Hitachi Highly WHP13300PSDPC8FQ Installation Manual page 17

H E A T P U M P W A T E R H E A T E R C O M P R E S S O R C R I T E R I A
（27）The fuse or/and breaker should be equipped in the main circuit.
(28) The thickness of the refrigerating system using tube
the tube thickness as followed
External diameter(mm)
Thickness
6.35 6.35～11.0
>=0.5 >= 0.5
11.0～13.0 13.0～15.0
>= 0.6 >=0.6
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15.0～19.0
>= 0.8