Dimensions and weight-per-inch values for standard US EI lamination sizes
Standard EI Proportions
T = tongue. Window width is T/2 across the legs, window height 3T/2 along them — three times the width. On an EI-75 that is a 0.375″ × 1.125″ window in a 2.25″ × 1.875″ lamination. Stack height is the thickness of the lamination pile, into the page.
EI Size Table
Core Weight = (Weight per inch from table) × (Stack height in inches)
EI Designation
Tongue Height (T)
Overall Dimensions
Magnetic Path (in)
Weight per Inch
Width (3T)
Height (2.5T)
lb
US EI Nomenclature
The EI designation number indicates the tongue (T) in hundredths of an inch (truncated):
EI-87 = 7/8" (0.875") tongue
EI-100 = 1" tongue
EI-150 = 1-1/2" tongue
Example Calculation
For a transformer using EI-100 laminations (1" tongue) with a 1.5" stack height:
Stacking factor: 0.95 (accounts for insulation and air gaps between laminations)
Magnetic path length: 6T (mean path around core)
Standard scrapless EI proportions, lamination upright: outer leg = T/2, yoke = T/2,
window width = T/2 across the legs and window height = 3T/2 along them — the height is
three times the width
Steel area = 6T² per lamination: 7.5T² overall less 1.5T² of window
Actual weights may vary ±5% depending on lamination grade and interleaving method