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