Transformer Sizing Calculator

This calculator helps determine the appropriate transformer size and protection based on connected loads and NEC 450.3 requirements.

Equipment

Transformer

Results

Connected Equipment

Equipment	kW Real Power	PF Power Factor	Qty	DF Diversity Factor	kVA =kW/(PF×Qty×DF)

Total Load (kVA):
Diversity Factor:
Total DF Load (kVA): = Total kVA × DF
Future Growth (%):
Required Transformer Size (kVA): = DF Load × (1 + Growth%)

Transformer Details

Primary Voltage (L-L, V):
Secondary Voltage (L-L, V):
Connection Type:
Transformer Impedance (%Z):
Efficiency at Full Load (%):
Efficiency at 1/4 Load (%):
Selected Transformer Size (kVA):
Protection Type:
Protection Method:

Calculation Results

Voltages

Primary Line Voltage (L-L): V_L-L = V_input		V
Primary Phase Voltage (L-N): V_L-N = V_L-L/√3 (Y) or V_L-L (Δ)		V
Secondary Line Voltage (L-L): V_L-L = V_input		V
Secondary Phase Voltage (L-N): V_L-N = V_L-L/√3 (Y) or V_L-L (Δ)		V

Currents

Primary Line Current: I_L = kVA×1000/(√3×V_L-L)		A
Primary Phase Current: I_φ = I_L (Y) or I_L/√3 (Δ)		A
Secondary Line Current: I_L = kVA×1000/(√3×V_L-L)		A
Secondary Phase Current: I_φ = I_L (Y) or I_L/√3 (Δ)		A

Transformer Losses

Copper Loss @ Full Load: P_cu = (16/15)×(Loss_FL-Loss_QL)×1000		W
Core Loss: P_core = Loss_FL×1000 - P_cu		W
Total Losses @ Full Load: P_total = P_cu + P_core		W
Total Losses @ 1/4 Load: P_total = (P_cu/16) + P_core		W

Protection Sizing (NEC 450.3)

Primary Protection Size: I_prot = I_L×1.25 (primary only) or ×2.5 (both)		A
Secondary Protection Size: I_prot = I_L×1.25		A

Transformer Educational Content

Transformer Fundamentals

Transformers are electrical devices that transfer energy between circuits through electromagnetic induction. They are essential for:

Voltage step-up/down for efficient power transmission
Isolation between circuits
Impedance matching

Turns Ratio: V₁/V₂ = N₁/N₂

Current Ratio: I₁/I₂ = N₂/N₁

Impedance Ratio: Z₁/Z₂ = (N₁/N₂)²

Transformer Types

Type	Description	Common Applications
Power Transformer	Large, high efficiency	Utility substations
Distribution Transformer	Medium size	Commercial buildings
Isolation Transformer	1:1 ratio	Equipment protection
Autotransformer	Single winding	Voltage adjustment

Connection Types

Connection	Primary Voltage	Secondary Voltage	Characteristics
Y-Y	Line-to-line	Line-to-line	Neutral available, 3-wire or 4-wire
Δ-Δ	Phase	Phase	No neutral, good for unbalanced loads
Y-Δ	Line-to-line	Phase	Step-down, no secondary neutral
Δ-Y	Phase	Line-to-line	Step-up, neutral available

Transformer Losses

Transformers have two main types of losses:

Copper Losses (I²R): Vary with load current (proportional to square of load)
Core Losses (Iron Losses): Constant regardless of load (hysteresis and eddy currents)

Efficiency: η = (Output Power) / (Output Power + Losses) × 100%

Typical Efficiency Values:

Transformer Size	Full Load Efficiency	1/4 Load Efficiency
15kVA	98-98.5%	97-97.5%
75kVA	98.5-99%	97.5-98%
300kVA	99-99.3%	98-98.5%

NEC 450.3 Protection Rules

Protection Type	Primary Protection	Secondary Protection
Primary Only	≤125% rated current	Not required
Primary + Secondary	≤250% rated current	≤125% rated current

Note: For transformers over 600V, different rules apply. This calculator covers common low-voltage applications.

Practical Example

Scenario:

Commercial building with 50kW lighting load at PF=0.95 and 30kW HVAC load at PF=0.85. Diversity factor=0.8, future growth=25%.

Calculation Steps:

Lighting kVA: 50kW / 0.95 = 52.63kVA

HVAC kVA: 30kW / 0.85 = 35.29kVA

Diversified Load: (52.63 + 35.29) × 0.8 = 70.34kVA

With Future Growth: 70.34 × 1.25 = 87.92kVA

Selected Size: Next standard size = 100kVA

Standard Transformer Sizes (kVA)

Single Phase	Three Phase
0.05, 0.1, 0.25, 0.5	3, 6, 9, 15
1, 1.5, 2, 3, 5	25, 30, 37.5, 45
7.5, 10, 15, 25	50, 75, 100, 150
37.5, 50, 75, 100	200, 250, 300, 500