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Three-phase circuit breaker size calculation help

T

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.
 
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why did you multiply 16.6 x 3 again?

a three phase circuit breaker will have 3 x MCB's linked at the switch (most likely)
 
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I would have thought the 16.6 is in amps per phase so if there are a total of three phases you have to multiply it by three ? (This is what was confusing me)

There are not going to be individual circuit breakers, just the one.
 
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ta-electrical said:
So, your saying a three-phase circuit breaker say 32 amp is actually 3x32amp MCBs fitted together?
Click to expand...

A basic 3 phase MCB is 3 single MCB's, formed together with a link to ensure isolation of all Live conductors occurs. More specific, higher rated MCB's will be specially manufactured.
 
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Okay, i understand but, just to make sure your saying in reference to my calculation in my original post, a 20 Amp breaker would be enough?
 
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Correct,

11500w on single phase = 11500/230 = 50Amp
11500w on three phase = 11500/√3 x 400 = 16.60Amps, so a 20Amp MCB should suffice (assuming no inrush or anything to be considered).
 
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It's an air compressor in a workshop, it wasn't a balanced load i.e. it required a neutral as well.
Origanally, i had used a 32 Amp type C but, it tripped instantly afterwards, i was told it requires a 50Amp breaker type C, which worked.
I'm not sure how 50Amp was calculated, the only way that made sense to me was to multiply by 3 (refer to original post), and this is where my confusion is.

Thanks for the replys, i hope you can clear my mind on this one.
 
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It is a very big compressor and it is very likely you may be right about it being per phase.
I will make sure to check it again to confirm, thanks.
 
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ta I have to assume that as it's a large compressor it is star-delta connected, When you say you fitted a 32amp C breaker and it tripped have you connected it right ie star-delta, because if you haven't and connected to the wrong contactor so it is delta-star it would easily trip.

If you have connected it correctly is the star-delta timer an adjustable one, again if it is then you could be running on star start up to long, so look at that, or even look at a faulty timer.

Finally as widdler suggest is it 11.5kw per phase which in pit terminology make it a FBC ............... F big compressor.
 
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i can't see the motor being 11.5kw /phase. the fact that a 32A C is tripping leads me to believe that the motor is goosed.
 
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@malcolm, yes it is star-delta and i would assume it is connected correctly otherwise the 50Amp breaker would not have worked either. I haven't confirmed whether it is 11kW per phase but, will do so soon as well as checking the timer.

@telectrix, sorry can you elaborate on what you meant when you said 'goosed'? Thanks.
 
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ta-electrical;319032 @telectrix said:
Well.its not in Wikipedia,so its understanble you may not know

I suspect words like knackered and toasted may be more familiar
icon7.png
Click to expand...
 
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@ 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.
 
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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 !
 
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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:
 
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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????
 
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Electro-tech said:
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 !
Click to expand...
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
 
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GLENNSPARK said:
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
Click to expand...

are you one of these people who argues in the wholesales?
 
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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.
 
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AndyL said:
I'm sure its only ever constructive criticism right Glen?
Click to expand...
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...
 
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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?
 
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ta-electrical said:
So, your saying a three-phase circuit breaker say 32 amp is actually 3x32amp MCBs fitted together?
Click to expand...
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
 
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HCgirl said:
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?
Click to expand...
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.
 
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Three-phase circuit breaker size calculation help
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