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Article 450: Transformers and Transformer Vaults

May 1, 2008
The operation of any facility depends on power distribution, which, in turn, depends on transformers. Safe and reliable operation of transformers is crucial that's where Art. 450 comes in. Part I of Art. 450 contains general requirements such as guarding, marking, accessibility, and ventilation. Part II contains the requirements for different types of transformers, and Part III provides the requirements

The operation of any facility depends on power distribution, which, in turn, depends on transformers. Safe and reliable operation of transformers is crucial — that's where Art. 450 comes in. Part I of Art. 450 contains general requirements such as guarding, marking, accessibility, and ventilation. Part II contains the requirements for different types of transformers, and Part III provides the requirements for transformer vaults.

Article 450 opens by telling you that it applies to the installation of all transformers, but then it immediately lists eight exceptions. Given the amount of exceptions, what's left? The three most common areas of application: power transformers, most kinds of lighting transformers, and transformer vaults. It also covers transformers that are dedicated to supplying fire pumps, except as modified by Art. 695.

Our discussion will focus on systems that are 600V or below (secondary side), although Art. 450 also applies to secondary systems of more than 600V.

Read the label

Every transformer comes with a nameplate that identifies:

  • Manufacturer.

  • Rated kVA.

  • Primary and secondary voltage.

  • Impedance (if 25kVA or larger).

  • Required clearances (if it has ventilating openings).

Manufacturers provide this nameplate to comply with 450.11. The information on the nameplate tells you what you're working with. Make sure the drawings and other references match that information. If there's a conflict, either you have the wrong transformer or your references are wrong.


Transformers must be readily accessible to qualified personnel for inspection and maintenance [450.13]; however, you don't need to make dry-type transformers readily accessible, if you locate them:

  • In the open on walls, columns, or structures (Fig. 1).

  • Above suspended ceilings or in hollow spaces of buildings. Transformers installed in that space, however, must be rated not more than 50kVA and can't be permanently closed in by the structure (Fig. 2).

Stay cool

Although overheating is a core issue with transformers, the NEC doesn't completely address it, because the NEC isn't a design manual [90.1].

Selecting the right transformer for the application is the first step in addressing the heat issue. The NEC doesn't tell you how to do this, because it assumes you have made the proper selection in the design of the system. With Art. 450, the NEC takes you to the next step in the process of protecting the transformer against overheating: providing overcurrent protection. The Code also addresses other transformer and vault factors that determine whether you have a safe installation.

Overcurrent protection

To protect the primary winding of a transformer against overcurrent, use the percentages listed in Table 450.3(B) and its applicable notes (see Table). Typically, the Code places the requirements for more than 600V installations after the less than 600V rules, but not for transformers. So be careful not to use Table 450.3(A) when you need to use Table 450.3(B). Remember that Art. 450 is for the protection of the transformer windings — not the conductors supplying the transformer or leaving the transformer. When using Table 450.3(B), you'll notice there are two main options: primary protection only, and primary and secondary protection. Secondary protection is seldom required, so we will concentrate on primary only protection in this discussion.

Where 125% of the primary current doesn't correspond to a standard fuse or nonadjustable circuit breaker, you can use the next higher rating of overcurrent device, as listed in 240.6(A). This note only applies to currents of 9A or more, however. Let's see if you can apply that table. Here's a practice question.

What primary overcurrent device rating and conductor size do you need for a 45kVA, 3-phase, 480V — 120/208V transformer that will operate at its full rating continuously? Terminals are rated 75°C (Fig. 3).

Step 1. Calculate the primary current.

I = VA ÷ (E x 1.732)

I = 45,000 VA ÷ (480V × 1.732)

I = 54A

Step 2. Find the phe primary overcurrent device rating [240.6(A)].

54A × 1.25 = 68A, next size up 70A [Table 450.3(B) Note 1]

Step 3. Size the primary conductor to carry 54A continuously (54A × 1.25 = 68A) [215.2(A)(1)]. Protect it with a 70A protection device [240.4(B)]. A 4 AWG conductor rated 70A at 75°C meets all of the requirements [110.14(C)(1) and 310.16].

Step 4. Calculate the secondary current.

I = VA ÷ (E × 1.732)

I = 45,000 VA ÷ (208V × 1.732)

I = 125A

If the secondary conductors are no longer than 25 feet and they terminate in an overcurrent device that doesn't exceed the ampacity of the conductors, size them at 125% of the continuous load [215.2(A)(1) and 240.21(C)(6)].

Note: All secondary conductors are required to terminate in an overcurrent device sized not greater than the rating of the conductor ampacity at 75°C [240.21(C)].

Ready to try another question? What size secondary conductor is required for a 45kVA, 3-phase, 480-120/208V transformer?

Step 1. Determine the secondary current rating.

I = VA ÷ (E × 1.732)

I = 45,000 VA ÷ (208V × 1.732)

I = 125A

Step 2. Size the secondary device rating at 125% of the secondary current rating.

125A × 1.25 = 156A, 175A overcurrent device

Step 3. Size the secondary conductor where they have an ampere rating of “not less” than the rating of the secondary device [240.21(C)(2)].

2/0 AWG rated 175A at 75°C 110.14(C)(1) and Table 310.16]

Secondary conductors must terminate in an overcurrent device that doesn't exceed the ampacity of the conductors [240.21(C)(6)]. 2/0 AWG, rated 175A at 75°C, terminating on a 175A overcurrent device meets this requirement.

Transformer ventilation

Transformer ventilation requirements [450.9] can be summed up as follows:

  • Provide enough ventilation so the transformer doesn't overheat.

  • Make sure the openings aren't blocked by walls or other obstructions.

  • Install per the manufacturer's instructions.

The Fine Print Notes (FPNs) in 450.9 help address the heating issue. The first FPN recommends a couple of related standards. The second FPN, as well as 450.3 FPN No.2, tells you that transformers can heat up beyond their rating because of odd triplen harmonic currents (3rd, 9th, 15th, etc).

The heating from harmonic currents is proportional to the square of the harmonic current. This means the third (3rd) harmonic currents (180 Hz) cause heat at nine times the rate of 60 Hz current (Fig. 4).

Vault ventilation

When designing and locating a transformer vault, do your best to ventilate it to the outside air without using flues or ducts [450.41]. Other considerations may apply, and design compromises might make some or all such ventilation impractical. In that case, the NEC allows you to use flues and/or ducts. Ventilating ducts must be of fire-resistant material [450.45(F)].

Locate ventilation openings as far as possible from doors, windows, and combustible material [450.45]. A vault ventilated by natural circulation must have no more than 50% of the total opening area near the floor, with the remainder of the opening area in the roof or sidewalls near the roof [450.45(B)]. For a vault ventilated by natural circulation, the total opening area must be at least 3 square inches per kVA capacity. In no case can the area be less than 1 square foot for 50kVA or less [450.45(C)].

Cover ventilation openings to avoid unsafe conditions. You can use durable gratings, screens, or louvers to provide this protection [450.45(D). If you have indoor ventilation openings, you must provide them with automatic closing fire dampers rated not less than 1½ hours [450.45(E)]. These must close in response to a vault fire. See ANSI/UL 555-1995, “Standard for Fire Dampers” [450.45 FPN].

Vault construction

The floors, walls, ceilings, and roofs of vaults must have adequate structural strength with a minimum fire resistance of 3 hours, such as 6-inch-thick reinforced concrete [450.42]. Provide each vault doorway with a tight-fitting door with a minimum fire-resistance rating of 3 hours [450.43(A)]. This minimum fire resistance (for the vault and the door) drops to 1 hour, where an automatic sprinkler system protects the vault.

Note: The NEC vault requirements only apply when a vault is required for transformer containing oil-insulation or rated over 35,000V.

Vault doors must [450.43(C)]:

  • Swing out.

  • Be equipped with panic bars or pressure plates so the door can open from inside under simple pressure.

  • Be provided with locks that are accessible only to qualified persons.

Not a storage space

Don't allow the vault to double as a storage room [450.48]. Many occupants believe that “electrical things” like lamps belong in a vault, and the “unused space” is ideal for storing paper files. Actually, that “unused space” is very much in use — for cooling the transformers.

Filling vault space with combustibles (or items that may explode) defeats one purpose of having a vault. Making it a storage room defeats another purpose of having a vault, which is keeping people and transformers separated. The deposit and retrieval aspects of storage mean unnecessary exposure to possible hazards, such as arc blast.

An architect might think the vault is a handy place to locate a water heater or phone closet. If so, get the drawing changed. Nothing unrelated to the transformers can go in a vault — not even piping, except for vault fire protection or cooling [450.47].

Most of the Art. 450 requirements are aimed at keeping transformers cool and away from people. That effort doesn't begin or end with the NEC, however. Understanding and correctly applying Art. 450 is a crucial part of that effort. When you begin determining where transformers will be installed, vaulted, or ventilated, evaluate your options against the requirements of Art. 450.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (, one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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