How To Count Total Time In Sql Server 2008 (Query)

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SQL-Server-2008R2

My Problem in sql query

my column TotalTime datatype nvarchar(50)=null

Total Time = '15:00' Per Day
Total Time = '15:00'Per Day
Total Time = '15:00' Per Day
Total Time = '15:00'Per Day

count final total = '12:00' (Show This Result For my Query )

but Actual result is '60:00'

Query :-

SELECT Convert(nvarchar(10), Datepart(HOUR, DATEADD(MINUTE, SUM(DATEDIFF(MINUTE, 00:00;, TotalHour)), 00:00)))+
Convert(nvarchar(10), Datepart(MINUTE, DATEADD(MINUTE, SUM(DATEDIFF(MINUTE,00:00, TotalHour)), 00:00))) as TotalHour
FROM tbl_EmployeeAttendance
where empId='5' and work_type=1
and DateName( month , DateAdd( month , DatePart(Month,convert(datetime,Attendance_Date,103)) , 0 ) - 1 ) ='November' and YEAR(convert(datetime,Attendance_Date,103))= '2014'

Problem :- IF Total time is greater than 24 hour then totaltime calculate start 00:00
but i m count the total time in hour...

forexp:-
total time 1st day 12:00
total time 1st day 13:00
total time 1st day 11:00
total time 1st day 11:00
total time 1st day 12:00

Final Total Time = 59:00


Please Help this problem