Wiring and Protection
What a Split-Phase Supply Means
Learn common center-tap voltage relationships and neutral imbalance.
You are applying “What a Split-Phase Supply Means” to a real system and need the concept, assumptions and verification boundary to stay clear.
Understand a North American-style 120/240 V split-phase secondary: each outer leg is about 120 V from the center-tap neutral, the legs are about 240 V apart, and neutral carries imbalance.
- Select or check first
- “Opposite legs” describes polarity, not two independent phases. The outer-to-neutral waveforms reach opposite polarities at the same instant because they are measured from opposite ends of one center-tapped secondary.
- Controlling assumption
- Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.
- Top risk
- Do not measure a service panel for learning. Both outer conductors are energized relative to neutral and earth, and opening a branch device does not isolate the service equipment. Available fault current can be extremely high.
- Proof
- Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Start with the basics Optional foundation before “What a Split-Phase Supply Means”
Electrical-engineering basics
Electrical behaviour becomes easier to reason about when every voltage has a reference and every current has a complete return path.
- Voltage
- Voltage is potential difference between two points. A voltage statement is incomplete unless both points or its reference are known.
- Current
- Current is charge flow through a path. It does not disappear after a component; in a complete circuit it returns to the source.
- Resistance and impedance
- Resistance opposes steady current. Impedance extends that idea to changing signals and can depend on frequency through capacitance and inductance.
- Power and heat
- Electrical power is the rate of energy transfer. Some becomes useful output and some becomes heat that must stay within component and enclosure limits.
- Rating versus operating point
- A rating is a limit or qualified condition, not a target. The operating point must remain inside all relevant voltage, current, power, temperature and timing limits.
How to use this article: Draw the source, load, reference and return path before calculating. Treat exposed or stored electrical energy as hazardous until isolation is proved.
On this page
Split phase is a single-phase supply arrangement commonly used for North American residential service. A center tap on a transformer secondary becomes the neutral reference. The two outer conductors are opposite ends of the same winding—not two unrelated sources and not a three-phase system. Other regions commonly use different nominal voltages and distribution arrangements, so this article describes a location-dependent example rather than a worldwide household standard.
One center-tapped winding provides two voltage relationships
Diagram description
The two half-winding voltages have opposite polarity when measured using the same neutral reference. The transformer photograph is generic and its visible terminals are not a connection guide.
Voltage relationship using one reference direction
V_AN = −V_BN; V_AB = V_AN − V_BN ≈ 240 V RMS
For nominal 120/240 V service, |V_AN| and |V_BN| are approximately 120 V RMS. Because their phasors are opposite, the magnitude between A and B is approximately 240 V. Actual service voltage varies within the utility’s allowed range and with loading and voltage drop.
- V_AN
- RMS phasor voltage from leg A to neutral
- V_BN
- RMS phasor voltage from leg B to neutral
- V_AB
- RMS phasor voltage from leg A to leg B
| Connection | Nominal voltage | Neutral load current involved? |
|---|---|---|
| Leg A to neutral | 120 V | Yes, for a line-to-neutral load |
| Leg B to neutral | 120 V | Yes, for a line-to-neutral load |
| Leg A to leg B | 240 V | No neutral current for a pure two-wire line-to-line load |
| Protective equipment conductor | Not a normal supply conductor | Must not be used as the intended load return |
Neutral carries the difference for matched resistive line-to-neutral loads
Diagram description
The arithmetic magnitudes apply to sinusoidal resistive loads on opposite legs. Loads with different phase angles or nonlinear current waveforms require phasor or time-domain addition rather than simple subtraction.
Special case: sinusoidal currents with the same waveform and phase character
I_N = |I_A − I_B|
The opposite source polarity makes equal line-to-neutral load currents arrive at the neutral in cancelling directions. This shortcut is not a universal neutral-current equation. Harmonics, different power factors, transient current and waveform distortion must be combined as actual instantaneous currents or phasors where applicable.
- I_N
- neutral RMS current for the qualified special case, A
- I_A
- leg-A line-to-neutral load current, A
- I_B
- leg-B line-to-neutral load current, A
| Loads | Leg currents | Neutral result |
|---|---|---|
| Equal resistive 120 V loads on opposite legs | Same RMS magnitude | Ideally cancels at the neutral |
| Unequal resistive 120 V loads | Different RMS magnitudes | Neutral carries the difference |
| One 240 V load between legs | Same series current in the two outer conductors | No load connection to neutral |
| Nonlinear electronic loads | Current may be pulsed and harmonic-rich | Calculate from waveforms; do not assume simple RMS subtraction |
A loose or open neutral can move the midpoint dangerously
With the neutral intact, each line-to-neutral load is held near the transformer center-tap potential. If a shared neutral opens while unequal 120 V loads remain connected across the two outer legs, those loads can behave like a series divider across approximately 240 V. The higher-impedance side can then see excessive voltage while the other side sees too little. Flickering, simultaneous brightening and dimming, or abnormal voltages across several circuits require prompt utility or qualified-electrician attention—not exploratory work inside a panel.
Why balancing loads does not reduce the energy they use
Moving compatible 120 V loads between legs can reduce neutral current and improve loading balance, but it does not cancel the power consumed by the loads. Their real powers add. Current cancellation occurs only in the shared neutral path because the return currents oppose there; energy still comes through the two outer conductors.
The difference between power and accumulated energy is developed in How Energy Meters and Home Distribution Work. For phasor and AC vocabulary, use the earlier AC-power lessons before extending the simple resistive subtraction rule.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for What a Split-Phase Supply Means?
“Opposite legs” describes polarity, not two independent phases. The outer-to-neutral waveforms reach opposite polarities at the same instant because they are measured from opposite ends of one center-tapped secondary.
Which assumption can change the conclusion?
Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.
What can go wrong if I copy the example directly?
Do not measure a service panel for learning. Both outer conductors are energized relative to neutral and earth, and opening a branch device does not isolate the service equipment. Available fault current can be extremely high.
How do I verify the result in the real system?
Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Sources and further reading
- NIST Technical Note 2238 — Split-phase residential measurement configurationNational Institute of Standards and Technology · 2026-10-13NIST technical source reviewed for the 120/240 V split-phase configuration, line-to-line and line-to-neutral relationships, and neutral current as the leg-current difference in the qualified arrangement.
- DOE Fundamentals Handbook: Electrical Science, Volume 4U.S. Department of Energy · 2026-10-13Government handbook reviewed for the center-tapped 120/240 V three-wire scheme and the distinction between insulated load conductors and the safety-ground path.
- Consumer Power Quality Problems: Troubleshooting by TelephoneNational Institute of Standards and Technology · 2026-10-13NIST guidance reviewed for the unequal-voltage hazard created by an open neutral in a 120/240 V customer installation with unbalanced loads.