Single-phase ↔ three-phase: the currents on the phases and on the neutral
Switch between single-phase (230 V) and three-phase star (400/230 V) and watch, in real time, how the currents split across the phases and what the neutral conductor carries — from balanced loads to imbalance.
What the neutral actually carries
Three situations every residential electrician meets daily.
Single-phase — the neutral carries the whole current
In a single-phase circuit, the current leaves on the line and returns fully on the neutral. The neutral conductor carries exactly the same current as the line, so it is not undersized.
I = P / (230 · cos φ) → I_nul = I_fază
Balanced three-phase — the neutral close to zero
With three equal phases, shifted by 120°, the return currents cancel each other out. The neutral stays close to zero, and the same power is split across three conductors — each phase carries less current.
I_nul = I_R + I_S + I_T (sumă fazorială, 120°) → 0
Unbalanced three-phase — the neutral carries the difference
When the loads on the phases are unequal, the cancellation is no longer complete: the neutral carries the difference. The greater the imbalance, the more the neutral current grows — which is why loads are balanced across the phases.
I_nul = √(I_R² + I_S² + I_T² − I_R·I_S − I_S·I_T − I_T·I_R)
Adjust and watch the currents
Change the regime and the phase powers — the diagram, the oscilloscope and the phasors recompute instantly.
400 V between phases · 230 V to neutral
Oscilloscope — instantaneous currents
Phasor diagram
Purely resistive loads; cos φ enters only through the equivalent resistance, not through reactive elements. The phase currents shown are RMS values; the oscilloscope shows the instantaneous values, fed directly by the simulation engine.
Where the neutral carries current — and where it does not
Each circuit’s neutral vs. the common neutral towards the service connection.
Every single-phase load returns on its own circuit’s neutral — that is where the WHOLE current of that circuit flows. All the neutrals join at the neutral bar; from there ONE single common neutral runs to the service connection. Only on that one do the three currents (shifted by 120°) overlap and cancel out.
- ●On EACH circuit’s neutral (load → bar) its full current flows — it is sized like the line.
- ●On the COMMON neutral (bar → service connection) the phasor sum I_R + I_S + I_T ≈ 0 flows at balance.
- ●The cancellation happens ONLY on the common segment; under imbalance, the difference appears on it too — which is why you balance the loads across the phases.
What to remember
- ●Single-phase: the neutral carries 100% of the load current — never thinner than the line.
- ●Balanced three-phase: the neutral ≈ 0 A, and the current on each phase drops — thinner conductors per phase can be used.
- ●Unbalanced three-phase: the neutral becomes the critical fourth conductor — it carries the difference between the phases.
- ●The cross-sectional area of the neutral depends on balance and on harmonics: under imbalance or with 3rd-order harmonics the neutral may require the same cross-sectional area as the phases.
Want to practise distributing loads across the phases, step by step?
Open the three-phase balance calculator