Mil Spec Std 275 - Trace Current Carrying
Capacity of Copper

Copper Weight defines the number of ounces of copper on a 1 sq. ft. area, such as 0.5, 1.0, 2.0, 3.0 oz, etc. Usually this parameter is used to specify the thickness of the copper on each layer of the board. It is very easy to work out the thickness of 1.0 oz copper on 1 sq. ft. board based on the copper density (8.9 g/cm3), which is 34.29 µm and rounded up to 35 µm.

• 0.5 oz means 17.5 µm copper thichness
• 1.0 oz means 35 µm copper thichness
• 2.0 oz means 70 µm copper thichness

Calculating maximum current capacity of conductors (usually called Ampacity) is not trivial. It involves many environmental parameters of the conductor, such as temperature ranges, insulation, frequency, etc. The table below represents conservative numbers that can be used in most (but not all) situations.

Temp Rise   C°

Copper weight
(oz/ ft²)

Trace Width
(inches)

Maximum Current Amps

.010

.015

.020

.025

.030

.050

.075

.100

.200

.250