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Thread: Sizing of Electrical components

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    Sizing of Electrical components

    Hey guys I have got a three phase drill presser here; on the name plate it written, Volt (400.), Ph(3), Pole (4), ,Hz (50),, kW (1.1), r.p.m. (1400), A(2.75), Cos (0.72), type (YS 90), Ins.Cl F, IP (54)& S6-60%. Now my 1st question what size 3 Phase C/B should I use?

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    Diamond Member AndyD's Avatar
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    The circuit breaker is sized according to the circuit, not the appliance. Can you give details of the circuit? Is it on a plug and socket or will it be the only appliance on the circuit and hard wired into a t/pole isolator?
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    Sorry for the late response Andy, it will be the only appliance connected. It will be 5meters away from the DB that I'll use to power it up. The cable will be run on a cable tray.

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    Does the drill press motor has its own overcurrent relay for in the case of an overcurrent? If so, it can be wired from the db with a 1 or 1.5sqmm surfix,protected by a 5A curve 1 mcb.

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    Thank you MullerR for your response, if I may ask what formular did you use to size the circurt breaker. If I calculate the current that my drill presser will draw (using P=root3*V*I) I get 1.59Amps.
    from there how do you size your breaker?

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    Your circuit breaker size is calculated from the cable size, the installation method, cable length etc etc. The circuit breaker protects the circuit, not the drill that's connected to it. The drill press motor should be protected by an appropriately sized thermal overload contained within the drill press itself.

    Also you can't calculate the run current of a motor from the kW rating of the motor. The kilowatt rating of a motor is an indication of its mechanical output, not its electrical input. In the good old days we sized motors in Horsepower and a 1 HP (Horsepower) motor meant it could lift the same weight at the same speed as an actual horse which is a measure of its output capabilities. Nowadays we use Kilowatts for the same measure and 1 horsepower is around 0.75kW. This has lead to endless confusion because electrical input is also measured in kilowatts but the electrical input will always be higher than the mechanical output because no motors are 100% efficient and free from losses.

    So basically you can't convert the kilowatt rating of a motor into a run current unless you also know its efficiency, it's power factor and any other losses including friction etc. If you want the run current rather refer to the data on the plate on the side of the motor or the loading graphs supplied by the manufacturer.
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    Quote Originally Posted by @SamboLindoh View Post
    Hey guys I have got a three phase drill presser here; on the name plate it written, Volt (400.), Ph(3), Pole (4), ,Hz (50),, kW (1.1), r.p.m. (1400), A(2.75), Cos (0.72),...
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    A rule of thumb I use to calculate starting current for a motor is 8:1. So if the motor is a 1Amp motor, the starting current will peak approximately at 8Amps. The period that it remains at 8Amps is directly linked to the load.

    But as Andy says the circuit breaker in the DB is there to protect the wiring, not your load.
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    The drill presser will be the only machinery connected to this circuit breaker there will be no other machinery or appliance connected. the drill presser will be "5m" away from the the DB and the cable will run on a cable rack.Please see the attached name plate for the drill presser.
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    Quote Originally Posted by Justloadit View Post
    A rule of thumb I use to calculate starting current for a motor is 8:1. So if the motor is a 1Amp motor, the starting current will peak approximately at 8Amps. The period that it remains at 8Amps is directly linked to the load.

    But as Andy says the circuit breaker in the DB is there to protect the wiring, not your load.
    With small motors the start current would generally only affect the selection of the curve of the circuit breaker. In SA most MCB's are C curve as standard which means they'll tolerate 5-10 times their rated load current without instantaneous tripping which will tolerate most motor start currents. In some EU countries and the UK for example most MCB's are installed as B curve as standard so they would be more likely to nuisance trip with a motor load.

    @Sambo, The fact it's such a small motor limits your circuit design options somewhat so I really wouldn't overthink this too much. Being a drill press it won't be started under any great load so I'd just go with MullerR's suggestion of a 1.5mm Surfix with a 5A C-curve MCB.
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