Three-phase circuit breaker size calculation help

[ta-electrical]

Hi, i'm a little confused about the way to calculate the amp of the circuit breaker for a three-phase system.

If i have a power of 11.5 kW and voltage of 400V, what size circuit breaker is needed for three-phase system.

My calculation was 11500/(1.732x400) = 16.6 Amps per phase ... 16.6 x 3 = 49.8 Amps, so i need a 50 Amp breaker.

Please, tell me if i worked this out correctly, if not show me how it should be calculated and state the required circuit breaker size i need.

Thanks in advance.

 

[Des 56]

Well.its not in Wikipedia,so its understanble you may not know

I suspect words like knackered and toasted may be more familiar

 

[Sparky Ninja]

Well.its not in Wikipedia,so its understanble you may not know

I suspect words like knackered and toasted may be more familiar

#

I'm thinking along similar lines with cream crackered and fudged!!

 

[malcolmsanford]

@ ta if it is just 16amps per phase as we originally thought then by putting in a c type 50 amp breaker on the circuit you could run 3 or perhaps 3.5 times the motor design and still not trip the MCB

Have you clamped the motor while it is running ? I agree it should trip out on overload, that is of course if it is connected properly.

 

[Electro-tech]

Then is the C type 50 amp Breaker suitable for Zs of the circuit ? As it seems if a C wont work then change it for D type is so common without any regard for the supply cable size or starter/ overload ect. As long as it works !

 

[Knobhead]

Motors aren’t rated per phase it’s the full rating. So work from that.

By the way where are you putting the clamp meter? It should be on the incoming to the starter, not one of the outgoing to the motor.

PS 3MW is a big compressor :sailor:

 

[Harris786]

Hi,

Why I can not get this value when I put this figure in my calculator? Regards

 

[tomfj]

I know this is an old thread but i had a similar problem at work and i cant work out how the motor vendor did his calculation .is there any good online calculators , the motor was a 55kw 400v 3 phase pf .81 squirrel cage motor DOL and the vendor said 60amp????

 

[Anirudra Ray]

Can you please elaborate? How can i choose the MCB then?

 

[GLENNSPARK]

Then is the C type 50 amp Breaker suitable for Zs of the circuit ? As it seems if a C wont work then change it for D type is so common without any regard for the supply cable size or starter/ overload ect. As long as it works !

i had to listen to this kind of thing a while back of ov someone in the wholesalers

it was a walk in fridge that kept tripping the MCB at a resteraunt...

so this fella comes in asking for a 16A d curve MCB.....eh i thought....to feed a chuffin fridge...
so i asks him what the Zs was....he didn`t know....hadnt measured it....
good start methinks..

anyway...
his argument was that the chef was going nuts over this fridge tripping out....so he was going to install this d curve..to `eliminate the problem`..and to `cover his back`...
christ...

i explained to him that he would be better `covering his back`..by ensuring he met disconnection times

didn`t listen...off he tootled to fit it

saw him again about a week later....same topic...same wholesalers..

turned out that someone had swapped the 16A MCB for a 6A...and that was the reason it was tripping...

he hadn`t even bothered to look at the rating of the existing MCB before this `upgrade`...

well. it take allsorts......i suppose

 

[ChrisElectrical88]

i had to listen to this kind of thing a while back of ov someone in the wholesalers

it was a walk in fridge that kept tripping the MCB at a resteraunt...

so this fella comes in asking for a 16A d curve MCB.....eh i thought....to feed a chuffin fridge...
so i asks him what the Zs was....he didn`t know....hadnt measured it....
good start methinks..

anyway...
his argument was that the chef was going nuts over this fridge tripping out....so he was going to install this d curve..to `eliminate the problem`..and to `cover his back`...
christ...

i explained to him that he would be better `covering his back`..by ensuring he met disconnection times

didn`t listen...if he tootled to fit it

saw him again about a week later....same topic...same wholesalers..

turned out that someone had swapped the 16A MCB for a 6A...and that was the reason it was tripping...

he hadn`t even bothered to look at the rating of the existing MCB before this `upgrade`...

well. it take allsorts......i suppose

are you one of these people who argues in the wholesales?

 

[GLENNSPARK]

are you one of these people who argues in the wholesales?

hmm....

sometimes...

 

[ChrisElectrical88]

Thought you might have been.

 

[GLENNSPARK]

Thought you might have been.

what gives you that idea?

cant understand it....

 

[ChrisElectrical88]

Well, at the moment your waiting for some poor unsuspecting soul to come on the forum so you can ambush them, as you have put in your other post, and you know your stuff. I'm not taking the pee or anything I love watching arguments in the suppliers.

 

[AndyL]

I'm sure its only ever constructive criticism right Glen?