NEC Article 450: Transformers and Transformer Vaults

Transformer questions appear on nearly every journeyman and master licensing exam, and the rules in Article 450 are some of the most commonly misread in the entire NEC. The trap is Table 450.3(B), which has two columns and two completely different sizing rules depending on whether you use only primary protection or both primary and secondary protection.

This guide breaks down every section of Article 450 likely to appear on a licensing exam, walks through Tables 450.3(A) and 450.3(B), and pairs the material with the Transformer Sizing Calculator so you can check your work.

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Article 450 Structure

Article 450 is organized into three parts:

• Part I, General Provisions (450.1 through 450.14)
• Part II, Specific Provisions Applicable to Different Types of Transformers (450.21 through 450.48)
• Part III, Transformer Vaults (450.41 through 450.48)

Most exam questions come from Part I.

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Part I: General Provisions

450.3, Overcurrent Protection (the most tested section)

Overcurrent protection of transformers depends on:

1. Voltage, over 1000 V (Table 450.3(A)) or 1000 V and less (Table 450.3(B))
2. Whether secondary protection is provided, primary only, or primary plus secondary

Exam writers test this with multi step problems. Read the question carefully to identify which combination applies.

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Table 450.3(B): Transformers 1000 Volts and Less

This is the table that drives most journeyman exam transformer questions.

Primary protection only (no secondary protection):

Primary and secondary protection both provided:

The exam trick: When the problem says "what is the maximum primary breaker," check whether secondary protection is provided. With secondary protection, primary can go up to 250 percent. Without secondary protection, primary is limited to 125 percent (or next higher standard size, never exceeding 250 percent for 9 A or more).

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Worked Example 1: 75 kVA Three Phase, Primary Protection Only

A 75 kVA, 480 V to 208Y/120 V transformer. No secondary OCPD.

Step 1, Rated primary current:

For three phase: I_primary = (kVA × 1000) / (1.732 × V) = (75 × 1000) / (1.732 × 480) = 90.2 A

Step 2, Apply 125 percent rule:

90.2 × 1.25 = 112.75 A

Step 3, Next standard size up (240.6 standard fuse and breaker sizes):

Next standard size above 112.75 A is 125 A.

Step 4, Maximum check:

125 A is below the 250 percent ceiling (90.2 × 2.5 = 225.5 A). The next standard size 125 A breaker is the answer.

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Worked Example 2: 75 kVA Three Phase, Both Primary and Secondary Protection

Same 75 kVA, 480 V to 208Y/120 V transformer, but a secondary main is now provided.

Step 1, Primary OCPD allowance (250 percent permitted because secondary protection is provided):

90.2 × 2.50 = 225.5 A

Next standard size below or equal to 225.5 A: 225 A primary breaker

Step 2, Secondary rated current:

I_secondary = (75 × 1000) / (1.732 × 208) = 208.2 A

Step 3, Secondary OCPD at 125 percent:

208.2 × 1.25 = 260.3 A

Next standard size up: 300 A secondary main breaker.

This is the realistic configuration in a commercial install. Notice how the same transformer can be protected by a 125 A primary (no secondary OCPD) or a 225 A primary plus a 300 A secondary main. The 250 percent allowance with secondary protection is what makes typical commercial transformer feeders practical.

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Worked Example 3: Small Single Phase Transformer Under 9 A

A 5 kVA, 480 V single phase to 240 V transformer.

Step 1, Rated primary current:

I_primary = (5 × 1000) / 480 = 10.4 A

This is 9 A or more, so the standard primary only rule applies: 125 percent, next size up, max 250 percent.

10.4 × 1.25 = 13 A

Next standard size: 15 A primary breaker.

If the same transformer were 2 kVA, the primary would be 2000 / 480 = 4.17 A, which is less than 9 A but more than 2 A. The 167 percent rule applies: 4.17 × 1.67 = 6.96 A, next standard size 10 A (the lowest standard inverse time circuit breaker).

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450.4, Autotransformers

Autotransformers shall be protected by overcurrent devices in each ungrounded line.

The overcurrent device cannot exceed 125 percent of the rated full load input current.

450.6, Secondary Ties

Permitted in industrial occupancies where parallel feeders or ties between secondary windings of separate transformers are required for load sharing. Rare on residential exams, commonly tested on master level industrial questions.

450.9, Ventilation

Ventilation is required to dispose of transformer full load losses without exceeding the rated temperature rise.

Ventilation openings cannot be blocked. Where transformers are equipped with thermometers or temperature monitoring, those readings are used to confirm ventilation adequacy.

450.10, Grounding

Exposed non current carrying metal parts of transformers shall be grounded under the conditions and in the manner prescribed in Part VII of Article 250.

For a separately derived transformer, the system bonding jumper and grounding electrode conductor are sized per 250.30, with the GEC sized per Table 250.66 based on the largest ungrounded secondary conductor.

450.13, Accessibility

Transformers must be readily accessible to qualified personnel for inspection and maintenance.

Exception 1: Dry type transformers rated 1000 V or less and located in the open on walls, columns, or structures do not need to be readily accessible.

Exception 2: Dry type transformers rated 1000 V or less and not exceeding 50 kVA are permitted in fire resistant hollow spaces of buildings, where ventilated to the open space.

450.14, Disconnecting Means

A disconnect for the transformer is required and must be in sight of the transformer or capable of being locked open. The disconnect must be marked to identify it.

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Working Space Around Transformers

Working space around transformers operating at 1000 V or less follows NEC 110.26:

Minimum width: 30 inches or the width of the equipment, whichever is greater.

Minimum height: 6 feet 6 inches in front of all electrical equipment.

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Part III: Transformer Vaults

450.41 through 450.48, Vault Construction

Vaults are required for indoor installations of oil insulated transformers above certain ratings (varies by transformer type), and may be required by AHJ for dry type transformers based on local fire codes.

Vault construction requirements:

Vault floors built on the ground are commonly poured concrete. Where the floor is not in contact with the earth, it must also provide the same fire resistance as the walls.

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Quick Reference: Key Numbers for Exam Day

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Related Reading

• Transformer Sizing Calculator
• NEC Article 220: Load Calculations
• NEC Article 230: Services
• NEC Article 240: Overcurrent Protection
• NEC Article 250: Grounding and Bonding
• Electrician Exam Math: Complete Formula Guide
• Start Your Practice Test