Refer Operating temperature Vs Watt density graph to find max permissible clearance.
Dia
mm
6.5
8
10
12.5
16
19
25
inch
1/4
5/16
3/8
1/2
5/8
3/4
1
Lenght
Min mm
35
35
40
40
50
60
75
Max mm
250
250
500
500
1500
1500
1500