1. Why layer primary/secondary/tertiary distribution? Mandatory Requirements of JGJ 46-2005
Chapter 7 of JGJ 46-2005, Safety Technical Code for Temporary Power on Construction Sites, explicitly requires:Temporary power on construction sites must use a three-tier distribution system — main distribution box (primary) → distribution box (secondary) → switch box (tertiary).
Why three tiers? Two purposes:
- Limit the scope of accidents: an end-of-line residual current trip only affects the last tier, not the upstream main incoming line
- Enable tiered protection: each tier has different residual current / short-circuit settings (residual current 30mA / 300mA / none, short-circuit small at the end → large at the primary), forming selective protection
Primary: main incoming line (transformer side), residual current optional (or none), repeated grounding ≤ 4Ω;Secondary: distribution box (workshop/floor), 300mA time-delayed residual current, repeated grounding ≤ 10Ω;Tertiary: switch box (equipment end), 30mA residual current, trips in ≤ 0.1s.
2. Standard Configuration for Secondary Distribution Boxes: Incoming/Outgoing Lines + Residual Current + Repeated Grounding Typical Single-Line Diagram
The standard configuration of the single-line diagram for a secondary distribution box, from incoming to outgoing, is as follows:
| No. | Component | Model Selection | Function |
|---|---|---|---|
| 1 | Incoming isolation switch | QS 250/400/630 A | Isolation for maintenance, not operated under load |
| 2 | Incoming circuit breaker | NS molded case 250/400/630 A | Main incoming short-circuit / overload protection |
| 3 | Main residual current device | 300 mA, 0.3 s delay | Second-level main residual current protection with time delay to prevent upstream tripping |
| 4 | Outgoing circuit breakers (×N circuits) | DZ47 or C65 series, 1P/3P | Independent protection for each outgoing circuit |
| 5 | Outgoing residual current protection (optional) | 30 mA, ≤0.1 s, critical circuits only | Add residual current protection to selected end circuits |
| 6 | N bar + PE bar | Separate and independent | Complies with GB 50054 Clause 5.2.7 |
Capacity selection rule of thumb:Second-level main incoming = sum of downstream third-level distribution box capacities × 1.2 (simultaneity factor) + 0.3 (demand factor). Example: a workshop has 8 third-level distribution boxes at 50 A each. Second-level main incoming = 8×50×1.2×0.3 ≈ 144 A; select a 160 A molded case breaker.
3. Typical second-level distribution box configurations: 6 / 8 / 10 / 12 outgoing circuits Choose the circuit count based on the application
| Outgoing circuits | Cabinet W×D×H | Main incoming line | Per outgoing circuit | Application |
|---|---|---|---|---|
| 6 circuits | 600 × 400 × 1200 | 160 A molded case | 32 A 1P/3P | Small projects / site zoning |
| 8 circuits | 700 × 400 × 1400 | 250 A molded case | 40 A 3P | Standard workshop / floor |
| 10 circuits | 800 × 600 × 1600 | 400 A molded case | 63 A 3P | Medium workshop |
| 12 circuits | 1000 × 600 × 1800 | 630 A molded case | 100 A 3P | Large workshop / main floor distribution |
More outgoing circuits on a second-level box is not always better.Rule of thumb: one outgoing circuit per third-level distribution box, plus 1–2 spare circuits.Example: 8 machines + 2 spares = 10 circuits. If loads vary widely between machines, split the main line into two sections, each feeding 5–6 third-level boxes.
4. Common mistakes: shared N bars / residual current devices tripping out of sequence Avoid these pitfalls
On-site commissioning of secondary distribution boxes:Three common problem categories:
1. Shared N busbar / shared N and PE busbar
In a TN-S system, N (neutral) and PE (protective ground) must be separated starting at the transformer, and a secondary distribution box must have an independent N busbar and PE busbar.Sharing them causes nuisance tripping of ground fault protection, defeats the purpose of TN-S, and causes abnormal operation of some sensitive equipment. The inspection will be rejected.
2. Ground fault protection trips upstream
If the time difference between the ground fault protection in the tertiary switch box (30mA ≤0.1s) and the secondary main ground fault protection (300mA) is insufficient, a ground fault at the end of the line will trip the secondary main breaker.Solution: add a 0.3s delay to the secondary main ground fault protection to create a time difference from the 0.1s at the end of the line, achieving selective protection.
3. Missing supplementary grounding
JGJ 46 Clause 5.4.2 requires the PE busbar to have supplementary grounding, with grounding resistance ≤10Ω (≤4Ω for primary, ≤10Ω for secondary, reliable PE continuity at the end of the line). On site,the most commonly missed item is grounding of the PE busbar in the secondary box — because the secondary box is often installed mid-floor, and the grounding conductor must run to the PE busbar of the main box on the floor above or to the building main ground.
V. Factory photos: 4 details of the secondary distribution box (workshop photos)
FAQ FAQ
Should a secondary distribution box include surge protection (SPD)?
Yes, recommended.GB 50054 Clause 6.4.1: a surge protective device shall be installed at the building power supply entry. If the secondary box is within 30 m of the building entry, the SPD can be omitted from the secondary box; if the secondary box is far from the entry, add a Type 2 SPD (In=20kA, Imax=40kA, Up≤1.5kV).
How do you select the 300mA main ground fault protection for a secondary box?
The secondary box main ground fault protection has two key parameters:① Rated residual operating current 300mA (main incoming line; a lower value easily causes upstream tripping); ② 0.3s time delay (offset from the 30mA 0.1s at the end of the line). Recommended: Schneider Acti9 Vigi iC60, or ABB System pro M compact series.
How many circuits should a secondary distribution box have?
Rule of thumb: 8 circuits is most common, 10–12 circuits is mainstream, 6 circuits suits small projects or site zoning, and more than 12 circuits requires sectionalizing. Each tertiary switch box gets 1 circuit, plus 1–2 spare circuits. Zhejiang Golden Unicorn standard is 8 circuits, customizable to 6/10/12 circuits.
What size should the incoming breaker of a secondary box be?
It depends on the total tertiary capacity. Formula:Secondary main incoming = tertiary total capacity × simultaneity factor 0.6–0.8 × demand factor 0.3. Example: 8 tertiary boxes at 50 A each, secondary main incoming ≈ 8×50×0.7×0.3 ≈ 84 A, so a 100 A molded case breaker is sufficient; if the tertiary loads are all large motors starting simultaneously, add 50% margin and select 160 A.
Should a secondary distribution box use GGJ or GGD?
GGJ is a derived cabinet typedesigned specifically for secondary distribution, with cabinet height ≤ 1800 mm and an operating panel below the front door, suitable for wall mounting or low-profile floor standing; GGD is larger and deeper, typically floor mounted.For 6–8 circuits, use GGJ; for 10–12 circuits, use GGD.Zhejiang Golden Unicorn builds both, so you can choose based on the space available on site.
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