How to add exceptions to functions using IF statements

Hello all.

I am trying to make a doc that features current draw for electrical components for a live A/V production I'm working on.

Currently, Column A is listing the Item names, Column B lists the quantity of each given item, Column C shows if the item features automatic voltage selection in a drop down menu (yes, no, N/A), Column D shows the per/unit current draw and Column E shows the total current draw for that item type.

I have Column E on that function:

=PRODUCT B3:B30*D3:D30

What I would like to do next is have Column D and E read N/A when Column C's N/A option is selected in the dropdown list.

I can't seem to find the correct IF statement without getting #VALUE or #SPILL errors in the column E