6.4 Transformers, Generators & Misc Equipment (0504)

Transformers (NEC Article 450)

A transformer is a stationary induction device that transfers electrical energy between circuits with no moving parts. NEC 450 governs transformer installation and protection.

Overcurrent Protection — Table 450.3(B) (≤1000 V)

Where the calculated device rating does not correspond to a standard size in 240.6(A), Table 450.3(B) Note 1 permits the next higher standard rating for the 125% and secondary-side cases. These are maximums — a smaller device may always be used, and the 250% primary allowance is available only because the secondary device is doing the overload protection.

Transformer Installation

• 450.13 Accessibility — all transformers and transformer vaults must be readily accessible to qualified personnel for inspection and maintenance. 450.13(A) permits dry-type transformers 1000 V or less located in the open on walls, columns, or structures to be simply accessible; 450.13(B) permits dry-type transformers 1000 V or less and not exceeding 50 kVA to be installed in hollow spaces of buildings not permanently closed in, provided they are listed for the location and ventilation requirements are met.
• 450.21(A) Dry-type, not exceeding 112½ kVA — must be separated from combustible material by not less than 12 in. (305 mm), unless separated by a fire-resistant, heat-insulating barrier, or the transformer is rated 1000 V or less and is completely enclosed except for ventilating openings.
• 450.21(B) Dry-type, over 112½ kVA — must be installed in a transformer room of fire-resistant construction, unless the transformer has Class 155 or higher insulation and is either separated from combustible material by a fire-resistant heat-insulating barrier or by at least 6 ft horizontally and 12 ft vertically, or is completely enclosed except for ventilating openings.
• 450.22 Dry-type outdoors — a dry-type transformer installed outdoors must have a weatherproof enclosure, and those exceeding 112½ kVA must not be located within 12 in. of combustible materials of buildings unless they carry Class 155 or higher insulation and are completely enclosed except for ventilating openings.

Worked Example: Transformer Primary OCPD Sizing

A 45 kVA, 480 V - 208/120 V, 3-phase transformer, primary-only protection.

1. Primary current = 45,000 VA / (480 V × √3) = 45,000 / 831.4 = 54.1 A.
2. Primary > 9 A → Table 450.3(B) maximum OCPD = 125% × 54.1 = 67.6 A.
3. Next standard size per 240.6(A) above 67.6 A = 70 A.
4. Secondary current = 45,000 / (208 × √3) = 125 A (for reference, since secondary protection is not used here). With primary-only protection, a 70 A primary OCPD protects the transformer.

If primary and secondary protection were used, the primary could be 250% × 54.1 = 135 A → 150 A standard, with a secondary OCPD of 125% × 125 = 156 A → 160 A.

Generators (NEC Article 445)

• 445.11 Nameplate — each generator shall be marked with maker, voltage, kW, amperes, power factor, phase, RPM, and frequency.
• 445.13 Ampacity of conductors — conductors from generator terminals to the first overcurrent device shall have an ampacity not less than 115% of the nameplate current rating of the generator.
• 445.18 Disconnecting means — a disconnecting means shall be provided to disconnect the generator and all protective and regulatory control devices from the load.
• 445.11 Marking — each generator must carry a nameplate giving the manufacturer, rated frequency, power factor, number of phases (for AC), rating in kilowatts or kilovolt-amperes, normal volts and amperes corresponding to the rating, rated ambient temperature, and rated temperature rise.
• 700.6(D) — for an emergency system supplied by a solidly grounded wye generator of more than 150 volts to ground with a disconnect rated 1000 amperes or more, ground-fault indication (not automatic disconnection) is required: audible and visual signal devices to warn of a ground fault without shutting the emergency source down.
• Standby/Emergency systems — for legally required standby (Art 701), optional standby (Art 702), and emergency systems (Art 700), the generator shall also meet the transfer, signaling, and capacity rules of those articles.

Capacitors (NEC Article 460)

Article 460 Part I covers capacitors 1000 volts, nominal, and under; Part II covers those over 1000 volts, and the two parts carry different discharge times — a favorite exam discriminator.

• 460.6(A) Time of discharge — the residual voltage of a capacitor must be reduced to 50 volts, nominal, or less within 1 minute after the capacitor is disconnected from the source of supply. (For capacitors over 1000 volts, 460.28(A) allows 5 minutes to reach 50 volts.)
• 460.6(B) Means of discharge — the discharge circuit must be either permanently connected to the capacitor terminals or provided with automatic means of connecting on removal of voltage; manual switching or contactors may not be used.
• 460.8(A) Conductors — the ampacity of capacitor circuit conductors must be not less than 135% of the rated current of the capacitor, and where connected to a motor circuit, not less than one-third the motor circuit conductor ampacity.
• 460.8(B) Overcurrent protection — an overcurrent device is required in each ungrounded conductor for each capacitor bank, rated as low as practicable, except where the capacitor is connected on the load side of the motor overload device.
• 460.8(C) Disconnecting means — a disconnecting means is required in each ungrounded conductor for each capacitor bank, capable of being opened independently of the motor controller and rated at not less than 135% of the capacitor rated current, except where the capacitor is connected on the load side of the motor overload device.

Storage Batteries (NEC Article 480)

• Disconnecting means — a disconnecting means is required for all ungrounded conductors of a stationary standby battery system operating at over 60 V dc, located as required by Article 480 and readily accessible, so that the battery can be isolated from all other sources.
• Working space and ventilation — the working space about battery systems must comply with 110.26, and the location must provide ventilation sufficient to prevent an accumulation of an explosive mixture of hydrogen evolved by flooded lead-acid cells.
• Battery locations — batteries must not be exposed to physical damage or to temperatures outside their rating, the enclosure or room must be suitable for the environment, and spill containment is required for vented cells.

An E2 inspector meets Article 480 most often at the battery bank of a UPS or an emergency lighting inverter, where the working-space and ventilation checks are the practical items.

Cross-References for Inspection