NEC Art 450

[Paul Asselin]

NEC Article 450.21 (B) states that a transformer over 112 1/2 KVA shall be installed in a room of fire resistant construction. There is an excetion that states if the transformer has an insullation class of 155 or higher the fire resistant room is not needed. My question is what is a Class 155 insulation system as related to transformers???

 

[earlydean]

UL 1561 has the information you want.

Dry-type transformers have insulation ratings based on temperature.
Those with a rating above 155 do not always require a vault or fire-rated room because of the fire resistant characteristics of high-temperature insulations.

The classes of insulation are tested for their ability to withstand a stated temperature rise (between 150 to 55 degrees C.) at 200% of the rated load for 20-30 minutes. Class 155 transformer insulation systems are tested for 30 minutes at 200% current, and are allowed a 85 degree C. temperature rise.
Class 220: 150 degrees C. for 20 minutes; Class 105: 55 degrees C. for 30 minutes.