# Circular mil

A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil (one thousandth of an inch). It corresponds to 5.067×10-4 mm². It is a unit intended for referring to the area of a wire with a circular cross section. As the area in circular mils can be calculated without reference to π, the unit makes conversion between cross section and diameter of a wire slightly easier.

The area in circular mils, A, of a circle with a diameter of d mils, is given by the formula:

$A= d^2$

Electricians in Canada and the United States are familiar with the circular mil because the National Electrical Code (NEC) uses the circular mil to define wire sizes larger than 0000 AWG. In many NEC publications and uses, large wires may be expressed in thousands of circular mils, which is abbreviated in two different ways: MCM[1] or kcmil.[2] For example, one common wire size used in the NEC has a cross-section of 250,000 circular mils, written as 250 kcmil or 250 MCM, which is the first size larger than 0000 AWG used within the NEC.

## Equivalence to other units of area

Although square mils are rarely used, it is convenient to convert between square inches and circular mils. As a classic example taken from the NEC, a 0000 AWG solid wire is defined to have a diameter of exactly 0.46 inch.

Formula 1: Circular Mil

Note: 1 inch = 1000 mils
$d= 0.46$ inch = 460 mils
$A= d^2$
$A= 460^2$ = 211,600 circular mils
(This is the same result as the AWG circular mil formula shown below for n=-3)

Formula 2: Square Mil

$d= 0.46$ inch = 460 mils
$r= {d \over 2}$ = 230 mils
$A= \pi r^2$
$A= \pi \times 230^2 = 52,900 \pi \approx 166,190.25$ square mils

Formula 3: Square Inch

$d= 0.46$ inch
$r= {d \over 2}$ = 0.23 inch
$A= \pi r^2$
$A= \pi \times (0.23)^2 = .0529 \pi \approx 0.16619$ square inches

Formula 4: Solving for Circular Mil

$A= A$
$211,600$ circular mils$= 52,900 \pi$ square mils
1 circular mil $= {52,900 \pi \over 211,600}$ square mils
1 circular mil $= {\pi \over 4}$ square mils

Formula 5: Solving for Square Mil

1 square mil $= {4 \over \pi}$ circular mils

Therefore, the following conversions apply:

• To obtain square mils   ⇒ (# of circular mils) × π ÷ 4
• To obtain square inches ⇒ (# of circular mils) × π ÷ 4,000,000
• To obtain circular mils ⇒ (# of square mils)   × 4 ÷ π
• To obtain circular mils ⇒ (# of square inches) × 4,000,000 ÷ π

1 circular mil is approximately equal to:

• 0.7854 square mils (1 square mil is about 1.273 circular mils)
• 7.854×10-7 square inches (1 square inch is about 1.273 million circular mils)
• 5.067×10-10 square metres
• 506.7 μm²

1000 circular mils = 1 MCM or 1 kcmil, and is (approximately) equal to:

• 0.5067 mm², so 2 kcmil ≈ 1 mm² (a 1.3% error)

Therefore, for practical purposes such as wire choice, 2 kcmil ≈ 1 mm² is a reasonable rule of thumb for many applications.

When large diameter wire sizes are specified in kcmil, such as the ubiquitous 250 kcmil or 350 kcmil wires, the wire's diameter can be easily determined with the following formula:

Formula 6: Diameter

Note: We first convert from kcmil to circular mil
$A= 250$ kcmil
$A= 250,000$ circular mils
$d= \sqrt{A}$
$d= \sqrt{250,000}$ = 500 mils = 0.500 inch

Thus, this wire would have a diameter of a half inch or 12.7 mm.

## AWG circular mil formula

The formula to calculate the circular mil for any given AWG (American Wire Gauge) size is as follows. An represents the circular mil area for the AWG size n.

$A_n =(5 \times 92^\frac{36-n}{39})^2$
• For example, a number 12 gauge wire would use n = 12; and the calculated result would be 6529.946789 circular mils

## Standard sizes

Standard sizes are from 250 to 400 in increments of 50 kcmil, 400 to 1000 in increments of 100 kcmil, and from 1000 to 2000 in increments of 250 kcmil.[3]

The diameter in the table below is that of a solid rod with the given conductor area in circular mils. Stranded wire is around 5% larger in diameter to allow for gaps between the strands, depending on the number and size of strands.

Standard kcmil wire sizes
& solid copper equivalents
kcmil
MCM
mm² Diameter NEC copper wire
ampacity with
60/75/90 °C
insulation (A)[4]
in. mm
250 126.7 0.500 12.70 215 / 255 / 290
300 152.0 0.548 13.91 240 / 285 / 320
350 177.3 0.592 15.03 260 / 310 / 350
400 202.7 0.632 16.06 280 / 335 / 380
500 253.4 0.707 17.96 320 / 380 / 430
600 304.0 0.775 19.67 355 / 420 / 475
700 354.7 0.837 21.25 385 / 460 / 520
750 380.0 0.866 22.00 400 / 475 / 535
800 405.4 0.894 22.72 410 / 490 / 555
900 456.0 0.949 24.10 435 / 520 / 585
1000 506.7 1.000 25.40 455 / 545 / 615
1250 633.4 1.118 28.40 495 / 590 / 665
1500 760.1 1.225 31.11 520 / 625 / 705
1750 886.7 1.323 33.60 545 / 650 / 735
2000 1013.4 1.414 35.92 560 / 665 / 750