As per sub rule 2 of Rule 8D, the expenditure in relation to income which does not form part of the total income shall be the aggregate of following amounts –
(i) the amount of expenditure directly relating to income which does not form part of total income;
(ii) in a case where the assessee has incurred expenditure by way of interest during the previous year which is not directly attributable to any particular income or receipt, an amount computed in accordance with the following formula -
A X B/C
A = amount of expenditure by way of interest (other than the amount of interest directly relating to income which does not form part of total income) incurred during the previous year.
B = the average of value of investment, income from which does not or shall not form part of the total income, as appearing in the Balance Sheet of the assessee, on the first day and the last day of the previous year.
C = the average of total assets as appearing in the Balance Sheet of the assessee, on the first day and the last day of the previous year.
(iii) an amount equal to 0.5% of the average of the value of investment, income from which does not or shall not form part of the total income, as appearing in the balance sheet of the assessee, on the first day and the last day of the previous year.
Further, the term ‘total assets’ has been defined to mean total assets as appearing in the Balance Sheet excluding the increase on account of revaluation of assets but including the decrease on account of revaluation of assets.
The AO can invoke rule 8 D only when he is not satisfied with the claims made by assessee.
However if only own funds are used for earning exempted income then the value under Rule 8D (ii) will be nil, resulting in reduction of empenditure on exempted income to be disallowed by the AO. But value will be calculated as per clause (i) & (iii).
Section 14A provides that for computing the income of an assessee, no deduction shall be allowed in respect of expenditure incurred by the assessee in relation to income which does not form part of the assessee’s total income under the IT Act. As per your query if the assessee is not claiming any expenditure at all, then the AO cannot invoke section 14A itself.