Calculation of wear and tear

All allowances in respect of wear and tear under sections 11(1) (c) and 12 of the Act shall be calculated on the written down or diminished value of the machinery or plant. Such written down value shall be arrived at by deducting year by year from the original cost of the machinery or plant, the annual wear and tear, calculated at the percentage rates allowed by the Director General, which has occurred since the purchase of such machinery or plant, and the allowance for wear and tear when so arrived at shall be included as a trade expense of the year of the profits and gains of which are chargeable under the Act, and in which year such machinery or plant were in actual use.