MCB Size Calculator
Enter the load in kW, the supply voltage and phases, the power factor, the load type and the cable size. The calculator finds the design current, the next standard MCB rating and checks the cable.
- MCB rating
- Trip curve
- Cable check
Cable ratings here are for twin and earth clipped direct.
Ib = P ÷ V, then pick an MCB rated at or above it and no bigger than the cable. A 3 kW heater on 230 V draws 13 A, so a 16 A type B MCB suits it. A 2.5 mm² cable carrying about 27 A is fine.
MCB size chart
Single phase, 230 V. Search for a load.
| Load | Current | MCB | Cable |
|---|---|---|---|
| 1 kW | 4.3 A | 6 A | 1.0 mm² |
| 2 kW | 8.7 A | 10 A | 1.0 mm² |
| 3 kW | 13 A | 16 A | 1.5 to 2.5 mm² |
| 4.6 kW | 20 A | 20 A | 2.5 mm² |
| 7 kW | 30.4 A | 32 A | 6 mm² |
| 9.5 kW | 41.3 A | 50 A | 10 mm² |
| 11 kW | 47.8 A | 50 A | 10 mm² |
Amps.
Load and MCB
230 V single phase. Slide the load.
The MCB rating is shown on the breaker.
How to size an MCB
Load to breaker. Step through it.
Load
kW and power factor.
The breaker protects the cable, so never fit one bigger than the cable rating.
Type B, C or D
Trip curves. Compare them.
Homes.
Commercial.
Industrial.
Use C or D only where inrush currents trip a B.
Before you fit it
MCB checks. Tick each item.
Tick each item as you go.
Only a qualified electrician should fit or change breakers.
What you get
MCB sizing. Each tile explains one result.
Amps.
B, C or D.
Checked.
Design.
Single or three.
An oversized MCB will not protect the cable from overheating.
Frequently Asked Questions
No question matches that search.
What size MCB do I need?
Work out the load current and pick the next standard rating above it.
What size MCB for a three kilowatt heater?
A sixteen amp type B breaker on two and a half square millimetre cable.
What is the difference between B and C type MCBs?
C types allow a bigger surge before tripping, for motors and lighting.
Can I fit a bigger MCB to stop it tripping?
No. The breaker must protect the cable, so find the cause instead.
What size MCB for an electric shower?
Often forty or forty five amps on ten square millimetre cable, check the shower rating.
Does cable length affect MCB size?
Long runs may need a bigger cable for voltage drop and fault current.
Protected circuits
Match the breaker to the load and the cable and the circuit stays safe.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.