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.

 

[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

 

[telectrix]

max. Zs for a 32A type C is 0.7 ohms.