6.3 Transformers & Generator Installations

6.3 Transformers & Generator Installations

Quick Answer: Calculate transformer current first, then perform three separate checks: transformer protection under Article 450, conductor protection under Article 240, and grounding/bonding under Article 250. Keep transformer ventilation openings unobstructed and follow marked clearances; NEC 450.9 does not itself state a universal six-inch wall gap. For a generator, determine whether the neutral is switched before deciding whether the source is separately derived and where the system bond belongs.

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1. Transformer Current

Use nameplate voltages and kVA:

[ I_{1phi}=rac{ ext{kVA} imes1000}{V} ]

[ I_{3phi}=rac{ ext{kVA} imes1000}{1.732 imes V} ]

For a 45 kVA, 480-to-208Y/120-volt, three-phase transformer:

[ I_P=rac{45,000}{1.732(480)}=54.1 ext{ A} ]

[ I_S=rac{45,000}{1.732(208)}=124.9 ext{ A} ]

Do not use 120 volts to calculate total three-phase secondary current; 208 volts is the line-to-line value in the three-phase formula.

2. Transformer Overcurrent Protection

Table 450.3(B) governs transformers rated 1000 volts or less. Under the commonly tested primary-only method, a primary current of 9 amperes or more has a 125% maximum, with the table note permitting the next higher standard overcurrent rating when the calculated value is not standard. Smaller-primary-current rows have different percentages.

Where both primary and secondary protection are provided, the table permits a larger primary percentage while limiting the secondary device. Read the row, current range, and notes; “250% primary” is not a stand-alone permission detached from secondary protection.

For the 45 kVA example:

[ 54.1(1.25)=67.6 ext{ A} ]

Under the applicable table note, a 70-ampere standard primary device can be selected for primary-only transformer protection.

Article 450 protects the transformer. It does not automatically prove that the conductors on either side are protected, that inrush will coordinate, or that the equipment short-circuit rating is adequate.

3. Secondary Conductors

NEC 240.4(F) recognizes primary protection for a limited two-wire secondary relationship. A typical 208Y/120-volt, 3-phase, 4-wire secondary is not protected by the primary breaker. A line-to-neutral secondary fault or overload does not necessarily reflect enough current to operate the primary device before a secondary conductor is damaged.

Apply one of the transformer-secondary-conductor rules in 240.21(C). The permitted method depends on length, location, physical protection, conductor ampacity, transformer ratio, and the device at the termination. Key choices include:

• conductors not over 10 feet;
• conductors not over 25 feet;
• conductors outdoors;
• conductors from a feeder-tapped transformer arrangement; and
• specific industrial installation provisions.

Do not paraphrase the 25-foot formula from memory. The rule relates secondary-conductor ampacity to the primary overcurrent-device rating after applying the primary-to-secondary voltage ratio, and it also includes load, termination, and physical-protection conditions. Write the ratio with units before choosing an answer:

[ ext{primary OCPD} imesrac{V_P}{V_S} ]

Then apply the fraction and other conditions stated in the tested subsection. Secondary conductors terminate in a single overcurrent device or set of fuses as the selected rule requires; one unprotected tap does not feed another unprotected tap.

4. Grounding a Separately Derived Transformer

A transformer secondary with no direct electrical connection to another source is a separately derived system. Establish the grounded conductor, system bonding jumper, equipment bonding, grounding electrode conductor, and electrode connection under 250.30.

The system bonding jumper is installed at the source or first system disconnecting means as permitted, but not at both locations in a way that creates parallel neutral paths. On the load side, isolate grounded conductors from equipment grounding conductors. Size the grounding electrode conductor and bonding jumper from their own tables and sections; do not use the branch-circuit equipment-grounding table for every conductor.

5. Installation and Ventilation

NEC 450.9 requires ventilation adequate to dispose of transformer full-load losses without creating excessive temperature rise. Ventilation openings must not be blocked by walls or other obstructions. The Code section does not supply a universal “six inches from every wall” dimension. The actual clearance comes from the listing, nameplate, installation instructions, or a design that demonstrably maintains the required airflow.

Keep combustible material away as required, protect the transformer from physical damage, and provide access for inspection and maintenance. Above-ceiling placement is permitted only for transformers within the scope and rating of the specific 450.13 provision and only where the space and access meet its conditions. Do not assume every 50 kVA transformer belongs above a suspended ceiling.

6. Generator Output Conductors and Disconnects

Article 445 covers generator installation. Where design and operation do not prevent overload, conductors from generator terminals to the first distribution device containing overcurrent protection generally have ampacity not less than 115% of nameplate current. Apply the exception and equipment rating exactly when the design prevents overloading or when conductors are protected by an integral device.

Provide the required disconnecting and emergency-shutdown means for the generator type and location. A disconnect must open the required ungrounded conductors and be lockable where specified. Outdoor one- and two-family generators have emergency-shutdown rules that are distinct from the service emergency-disconnect rules.

7. Neutral Switching and Source Bonding

The decisive question is whether any direct solid connection remains between generator circuit conductors and another source:

• With a solid neutral transfer arrangement, the generator neutral remains connected to the service neutral. The generator is not separately derived, and a second neutral-to-frame bond would create an objectionable parallel path.
• With a switched neutral that isolates all circuit conductors from the normal source, the generator is separately derived. A system bonding jumper and grounding electrode connection are then installed under 250.30 at a permitted point.

Do not say a generator “must always float” or “must always be bonded.” Portable-generator rules, cord-and-plug use, transfer equipment, onboard devices, and separately derived status affect the answer. Trace the neutral on the one-line diagram.

Transfer equipment prevents inadvertent interconnection of normal and alternate sources unless it is specifically listed and controlled for an intentional parallel or closed-transition operation. Before energizing, verify phase sequence, system voltage, neutral switching, overcurrent coordination, grounding, and the transfer sequence.