NEC Commercial Electrical Practice Test

Dry-type transformers exceeding 112 1/2 KVa must be kept at least how many inches from combustible material unless they have a class 155 insulation system or higher?

1. 10 inches

2. 12 inches

3. 15 inches

4. 18 inches

The correct answer is: 12 inches

Transformers play a crucial role in electrical systems, and their installation must adhere to safety codes to prevent fire hazards. The National Electrical Code (NEC) stipulates separation distances between certain electrical equipment and combustible materials to mitigate the risk of fire. In the case of dry-type transformers exceeding 112 1/2 kVA, maintaining a distance of at least 12 inches from combustible materials is essential unless the transformer is equipped with a class 155 insulation system or higher. This requirement is in place because dry-type transformers can generate heat during operation, and if they are placed too close to combustible materials, there is an increased risk of those materials igniting due to the heat emitted. The specification for 12 inches is grounded in safety standards that assess the heat buildup in various types of transformers and the materials used in construction. If a transformer has a higher class of insulation, such as class 155, it can operate at elevated temperatures without igniting adjacent materials, allowing for a potentially closer installation without compromising safety. Thus, adhering to these clearance requirements is essential for safe electrical practices and is a critical aspect of transformer installation within NEC guidelines.