wewire2;I may be mis-reading the post, so please inform of any detours...

A:

In Chapter 9, table 8 of the NEC I'm curious as to why the conversions from mm2 to Circular mils are missing for wire sizes 250MCM to 2000MCM.

If I am reading this query correctly, the AWG sizes => 250 MCM will append with Three Zeroes;

i.e.: 250,000 CM to 2,000,000 CM

"MCM" = 1,000 Circular Mills - being synonymous with "Kcmil", also meaning 1,000 Circular Mills.

Old English / Roman Numerals (IIRC) term "Mill" for 1,000, so "MCM" = "Mill, Circular Mills" - or 1,000 Circular Mills...

Translation; 250 MCM = 250,000 Circular Mills.

The first "NON-MCM" size downward from the first MCM is:

4/0 AWG; which is 211,600 CM

B:

It's easy to do the conversions but can't figure why they are left out

The AWG (CM) to Square Millimeters conversion factor appears to be apx. 1,974.34 CM = 1.0 mm2.

Using the above values with 250 MCM:

250,000 / 1,974.34 = 126.6245935 mm2.

Rounded to nearest "One" makes the converted value "127 mm2", which is listed in Table 8

C:

Also 250.122(B) Says to use the circular mil area of the conductor for proportionate up-sizing. If it's proportionate then what's the reason for not using mm2 figures in the calculations?

Math-wise, the equation may be compiled using either Circular Mills ("CM"), or Square Millimeters ("mm2") for expressions.

The resultant - if mm2 was used, would be the equivalent Conductor Size per the American Wire Gauge (AWG).

The Conductor's Size - in AWG listing, would be the closest related CM -vs- mm2 size.

Example:

A 100 Amp Feeder, originally planned to use (3)#1 THHN CU + (1)#8 CU EGC, is up-sized to use (3)#2/0 THHN CU., for Voltage Drop compensation.

What is the corresponding size for the Equipment Grounding Conductor (EGC), using Metric Values.

a.: Conductor Area - Values For Expressions:

#8 = 8.367 mm2 (16,510 CM)

#1 = 42.41 mm2 (83,690 CM)

#2/0 = 67.43 mm2 (133,100 CM)

b.: Ratio Equation:

C = B/A where:

"A" = Original Conductor Size (#1 THHN)

"B" = New Conductor Size (#2/0 THHN)

"C" = Multiplier.

Using the Metric equivalents:

(B) --2/0-- = 67.43 mm2

Divided By

(A) --#1-- = 42.41 mm2

Equals the Multiplier

(C) --1.589--

Multiply the equivalent Metric Value of #8, by the Multiplier achieved above:

8.367 mm2 * 1.589 = 13.30 mm2

The new adjusted EGC Size is 13.30 mm2

Multiply 13.30 mm2 by 1,974.34 to derive the CM Equivalent, which is 26,259 CM - slightly higher than the listed CM of #6, which is 26,240 CM.

Table 8 shows 13.30 mm2 to be equivalent to #6 AWG, so the Equation using Metric Expressions arrived at the same end value.

In conclusion;

An Equation to find the EGC's new increased size may be comprised by using either:

a.: Circular Mills for the Expression,

Or,

b.: Square Millimeters for the Expression.

Either way, the resultant value will be applied to a relevant AWG size.

(such as the equivalent sizes listed per NEC Chapter 9, Table 8)There 'Ya go, nice and clear!;

Clear as Mud / Thick, Dense Fog / Pea Soup, etc...

Let me know if the above "Math = Mud Transparency" stuff came close to answering your queries, or if there are any additional questions.

--Scott (EE)