Low voltage distribution transformers typically supply panelboards and utilization equipment. Three phase 480-208Y/120 VAC, delta-wye, dry-type transformers are among the most frequently encountered. The primary delta winding eliminates the need for a neutral conductor in the distribution system and the secondary wye winding establishes a separately derived system by bonding the neutral (X0) to ground. Establishing this separately derived system ensures the utilization voltage on the transformer secondary is fixed with respect to ground.
The 208Y/120 VAC wye secondary is versatile. Three-phase 208 VAC, single-phase 208 VAC, and single-phase 120 VAC loads can all be fed. Single-phase 208 VAC loads are connected across two of the three phases (line-to-line, A-B, B-C, or C-A) and require two-pole circuit breakers.
Supplying larger 208 VAC loads with a three-phase circuit is preferable to a 208 VAC single-phase circuit to maintain balanced loading across the phases. The unbalanced current on the wye secondary will be carried on the neutral conductor. Unless the load is guaranteed to be all linear upsizing the neutral to accommodate non-linear loads is necessary. As the current on the neutral conductor can reach 173% of the phase current under non-linear loading conditions 200% rated panelboard neutrals are common.
Ampacity
The ampacities of nominally sized low voltage distribution transformers is tabulated below. These values are necessary to size overcurrent protection and conductor size.
480V-208V Transformer Nominal Current
Primary and secondary line-to-line current for nominally-sized 480V-208V transformers.| Size [kVA] | Primary, Line [A] | Primary, Phase [A] | Secondary, Line [A] |
|---|---|---|---|
| 5 | 6 | 3 | 14 |
| 10 | 12 | 7 | 28 |
| 15 | 18 | 10 | 42 |
| 30 | 36 | 21 | 83 |
| 37.5 | 45 | 26 | 104 |
| 45 | 54 | 31 | 125 |
| 75 | 90 | 52 | 208 |
| 112.5 | 135 | 78 | 312 |
| 145 | 174 | 101 | 402 |
| 150 | 180 | 104 | 416 |
| 225 | 271 | 156 | 625 |
| 300 | 361 | 208 | 833 |
| 500 | 601 | 347 | 1388 |
| 750 | 902 | 521 | 2082 |
| 1000 | 1203 | 694 | 2776 |
Protection
NEC Article 450.3