2P 63A mcb Rating help?

[bk81]

Hi Everyone,

I have two guys telling me different things on a 2P 63A mcb.

1 guy says it's rated to 125A (effective 2x63)

1 guy says it's 80% of this (100A)

Anybody know who's right and why?

Cheers.

 

[Richard Burns]

Please explain what you are intending to convey by rating?

 

[bk81]

I mean what current rating the mcb should be used for, should I use 125A or 100A for my calculations?

At the moment I'm using both values and taking worst case scenarios so I'm covered, but, it would be good to know what the real rating is to save from doing two lots of calculations.

 

[Richard Burns]

You use a 63A MCB with a current rating of 63A, the clue is in the name?

This would be 63A per phase if you were using two phases (not a usual occurrence) or 63A overall if you were double pole protecting a single phase line and neutral.

 

[bk81]

Richard,

If only it was that simple .

I have just found this data from GE, turns out both are wrong. A 2P 63A mcb has a current rating of 107.45A.

 

[telectrix]

something wrong here. all ratings have that figure of 107.45. must be something else

 

[bk81]

Good point, I should read the whole part!

 

[bk81]

What a *** I am, that's the price. Need to find my bag of excuses now...

 

[telectrix]

bloody expensive then, unless that price is in polski zloty.

 

[telectrix]

a 63AMCB has a rating of 63A ( surprise). it will happily run at 91A for approx 1 hour before tripping ( maybe that's where the confusion arises). so, your circuit design load ( Ib ) needs to be < 63A. but the cable's max current capacity Iz, needs to be >63A.

 

[bk81]

These guys are putting a 50mm cable onto a 2P 63A mcb and using it as an alternative to 1.5P 125A mcb!

I've emailed GE to say what they have to say about their MCB before giving out the bad news, hope they get back to me soon!

 

[Darkwood]

I see whats happening here .... by any chance have we got parallel cables here and they are protecting each conductor with its own O/L protection so in effect the cores are supplying 125amps but shared over 2 x 63amp mcbs of a 2 pole device hence they achieve a 125amp between them???

Confirm before i comment any further.

 

[Darkwood]

You also state on of the guys states 80% .... due to it been DP and possibly other mcb's in align with them then a mcb derating factor of must be applied i.e.... if 6 mcb's are fitted adjacent to each other and Ib is greater than O.85% of the mcb's then a grouping factor of 0.85 would be applied ...if the mcb in your example was on its own then a derating factor of 0.95% needs to be applied as the mcb's will increase the ambient temp of the enclosure and will reduce the In rating of your mcb.

Mcb's are usually factory tested at 30-degrees if the environment is higher than this then a derating fator again is required to apply to the mcb..... a 63amp mcb at 35-degreesonly has a rating of 59 amps.

 

[bk81]

Darkwood,

The cables are not paralleled, 1 x 50mm instead of 2 x 25mm, but in you are correct in essence, a 50mm cable is being connected in parallel over 2 63A mcbs with a mini busbar/comb on the load side.

So with the MCB's being in parallel then the current rating would be more around 125A than 63A?

 

[yellowvanman]

So your trying to get a 125A supply from a DB by using 2 x 63A MCBs?

Wouldn't recommend it! Can see all sorts of issues with it!