Distribution Board Rating

[Jeffmo65]

Hi , quick question , in my thoughts requires a quick answer
160amp fused switch feeding existing Hagar panel board fine no problem with that
My superior wants to install a new 125amp rated DB fed from same switch ?
I don't I would want to see a board rated at 160amp or above
Thoughts please

 

[Engineer54]

Hi , quick question , in my thoughts requires a quick answer
160amp fused switch feeding existing Hagar panel board fine no problem with that
My superior wants to install a new 125amp rated DB fed from same switch ?
I don't I would want to see a board rated at 160amp or above
Thoughts please

Why?? Just change the fuses out to 125A.
The size of the fuse switch is basically just a frame size, same as a DNO's service cut out, where the max fuse rating is say a 100A, but can accomodate anything from say 20A to 100A

 

[telectrix]

think OP means a 2nd 160A DB parallelled with an existing 160A DB, both on a 160A fuse

 

[Jeffmo65]

Correct . Can't change the fuse . The rating as far as I gather is the bus bars in the board . Surely they must be rated to the protective device ?

 

[Engineer54]

think OP means a 2nd 160A DB parallelled with an existing 160A DB, both on a 160A fuse

If that's the case, still can't see any real problem, if the 160A switch fuse supply is sufficient to cover the needs of both DB's.

 

[Darkwood]

The 160amp fuse is there to protect the submain cable, any switchgear attached should be rated to the upstream fusing i.e. 160A in this case unless due to design parameters where the board cannot pull more than the main switch can handle due to say the sum of the protective devices downstream limits possible FLC (Although its good practice to match the switchgear even in this case to allow for expansion and alteration).

2 x 160amp off one is fine if demand has been worked out to satisfy this set up, although I would have the 160amp front end feed a small busbar chamber and 2 individual Isolators for the DB's

 

[Engineer54]

Correct . Can't change the fuse . The rating as far as I gather is the bus bars in the board . Surely they must be rated to the protective device ?

Why can't you change the fuse(s)??

Would you design an installation where the total current expected of the individual circuits of a DB is going to be in excess of the rating of the DB, i HOPE NOT!!

 

[Jeffmo65]

Why can't you change the fuse(s)??

Would you design an installation where the total current expected of the individual circuits of a DB is going to be in excess of the rating of the DB, i HOPE NOT!!

Eng I havnt designed anything this is an existing supermarket with existing distribution connected and yes your right about the design aspect . However someone far more worthy than me has done this previous . This scenario is an add on which I am arguing with my gaffer about . In essence I see the bussbar as just a link of copper so therefore the fused switch device is there to protect the sub feed and the links through the DB . I appreciate that some maths wizard will whistle of some equation about withstand and PSCC and all that which would be fine . But trust me my boss would look at it and throw it in the bin cos he wouldn't understand it . The design with regards to bussbar chamber I had thought of . Costs prevail unfortunately . Thanks guys

 

[Engineer54]

The 160amp fuse is there to protect the submain cable, any switchgear attached should be rated to the upstream fusing i.e. 160A in this case unless due to design parameters where the board cannot pull more than the main switch can handle due to say the sum of the protective devices downstream limits possible FLC (Although its good practice to match the switchgear even in this case to allow for expansion and alteration).

2 x 160amp off one is fine if demand has been worked out to satisfy this set up, although I would have the 160amp front end feed a small busbar chamber and 2 individual Isolators for the DB's

Providing the 160A switch fuse supply is sufficient to supply both of these DB's then like you the inclusion of a busbar chamber and isolators/switch fuses for the DB's is the way i'd go too. However i can't see why you'd need both DB's to be rated to the 160A switch fuse....

 

[Jeffmo65]

Thanks Eng . Like you the buss chamber with the two isolators is the way forward . Another option maybe would be to have another sub feed from the existing switch feeding an 125amp switch fuse then into an 125amp rated DB ? . I understand the outgoing would in theory limit the draw but knowing how they work someone else will probably come along and drop another say 63amp breaker in there for some humongous bit of kit and I would worry the board would not be sufficiently rated and guess what it's my arse that's on the dotted line . Cheers

 

[Engineer54]

Eng I havnt designed anything this is an existing supermarket with existing distribution connected and yes your right about the design aspect . However someone far more worthy than me has done this previous . This scenario is an add on which I am arguing with my gaffer about . In essence I see the bussbar as just a link of copper so therefore the fused switch device is there to protect the sub feed and the links through the DB . I appreciate that some maths wizard will whistle of some equation about withstand and PSCC and all that which would be fine . But trust me my boss would look at it and throw it in the bin cos he wouldn't understand it . The design with regards to bussbar chamber I had thought of . Costs prevail unfortunately . Thanks guys

As stated above the 160A fuses in the switch fuse is there to protect the sub main cable. It's not there to provide CCC of downstream DB's, if such DB protection is required /needed then it should be provided (see above post) . It would be an extremely badly designed system if a 125A DB was installed to supply say 160A of circuits.

 

[Darkwood]

Eng I havnt designed anything this is an existing supermarket with existing distribution connected and yes your right about the design aspect . However someone far more worthy than me has done this previous . This scenario is an add on which I am arguing with my gaffer about . In essence I see the bussbar as just a link of copper so therefore the fused switch device is there to protect the sub feed and the links through the DB . I appreciate that some maths wizard will whistle of some equation about withstand and PSCC and all that which would be fine . But trust me my boss would look at it and throw it in the bin cos he wouldn't understand it . The design with regards to bussbar chamber I had thought of . Costs prevail unfortunately . Thanks guys

Firstly dependent on the board you buy... lets say a Schneider for arguments sake.... th 3ph usually have a standard main switch of 125amp rated and the switch to busbar links are only rated at this, the main busbar riser that connects to your mcb/rcbo's are actually rated for the largest switch that can be fitted to the board. When you upgrade these boards to 160A main switch you are given a new set of appropriately rated links and an expansion chamber to house the larger unit - so no the board may not be suited to be on a 160amp fuse.

I don't understand your comment about cost?... to do the job correctly this is the method and not sure how you are going to acheive it otherwise, the existing board and main sub-main isolator will not be designed to receive 2 cables of 160amp rating unless pre-designed to do so like parallel supplies but often expension boxes fitted to allow this, to do so on an existing system designed for one cable would be cowboy land IMHO... if your boss cannot design and/or price the work to be done correctly then he shouldn't be taking on these jobs and should stick to housebashing which would seem to be his norm' judging by his requests to you.

Its a supermarket and poor design can have consequences...

-Each board should be able to be individually isolated for maintenance and emergency without other DB's been effected, this is a business and could have large costs if you directly piggy back the cable.
-The 125amp switch with not be designed to fit the cable you need to rate for the 160amp front end and it may also be difficult to sweep in without expansion box meaning excessive sideforces with be on the mainswitch, this can lead to poor internal contact and burnout.

So exactly how are you proposing to piggy back the cable and maintain the rating without compromising the integrity of the new 125a main switch.... without a busbar chamber to split for isolation, to note a busbar chamber could also let you fit a switch/fuse of 100amp for the second board, keeping everything within regulation for protection as well as isolation requirements.

 

[Engineer54]

Thanks Eng . Like you the buss chamber with the two isolators is the way forward . Another option maybe would be to have another sub feed from the existing switch feeding an 125amp switch fuse then into an 125amp rated DB ? . I understand the outgoing would in theory limit the draw but knowing how they work someone else will probably come along and drop another say 63amp breaker in there for some humongous bit of kit and I would worry the board would not be sufficiently rated and guess what it's my arse that's on the dotted line . Cheers

As above post.

You could say the same about an additional 63A breaker being installed on a 160A DB that is already close to it's max rating. You can't realistically design for the idiots in this world. So long as the installation meets it's current design needs all is well....

 

[Darkwood]

Just going slightly off topic Jeff

Your boss is an idiot
You will never really learn anything or advance you career where you work
You probably will never be heard by your boss regardless if your right

I would in the short term just express your concerns and do the work he asks, in the long term I would look for a better company to work for and one thats goign to teach you the trade and excel your knowledge and appreciate your work... go for one with industrial and commercial work, this is a safer knowledge to gain in economic terms, if the housing market is slow or saturated by short course wonders then you have the skill to tap the industrial and commercial sector giving a good future prospect of always been employable.

 

[Jeffmo65]

In essence Darkwood that is exactly the answer I was wanting to get to . Yes I agree with you about terminations on the existing switch . The existing cable according to drawings are 35mm . So what you are maybe theoretically proposing is i
1. Main incomer switch feeding busbar
2. Switch for existing fed from bus
3. Switch for new DB fed from bus
so another 2 switches required plus bus chamber and standard let's say 125amp Schneider DB .
This is the right way to me . To him is another matter . I am currently the QS for the company and come from a commercial/ light industrial background , my boss ? You've guessed it . He still has QS STATUS so he can sign for it then . Cheers Darkwood

 

[Jeffmo65]

Just so you don't think I'm an idiot . I have installed many bus chambers and designed and installed many panels including ATI panels changeover panels . This was a general question about wether I was loosing it or not as we do work for a number of large supermarket chains and if he isn't going to listen to me then what's the point of my position . I've always installed panels at the fuse rating as a matter of course and as Tony says rectum protectum . Your replies have been very helpful and my boss is due a few lessons maybe and a nudge to read his regs? . Cheers again boys/ girls

 

[Darkwood]

In essence Darkwood that is exactly the answer I was wanting to get to . Yes I agree with you about terminations on the existing switch . The existing cable according to drawings are 35mm . So what you are maybe theoretically proposing is i
1. Main incomer switch feeding busbar
2. Switch for existing fed from bus
3. Switch for new DB fed from bus
so another 2 switches required plus bus chamber and standard let's say 125amp Schneider DB .
This is the right way to me . To him is another matter . I am currently the QS for the company and come from a commercial/ light industrial background , my boss ? You've guessed it . He still has QS STATUS so he can sign for it then . Cheers Darkwood

There is another thread selling a 200amps busbar 750mm and 160 amp switch at a very good price, maybe you want to check it out, you can get cheaper poor quality tat but you will learn quickly that its not worth it in the long term and yes thats the way you should design it if using common upstream isolator, you could as ENG stated cut several hundrend quid off the cost by running a second supply from the 160 sw/f to a 125/100 sw/f and protect your second board correctly but this isn't what would be classed as good design practice but does achieve compliance at a lower cost, alternatively change the 125amp main switch for a 125amp mccb if the brand has the option?

PS 160amp on 35mm cable sounds wrong, are you sure the fuses haven't been de-rated in the 160amp switchgear? What cable and install method isthe existing?

 

[Jeffmo65]

Basically Darkwood the switch fuse is directly next to the cut out and the main panel existing is directly above it . From the photos it is all ran in Galv trunking so would imagine supply side of switch D/I tails and singles from topside to panel . Can't answer on the fuse size but as fitted drawings state 160 . Cheers .

 

[Engineer54]

35 mm on 160A supply, that's got to be right on the limit, depending on the cable and installation method, but i've nothing here to check/confirm...

 

[Darkwood]

Even tri-rated 35mm is going to be a push IMHO without looking it up, I would have thought 50mm tri- as a minimum but I cannot look it up right now, and it is the existing so not your direct concern but may be a point to look into and to be brought up if it doesn't comply, this is why I asked if they had been derated when you said 35mm....