What Is Circular Mil?
A circular mil is a special unit used to measure the area of a circle, mainly for the thickness of electrical wires. It tells you how big the cross section of a round wire is.
Definition
A circular mil is the area of a circle whose diameter is exactly one mil.
A mil is one thousandth of an inch.
- 1 mil = 0.001 inch
- 1 circular mil is the area of a circle with diameter 0.001 inch
Because of how it is defined, if a round wire has a diameter of d mils, its area in circular mils is simply:
Area in circular mils = d × d
For example, if a wire is 10 mils wide, its area is 10 × 10 = 100 circular mils.
History / Origin
The circular mil was created in the United States when early electricians and engineers needed an easy way to compare the sizes of round wires.
- They often worked with inches and mils, not metric units.
- Round wires are circles, so they needed a circle based area unit.
- Using circular mils let them avoid using the number pi in everyday math.
With circular mils, the area of a round wire is just the diameter in mils squared, so calculations became faster and easier for designing cables and power systems.
Symbol & Abbreviation
The circular mil is usually written in short form:
- cmil is the most common symbol.
- Sometimes people write just cm in wire tables, but this can be confusing with centimeters, so cmil is safer.
- kcmil means one thousand circular mils.
- MCM is an older way to write thousands of circular mils. Today kcmil is preferred.
Current Use Around the World
The circular mil is still used mainly in North America, especially in the electrical and building industries.
- In the United States and Canada, circular mils and kcmil are common in wire and cable catalogs.
- It is often used along with AWG American Wire Gauge sizes to describe conductor size.
- Power companies and electrical engineers use it when sizing large copper or aluminum conductors.
In most other countries, people use metric units, especially square millimeters, to measure wire area. So the circular mil is not an SI unit and is rarely used outside places that follow inch based systems.
Example Conversions
Here are some helpful conversion facts using rounded values for easier learning.
- 1 inch = 1000 mils
- 1 circular mil ≈ 0.000000785 square inches
- 1 square inch ≈ 1,273,240 circular mils
- 1 circular mil ≈ 0.0005067 square millimeters
- 1 square millimeter ≈ 1,974 circular mils
Simple examples
- If a wire is 20 mils in diameter, area in circular mils = 20 × 20 = 400 cmil.
- If a wire is 250 mils in diameter, area = 250 × 250 = 62,500 cmil.
- A 1 kcmil wire has 1,000 circular mils of area.
Conversion example to square inches
Find the area of 10,000 circular mils in square inches.
- 1 circular mil ≈ 0.000000785 square inches
- 10,000 circular mils ≈ 10,000 × 0.000000785 ≈ 0.00785 square inches
Conversion example to square millimeters
Find the area of 2,000 circular mils in square millimeters.
- 1 circular mil ≈ 0.0005067 square millimeters
- 2,000 circular mils ≈ 2,000 × 0.0005067 ≈ 1.01 square millimeters
Related Units
The circular mil is connected to several other area and wire size units.
- Mil length unit equal to 0.001 inch, used to define the circular mil.
- Square inch in² area unit in the inch system.
- Square mil the area of a square that is 1 mil on each side.
- Square millimeter mm² common metric area unit for wire size.
- kcmil one thousand circular mils, used for large wire sizes.
- AWG American Wire Gauge a system that labels wire sizes, often converted to circular mils.
FAQs
Q: What is the difference between a mil and a circular mil?
A: A mil measures length and is 0.001 inch. A circular mil measures area and is the area of a circle that has a diameter of one mil.
Q: Why do electricians use circular mils?
A: Circular mils make math for round wires easier. The area is just the diameter in mils squared, so they can quickly compare wire sizes and choose safe conductors for carrying current.
Q: Is the circular mil an SI metric unit?
A: No. The circular mil belongs to the inch based system. The SI metric unit for wire area is the square millimeter.
Q: How do I change circular mils to square millimeters?
A: Multiply the number of circular mils by about 0.0005067 to get square millimeters. For a quick estimate you can think of 2,000 cmil as about 1 mm².
Q: What is kcmil or MCM?
A: Both mean thousands of circular mils. For example 250 kcmil means 250,000 circular mils of area. Today kcmil is more common than MCM.
Q: Do other countries use circular mils?
A: Circular mils are mostly used in the United States and Canada. Many other countries use square millimeters only, so knowing how to convert between them is helpful.