Circuit Load and Breaker Calculator
Design current, breaker loading and suggested ratings.
Inputs
Result
Formula
| Diversified load | load × diversity |
| Design current | Pc ÷ (U × cosφ) |
| Loading | current ÷ breaker rating |
| Lower limit | ≥ 1.1 × current |
| Upper limit | ≤ cable capacity |
| Cable | capacity ≥ breaker rating |
3000 W at 220 V, 0.9 power factor, 0.8 diversity and a 16 A breaker: 2400 W diversified, 12.12 A and 75.7% loading — sensible, with about 760 W spare.
FAQ
Is a bigger breaker better?
The opposite. The breaker protects the cable, so its rating must be below the cable capacity. A 16 A breaker on 1.5 mm² (19 A) is sound; a 32 A breaker on the same cable will not trip at 25 A, and the conductor heats until the insulation melts and burns. That is the commonest origin of domestic electrical fires.
How is the diversity factor chosen?
It captures the gap between what is installed and what runs at once: 0.7-0.9 for domestic sockets, 0.9-1.0 for lighting, and 1.0 for lifts and fire equipment that must all work together. Too low and breakers trip; too high and you waste cable and money. A whole building's incoming supply can use 0.4-0.6, since no two households peak together.
Why must socket circuits have RCD protection?
Because sockets are what people touch, so a fault becomes a shock. The 30 mA threshold is set below the level that causes ventricular fibrillation, tripping within 0.1 s. Lighting circuits, mounted overhead and out of reach, may go without. An RCD does not replace earthing — they are complementary and both are needed.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.