Circuit Load and Breaker Calculator
Design current, breaker loading and suggested ratings.
Quick answer
With the default inputs: Connected load 3000 W · Voltage 220 V · Power factor 0.9 etc.Design current 12.12 A
Edit the inputs below to recalculate.Inputs
Result
Design current
12.12A
| Diversified load | 2400W |
| Diversity factor | 0.80 |
| Breaker rating | 16A |
| Loading | 75.8% |
| Verdict | Loading is sensible |
| Suggested breaker | 15A |
| Matching cable (conduit) | 1.5mm² |
| Spare capacity | 3.88A |
| Spare load | 768W |
A breaker must satisfy two conditions at once: no larger than the cable capacity, so it protects the conductor, and at least 1.1-1.25 times the design current, so it does not nuisance trip. Sustained loading above 85% ages the device; below 40% short-circuit protection is insensitive; 60-80% is the sweet spot. Socket circuits need 30 mA residual current protection.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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.
How to work it out
- Count the load that will realistically run at the same time on this circuit — not the whole-house total, and not a naive sum of every appliance nameplate.
- Apply a diversity factor: 0.6-0.8 for dwellings, 0.7-0.9 for offices. A 3 kW induction hob, 2 kW air conditioner and 1.5 kW water heater total 6.5 kW, but at 0.8 that is 5.2 kW — and that is the design figure.
- Compute the design current I = Pc ÷ (U × cosφ). 5.2 kW at 220 V with cosφ 0.9 gives 26.3 A.
- Size the breaker: lower limit ≥ 1.1 × I to ride through normal fluctuation and small inrush, upper limit ≤ the cable capacity. 26.3 × 1.1 = 28.9 A, so a C32 fits.
- Check loading: 26.3 ÷ 32 = 82%. The comfortable band is 50-80%. Above 90% it runs hot and trips; below 40% the breaker is oversized and protects nothing.
- Size the cable last: capacity ≥ 32 A breaker rating. 4 mm² in conduit carries only 25.6 A, so use 6 mm² (about 32 A in conduit) or run it in free air.
Where it goes wrong
- Summing every nameplate as the design load. That gives 6.5 kW and 32.8 A, drives a C40 selection, and quietly abandons overload protection for a combination that almost never runs flat out.
- Oversizing the breaker and calling it safety margin. A C40 on 2.5 mm² in conduit (20 A) means the cable burns before the breaker trips — the single most common cause of residential electrical fires. The code requires the breaker rating not to exceed the cable capacity.
- Undersizing the breaker and ignoring inrush. Air-conditioner compressors draw 5-7 times rated on start; a type C curve at 5-10 times barely copes. Pumps and compressors need type D or they trip on every start.
- Using an RCD as a breaker. An RCD only responds to earth leakage, not overload. A 32 A RCD cannot replace a 16 A breaker protecting a 2.5 mm² circuit — the two functions are separate devices.
Material and cost
C16/C20 breakers run 20-40 CNY each for domestic brands and 60-120 CNY for Schneider or ABB. A 30 mA RCD is 80-200 CNY. A 12-way empty enclosure is 150-400 CNY. Devices only — wiring and internal dressing excluded.
On site
- Too many sockets on one circuit is the standard defect: eight living-room sockets on a single circuit will trip the moment a heater and a kettle run together. Keep to about ten sockets per circuit.
- Bathrooms, kitchens and air conditioning must be on dedicated circuits with RCD protection. This is a mandatory requirement, not a preference.
- Torque breaker terminals to the manufacturer figure, typically 2-2.5 N·m. Hand-tight terminals are the number one cause of overheating and blackened lugs in the field.
- In older buildings with aluminium service conductors, oxidised joints are the real hazard. Replacing the whole run with copper is safer than splicing, which just creates more dissimilar-metal contacts.
Basis
- GB 50054-2011 Code for design of low voltage distribution, circuit protection and device selection
- GB 50096-2011 Code for design of residential buildings, electrical chapter
- JGJ 242-2011 Code for electrical design of residential buildings
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Circuit Load and Breaker Calculator. 2026-10-25. https://jianzhujisuan.com/en/circuit-load
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.