Three-Phase Neutral Current Calculator
Enter the three phase currents and the third-harmonic content to get neutral current and the minimum neutral conductor.
Step-by-step
- Enter valid values to begin.
Fundamental currents are assumed 120° apart. Triplen harmonics (3rd, 9th, 15th) are in phase across the three conductors and add arithmetically in the neutral; only the third harmonic is modelled here. Use a true-RMS power-quality meter to measure real harmonic content. Ampacity from NEC Table 310.16.
Three-Phase Neutral Current: Unbalance and the Harmonic Surprise
Core Engineering Principles
In a balanced three-phase four-wire wye system, the three phase currents are equal and 120° apart, so they add to zero at the neutral. The neutral does nothing, and in a perfectly linear plant it could be the size of a pencil. In a real panel the phases are never equal, and what comes back down the neutral is the vector difference. The formula is IN = √(Ia² + Ib² + Ic² − IaIb − IbIc − IcIa). With 100, 90 and 80 A on the phases, the neutral carries only 17 A.
Then somebody installs switch-mode power supplies, LED drivers and drives, and the picture changes. These loads draw current in short pulses, which are rich in odd harmonics. The triplen harmonics, 3rd, 9th and 15th, are all in phase with each other in all three conductors. They don’t cancel in the neutral, they add. Three phases each carrying 35% third-harmonic put 105% of one phase’s current into the neutral, arithmetically. That’s how a neutral ends up with more current than any phase, in a panel that looks half loaded.
Total IN = √(IN,fund² + IN,3rd²)
NEC & Standard References
NEC 220.61(A) says the feeder or service neutral load is the maximum net computed load between the neutral and any one ungrounded conductor. 220.61(B) allows a demand reduction of 70% for the part over 200 A, but 220.61(C)(2) says no reduction for the portion of the load made up of nonlinear loads in a 4-wire wye system. NEC 310.15(E) says that on a 3-phase 4-wire wye system where the major part of the load is nonlinear, the neutral is counted as a current-carrying conductor for derating. IEEE 519 limits current distortion at the point of common coupling, and IEEE C57.110 covers transformer derating for harmonics. 4-wire delta systems have their own rules: the high-leg neutral is not the same as a wye neutral.1. Fundamental neutral: √(100² + 90² + 80² − 100×90 − 90×80 − 80×100) = √300 = 17.3 A.
2. Third-harmonic per phase: 0.35 × (100, 90, 80) = 35, 31.5, 28 A; they add in the neutral: 94.5 A.
3. Total neutral: √(17.3² + 94.5²) = 96.1 A, which is 107% of the 90 A average phase current.
4. A 3 AWG copper neutral (100 A at 75°C) is the minimum; most engineers would pull a full-size neutral or a 200% neutral on an IT panel feeder.
- Clamp-on meters lie. An average-reading meter underestimates harmonic current. Use a true-RMS meter and check the neutral separately.
- Neutral overheating is silent. Breakers don’t protect a neutral, so a burned neutral shows up as insulation damage or a failed lug.
- Shared neutrals are risky. Multi-wire branch circuits with nonlinear loads can overload the shared neutral.
- Transformers pay too. The third-harmonic current circulates in a delta primary and heats the winding. Use K-rated transformers when nonlinear loads exceed about 30%.
- Loose neutral = overvoltage. If the neutral opens, single-phase loads can see 208 V instead of 120 V.