8.3 kVA 3 = 2.8 kVA The next larger common size single-phase transformer is 3 kVA. Therefore, three single-phase, 3-kVA, 240-V-to-480-V step-up transformers can be connected to form a 3-phase transformer bank. The transformer bank will have a 3-phase rating of 9 kVA. OVERCURRENT PROTECTION Wiring a transformer circuit is one of the most diffi-cult of wiring tasks, unless the installer understands transformer fundamentals. This unit deals with dry-type transformers operating at 600 V and less. Rules for siz-Unit 14 Transformers 285
ing overcurrent protection for this type of transformer are covered in NEC Section 450-J(b). It must be noted that these rules apply only to the transformer itself, and not necessarily to the input and output circuit wires. Sizing and protecting transformer input and output wires is cov-ered in the next section. Three methods of providing overcurrent protection for transformers is covered by the National Electrical Code. Both the primary and the secondary windings must be protected. The procedure begins by calculating the primary and the secondary full-load current, using Equation 14.4 for single-phase transformers, and Equa-tion 14.5 for 3-phase transformers. A transformer can be protected by one overcurrent device on the primary side rated at not more than 1.25 (125%) times the primary full-load current, Figure 14-24. This overcurrent device can be a set of fuses in a panelboard, a fusible switch, or a circuit breaker. Consider that a 25-kVA, single-phase transformer has a 480-V primary and a 120/240-V, 3-wire secondary. The primary full-load current is 52 A. Primary full-load current 25kVAX1000 =52 A 480 v Maximum ~iz~ = 52 A X 1.25 = 65 A overcurrent device The maximum size overcurrent device is 65 A. Check-ing a fuse catalog or NEC Section 240-6, 65 A is not a Primary Disconnect ~ Maximum size / 125'Yc, of primary current (see exceptions in text) Primary protection may be placed here to also protect feeder wire 125'Yo of primary current Transformer 2-wire secondary Transformer 2-wire secondary Figure 14-24 A transformer may be protected with one overcurrent device on the primary, and sized at not more than 125% of the primary full-load current 286 Unit 14 Transformers standard size. However, Exception No. 1 to NEC Section 450-J(b)(l) permits the next higher standard size over-current device to be chosen. Therefore, the maximum size overcurrent device permitted for this situation is 70 A. A smaller size overcurrent device could have been used; for example, 60 A. In fact, the overcurrent device can be as small as desired as long as it is large enough to satisfy the load requirements. If the primary full-load current is less than 9 A, the primary overcurrent device is not permitted to exceed 1.67 (167%) times the primary full-load current, NEC Section 450-3( b )(I), Exception No. 1. If the primary cur-rent is less than 2 A, the overcurrent device is not per-mitted to exceed 3.0 (300%) times the primary full-load current, NEC Section 450-J(b)( 1).
- Spring '16
- HEAVY CURRENT(ELECTRICAL)
- Transformer Description, NEC Section