Hey guys, what does for example 300A rating on a 3 pole MCB means?

[monira]

Hey guys, what does for example 300A rating on a 3 pole MCB means? Does it mean each phase carries 300A i.e. (Current in R=300A, Y=300A, B=300A) or it represents total currents in all the 3 phases i.e.(Current in R+Y+B=300A)?

Please assist, it is confusing me a lot with my calculations of breaker ratings, thanks to all helping hands I advance.

 

[James]

300A per phase.
just think of it as 3 x 100A single phase breakers joined Together.

 

[Risteard]

300A per phase.
just think of it as 3 x 100A single phase breakers joined Together.

3*300A!

 

[monira]

300A per phase.
just think of it as 3 x 100A single phase breakers joined Together.

Hi thanks for ur reply but now I am even getting more confused!
Here is why,
I have a 300A 3 pole MCB connected breaker which has a load current per phase as follows:-
R=101A, Y=98A, B=118 all summed up totalling to 317A ... you said I should think of it as a 100A per phase system and NOW I have above issue at hand where R and B phase is above the 100A and the MCB is tripping after some times of operation, not frequent but it's nuisance sometimes once in a week or even once in a fortnight, but one time it tripped three times in a week!!!

The question is, does it mean the R and B phase is loaded leading to Overload????? The breaker is rated 300A. Thanks once again.

 

[James]

Oops, as @Risteard pointed out my typing mistake should read 3 x 300A

you either have a faulty breaker or the load on at least one phase is going over 300A by a considerable margin.

 

[ElectroChem]

Hi thanks for ur reply but now I am even getting more confused!
Here is why,
I have a 300A 3 pole MCB connected breaker which has a load current per phase as follows:-
R=101A, Y=98A, B=118 all summed up totalling to 317A ... you said I should think of it as a 100A per phase system and NOW I have above issue at hand where R and B phase is above the 100A and the MCB is tripping after some times of operation, not frequent but it's nuisance sometimes once in a week or even once in a fortnight, but one time it tripped three times in a week!!!

The question is, does it mean the R and B phase is loaded leading to Overload????? The breaker is rated 300A. Thanks once again.

James had an unfortunate typo in his response. The rating on circuit breakers is per phase, so a 300A is 300A on each of L1/2/3.

On a side note, it's very unlikely your breaker is an MCB (Miniature Circuit Breaker). They top out at about 100-120A, you are probably dealing with an MCCB (Moulded Case Circuit Breaker).

 

[James]

Not sure if it was a typo or a brain fart moment!
was thinking 3 x 300 but somehow 3 x 100 got on the page And I didn’t notice.

top marks to @davesparks for calling me out on it, I don’t get many disagree marks but that one was well deserved.

 

[monira]

Not sure if it was a typo or a brain fart moment!
was thinking 3 x 300 but somehow 3 x 100 got on the page And I didn’t notice.

top marks to @davesparks for calling me out on it, I don’t get many disagree marks but that one was well deserved.

Not sure if it was a typo or a brain fart moment!
was thinking 3 x 300 but somehow 3 x 100 got on the page And I didn’t notice.

top marks to @davesparks for calling me out on it, I don’t get many disagree marks but that one was well deserved.

Thanks @James & everyone, now I get it 100%. (300A per phase for a 300A 3 pole rated MCCB, it's actually MCCB not MCB as i mentioned b4) Thanks so much once again for support and response from everyone

 

[monira]

James had an unfortunate typo in his response. The rating on circuit breakers is per phase, so a 300A is 300A on each of L1/2/3.

On a side note, it's very unlikely your breaker is an MCB (Miniature Circuit Breaker). They top out at about 100-120A, you are probably dealing with an MCCB (Moulded Case Circuit Breaker).

Thanks a lot @ElectroChem , I've got it clearly. It's actually MCCB not MCB as I mentioned b4