Transformers under Article 450

Delgado Fabrication's 480-volt service had one gap: the office, the break room and the shop's convenience receptacles all needed 120 volts. Ray's plan was a 75 kVA dry-type transformer, 480 volts delta to 208Y/120 volts, mounted on the shop wall and feeding a new 208Y/120-volt panel. Dana's job was to size the overcurrent protection and the conductors on both sides. Transformer questions are among the most feared on master exams because they combine three-phase arithmetic, a table of percentages, rules for rounding that change depending on the side of the transformer, and the feeder tap rules of 240.21. Taken one step at a time, they are manageable.

This lesson covers transformer currents, Article 450 overcurrent protection for transformers of 1,000 volts or less, the difference between primary-only and primary-and-secondary protection, the secondary conductor rules of 240.21(C), the special case of 240.4(F), grounding of the separately derived system, and installation rules for ventilation, accessibility and working space.

Transformer currents

Every transformer problem starts with the rated current on each side, found from the kVA rating and the voltage. For a three-phase transformer, divide the volt-amperes by the line-to-line voltage times the square root of 3; for a single-phase transformer, divide by the voltage alone.

Protecting the transformer: Table 450.3(B)

Section 450.3 requires overcurrent protection for transformers, and Table 450.3(B) gives the maximum ratings for transformers rated 1,000 volts or less as a percentage of rated current. The table offers two methods. With primary-only protection, the primary device protects the transformer by itself. With primary and secondary protection, a larger primary device is allowed because a secondary device also protects the transformer. In this course's illustrative values for currents of 9 amperes or more, the percentages are these.

The table's notes add the rounding rule that makes these problems tricky. Where 125% of the current does not correspond to a standard rating, the next higher standard rating is permitted. The 250% primary value in the second method is a maximum that may not be rounded up. Smaller transformers, with currents under 9 amperes or under 2 amperes, have higher percentages in the table, and exams sometimes test them with control-size transformers.

Why choose the second method? A primary device at 125% sometimes trips on the transformer's magnetizing inrush when it is energized, which can be ten or more times rated current for a fraction of a second. The higher primary rating allowed with secondary protection avoids nuisance trips. Many designers use the second method for that reason, accepting the cost of a secondary main breaker in the new panel.

Protecting the conductors: 240.21(C)

Transformer protection under Article 450 protects the transformer. The conductors still need protection under Article 240, and the secondary conductors are the hard part, because the primary device is on the other side of the transformer. Section 240.21(C) lists the situations in which secondary conductors may be connected without overcurrent protection at the secondary terminals, each with conditions. The two most often tested are these.

• The 10-foot rule, 240.21(C)(2): secondary conductors not over 10 feet long, with an ampacity not less than the rating of the device they terminate in, enclosed in a raceway or otherwise protected, and ending in a single circuit breaker or set of fuses that limits the load to the conductor ampacity.
• The 25-foot rule for industrial installations and the general 25-foot rule, 240.21(C)(3) and (C)(6): secondary conductors not over 25 feet, where the ampacity of the secondary conductors, multiplied by the secondary-to-primary voltage ratio, is at least one-third of the rating of the primary device, and the conductors end in a single device rated not more than the conductor ampacity, with physical protection required.

In practice, the termination requirement usually controls the size, and the one-third check matters only when the primary device is large compared with the secondary conductors. Exams like the one-third calculation because it combines the voltage ratio and the fraction, so practice it until it is automatic.

The 240.4(F) exception

Section 240.4(F) allows the conductors on the secondary of a single-phase transformer with a two-wire secondary, or a three-phase delta-delta transformer with a three-wire secondary, to be protected by the primary device, provided the primary device does not exceed the secondary conductor ampacity multiplied by the secondary-to-primary voltage ratio. It does not apply to wye secondaries like Delgado's 208Y/120 volts, because a four-wire secondary carries unbalanced loads and neutral current that the primary device cannot sense. On exams, the question usually describes the transformer, and the answer depends on whether the secondary is two-wire or delta three-wire.

Grounding the separately derived system

A transformer with no direct electrical connection between primary and secondary, like Delgado's delta-wye unit, creates a separately derived system. Section 250.30 requires the secondary to be grounded with a system bonding jumper connecting the neutral to the equipment grounding conductors and the enclosure at one point, a grounding electrode conductor to a grounding electrode, and a supply-side bonding jumper sized from the secondary conductors. MELU-104 covers the sizing in detail. On an Article 450 exam question, the key fact is that the neutral and ground are bonded at the transformer or at the first disconnect, not at both, and not again downstream.

Installation rules

Article 450 also covers where and how transformers are installed. The rules most often tested are these, in general terms.

• Ventilation, 450.9: transformers with ventilating openings must be installed so the openings are not blocked, with the clearances marked on the transformer.
• Accessibility, 450.13: transformers must be readily accessible to qualified personnel, with exceptions for dry-type units of 1,000 volts or less located in the open on walls, columns or structures, and for units in suspended ceiling spaces up to a stated size.
• Dry-type transformers installed indoors, 450.21: separation from combustible material and, above a stated size, installation in a transformer room of fire-resistant construction.
• Grounding of exposed non-current-carrying metal parts, 450.10, and guarding of live parts, 450.8.
• Working space around the transformer and its disconnects, under 110.26.

import math
STANDARD = [90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400]
next_up = lambda x: min(s for s in STANDARD if s >= x)
round_down = lambda x: max(s for s in STANDARD if s <= x)

kva, vp, vs = 75, 480, 208
ip, isec = kva * 1000 / (math.sqrt(3) * vp), kva * 1000 / (math.sqrt(3) * vs)
print(f"Primary {ip:.1f} A, secondary {isec:.1f} A")
print("Primary only: 125% =", round(1.25 * ip, 1), "A ->", next_up(1.25 * ip), "A")
print("Primary and secondary: primary 250% =", round(2.5 * ip, 1), "A ->", round_down(2.5 * ip), "A;",
      "secondary 125% =", round(1.25 * isec, 1), "A ->", next_up(1.25 * isec), "A")
third = 125 * vp / vs / 3
print("25 ft rule: one third of the 125 A primary device referred to the secondary =", round(third, 1), "A")

Primary 90.2 A, secondary 208.2 A
Primary only: 125% = 112.8 A -> 125 A
Primary and secondary: primary 250% = 225.5 A -> 225 A; secondary 125% = 260.2 A -> 300 A
25 ft rule: one third of the 125 A primary device referred to the secondary = 96.2 A

Single-phase transformers and buck-boost units

Single-phase transformers follow the same Table 450.3(B) percentages, with the current found from the kVA divided by the voltage. They appear on exams most often as a small unit supplying 120/240-volt loads from a 480-volt system, or as a buck-boost transformer connected as an autotransformer to raise or lower a voltage slightly.

A buck-boost transformer is a small dry-type transformer whose windings are connected so the secondary adds to or subtracts from the supply voltage, such as raising 208 volts to about 230 volts for equipment that runs poorly at 208. When connected this way it is an autotransformer, and 450.4 and the related rules for autotransformers apply: the overcurrent device is based on the input current, and the system grounding is not changed, because an autotransformer is not a separately derived system. On exams, the key facts are that an autotransformer does not create a new grounded system and that the kVA of the load it serves can be many times its nameplate kVA.