Count every circuit, add spare ways (many specifications ask for 20–30%, but follow your local code or tender), choose the incomer and busbar rating from the calculated maximum demand, check the board’s short-circuit withstand against the fault level at the installation point, then pick enclosure material and IP rating for the environment.
Key facts
- A “way” is one module position (usually 18 mm). A 1-pole MCB takes one way; an RCCB or 2-pole isolator takes two.
- CHINT DB2 and NX2 boards are available from 4 to 36 ways, in metal or plastic.
- Spare capacity is a design decision. Ask the project specification before you assume a figure.
Method
- Write the circuit schedule. List every circuit — lighting, sockets, air-conditioning, water heater, pump — with its load and cable.
- Estimate maximum demand. Not every load runs at once; apply diversity according to your local code to arrive at the incomer current.
- Choose the incomer. An isolator, MCB or MCCB rated above the maximum demand, with a breaking capacity that suits the fault level.
- Count the modules. Add up the ways needed by the incomer, RCDs and every outgoing device.
- Add spare ways. Then round up to the next available board size.
- Decide the RCD arrangement. Split-load boards use a few RCCBs, each covering a group; RCBOs protect each circuit separately and limit nuisance tripping to one circuit.
- Check the short-circuit rating. The board and its devices must withstand the prospective fault current where they are installed.
- Choose the enclosure. Metal or plastic, flush or surface, and the IP rating for the location — for example IP54 for outdoor villa-compound use.
Worked example (illustrative)
| Item | Ways |
|---|---|
| 16 single-pole MCBs for lighting and sockets | 16 |
| 2 two-pole RCCBs (split load) | 4 |
| 1 two-pole incomer isolator | 2 |
| Subtotal | 22 |
| Plus 25% spare (about 5.5, rounded up to 6) | +6 |
| Required | 28 — choose the next board size up |
This is an illustration of the method, not a design: your circuit count, spare policy and protection scheme come from your own design and local code.
Common mistakes
- Forgetting that RCCBs and isolators take two ways.
- Choosing a board for today’s circuits with no room to expand.
- Selecting plastic enclosures for hot, sunlit outdoor locations without checking the rating.
- Quoting the board without the devices that go inside — one reason we consolidate boards and breakers in one shipment.
Frequently asked questions
How many spare ways should a distribution board have?
Many specifications ask for 20–30%, but the right number comes from your local code and the project specification.
Metal or plastic distribution board?
Plastic suits most indoor residential use; metal is chosen for robustness, higher fault levels or where the specification requires it. Check the IP rating either way.
Do I need RCBOs or RCCBs?
RCCBs are economical for grouped circuits; RCBOs combine overcurrent and earth-leakage protection per circuit, which isolates faults better. See RCCB types and the RCCB / RCBO range.
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Reviewed by the HAZAM technical team, October 2026. CHINT, DELIXI and TENGEN are trademarks of their respective owners. HAZAM is a first-tier authorised distributor of CHINT products; the authorisation certificate is available on request from our sales manager. Ratings and designations refer to published manufacturer documentation; the certificate for your exact ordering code governs.