Found these discussions and Q&As online around this and am trying to determine how/ when one would decide to choose which way to go?


Type B - trips between 3 and 5 time full load current

Type C - trips between 5 and 10 times full load current

Type D - trips between 10 and 20 times full load current

I will quote more information from the above thread as we begin to understand it more.

A simpler Q&A not as detailed assessment of factors as above discussion thread.


Short-circuit relay trip-current setting (Im)

Short-circuit tripping relays (instantaneous or slightly time-delayed) are intended to trip the circuit-breaker rapidly on the occurrence of high values of fault current.

There is some insight on this question but its brief and specific and does not clarify or explain which to choose in other scenarios & why?

Working house main Control Miniature Circuit Breaker(25A) instead of transfuse (16A)?

Also will be sharing photos of existing ELCD/ MCB scenario at location for which the above fundamentals / factors need to be understood and then to arrive at a decision.

  • \$\begingroup\$ Your "full load current" in the B, C and D definitions should probably be "rated current" as full load current implies the maximum current which will be well above the MCB's nominal rating. \$\endgroup\$
    – Transistor
    Commented Nov 11, 2017 at 13:40

2 Answers 2


Your specifications on type B, C and D MCBs are incomplete. The crucial detail is that these are the overload currents they will tolerate for a maximum of 5 seconds. All of them should trip if they experience a current above their rating for longer.

The MCB type you would choose for a circuit therefore depends on the peak transient current you expect your load to draw. For most applications type B is perfectly adequate, but for circuits that supply very reactive loads such as air conditioners, motors and other machinery, large switch-on currents would force you to use C or D. The manufacturers of such devices will normally specify details such as supply cable size, MCB capacity and type and local wiring regulations may also restrict some of these choices.


You want to choose a circuit breaker that will protect your circuit but not nuisance trip to a non fault condition.

To answer your question you need to know the highest current for the circuit you are trying to choose the circuit breaker for. This isn't as simple as adding the rated loads together and adding 20% as is typically done to choose a 15 amp versus a 20 amp circuit breaker.

A good starting point would be to clamp on a current meter onto your circuit on a min max setting to capture the highest current draw (max) when all your loads turn on.

I believe that B,C,D circuit breakers will trip in a sustained overload condition but will take some time to respond. This could be seconds or minutes at loads in the 1.5 X range.

The B,C,D trip curves have more to do with the instantaneous trip response. According to the attached trip curve image - a 4 X current fault on a B trip curve will trip the breaker at somewhere within 1/2 second. The same fault on a C trip curve breaker will take 10 seconds or more for the breaker to respond.

Your next question in making your descision is what can your wiring infrastructure handle? You indicate that this is a Home. Will the wiring from the home, which is connected to the breaker be damaged by a 10 second interval of 100 Amps (25 amp rating X 4 = 100 amps). I am thinking that the wire and insulation for a 14 awg or 12 awg circuit may start to get hot during an event such as this.

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