Cable Tray Sizing

[blues]

In NEC Table 392.22(A) what is the purpose of column 2. I understand the (1.2Sd) is equal to sum of the diameters of 4/0 and larger cables x 1.2. But I don't understand for example the 42-(1.2Sd) for 36inch cable tray.

Thanks

 

[Smart $]

In NEC Table 392.22(A) what is the purpose of column 2. I understand the (1.2Sd) is equal to sum of the diameters of 4/0 and larger cables x 1.2. But I don't understand for example the 42-(1.2Sd) for 36inch cable tray.

Thanks

For 36" tray, note Column 1 permits 42.0 in? when all cables are smaller than 4/0. When some cables are 4/0 and larger, Column 2 simply says you have to subtract 1.2Sd from the 42.0 in? to determine the cross sectional area permitted for those cables that are smaller than 4/0.