How to Decide the Suitable MCB Size?

Re: Sizing MCB

Re: Sizing MCB

Re: Sizing MCB

Sizing MCB

Please read all above CR4 threads on this subject.

Once again, it depends entirely upon the wiring arrangements.

And once again, British Standard 7671 and its like in other countries gives full protocols.

You have the Power, Power factor & voltage. you already have the formula.

all u have to do is calculate. Once you find the current ,choosing an MCB for a particular application, four major factors need to be considered. These are

- the standard to which the MCB conforms

- its current rating,

- the type of trip characteristic and

- its breaking capacity.

Breakers for use should always conform to the appropriate standard, as this will guarantee that they correctly carry out their protection functions.

The current rating of an MCB is the maximum current that it will carry continuously without tripping. MCBs should always be chosen so that their current rating matches, as closely as possible, the maximum load current of the circuit they are protecting

Trip Characteristics

On the face of it, an MCB that trips as quickly as possible under fault conditions sounds ideal. In fact, this is what is required for short-circuit faults but, for overload protection, the situation is more complicated. Manyitems of equipment, such as fluorescent lights, transformers and motors, draw a high peak current for a short period when they are switched on.

An MCB, which reacts instantaneously, would trip every time such a peak occurred, which would make it unusable. Fortunately, the thermal element in MCBs does not react instantaneously, as the bi-metal strip takes time to heat up. It is, therefore, hardly affected by short-term current peaks. By changing the design of the bimetal elements, MCB manufacturers can determine what size of peak current a particular MCB will ignore, and for what length of time. This relationship between current and tripping time is usually shown as a curve, known as the MCB's trip characteristic.

To avoid the need for users to work with the curves, BS EN 60898 defines several types of standard

characteristic, the most important of which are Types B, C and D. In most cases, it's easy to choose one of these types to match the application in hand, and only in really specialized applications will users need to work with the full characteristic curves.

Type B MCBs react quickly to overloads, and are set to trip when the current passing through them is between 3 and 4.5 times the normal full load current. They are suitable for protecting incandescent lighting and socketoutlet circuits in domestic and commercial environments, where there is little risk of surges that could cause the MCB to trip.

Type C MCBs react more slowly, and are recommended for applications involving inductive loads with high inrush currents, such as fluorescent lighting installations. Type C MCBs are set to trip at between 5 and 10 times the normal full load current.

Type D MCBs are slower still, and are set to trip at between 10 and 20 times the normal full load current. They are recommended only for circuits with very high inrush currents, such as those feeding transformers and welding machines.

Breaking Capacity

The final factor to be considered when choosing an MCB is its breaking capacity. This must always be greater than the prospective short-circuit current (PSCC) at the point where the MCB is installed.

It should not be difficult to make sure that this is the case, as part of the testing required on electrical Installations involve measuring the source impedance of the supply, from which the PSCC can easily be calculated. Many modern installation testers carry out this calculation automatically.

Thanks for the information. But what i worried is currently we assume power factor based on PCs. but in any case if the will be other equipments added all the pf and current rating will be different. In UK standard do we have any standard rating when we choose the MCB?

So. Just north of 45A. The upstream feeder and its protective device will be rated somewhat more than this, clearly.

Now, what does the downstream wiring look like <rhetorical question>?

The receptacles on the back of BS1363 13A sockets won't take more than 2x4mm² physically as they are not sized for it. It would be best to break the supply down into three sub-circuits: 2no 32A ring circuits in 2.5mm² and 1no 6A lighting circuit in 1.5mm² so it is compatible with any ceiling/loft insulation, OR a 32A feeder in 6mm² for the server, a ring in 2.5mm² for the computers and a 6A 1.5mm² circuit for the lighting. So, why is an MCB required at all <rhetorical question>? All that is needed is a latest-regs-edition 4-way one of these hanging on the bottom end of the feeder cable

fitted with the appropriate breakers, then wired out to the sockets connecting to the computer equipment and to the lighting circuit. This is bread-and-butter stuff for the local electrical installer, so farm the job out to someone that knows how to do it properly, and stay safe.