Sizing a smart breaker is not harder than sizing a conventional MCB — but buyers get it wrong more often because the product is sold by connectivity first and electrical characteristics second. Three numbers decide everything: load current, supply voltage and phase, and the curve type.
Step 1 — Calculate the load current
For single-phase loads, divide power by voltage and power factor. A 2.2 kW water pump at 230 V with a power factor of 0.85 draws roughly 11.2 A. The rule is to select the next standard rating up — 16 A — never a rating below the calculated current, and never oversized by more than one step, because the overload protection curve shifts with it.
Step 2 — Match the supply: single phase or three phase
Most export markets run 230 V single phase for small loads and 380–400 V three phase for anything above about 3 kW. Three-phase pumps and compressors need a breaker that switches all phases; if the system also has a neutral through the breaker, you need a 4P unit, and if the neutral is bonded separately, a 3PN unit. The difference trips up even experienced buyers — we wrote a separate guide on true 4P versus 3P+N switching.
Step 3 — Pick the right curve
Curve B trips at 3–5× rated current, curve C at 5–10×, curve D at 10–20×. Pumps, fans and compressors have inrush currents of 5–7× rated current for a few hundred milliseconds, which makes curve C the default for motor loads. Lighting and resistive heating are fine on curve B. If your breaker nuisance-trips at every start, the curve — not the rating — is usually the culprit.
Step 4 — Decide whether you need metering
A metering smart breaker measures voltage, current, power and accumulated energy in the app. For rental equipment, irrigation districts and tenant billing this pays for itself immediately; for a simple on/off lighting circuit it is unnecessary. Our SMR1 metering smart RCBO holds a class-1 energy accuracy and logs hourly consumption for 12 months.
Step 5 — Choose the network: Wi-Fi or 4G
If the installation site has stable broadband, Wi-Fi is free to run and simple. If it does not — fields, billboards, construction sites, holiday homes — a 4G unit with an included data card is the only practical choice. We compared both in detail in the 4G vs Wi-Fi selection guide.
The three mistakes we see most
- Sizing from the generator or inverter rating instead of the actual load.
- Ignoring inrush: a correctly rated curve-B breaker that trips on every motor start.
- Forgetting ambient derating: above 40 °C inside a closed cabinet, derate by roughly 1% per °C or move to the next rating.
Send us the load nameplate — power, voltage, phase and starting method — and our engineers will confirm the rating, curve and model the same day. Request a free sizing check here.