A floor of computers and LED drivers is balanced across three phases, yet the neutral reads MORE than any phase conductor. Most likely cause?

B. Third-harmonic (triplen) currents adding in the neutral

D. The meter is reading the peak

The shared neutral of a three-phase, four-wire feeder comes loose. What happens to the 120 V loads?

B. The line-to-neutral voltages shift — lightly loaded phases go high, heavily loaded phases go low

C. Nothing — the ground carries the neutral current safely

D. Only the motors are affected

Why is a balanced three-phase load easier on the supply than the same power taken from one phase?

B. The current is shared by three conductors and the neutral carries little or none

C. It has a better power factor automatically

D. Balanced loads do not heat conductors

Troisième année Lesson 2 of 13 · about 13 min

Unbalanced loads and neutral current

What you’ll learn

Why three equal phase currents cancel at the neutral, so a balanced wye neutral carries nothing.
How to work out neutral current when the three phases are loaded unevenly.
Why split-phase neutrals subtract but three-phase neutrals need a square root.
How third-harmonic currents break the rule, and what an open neutral does to your 120 V loads.

Before this: Wye and delta

On a four-wire wye — 208Y/120, 480Y/277, 600Y/347 — each line-to-neutral load sends its current back on the neutral. But the three phase currents are 120° apart, so on the neutral they partly cancel. Perfectly balanced, they cancel completely and the neutral carries nothing. Unbalanced, it carries whatever is left over.

Three unequal line-to-neutral loads. The neutral does not carry the sum of the three (60 A), and not zero either: 17.3 A, what is left after three currents 120° apart partly cancel.

Balanced: nothing on the neutral

Picture the three currents as three arrows of equal length, 120° apart — like a Mercedes badge. Add them tip to tail and you end up exactly where you started. That sum is the neutral current, so on a perfectly balanced load it is zero. The current from phase A returns on phases B and C; the neutral is just there for the moments the load is not balanced.

That is the whole reason panel schedules spread single-phase circuits across A, B and C: the better the balance, the less the neutral carries and the less every conductor heats.

Unbalanced: the leftover comes home

When the arrows are different lengths they no longer close, and the gap is the neutral current. For loads at the same power factor (resistive loads are the easy case), there is a formula that does the arrow-adding for you:

IN = √(Ia² + Ib² + Ic² − Ia·Ib − Ib·Ic − Ic·Ia)three-phase, four-wire wye, loads at the same power factor
IN = I1 − I2120/240 V split-phase: the legs are 180° apart

Two quick checks keep you honest. All three equal gives zero. Only one phase loaded gives that phase’s current, because there is nothing to cancel against. Two phases loaded equally and the third empty gives the same current as either one — not zero, and not double.

Worked: 30, 20 and 10 A on a 208Y/120 panel

Squares: 30² + 20² + 10² = 900 + 400 + 100 = 1,400.
Cross products: 30×20 + 20×10 + 10×30 = 600 + 200 + 300 = 1,100.
IN = √(1 400 − 1 100) = √300 = 17,3 A.
Now move 10 A of load from A to C, so the phases read 20, 20, 20: IN = √(1,200 − 1,200) = 0 A. Same total load, balanced, nothing on the neutral.

Worked: two phases loaded, one empty

A 600Y/347 V lighting panel with 16 A on phase A, 16 A on phase B and nothing on C.
IN = √(16² + 16² + 0 − 16×16 − 0 − 0) = √(256 + 256 − 256) = √256 = 16 A.
Not 32 A and not 0 A. That surprises people the first time they clamp it.

Where the rule breaks: triplen harmonics

The cancelling only works for current at 60 Hz. Computers, LED drivers, VFDs and other electronic loads draw current in short pulses, and those pulses contain harmonics — currents at multiples of 60 Hz. The third harmonic (180 Hz) and its multiples (the triplens) are in step on all three phases, so on the neutral they add. A neutral feeding a floor of electronic loads can carry more current than any phase conductor, with the phases perfectly balanced. The power quality lesson goes deeper.

The open neutral

Lose the shared neutral on a four-wire system and the three sets of 120 V loads end up in series with each other across the line voltage. The voltage divides according to the loads: the lightly loaded phase goes high — sometimes close to line voltage — and the heavily loaded phase goes low. Electronics on the high phase fail, often all at once. Flickering lights that get brighter when something else switches on are the classic warning sign.

Sur le chantier

Tips

Clamp all three phases and the neutral when you open a panel you are adding to. The phase with the least current is where the new circuit goes.
Use a true-RMS clamp on neutrals. An average-responding meter under-reads the pulsed currents electronic loads draw.
Neutral current higher than the phase currents on a balanced panel is harmonics until proven otherwise.
On a multiwire branch circuit, the handle tie (or common-trip breaker) is what stops someone working on one circuit while the shared neutral is still carrying the other’s current.

Common mistakes

Adding the three phase currents and expecting the neutral to carry the total.
Using split-phase subtraction on a three-phase panel.
Opening a shared neutral with circuits still live — the current it was carrying finds a new path, possibly you.
Assuming a balanced panel means a quiet neutral on electronic loads.
Letting the neutral lean on the bonding conductors. Neutral current belongs on the neutral.

Check yourself

0 right of 0 answered 7 questions · pick one answer each

1.A 208Y/120 V panel feeds 20 A of resistive load on each of the three phases, all to neutral. What does the neutral carry?
2.Resistive loads of 30 A, 20 A and 10 A on the three phases of a 120/208 V wye. About how much current is on the neutral?
3.Only ONE phase of a three-phase, four-wire panel has load on it: 25 A to neutral. The neutral carries…
4.On a 120/240 V single-phase (split-phase) multiwire circuit, one leg draws 15 A and the other 9 A. What is on the shared neutral?
5.A floor of computers and LED drivers is balanced across three phases, yet the neutral reads MORE than any phase conductor. Most likely cause?
6.The shared neutral of a three-phase, four-wire feeder comes loose. What happens to the 120 V loads?
7.Why is a balanced three-phase load easier on the supply than the same power taken from one phase?

Answers

C. About 0 A — Three equal currents 120° apart cancel exactly at the neutral point. A perfectly balanced wye load sends nothing back on the neutral — the currents return on the other two phases.
B. 17.3 A — IN = √(Ia² + Ib² + Ic² − IaIb − IbIc − IcIa) = √(900 + 400 + 100 − 600 − 200 − 300) = √300 = 17.3 A. Adding the currents as plain numbers (60 A) is the classic mistake — they are 120° apart.
C. 25 A — With nothing on the other two phases there is nothing to cancel against, so all 25 A comes back on the neutral — exactly like a two-wire single-phase circuit.
B. 6 A — The two legs of a split-phase supply are 180° apart, so their neutral currents subtract: 15 − 9 = 6 A. (Three-phase is 120°, which is why it needs the square-root formula instead.)
B. Third-harmonic (triplen) currents adding in the neutral — Electronic power supplies draw pulses of current rich in the 3rd harmonic. Triplen harmonics from the three phases are IN step with each other, so they add in the neutral instead of cancelling. Use a true-RMS meter to see it.
B. The line-to-neutral voltages shift — lightly loaded phases go high, heavily loaded phases go low — Without the neutral holding the centre point, the loads end up in series across 208 V, and the voltage divides by their impedance. Lightly loaded phases can see well over 120 V and burn out equipment. Never rely on the grounding path to carry neutral current.
B. The current is shared by three conductors and the neutral carries little or none — Spread evenly, each line carries a third of the load and the neutral almost nothing, so conductor heating and voltage drop are lowest. Stack it on one phase and one line plus the neutral carry it all.

Questions fréquentes

Can the neutral carry more current than the phase conductors?

Yes, on a four-wire wye feeding lots of electronic loads. Their third-harmonic currents are in step on all three phases, so they add in the neutral instead of cancelling. That is why some feeders serving computers and LED lighting get a full-size or oversized neutral — how big is a design and code-book question.

Why does my clamp-on show neutral current on a panel I thought was balanced?

Because real panels are rarely balanced to the amp, and the loads change all day. A few amps of neutral current is normal. If the neutral reads near or above the phase currents on a balanced panel, suspect harmonics and check with a true-RMS meter.

Is it true that a neutral carries "the difference"?

On a 120/240 V split-phase circuit, yes — the two legs are 180° apart, so the neutral carries the plain difference. On three-phase the currents are 120° apart and the difference is worked out with the square-root formula in this lesson, not by subtraction.

Voir aussi

Wye and delta: line vs phase — the lesson before this one
Power quality and harmonics — the triplen story in full
Calculateur de déséquilibre de tension
Reading a panel schedule — where phase balance is planned
Hot, neutral, ground — the neutral by function

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