Three phase motor mcbs and rating

[Buzz1980]

Hi all a bit confused and need some advice on what size of mcbs I require for wiring a three phase motor just wanted to get some some info on it. So if I have a three phase induction motor with full load current at say 30amps on the nameplate at voltage 415volts. What size of breaker will I require? As I know that three phase mcbs say rated at 30amp are rated for 30 amp on each pole of the mcb so makes 90amps can pull threw that mcb. So if the motor is only requiring 30amps could you install a 10amp mcb which will have 10amp on each phase going to the the motor to make 30amp. On the motor nameplate where it states 30 amp is this 30 amp required on each phase or in total for the motor? Or do I install a 30 amp mcb for the motor? Any info will be much appreciated? Thanks trig

 

[telectrix]

generally the current specced on the rating plate is per phase. so you would need a MCB with In>30A, a type C or possibly D. what's the kW rating? the kW is specced for all 3 phases, wher ethe current is for each phase.

 

[Marvo]

As tel says the run current of the motor is stated per phase. Also the MCB isn't specified to protect the motor, it's specified according to the size of the cable it's supplying. Motor protection itself is an additional requirement on top of the MCB. Depending on what type of motor and what application it is you may require special fuses, you may need a thermal overload or manual motor starter. If motor protection is an area you don't have much experience in then I'd suggest you don't jump in at the deep end, it can turn into a very expensive learning curve if you're not careful.

 

[Simonslimline]

Hi all a bit confused and need some advice on what size of mcbs I require for wiring a three phase motor just wanted to get some some info on it. So if I have a three phase induction motor with full load current at say 30amps on the nameplate at voltage 415volts. What size of breaker will I require? As I know that three phase mcbs say rated at 30amp are rated for 30 amp on each pole of the mcb so makes 90amps can pull threw that mcb. So if the motor is only requiring 30amps could you install a 10amp mcb which will have 10amp on each phase going to the the motor to make 30amp. On the motor nameplate where it states 30 amp is this 30 amp required on each phase or in total for the motor? Or do I install a 30 amp mcb for the motor? Any info will be much appreciated? Thanks trig

Your'e profile says "handyman". You should get a suitably qualified & experienced electrician to do carry out the work. It could all go horribly wrong trying to guess what to do IMO.

 

[Darkwood]

If the plate says FLC 30amps then its unlikely you can protect this motor with a mcb even if you had it star/delta starting...

If as above you are a Handyman then keep well away from trying to design a motor supply circuit, this is often messed up by experienced Electricians to give you an idea how many factors need to be taken into account when designing a power and starting circuit for a motor - at a guess you have a 15kw motor there and I wouldn't be looking at MCB's as a means of circuit protection or your asking for trouble.

I wire and design motor circuits routinely and depending on how you design it you can happily have a type (b) covering a smaller motor than yours without any issues, this type 'b' for this, type 'c' for this and 'd' for that is a very generalised rule of thumb that gets taught as an introduction in college.

Type 'B' - general use (resistive and light inductive loads)
Type 'C' - moderate inductive loads
Type 'D' - high inductive loads

If a MCB is suitable sized above the load demand and ELI is still met then type 'B''s can be chosen for some motors, this is useful when meeting ELI of a type 'C' or 'D' may be problematic.

I have had this chat with you before, you are well out of your depth Trig, you may question how I know your well out of your depth and how I know what your experience is ... well your profile lists all your threads you start and posts you do, the amount of motor related questions is clearly evident you are getting involved in things you should not be. Getting a motor running running is the easy part, knowing that in various fault scenarios that motor will be protected and the equipment it is used for plus the safety of any user or local persons will not be put at risk is where competence comes in.

 

[Knobhead]

This member should be in the DIY section.

As a handyman he shouldn’t be touching anything like this and I’m surprised at some of the members that have given advice.

 

[ipf]

Looking through previous....seems some fair size plant involved.....hope you are in tandem with someone of experience and aren't 'sorting things out' all by yourself. Can't 'do and learn off the cuff' with this sort of stuff.