Consumer unit for 8 outlet circuits - module count and board size

    Size a consumer unit from your circuit count. The calculator adds up breaker modules, RCDs, the main switch and any extras, applies a spare-capacity margin, and recommends the smallest standard enclosure that fits.

    Planning an installation with 8 outlet circuits means your consumer unit needs at least 8 breaker modules just for sockets - before you count lighting, dedicated appliances or RCDs. A typical setup with 8 outlet circuits also includes 3-5 lighting circuits and 2-4 dedicated runs (oven, hob, EV charger), pushing the real module count well beyond 8. Enter your full circuit breakdown below and the calculator recommends the smallest standard enclosure - 12, 24, 36, 48 or 60 modules - with enough spare capacity for future expansion.

    Parameters

    Enter data for calculations

    Each = 1 module (B10A breaker).

    Each = 1 module (B16A breaker).

    Each = 1 module (B20A-B32A breaker).

    Each = 2 modules (RCD 30 mA).

    Main isolator in the consumer unit.

    SPD, contactors, timers, meters etc.

    Affects installation method and appearance.

    Empty modules for future expansion.

    Optional - for cost estimate.

    Optional - for cost estimate.

    Form progress0 / 7 fields

    💡 Fill in all required fields to unlock the calculate button

    What this calculator does

    A distribution board (consumer unit) holds every breaker, RCD and ancillary device that protects the circuits in a building. Getting the enclosure size wrong is expensive: too small and you run out of DIN-rail space before the last circuit is wired; too large and you overspend on a box that stays half-empty for years. This calculator counts the modules for you. Feed in your lighting, outlet and dedicated circuit numbers, pick the RCD strategy and spare-capacity margin, and it recommends the smallest standard enclosure - 12, 24, 36, 48, 60 or 72 modules - that fits everything with room to grow.

    Quick-start example

    Typical three-bedroom house
    5 lighting circuits + 7 outlet circuits + 3 dedicated (oven, hob, EV charger) = 15 breakers (15 modules).
    Auto RCDs: ceil(15/6) = 3 RCDs (6 modules). Main switch 3P (3 modules). SPD Type 2 (2 modules).
    Net: 26 modules. With 30% spare: ceil(26 x 1.3) = 34 modules. Recommended: 3x12 (36-module) flush-mount board with 10 spare slots.

    How to use this calculator - step by step

    1. Lighting circuits - each takes 1 module (B10A breaker). One circuit per 1-2 rooms.
    2. Outlet circuits - each takes 1 module (B16A breaker). One per room group, separate for kitchen/bathroom.
    3. Dedicated circuits - each takes 1 module (B20A-B32A). Oven, hob, boiler, AC, EV charger.
    4. RCDs - each takes 2 modules. Auto mode assigns 1 RCD per 6 circuits. Override if your design groups differently.
    5. Main switch - 3-pole isolator (3 mod.) for three-phase, 1-pole (1 mod.) for single-phase, or none if the main switch is in the meter box.
    6. Additional modules - surge protectors, contactors, timers, DIN-rail meters. Enter the total module count.
    7. Mounting type - flush (recessed) or surface (on wall face).
    8. Spare capacity - 30% recommended, 40% for new builds expecting future loads (AC, solar, EV).
    9. Read results: total modules, recommended enclosure size, equipment table and optional cost estimate.

    Standard enclosure sizes

    Size Modules DIN rows Typical use
    1x12 12 1 Studio, small sub-board
    2x12 24 2 Apartment up to 70 m2
    3x12 36 3 House 80-140 m2
    4x12 48 4 House 140-200 m2, commercial
    5x12 60 5 Large house with solar, EV, AC
    6x12 72 6 Large commercial, multi-zone

    Scenarios

    Scenario 1: One-bedroom apartment, 45 m2
    2 lighting + 3 outlet + 1 dedicated = 6 circuits. 1 RCD (2 mod). 1P switch (1 mod). Net: 9 modules, 30% spare = 12. Board: 1x12.
    Scenario 2: Three-bedroom apartment, 95 m2
    4 lighting + 5 outlet + 3 dedicated = 12 circuits. 2 RCDs (4 mod). 3P switch (3 mod). Net: 19 modules, 30% = 25. Board: 3x12 (36). Spare: 17 slots.
    Scenario 3: Detached house, 160 m2, EV charger + AC
    6 lighting + 8 outlet + 5 dedicated = 19 circuits. 4 RCDs (8 mod). 3P switch (3 mod). SPD Type 2 (2 mod). Net: 32 modules, 30% = 42. Board: 4x12 (48). Spare: 16 slots.
    Scenario 4: Commercial unit, 250 m2
    8 lighting + 10 outlet + 6 dedicated = 24 circuits. 4 RCDs (8 mod). 3P switch (3 mod). SPD 1+2 (4 mod). Meter (4 mod). Net: 43 modules, 30% = 56. Board: 5x12 (60).
    Scenario 5: Garage sub-board
    1 lighting + 2 outlet + 1 dedicated = 4 circuits. 1 RCD (2 mod). No main switch (fed from house board). Net: 6 modules, 30% = 8. Board: 1x12, surface-mount.

    FAQ - Frequently asked questions

    How many RCDs does a house need?
    The standard rule is max 6 circuits per RCD. A 15-circuit house needs at least 3 RCDs. Some electricians prefer one RCD per room type (lighting, sockets, dedicated) so a single trip does not black out the whole house. Each RCD takes 2 DIN modules.
    What is the difference between flush-mount and surface-mount boards?
    Flush-mount boards sit inside the wall (a cavity is chased or built during construction). They look neater and are standard in living areas. Surface-mount boards are screwed onto the wall face - faster to install and easier to access, but they stick out 10-15 cm. Common in garages, basements and utility rooms. Both accept the same DIN-rail modules.
    Why leave 30% spare capacity?
    Future additions - air conditioning, EV charger, home battery, solar inverter disconnect - each need 1-4 extra modules. Adding spare modules during initial installation costs a few dollars. Replacing the entire board later means re-chasing the wall, re-terminating every cable and paying an electrician for a full day. Industry standard is 30%; new builds should aim for 40%.
    Do I need a main switch in the consumer unit?
    If the meter box has a main isolator that can disconnect the entire installation, a second one inside the consumer unit is optional. Most three-phase houses include a 3-pole isolator (3 modules) in the board for convenience - it lets you kill all power without going to the meter. Single-phase apartments often use a 1-pole isolator (1 module).
    What goes into the "additional modules" field?
    Anything beyond breakers, RCDs and the main switch: surge protector SPD Type 1+2 (4 modules), SPD Type 2 (2 modules), lighting contactor (1-2 modules), timer switch (2 modules), DIN-rail energy meter (4-6 modules), priority relay (2 modules). Add up the modules for all planned devices and enter the total.
    Can I use a 24-module board for a house?
    Only for very small houses (under 80 m2) with minimal dedicated circuits. A typical 120 m2 house with 12-15 circuits, 2-3 RCDs, a main switch and an SPD already occupies 22-25 modules net. With 30% spare that pushes to 30+, which means a 36-module (3x12) board minimum. Starting with 24 modules almost guarantees a board swap within a few years.

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    See also

    3 Outlet Circuits - What Size Consumer Unit Do You Need?4 Outlet Circuits - What Size Consumer Unit Do You Need?5 Outlet Circuits - What Size Consumer Unit Do You Need?6 Outlet Circuits - What Size Consumer Unit Do You Need?7 Outlet Circuits - What Size Consumer Unit Do You Need?10 Outlet Circuits - What Size Consumer Unit Do You Need?12 Outlet Circuits - What Size Consumer Unit Do You Need?

    Calculator verified by the LiczGrupa.pl team

    Content, formulas and results have been reviewed for accuracy and relevance by our team of specialists.

    Patryk Matyjasik

    Reviewed by: Patryk Matyjasik

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