I may be mis-reading the post, so please inform of any detours...
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
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
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.
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
(A) --#1-- = 42.41 mm2
Equals the Multiplier
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.
An Equation to find the EGC's new increased size may be comprised by using either:
a.: Circular Mills for the Expression,
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.