"Leave 30% spare in the board." Every sparky says it. Most homeowners nod along. And five years later half of them are paying to swap the enclosure because an EV charger, a heat pump or a home battery turned those spare slots into a wishful memory. The rule is not wrong. The problem is that people apply it to the wrong number.
Thirty percent of 12 modules is 3.6 slots. Thirty percent of 36 modules is 10.8 slots. A 12-way board with 30% spare absorbs exactly one future appliance before it is full. A 36-way board absorbs four or five. The percentage is identical but the practical headroom is wildly different.
That is the first myth. There are more.
Myth 1: "A 12-way board is enough for any flat"
A one-bedroom flat with 2 lighting circuits, 3 outlet circuits and 1 dedicated circuit needs 6 breakers. Add 1 RCD (2 modules) and a 1-pole isolator (1 module) and you are at 9 modules occupied. A 12-way board leaves 3 spare slots. Install an induction hob or a bathroom heater and you are at 11. One more anything and the board is full.
Run the numbers yourself:
| Outlet circuits | Board size needed (30% spare) | Try it |
|---|---|---|
| 3 outlet circuits | 12-way | Studio flat |
| 4 outlet circuits | 24-way | 1-2 bed flat |
| 5 outlet circuits | 24-way | 2-bed flat |
| 6 outlet circuits | 36-way | 3-bed house |
| 7 outlet circuits | 36-way | 3-4 bed house |
| 8 outlet circuits | 36-way | 4-bed house |
| 10 outlet circuits | 48-way | 5+ bed / commercial |
| 12 outlet circuits | 48-way | Large house + EV + AC |
Myth 2: "You can estimate cable by the roll"
The second most common shortcut is ordering cable by feel. "About 200 metres should do for the house." But 200 metres of what? A two-storey house with 15 circuits and an 8 m average run needs roughly 249 m of cable - split across four different cross-sections. Buying 200 m of 2.5 mm2 leaves you with zero 1.5 mm2 for lighting and zero 4 mm2 for the oven circuit.
The actual breakdown for that house: 74 m of 1.5 mm2, 123 m of 2.5 mm2, 38 m of 4/6 mm2 and 14 m of 10 mm2 for the main feed. Four separate orders. Miss one and the electrician stops mid-job.
How much does average run length change the total? More than most people think:
| Average run | Total cable (15 circuits, 15% waste) | Try it |
|---|---|---|
| 3 m | ~100 m | Compact flat |
| 4 m | ~130 m | Small flat |
| 5 m | ~160 m | Medium flat |
| 6 m | ~185 m | Small house |
| 7 m | ~215 m | Mid-size house |
| 8 m | ~249 m | 3-bed house |
| 10 m | ~310 m | Large house |
| 12 m | ~370 m | Detached / commercial |
| 15 m | ~460 m | Large commercial |
Myth 3: "Plasterboard needs a metal frame"
Metal stud framing (CD/UD profiles) is the default method for mounting plasterboard over insulation. But it is not the only one. Disc anchor systems mount the board directly through insulation into the masonry behind - no profiles, no thermal bridging, roughly twice the installation speed. A 2.8 x 2.8 m wall at 60 x 40 cm spacing takes 39 disc anchors (2 packages of 21) and about 45 minutes. The same wall with metal profiles needs a full frame first.
The trade-off is familiarity. Most builders know profiles. Disc anchors require a different technique and a specific drill bit. But the material savings and thermal performance make the system worth considering for interior insulation and attic builds.
Myth 4: "15% waste allowance is overkill"
On simple straight runs in a modern flat with few penetrations, 10% might be enough. But on a typical house rewire with wall pass-throughs, junction box tails (15-20 cm each end), stairwell risers and the occasional wrong cut, 15% disappears fast. The cable calculator adds waste to each cross-section independently, so the actual extra metres scale with circuit count. On a 15-circuit house at 8 m runs, 15% adds about 34 m. At an average price of $1.50/m, that is $51 of insurance against a second delivery trip and two hours of downtime.
Myth 5: "RCDs are all the same"
An RCD protects against electric shock by tripping when it detects current leaking to earth. But putting all circuits behind a single RCD means one fault blacks out the entire house. The standard rule is max 6 circuits per RCD. A 15-circuit house needs at least 3 RCDs (6 DIN modules). Some electricians split by function: one RCD for lighting, one for sockets, one for dedicated circuits. That way a tripping toaster does not kill the lights.
The distribution board calculator handles RCD grouping automatically in auto mode - or you can override it and set the exact number.
The one myth that is actually true
"A bigger board is always cheaper than replacing a small one." This one holds up. A 36-module enclosure costs about $25 more than a 24-module one. Swapping a full 24-module board for a 36-module one later costs $200-400 in labour plus the hassle of re-chasing the wall cavity. The 30% spare rule works - you just need to apply it to the right starting number.
Tools discussed in this article
Distribution Board Calculator - size your consumer unit by circuit count with automatic RCD grouping and spare capacity.
Electrical Cable Length Calculator - total cable metres split by 1.5, 2.5, 4 and 10 mm2 cross-section with waste allowance and cost estimate.
Drywall Anchor Calculator - disc anchor and screw count for walls, ceilings and attics with package quantities and layout preview.
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