1. What Is a Three-Phase Four-Wire System? (L1/L2/L3/N/PE)
Three-phase four-wire system = three phase conductors (L1/L2/L3) + one neutral conductor (N), plus a protective ground conductor (PE) makes it "three-phase five-wire." Line voltage 380V, phase voltage 220V. Inside the junction box, L1/L2/L3 are arranged by phase sequence, N connects to the neutral bar, and PE connects to the protective bar.
2. Wye Connection (Y) (3 voltage levels)
Wye connection: the tail ends of three coil groups are connected together to form aneutral point, with L1/L2/L3 brought out from the head ends. Bringing the N conductor out from the neutral point gives the three-phase four-wire system:
| Parameter | Value |
|---|---|
| Line voltage | 380V |
| Phase voltage | 220V |
| Neutral N | Yes |
| Application | Mixed power and lighting loads |
3. Delta Connection (△) (380V only)
Delta connection: the head and tail ends of three coil groups are connected end-to-end in a closed triangle, with L1/L2/L3 brought out from the three connection points.No neutral conductor, only three phase conductors; loads must be balanced.
| Parameter | Value |
|---|---|
| Line voltage | 380V |
| Phase voltage | 380V |
| Neutral N | None |
| Application | Balanced three-phase motor load |
4. Phase sequence arrangement inside the junction box (Left to Right)
Horizontal arrangement, left to right:L1 (yellow) → L2 (green) → L3 (red) → N (blue) → PE (yellow-green). Vertical arrangement, top to bottom: L1 → L2 → L3 → N → PE. The N row and the PE rowmust be separate, the PE row is bonded to the sheet metal enclosure and grounded at an additional point.
5. Junction box selection: by current (4 Tiers)
| Load current | Cable entry hole diameter | Box dimensions | Material |
|---|---|---|---|
| ≤ 16A | PG11 | 100×100×60 | ABS |
| 16–32A | PG13.5 | 150×150×80 | ABS / steel |
| 32–63A | PG21 | 200×200×100 | Steel / stainless steel |
| 63–100A | PG29 | 300×300×150 | Stainless steel |
FAQ FAQ
What is the difference between three-phase four-wire and three-phase five-wire?
Three-phase four-wire = L1/L2/L3 + N; three-phase five-wire = L1/L2/L3 + N + PE. New projects under Chinese national standards must use three-phase five-wire (PE run separately).
How do I choose between star and delta connection?
Mixed motor and lighting loads use star (Y) to get 220V; a purely balanced three-phase motor load can use delta (△).
Can the N and PE conductors be combined?
No. The N conductor is a current-carrying conductor (it carries current); the PE conductor is a protective conductor (normally carries no current). Combining them will energize the enclosure.
What happens if the phase sequence is wired wrong inside the junction box?
The motor will run in reverse, and a three-phase imbalance will drive current up. Always verify with a phase sequence meter before energizing.
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