Module 2 · 30 minutes
Understanding the residential system
Follow power from the transformer to the load and understand a 120/240 V split-phase system.
Learning objectives
By the end, you should be able to:
- Trace the power path from utility transformer to branch-circuit load
- Explain the relationship among L1, L2, and neutral
- Distinguish neutral from equipment grounding conductors
Follow the power path
Utility transformer → service conductors → meter → service disconnect → panel → breaker → branch circuit → load
A typical dwelling receives a 120/240 V single-phase, 3-wire supply. The center-tapped transformer secondary gives us two ungrounded conductors and one grounded conductor.
| Measurement | Nominal voltage |
|---|---|
| L1 to neutral | 120 V |
| L2 to neutral | 120 V |
| L1 to L2 | 240 V |
Make it intuitive
A straight 240 V load uses L1 and L2 and ordinarily does not need the neutral. Equipment such as a range may need a neutral because some internal components operate at 120 V even though the heating elements use 240 V.
The neutral is a normal current-carrying circuit conductor. The equipment grounding conductor normally carries no current; it provides an effective path during a ground fault.
Field check
Sketch a service, panel, two-pole breaker, and straight 240 V water heater. Label every conductor and explain why the heater does or does not require a neutral.