Sheet CALC-21 · Design Guide & Calculator
Distribution Boards & Consumer Units: Size the Board, Not Just the Breakers
Build the outgoing circuit schedule, apply transparent diversity assumptions, balance single-phase circuits, test ways and spare capacity, screen incomer and assembly current ratings, compare prospective fault current with breaking capacity, and document RCD, RCBO and SPD decisions.
The Guide
A Distribution Board Is an Assembly
A board is not compliant merely because every outgoing breaker has the right ampere rating. The assembly has its own rated current, short-circuit performance, temperature-rise limits, dielectric performance, enclosure requirements, protective-circuit requirements and verification obligations. The selected devices then have to coordinate with the circuits and with each other.
CALC-21 keeps load calculation separate from assembly verification. It calculates circuit demand, phase loading, logical ways and basic rating checks. Manufacturer verification still governs the final assembly.
The Design Sequence
Build the circuits
List real outgoing circuits, connected load, power factor, phase and protective-device arrangement.
Assess demand
Apply diversity only where justified. Keep every assumption visible and editable.
Balance phases
Distribute single-phase circuits so the highest phase current is controlled.
Size the assembly
Check board rating, main device, incoming conductor and spare capacity against maximum demand.
Check fault duty
Compare prospective fault current with assembly and device capability, including verified conditional ratings where used.
Coordinate protection
Review RCD/RCBO type, SPD, selectivity, back-up protection, earthing and final verification.
Current Standards Basis
| Standard | Role |
|---|---|
| IEC 61439-1:2020 + corrigenda | General rules and verification framework for low-voltage assemblies. |
| IEC 61439-3:2024 | Current requirements for distribution boards intended to be operated by ordinary persons. |
| IEC 61439-2:2020 | Current requirements for power switchgear and controlgear assemblies. |
| IEC 60364-1:2025 | Current fundamental principles for low-voltage electrical installations. |
| IEC 60364-4-43:2023 | Protection against overcurrent and coordination of overcurrent protective measures. |
| IEC 60364-5-53:2019 + AMD1:2020 + AMD2:2024 | Selection and erection of protective, isolation, switching and control devices. |
| IEC 60947-2:2024 | Current circuit-breaker standard for instructed/skilled-person applications. |
| IEC 61009-1:2024 | Current general requirements for household and similar RCBOs. |
| IEC 61643-11:2025 | Current AC low-voltage SPD requirements and tests. |
| BS 7671:2018+A4:2026 | Current UK installation standard where the BS/IET framework is adopted. |
Core Relationship
Ib, In and Iz Are Different Quantities
Ib is the circuit design current. In is the protective-device rated current or setting. Iz is the conductor current-carrying capacity after applicable correction factors. CALC-21 checks whether the entered breaker rating is at least the calculated design current. It does not certify the cable, because installation method, material, grouping, ambient temperature and other factors belong in the cable-sizing calculation.
Common Design Failures
What Usually Goes Wrong
Breaking capacity chosen by habit. A 6 kA breaker is not automatically suitable because it fits the board. Determine prospective fault current and verify the assembly and device rating.
Diversity hidden inside a spreadsheet. Keep connected load and assessed demand side by side so the assumptions can be reviewed.
A balanced total with an unbalanced board. Three-phase kW can look acceptable while one phase is overloaded by single-phase circuits.
Spare ways but no spare capacity. Empty modules do not help if the busbar and incomer are already at their limit.
One RCD type everywhere. Electronic equipment, EV chargers and drives can change residual-current waveforms. Match residual protection to the circuit and equipment.
Related Tools
Where This Goes Next
Use the diversified board demand in the load schedule. Verify feeders and final circuits in the cable sizing calculator and check protective devices in the circuit breaker calculator. Coordinate surge exposure with the lightning protection calculator.
Beyond the Calculator
Need a Board Schedule or Full Electrical Design?
We prepare load schedules, distribution-board schedules, single-line diagrams, cable calculations, protection coordination and specifications as part of complete electrical design packages.