1. What Does a Secondary Distribution Box Look Like? (4 Structural Elements)
Typical secondary box structure: incoming disconnect switch + main ground fault protection + 3–6 outgoing circuits (each with branch isolation + branch ground fault protection) + N/PE bars. Sheet metal enclosure thickness ≥ 1.5mm, outdoor ≥ IP65.
2. Internal Layout Standards (Top to Bottom)
| Location | Component |
|---|---|
| Top — Incoming Section | Incoming cable + incoming disconnect switch |
| Middle — Main Switch Section | Main ground fault protection + main circuit breaker |
| Bottom — Outgoing Section | Branch circuit breakers (3–6 circuits) |
| Base — Grounding Section | N bar + PE bar + grounding terminals |
3. Circuit Labeling Standards (Every Circuit Must Be Labeled)
- Circuit number: 1L1, 1L2, 1L3… (circuit + phase sequence)
- Destination label: indicates which final-stage box or equipment the circuit feeds
- Breaker setting value: affixed next to the switch, e.g. C32 / 40A
- Phase color code: L1 yellow / L2 green / L3 red / N blue / PE yellow-green
4. How to Set Branch Ground Fault Protection in a Secondary Box (100–300 mA)
RCD rated residual operating current for secondary distribution box branch circuits 100–300mA, time-delay type 0.2 s, for line protection and ground fault protection. Only the final level (tertiary box) uses ≤ 30 mA for personnel protection.Do not install 30 mA in the secondary distribution box— it will race with the final level and cause nuisance tripping.
5. Secondary Distribution Box Selection Table (by number of branch circuits)
| Branch circuits | Enclosure size | Incoming breaker | Branch breaker |
|---|---|---|---|
| 3 circuits | 500×400×200 | 63A | 3× C32 |
| 4 circuits | 600×400×200 | 100A | 4× C40 |
| 6 circuits | 800×600×250 | 160A | 6× C63 |
| 8 circuits | 1000×600×250 | 250A | 8× C80 |
FAQ FAQ
What mA rating for the RCD in a secondary distribution box?
100–300mA, time-delay type. Only the final level uses ≤ 30 mA.
Can a secondary distribution box be installed outdoors?
Yes, with ingress protection ≥ IP65, a rain hood, and at least 1.2 m above ground.
What is the difference between a secondary distribution box and a primary distribution box?
The primary box is the main distribution point (main breaker + main metering); the secondary box distributes (branch circuits to each zone); the tertiary box is the final level (connects to equipment).
Can branch circuits in a secondary distribution box feed receptacles?
Yes, but each receptacle circuit must have its own RCD rated ≤ 30 mA.
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