Three-phase circuit breaker size calculation help

[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?

 

[GLENNSPARK]

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

oh aye...

just thought it a bit disconcerting that this fella thought it was cosha to start altering the characteristics of a circuit...

without consulting BS7671, looking at the existing to verify or getting some values first to confirm what he was going to do was acceptable...

very strange...

 

[John Dickson]

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????

Kilowatts to amps (A) conversion calculator plenty calculators here Strange how he got to 60 amp when closer to 100 amp would be the answer.

 

[Andy78]

......

 

[DPG]

Kilowatts to amps (A) conversion calculator plenty calculators here Strange how he got to 60 amp when closer to 100 amp would be the answer.

20A, 60A or 100A ?? Daz

 

[Darkwood]

Kilowatts to amps (A) conversion calculator plenty calculators here Strange how he got to 60 amp when closer to 100 amp would be the answer.

Welcome to the Forum John, if you wish to divulge your Electrical level (DIY, Trainee, fully qualified etc) we can give you access to the relevent areas of the forum to suit, if not then enjoy anyway you'll be made more than welcome.

 

[Dan]

Bump

 

[HCgirl]

Good day Guys

Read all the comments. Similar issue, I have a 32A MCB Hager NF332A for battery charger to charge Airplane Luggage Tuggers. Recently been tripping out a lot, measured and found 25A on each phase. Nothing has changed but the vendor purchased new units. so my calculation as per root3 method comes to18,475A max, which would explain the tripping. to fix this i would need a 43,3A minimum which i would need a 50A breaker. this really seems like too much and i am thinking we going to burn the cabling or connection points as they are all rated at 32A.

As per your guys comment the 32A should be sufficient to keep the 25A load, as is. Might be something else causing the trip?

 

[HCgirl]

So, your saying a three-phase circuit breaker say 32 amp is actually 3x32amp MCBs fitted together?

i am highly thinking this is the case. As per the OEM (Technical Properties NF332A - https://www.africa.hager.com/nf/products/energy-distribution/protection-devices/miniature-circuit-breakers/mcbs-ne-nf/nf332a/20543.htm) its rated at 32A not specifying whether its per phase or not. i do not think they will design a unit that first needs calculation to install. opens up for too many ways for error. so i am with you on this one. would of been great to hear from someone working at breaker OEMs

 

[telectrix]

Good day Guys

Read all the comments. Similar issue, I have a 32A MCB Hager NF332A for battery charger to charge Airplane Luggage Tuggers. Recently been tripping out a lot, measured and found 25A on each phase. Nothing has changed but the vendor purchased new units. so my calculation as per root3 method comes to18,475A max, which would explain the tripping. to fix this i would need a 43,3A minimum which i would need a 50A breaker. this really seems like too much and i am thinking we going to burn the cabling or connection points as they are all rated at 32A.

As per your guys comment the 32A should be sufficient to keep the 25A load, as is. Might be something else causing the trip?

32A should be correct for a 25A load .. might need a C type, dependent on Zs and disconnection times being met. also check that cable size is sufficient for 25A. installation ref. methods may lead to derating of the cable.

 

[HCgirl]

32A should be correct for a 25A load .. might need a C type, dependent on Zs and disconnection times being met. also check that cable size is sufficient for 25A. installation ref. methods may lead to derating of the cable.

Yes it is. A C type breaker, will check Z,s n times