Ventilated dry-type transformers preferably should be stored in a warm dry
location with uniform temperature. Ventilating openings should be covered
to keep out dust. If it is necessary to leave a transformer outdoors, it should
be thoroughly protected to prevent moisture and foreign material from entering.
Condensation and the absorption of moisture can be prevented or greatly
reduced by the immediate installation of space heaters or other small electric
heaters. If more convenient, incandescent lamps may be substituted for the
space heaters. For three-phase transformer rated at 750 kVA and below,
use six 150 W lamps; above 750 kVA three phase, use six 300 W lamps or the
equivalent. Two lamps should be located under each coil, one on each side of
the core. Lamps or heaters should be kept 4–6 in. from transformer coils and
should never be allowed to come in contact with transformer coil insulation.