Question 1

Transformers rated over 112.5 kVA installed indoors shall be installed in a fire-resistant vault unless they are:

Accepted Answer

NEC 450.21(B) Exception allows dry-type transformers >112.5 kVA to be installed without a vault if they have Class 155 or higher insulation and are separated from combustible material by a fire-resistant heat-insulating barrier or 6 ft horizontal/12 ft vertical distance.

Question 2

Calculate the maximum primary overcurrent protection rating for a 480V/120V, 50 kVA transformer (Primary Current = 60A) using the 'Primary Only' protection method.

Accepted Answer

Per Table 450.3(B), Primary Only protection for transformers 9A or more is 125%. 60A x 1.25 = 75A. Since 75A is a standard overcurrent device size, the maximum rating is 75A.

Question 3

What is the maximum rating for the secondary overcurrent device of a transformer (600V or less) when using 'Primary and Secondary' protection, if the secondary current is 100A?

Accepted Answer

Table 450.3(B) Secondary Protection column (when Primary is up to 250%): Secondary max is 125% (Note 1 allows next size up). 100 * 1.25 = 125A. Since 125A is standard, it is the max (next size up only applies if non-standard, but since 125 is standard, it stops there).

Question 4

Dry-type transformers installed indoors and rated 112.5 kVA or less must be separated from combustible material by at least:

Accepted Answer

NEC 450.21(A) requires dry-type transformers 112.5 kVA or less to be separated from combustible material by 12 inches (unless separated by a fire-resistant heat-insulating barrier).

Question 5

Transformer vaults shall have a fire resistance rating of not less than:

Accepted Answer

NEC 450.42 requires the walls and roof of a transformer vault to have a fire resistance of 3 hours (unless protected by automatic sprinkler, then 1 hour is permitted). The standard rule is 3 hours.

Question 6

A door sill or curb is required for a transformer vault. The minimum height is:

Accepted Answer

NEC 450.43(B) requires a door sill or curb of sufficient height to confine the oil from the largest transformer, but in no case less than 4 inches.

Question 7

Ventilation openings for transformer vaults shall have a net area of not less than _____ square inches per kVA of transformer capacity.

Accepted Answer

NEC 450.45(C) requires 3 square inches per kVA of transformer capacity, but never less than 1 sq ft total.

Question 8

Askarel-insulated transformers installed indoors:

Accepted Answer

NEC 450.25 requires Askarel-insulated transformers installed indoors to be rated for such use and provided with a pressure-relief vent vented to the outside (gas is toxic).

Question 9

When a transformer supplies a 120/240V single-phase panelboard, the grounded conductor (neutral) must be bonded to:

Accepted Answer

For a separately derived system, the system bonding jumper connects the neutral to the equipment grounding conductor (case) and the grounding electrode conductor (building steel/electrode).

Question 10

Autotransformers are permitted to be used for:

Accepted Answer

NEC 210.9 (and 450.4) permits autotransformers only if the system supplied has a grounded conductor that is electrically connected to the grounded conductor of the system supplying the autotransformer (common neutral).

Question 11

A dry-type transformer is installed in a hollow space of a building (e.g., suspended ceiling). It must be rated not more than _____.

Accepted Answer

NEC 450.13(B) allows dry-type transformers rated 50 kVA or less to be installed in hollow spaces of buildings not permanently closed in by the structure.

Question 12

The disconnecting means for a transformer must be:

Accepted Answer

NEC 450.14 requires a disconnecting means that is lockable in the open position (in accordance with 110.25) to be located in sight of the transformer, unless the location is remote and the disconnect is lockable/labeled.

Question 13

Parallel operation of transformers is permitted only if:

Accepted Answer

NEC 450.7 states that transformers may be operated in parallel and switched as a unit provided they have the same transformation ratio, impedance, and other characteristics suitable for parallel operation.

Question 14

Pipes or ducts foreign to the electrical installation shall not pass through a transformer vault unless:

Accepted Answer

NEC 450.47 states that pipes or ducts foreign to the electrical installation shall not pass through the vault.

Question 15

Calculate the primary OCPD rating for a 600V transformer (Primary Current 150A) where secondary protection is NOT provided.

Accepted Answer

Table 450.3(B) Primary Only protection: 125% of 150A = 187.5A. Note 1 allows the next higher standard rating. The next standard rating after 187.5A is 200A.

Question 16

What is the minimum height of a curb required around an oil-filled transformer installed indoors?

Accepted Answer

NEC 450.43(B) requires a sill or curb of at least 4 inches for transformer vaults to contain oil.

Question 17

A zigzag (interconnected star) grounding autotransformer shall not be used on:

Accepted Answer

NEC 450.5 states that a zigzag grounding autotransformer shall not be installed on the load side of any system grounding connection, effectively prohibiting it on a 4-wire system that already has a neutral.

Question 18

Transformer ventilating openings shall be covered with:

Accepted Answer

NEC 450.45(D) requires ventilation openings to be covered with durable gratings, screens, or louvers to prevent entry of animals/foreign objects.

Question 19

When a secondary tie circuit connects two power sources, it must be protected by:

Accepted Answer

NEC 450.6(A)(3) requires a limiter (a special high-capacity fuse) at each end of the tie circuit for protection in network systems.

Question 20

Transformers shall be marked with which of the following?

Accepted Answer

NEC 450.11 requires transformers to be provided with a nameplate giving the name of the manufacturer, rated kVA, frequency, primary and secondary voltage, etc.

Question 21

Less-flammable liquid-insulated transformers installed indoors without a vault must meet listing requirements and the liquid must have a fire point of not less than:

Accepted Answer

NEC 450.23(A) requires the liquid to have a fire point of not less than 300°C.

Question 22

Are transformers permitted to be installed on the roof of a building?

Accepted Answer

NEC 450.22 (Dry Type Outdoor) and 450.23-26 imply outdoor installation is permitted. There is no prohibition against roof installation provided structural support and clearances (450.27 for oil-insulated outdoor) are met. Oil-insulated on roofs must respect fire escape clearances.

Question 23

Materials shall not be stored in transformer vaults.

Accepted Answer

NEC 450.48 explicitly states that materials shall not be stored in transformer vaults.

Question 24

A step-down transformer has a primary voltage of 480V and a secondary voltage of 120V. If 10A flows in the primary, what is the secondary current (assuming 100% efficiency)?

Accepted Answer

Power In = Power Out. 480V * 10A = 4800VA. 4800VA / 120V = 40 Amps.

Question 25

Terminal bars in transformers must be:

Accepted Answer

NEC 110.14(D) requires torque values to be achieved. NEC 450.12 requires the transformer (and terminals) to be accessible.

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