500MCM=500 x one thousand (Roman M means one thousand) circular mils=500kcmil is a dimension for the conductor size according to American Wire Gage (AWG)
References:
1. NFPA 70-1999 (National Fire Protection Standard also known as "National Electrical Code (NEC)"
2. Fink D. G., Beaty H. W. "Standard Handbook for Electrical Engineers," 14th Edition, McGraw-Hill Co., 2000, Page 4-20
cmils=in**2 x 1,273,200=mm*2 x 1973.5
k=M=1000

Keith,
THe CM in MCM is circular mils.  The M may mean Metric, at this I can only guess.  Check out neewc.com for info source.
Cheers!
Mark

Comment:
MCM=(M=Roman 1000, C=Circular, M=Mils)

Circular Mil's is the diameter expressed in thousandths of inches suqared. For larger conductors it is expressed in thousands of CM's either kCM or MCM. To find the actual area multiply by Pi and divide by 4*10E6. Multiplying this by 645 will give square mm.

Alpha wire tables give 3/0 as ~ 170kCM and 87 square mm (depends on stranding), so the 500 MCM is 30% high in my books.

Owen

Suggestion to the previous posting: Please, would you clarify what you mean by "500MCM is 30% high in my books."

HI KEITHC

TO ANSWER YOUR QUESTION ABOUT AMERICAN WIRE GAUGE CONDUCTOR SIZING.

A 3/0 COPPER CONDUCTOR HAS A CROSS SETIONAL CIRCULAR MIL AREA OF 167,800 CM. IN INCHES THE DIAMETER IS .470IN AND THE CROSS SECTIONAL AREA IS .173SQ IN.
A 500 MCM COPPER CONDUCTOR HAS A CROSS SECTIONAL CIRCULAR MIL AREA OF 500,000 CM OR 500 MCM. IN INCHES THE DIAMETER WOULD BE .813IN. AND THE CROSS SECTIONAL AREA WOULD BE .519SQ IN. I HOPE THIS INFORMATION HELPS.

Suggestion: Because of differences in numbers in above postings, see Reference 2 in my posting
Table 4-12 includes 4/0 AWG copper wire with the following data:
Diameter=0.4096 in
Cir mils=167,800
Sq in=0.1318
Table 4-17 includes 500MCM AWG copper cable with the following conductor data:
Diameter=0.811 in
Cir mils=500,000
Sq in=0.3927