Residential distribution transformer (utility pole): A single utility pole transformer feeding a typical home provides two 120 V “legs.” Which construction detail on the secondary winding is used to deliver these two phases (split-phase supply)?

Introduction / Context:
Many residential services (e.g., North American split-phase) use a single-phase transformer on the pole to provide two 120 V legs that are 180 degrees apart. Understanding how the transformer produces both legs helps with panel wiring, balancing loads, and troubleshooting voltage complaints.

Given Data / Assumptions:

• The primary is single-phase and connected to the distribution line.
• The secondary provides two 120 V legs and a neutral.
• Loads may be 120 V (leg to neutral) or 240 V (leg to leg).

Concept / Approach:
Split-phase service is obtained by center-tapping the secondary of a single-phase transformer. The two ends of the secondary provide opposite polarities with respect to the center tap (neutral). Thus, each end to neutral is about 120 V, while end-to-end is about 240 V. This arrangement does not require a two-phase primary; the “two phases” seen at the home are simply two halves of one secondary winding referenced to its center tap.

Step-by-Step Solution:
Use a single secondary winding with a center tap taken as neutral.Measure from either end to neutral → approximately 120 V.Measure end-to-end → approximately 240 V (legs are 180° apart).Therefore, the hardware feature enabling this is the center tap on the secondary.

Verification / Alternative check:
Service entrance diagrams show a three-wire service: L1, N (center tap), and L2. Meter and panelboards accept this configuration for both 120 V and 240 V circuits.

Why Other Options Are Wrong:
Two phases on the primary: utilities use single-phase primaries for such transformers.Separate ground for each phase: grounding does not create two legs.Small coefficient of coupling: unrelated to split-phase creation.

Common Pitfalls:
Calling split-phase “two-phase”; it is a center-tapped single-phase system.

Final Answer:
a center tap on the secondary