The city may not incur indebtedness by the issue of bonds or otherwise, in any sum which, including existing indebtedness, shall exceed ten per cent of the assessed valuation of the real and personal property within the city subject to taxation as shown by the last preceding assessment roll of the city; provided, however, that in the case of fire, flood or other calamity requiring an emergency fund for the relief of the inhabitants of the city or for the repairing or rebuilding of any municipal buildings, works, bridges or streets, the legislative body of the city may borrow money due in not more than five years and in the amount not exceeding three-eighths of one per cent of the assessed valuation of all the real and personal property in the city, notwithstanding such loan may increase the indebtedness of the city beyond the limitation fixed in this chapter; provided, further[,] that in computing the net bonded indebtedness for the purposes hereof, bonds issued in anticipation of the payment of special assessments, even though they are also a general obligation of the city, mortgage bonds which are secured only by a mortgage on the property or franchise of a public utility, and bonds issued to refund moneys advanced or paid on special assessments for water main extensions, shall not be included, and the resources of the sinking fund pledged for the retirement of any outstanding bonds shall also be deducted from the amount of the bonded indebtedness.