Branch Circuit: Wire · Breaker · Outlet
Use this chart to coordinate branch-circuit wire size, breaker and outlet ratings under Japanese electrical practice instead of selecting each item separately. The conductor-color section distinguishes formal requirements from conventions that must be checked on the drawing.
Wire ↔ breaker ↔ outlet (Article 149)| Breaker | Min. wire (annealed Cu) | Outlet |
|---|
| 15A | 1.6 mm (2 mm²) | ≤ 15A |
| 20A — MCCB (配線用遮断器) | 1.6 mm (2 mm²) | ≤ 20A |
| 20A — fuse (ヒューズ) | 2.0 mm (3.5 mm²) | 20A |
| 30A | 2.6 mm (5.5 mm²) | 20–30A |
| 40A | 8 mm² | 30–40A |
| 50A | 14 mm² | 40–50A |
Conductor colors (JIS C 0446)| Conductor role | Color | Status |
|---|
| Neutral / grounded (接地側) | White (白) | Required |
| Protective earth PE (接地線) | Green (緑) or green/yellow | Required |
| 1φ2W — line (非接地側) | Black (黒) | Recommended |
| 1φ3W — two lines | Black (黒) & Red (赤); neutral white | Recommended |
| 3φ3W — R/S/T | By convention / drawing (not unified) | Not fixed |
Wire sizes are the minimum for annealed copper (軟銅線); a breaker (配線用遮断器) allows a thinner wire than a fuse (ヒューズ) at 20A. Colors: WHITE (白) for the neutral/grounded conductor and GREEN (緑) for the PE earth are regulated; the three-phase R/S/T phase colors are NOT unified by the Interior Wiring Regulations — they follow company convention or the drawing. This is a guidance table; actual design follows the load, installation and applicable standard.
Frequently asked questions
Can the breaker rating exceed the wire ampacity?
No. The breaker must coordinate with and protect the branch wiring and connected equipment. Read wire, breaker and outlet information from the same chart row, then verify the installation conditions.
Which colors does the chart show for neutral and PE?
The conductor-color section identifies the grounded conductor and protective earth under JIS C 0446. Read the row for the exact function rather than inferring it from three-phase R/S/T colors.
Are three-phase R/S/T conductor colors standardized here?
Not within this chart's scope. R/S/T colors are shown as convention- or drawing-dependent; use that chart row and confirm the project documents before wiring.
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