Neutral
Triplen harmonics and neutral conductor overload
The one harmonic problem that cannot be solved by adding a bigger filter, because the current is in a conductor most designs never sized for it.
Triplen harmonics — the 3rd, 9th, 15th and their multiples — are in phase across all three phases, so they add arithmetically in the neutral instead of cancelling. In an installation dominated by single-phase non-linear loads, neutral current can reach 1.5 to 1.8 times the phase current.
Standard practice sizes the neutral at 100% of the phase conductor. In a triplen-heavy installation that is a thermal failure waiting for the load to grow.
Why the neutral is the problem
In a balanced three-phase load, the three fundamental currents are 120° apart. Their phasors sum to zero, so no current flows in the neutral. This is the assumption behind sizing a neutral at the same cross-section as a phase conductor, or sometimes smaller.
The 3rd harmonic is three times the fundamental frequency. Three times 120° is 360°, which is a full revolution — the three 3rd harmonic currents are therefore exactly in phase. They do not cancel. They add.
IA1 ∠0° + IB1 ∠−120° + IC1 ∠+120° = 0
# phase currents, 3rd harmonic
IA3 ∠0° + IB3 ∠−360° + IC3 ∠+360° = 3 × IA3
The same logic applies to every order divisible by three. The 9th, 15th and 21st all behave identically. Orders that are not multiples of three — 5th, 7th, 11th, 13th — cancel in the neutral as the fundamental does.
Where the current comes from
Triplen harmonics are produced by loads that draw current in narrow pulses around the voltage peak, asymmetrically. The classic sources are single-phase switch-mode supplies, and they are now everywhere.
| Load | Typical 3rd harmonic content | Why |
|---|---|---|
| Desktop and server power supplies | 60–90% of fundamental | Capacitor-input rectifier without PFC |
| LED drivers, passive | 30–60% | Peak-charging front end |
| Single-phase UPS | 25–40% | Rectifier input |
| Fluorescent electronic ballasts | 20–40% | Rectifier input |
| Laptop and phone chargers | 80%+ | Very small fundamental, large pulse |
| Office air conditioning inverters | 10–25% | Single-phase rectifier |
An office building with 200 workstations may have a phase current dominated by fundamentals while its neutral carries more current than any single phase. This is why modern office fit-outs increasingly specify a neutral at 173% or double the phase rating.
How to tell this is happening
- Neutral bar or neutral cable noticeably hotter than the phase conductors — the definitive symptom
- Neutral link or terminal blocks discoloured, or insulation cracking at the neutral
- Nuisance tripping of RCDs or RCBs with no obvious earth fault
- Transformer overheating at moderate load, with the hot spot not explained by phase current
- Phase currents balanced but total neutral current well above zero
- Neutral current with a waveform that is visibly triple-frequency, if you have a scope
A clamp meter reading the neutral RMS value is the fastest diagnostic and is usually enough to justify a proper recording. If neutral RMS exceeds phase RMS on a balanced load, the answer is established.
The four fixes, in order of cost
| Approach | What it does | When it applies |
|---|---|---|
| Upsize the neutral | Removes the thermal risk | New builds and rewires. Does not reduce the harmonic, only survives it. |
| Specify PFC power supplies | Eliminates most of the 3rd harmonic at source | Procurement decision. Cheapest per unit but only works for new equipment. |
| Zone the loads by phase type | Prevents triplen accumulation on one neutral | Retrofit with separate neutrals per zone. Underused and often effective. |
| Four-wire active filter | Injects cancellation current into the neutral | Where the load cannot be changed. Must be a four-wire device with a full-rated neutral. |
The fourth row carries a specification trap. Many active filters are three-wire: they correct phase current distortion but have no neutral connection, so the neutral current is untouched. If the problem is neutral overheating from triplen harmonics, a three-wire filter will not solve it regardless of its rating.
Frequently asked
Why do 3rd harmonics add in the neutral instead of cancelling?
In a balanced three-phase system, the fundamental currents are 120 degrees apart and cancel in the neutral, which is why a balanced load needs no neutral current. Triplen harmonics — the 3rd, 9th, 15th and so on — are three times a multiple of the fundamental, so they are displaced by 360 degrees rather than 120. They are therefore in phase with each other in all three phases and add arithmetically in the neutral instead of cancelling.
Can neutral current exceed phase current?
Yes, and this is the defining characteristic of a triplen-dominated installation. Where the 3rd harmonic is large relative to the fundamental — typical of switch-mode power supplies, LED drivers and single-phase UPS units — the neutral current can reach 1.5 to 1.8 times the phase current. A neutral sized at 100% of phase conductor, which is normal practice, will run hot.
Does a standard active harmonic filter correct triplen harmonics?
Only if it is a four-wire device with a neutral connection. A three-wire active filter has no path to inject neutral current, so it cannot address triplen harmonics in the neutral even though it will reduce phase-current distortion. Specify a four-wire filter with a neutral rated for the full triplen current when single-phase non-linear loads dominate.
Neutral running hotter than the phases?
Send a recording that includes neutral current alongside the three phase currents. We will confirm whether the cause is triplen harmonics and specify a four-wire filter with the correct neutral rating.