how can select the fuse rating for a particular circuit?

Question

see a single phase circuit having a 94Amps, then 100Amps fuse is safest one or 125A. and why?

Answer 1

94 amps single phase sounds very large for what I assume is a domestic user.
The fuse is sized to suit the size of the load and type of load
Take a very extreme case of one motor with full load current of 94 amps (yes I know this is highly unlikely) then the starting current will be probably 500 amps and you would need a fuse a lot higher than 100 or 125 amps (you would not need 500 amps though because the fuse rating is the current the fuse will carry for ever not for the few seconds of starting current)
If your load is nearly all resistive (lights and heaters) there will not be starting current problems and 100 amps will be OK
The reality is that you will have a mixture of resistive loads and inductive loads (motors for fans, pumps, compressors) and you have to choose the fuse for whatever load mixture you have
The trial and error method is start with 100 amp fuse and test it by starting your largest motor, if fuse blows increase by one size until you reach the size which does not blow
Actually I think what you have is a number of single phase circuits and your total load is 94 amps so the individual cicuits will each have their own fuse and you want a fuse as backup for the total load, in this case the backup fuse probably needs to be twice the size of largest individual fuse
It is all a bit complicated and I think you need an experienced electrician to assess your particular case

Answer 2

100 Amps. You want to limit the current to prevent damage to the circuit. So if the current goes above 100 Amps, the fuse will blow. This way any sensitive components (which normally cost more than a fuse) will be protected.

Answer 3

its safest to stick as close to the working current, the 94 amps, with the 100 amp fuse. you are trying to protect the circuit from excessive current or you can use a slo-blo fuse rated at 100amps. the slo-blo will compensate for surges while a fast blo will pop at the first surge.

Answer 4

You size the wire to the maximum anticipated load. Normally you go to the next higher size to compensate for surges, such as motor start up. Then you use the fuse appropriate to the wire size. There are special situations where that isn't true so check with a qualified electrician if there is any doubt. Eletricity is not something to play around with.

Answer 5

Your fuse rating must be the closest to the predicted current in the circuit so it "fuses" before the current reaches an excessive value!