The delta connection is a common three-phase, three-wire system in which the voltage between each pair of line wires is the actual transformer voltage. This system is especially popular for use on ungrounded systems. (Grounding will be discussed later.) A delta system can be grounded, but this introduces some complications.
Balanced three-phase loads are connected directly to the three line wires, and their kVA loads are divided equally between the three phases of the transformer bank. Single-phase loads are connected between any two of the wires, but an attempt should be made to balance the single-phase loads across the various pairs of line conductors, so that none of the transformer phases are overloaded. If a bank of three single-phase transformers is used to supply an unbalanced group of single-phase loads, the most heavily-loaded transformer should be appropriately larger than the other two. When a three-phase transformer is used instead of a group of two or three single-phase units, the unbalanced coil currents (which are the natural result of the single-phase loads not being balanced) can cause abnormal heating.
The three-phase-rating must be reduced, accordingly.
Mixed single-phase 240/120-Volt and three-phase 240-Volt loads of moderate size can be served from a bank of two or three single-phase transformers connected in delta with one unit center-tapped and grounded. This unit supplies all of the 120-Volt single-phase loads and should be oversized accordingly. This is a four-wire, center-tap-grounded delta arrangement that is commonly used to serve churches, convenience stores, etc. Section 250-5(b)(3) of the NEC requires that this system be grounded at the center tap to serve the 120-Volt lights and convenience outlets which make up the bulk of the single-phase loads.
Note: If two transformers are used in "open delta," they can serve only 58% of the kVA load served by a bank of three similar transformers.