Home mortgage interest.
If your gross foreign source income (including income excluded on Form 2555 or Form 2555-EZ) does not exceed $5,000, you can allocate all of your interest expense to U.S. source mortgage interest (including points and income. Otherwise, deductible home qualified mortgage insurance premiums) is apportioned using a gross income method. Use the worksheet on page 14 of Instructions for 1116 to figure the amount .
Other interest expense.
These include investment interest, interest incurred in a trade or business, and passive activity interest. If you are a U.S. citizen, resident alien, or a domestic estate, and your gross foreign source income (including any income excluded on Form 2555 or Form 2555-EZ) does not exceed $5,000, you can allocate all of your interest expense to U.S. source income. Otherwise, each type of interest expense is apportioned separately using an "asset method" See Pub. 514 for more information.
Example. You have investment interest expense of $2,000. Your assets of $100,000 consist of stock generating U.S. source income (adjusted basis, $40,000) and stock generating foreign source income (adjusted basis, $60,000). You apportion 40% ($40,000/$100,000) of $2,000, or $800 of your investment interest, to U.S. source income and 60% ($60,000/$100,000) of $2,000, or $1,200, to foreign source income. In this example, you will enter the $1,200 apportioned to foreign source income on line 4b. You would not enter the $800 apportioned to U.S. source income on any line of Part I of Form 1116.
For more information, refer to the Form 1116 Instructions.