for 2-wire transformer secondaries

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