supplying sub main from a 3pole mcb?

[Motty]

Gents, your opinion please.

I am still finding my feet with commercial installation principles but this is ringing alarm bells ...

My customer wants a small DB for 6 single phase AC units to be fed from the last spare 3pole MCB in a 12way DB - " a sub-sub main" for want of a better description. He owns a small-medium sized AC installation firm who I am starting to to do sub-contracting for.

This would be fed by taking 3 x 10mm2 lines from a C-63A MCB to a small 6-way board with the loads split across the 3 phases; with neutral and earth connection back to the existing board. the units are 1x 3Kw, 3x 2.2kw and 2 x 1.5kw, total load of 12.6Kw, (approx 4.5 Kw per phase) total load of 52A, and the AC units being fed from C-16A and 10A MCBs.

My main concern, besides the hap-hazard arrangement, is having 2 MCBs connected in series e.g. nuisance tripping, and the neutral remaining connected to the source as and when the feed / 3phase MCB is turned off. A means of overcoming this is a 4-pole isolator connected between the boards, however this relies on the operator operating both the isolator and 3 ph MCB rather than just the 3ph MCB on its own. I think this is a bit gash tbh.

Also, the demand on the new board will be close to its 63A feed limit, and with a load of spare ways means anyone installing equipment in the future might overload it unintentionally. And labelling it with "do not connect further / additional equipment" just won't cut it imho.

Having looked at the job yesterday it would be straightforward to feed a new DB sub-main from the 11kvA transformer in the corner of the building, however this will quadruple+ the cost as its a 25 metre run to the new board position. There are 3 spare 400v ways in the switchgear cabinet, and existing cable ladder leading pretty much directly to the new board location.

your opinions, however fierce, would be appreciated. I am already thinking " we need to put a new sub main in; £**** this is what it costs, if you don't like it find another spark"

thanks in advance.

 

[Leesparkykent]

Gents, your opinion please.

I am still finding my feet with commercial installation principles but this is ringing alarm bells ...

My customer wants a small DB for 6 single phase AC units to be fed from the last spare 3pole MCB in a 12way DB - " a sub-sub main" for want of a better description. He owns a small-medium sized AC installation firm who I am starting to to do sub-contracting for.

This would be fed by taking 3 x 10mm2 lines from a C-63A MCB to a small 6-way board with the loads split across the 3 phases; with neutral and earth connection back to the existing board. the units are 1x 3Kw, 3x 2.2kw and 2 x 1.5kw, total load of 12.6Kw, (approx 4.5 Kw per phase) total load of 52A, and the AC units being fed from C-16A and 10A MCBs.

how far is the proposed new board from the existing?

My main concern, besides the hap-hazard arrangement, is having 2 MCBs connected in series e.g. nuisance tripping, and the neutral remaining connected to the source as and when the feed / 3phase MCB is turned off. A means of overcoming this is a 4-pole isolator connected between the boards, however this relies on the operator operating both the isolator and 3 ph MCB rather than just the 3ph MCB on its own. I think this is a bit gash tbh.

why are you worried about the neutral remaining connected?

Also, the demand on the new board will be close to its 63A feed limit, and with a load of spare ways means anyone installing equipment in the future might overload it unintentionally. And labelling it with "do not connect further / additional equipment" just won't cut it imho.

Load diversity?

Having looked at the job yesterday it would be straightforward to feed a new DB sub-main from the 11kvA transformer in the corner of the building, however this will quadruple+ the cost as its a 25 metre run to the new board position. There are 3 spare 400v ways in the switchgear cabinet, and existing cable ladder leading pretty much directly to the new board location.

your opinions, however fierce, would be appreciated. I am already thinking " we need to put a new sub main in; £**** this is what it costs, if you don't like it find another spark"




thanks in advance.

......

 

[Pete999]

No disrespect intended Motty, you describe yourself as a wannabe sparks, don't you think this sort of electrical work is best left to those who know their way around 3 phase work, sub the work out to a qualified Electrician.

 

[Engineer54]

Your over thinking and getting yourself in a right old pickle!!

 

[D Skelton]

You're over thinking it buddy.

Are you sure you're ready to be taking on such work if you're asking these types of questions?

This is a pretty bog standard job for any electrician and the 'problems' you describe are either not problems at all or they're easily overcome

 

[Spoon]

I agree with the above, in that you are over thinking this.
Answer the question in post 2 and think about it a little more mate.

 

[Darkwood]

Gents, your opinion please.

I am still finding my feet with commercial installation principles but this is ringing alarm bells ...

My customer wants a small DB for 6 single phase AC units to be fed from the last spare 3pole MCB in a 12way DB - " a sub-sub main" for want of a better description. He owns a small-medium sized AC installation firm who I am starting to to do sub-contracting for.

This would be fed by taking 3 x 10mm2 lines from a C-63A MCB to a small 6-way board with the loads split across the 3 phases; with neutral and earth connection back to the existing board. the units are 1x 3Kw, 3x 2.2kw and 2 x 1.5kw, total load of 12.6Kw, (approx 4.5 Kw per phase) total load of 52A, and the AC units being fed from C-16A and 10A MCBs.

My main concern, besides the hap-hazard arrangement, is having 2 MCBs connected in series e.g. nuisance tripping, and the neutral remaining connected to the source as and when the feed / 3phase MCB is turned off. A means of overcoming this is a 4-pole isolator connected between the boards, however this relies on the operator operating both the isolator and 3 ph MCB rather than just the 3ph MCB on its own. I think this is a bit gash tbh.

Also, the demand on the new board will be close to its 63A feed limit, and with a load of spare ways means anyone installing equipment in the future might overload it unintentionally. And labelling it with "do not connect further / additional equipment" just won't cut it imho.

Having looked at the job yesterday it would be straightforward to feed a new DB sub-main from the 11kvA transformer in the corner of the building, however this will quadruple+ the cost as its a 25 metre run to the new board position. There are 3 spare 400v ways in the switchgear cabinet, and existing cable ladder leading pretty much directly to the new board location.

your opinions, however fierce, would be appreciated. I am already thinking " we need to put a new sub main in; £**** this is what it costs, if you don't like it find another spark"

thanks in advance.

-Firstly from the off .. feeding sub mains from existing mcb boards is poor design as you will not get discrimation between you AC circuits mcb's and the 63amp sub mcb in the event of a short circuit. Having said this it happens all the time because the world is swamped with domestic wonders filtering into bigger jobs.
-Not sure why your worried about isolation of the N when you turn the submain supply off, its better and safer to have a solid link N for submains or if you did have a good reason to need to isolate N then the main 4 pole switch would have to have early make late break on the N to stop all manner of serious and dangerous issues.
-Demand is your issue to work out, in summer and winter it could be expected all AC units to work at the same time for long periods giving little if any diversity, if this is less than the sub-main fuse rating then ok - you task is to evaluate the load on the existing board and ensure it can cope with this high demand sub-main.
-Finally you have a god-send of a pre-installed cable ladder with a short 25m run and dampens all of the above worries, depending on the front end device and rating you should easily achieve full discrimination but you may have to go larger to do this, its not always about fitting the smallest cable possible.. sometimes your submains needs to be 2-3time you required needs to ensure full discrimination but as your AC units are small on this occasion it shouldn't effect you too much but would be looking at a minimun 100amp or higher to achieve full... again dependent on front end devices.


If this all seems out of your comfort zone then pull in a competent sparks and work along side but the minute they start running it off the existing sub knowing what has been said here then you know they are fobbing you off.

TBH - it depends on a chat with your customer with price for both options and even allowing future expansion if done from the origin even if it costs more initially.... if the customer is happy that a fault could take out the whole AC and dependent what feeds the existing 3ph board it could take out its fusing source before it tripped its own mcb... rare it maybe but it happens and I've seen it and rectified poor design so many times I lost count.

 

[Engineer54]

I don't think they'll be any problem with discrimination under short circuit conditions between the 10A and 16A MCBs and the the 63A supply MCB.

Providing the AC units have been correctly sized to the area's they are covering (and they should be, seeing as this is an AC installation company) and set to a sensible temp' level there will be a pretty substantial diversity factor for these units. It's only when idiots who have no idea, wind the temperature settings up/down to maximum will they be working (eg, compressor cooling/heating) for long periods of time.

Though i do understand what you are trying to get over to the OP!!

 

[Darkwood]

It really comes down to production cost if power is lost and if designed correctly as I was expressing (not what he would get away with) then in the OP's case it should be a sub-main to origin with spare capacity to ensure good design and full discrimination, I guessed the 100amp earlier but just checking up now if say you had a Square D - KQ 16amp mcb you would not achieve full discrimination on any front end device from the CD/CN/CH range of thermal magnetic trip units lower than 125amps....at 63amps you can only see full discrimination if the fault current does not exceed 500amps (partial discrimination) and as this is unlikely especially with the Tx sat in the corner of the building I can forsee losing the whole AC boards power if not taking out the supply to the existing board too.

Of course I'm teaching you to suck eggs here and have high respect for your knowledge E54, the explain is for the OP's benefit so he can understand better where he stands and understand our banter to each other.
PS; Yes I've put mcb's of submain to submain before but in knowing what the consequences are if the wrong mcb trips is really what the OP need to evaluate and discuss with the MD of said company.

As mentioned before, I've had to go in rectify others work many times to run new submains for this very reason of the thinking that a 63amp mcb can give full discrimination to say a 16amp which is not true and unlikely it would under most S/C faults.

 

[Motty]

All

Many thanks for the replies. Just to clarify a little I am the potential sub-contractor on this job, on labour only. The AC firm has priced the job and I have been prescribed the design / layout. Darkwood> I am glad I am not the only one who thinks a sub-main fed from an existing board is not ideal, and hence I am worried about the loading and overall integrity.

When I looked at the job yesterday I was thinking why did the AC firm not just price for a conventional sub-main given the cable ladder, straight run and spare 400v feeds at the main transformer/ switchgear. Its not my contract therefore I have not set any budgets etc and can only follow what the main contractor wants. As I am not convinced of the legitimacy of what I have been asked to do, I have posted my concerns here and value your comments.

Diversity: The AC I am assuming no diversity as the offices are internal cubicles inside a metal clad building - the condensors are not permitted to be sited outside (landlords restriction) so will be generating heat inside the warehouse and making it even warmer, so the AC will probably spend 3 months on full cooling 12 hours a day. Also concerning existing load on the DB they want me to feed the AC from, there were other 32A / 63A MCBs in this board to some pretty hefty lathes, milling machines etc. I am concerned that a high demand sub main will cause other equipment to trip, if not the existing DB 125A main switch, i.e. no discrimination at the single phase MCBs.

I wasn't sure on whether the triple pole mcb (i.e. Live only) switch would be acceptable on its own, hence questioning whether the Neutral needed to be simultaneously isolated. I stand corrected here to leave the neutral permanently connected.

Given the second thoughts on how I have been asked to fit the equipment, I shall probably pass the job up. And yes, my confidence (or rather lack of it) would rather sleep at night! Once again thanks for the replies

 

[D Skelton]

Don't pass up on the job, get a local contractor to split the job with you. Earn and learn.

With regards to discrimination issues, this was highlighted perfectly quite recently on a periodic we were doing up on the Thames. My apprentice shorted out the terminals of a switch in an awkward position with his voltage indicator. It was a switch fed by a sub db fed by a sub db fed by a sub db fed by the main panel. The short took the main 125A MCCB out which in turn took half the ship out.

Bad design leads to bad situations.

 

[Leesparkykent]

Don't pass up on the job, get a local contractor to split the job with you. Earn and learn.

With regards to discrimination issues, this was highlighted perfectly quite recently on a periodic we were doing up on the Thames. My apprentice shorted out the terminals of a switch in an awkward position with his voltage indicator. It was a switch fed by a sub db fed by a sub db fed by a sub db fed by the main panel. The short took the main 125A MCCB out which in turn took half the ship out.

Bad design leads to bad situations.

Maybe the OP should ask you, as your not to far.

 

[ipf]

Besides, it is an AC firm. I come across them regularly and invariably it's a case of throwing SY's in instead of SWA's or conduit. Most are excuses for electricians and the installations are badly designed.

 

[Darkwood]

Your certificate will stipulate designed by your contractor to which he will sign for the design...you can bring up any concerns regarding discrimination been partial at best but your lack of experience may be your downfall in making a strong case and to answer any questions put to you...

As long as they are willing to sign off the design then your only the installer so don't pass up on the job ... IMHO poor design, but bring it up as a query and not a challenge to their design if they are comfortable signing for it then so be it... as for milling machines and lathes, this is my area and you'd be surprised how much diversity can exist depending on what they are using them for and whether the machines are ever fully loaded ... the mcb's may be artificially high to allow for inrush on motors as an example an 11kw DOL motor may require a 63(c)mcb to allow for start-up but will never pull more than about 22amps and that will on be if it ever got stretch to its limits.

This is why commercial and Industrial gets very complicated very fast and indepth knowledge of most systems is a must to be able to evaluate the existing demand of a DB ... or just put an data logger to measure demand over a busy period....but that can be expensive and can price you out if you have to hire the unit.

 

[Motty]

After an almost sleepless night worrying about it, I will go back to the contractor and highlight concerns (strictly no plagiarism - I will tell them I have consulted the heavyweight sparks!)

I could'nt expect another firm to come in seeing as the money side of things isn't exactly lucrative to say the least - The MD reckons "it'll only take a day mate" ... (Existing DB MCB, connections to & new DB, 6x supply cables, 6 x isolators & Condensor connections, testing etc) so its priced on a wing and a prayer as it is, and technically I only have a days' labour at subbie rate. I'm just glad I went in before to look at it on my own time before I was committed. He also wanted me to take sole charge of the EIC so hence when I saw the design I started to worry ... With the best will in the world I'll think I'll give this one a miss. Once again I am thankful for your frank and very informative opinions, and will tread more cautiously in the future, and within my limits ...

cheers

 

[Engineer54]

Diversity: The AC I am assuming no diversity as the offices are internal cubicles inside a metal clad building - the condensors are not permitted to be sited outside (landlords restriction) so will be generating heat inside the warehouse and making it even warmer, so the AC will probably spend 3 months on full cooling 12 hours a day.

You're having a laugh here aren't you, never heard anything so ridiculous for a hell of a long time now!! Even if the compressor units were allowed outside these tiny AC units, in what sounds like open cubicles in a large open space building, would be knocking their guts out trying to cool/heat a vast open area. They would have been far better off installing sized AHU's and ducting to cool/heat such areas.... This is a AC installation company, ....Pull the other leg it's got bells on!! lol!!

Why is it we never get the full story in the OP's??

 

[Darkwood]

After an almost sleepless night worrying about it, I will go back to the contractor and highlight concerns (strictly no plagiarism - I will tell them I have consulted the heavyweight sparks!)

I could'nt expect another firm to come in seeing as the money side of things isn't exactly lucrative to say the least - The MD reckons "it'll only take a day mate" ... (Existing DB MCB, connections to & new DB, 6x supply cables, 6 x isolators & Condensor connections, testing etc) so its priced on a wing and a prayer as it is, and technically I only have a days' labour at subbie rate. I'm just glad I went in before to look at it on my own time before I was committed. He also wanted me to take sole charge of the EIC so hence when I saw the design I started to worry ... With the best will in the world I'll think I'll give this one a miss. Once again I am thankful for your frank and very informative opinions, and will tread more cautiously in the future, and within my limits ...

cheers

Well only you know your limits and whether to pass it on, if they expected you to sign for the design of the install too then thats when I personally would have said no, if they want it done their way for whatever reason that's fine as long as they sign for design then they are the ones who will be challenged come any issues in the future that may arise and not you. I've seen plenty of these firms and knowing my stuff I tell them straight my way or not at all - if they want to cut corners my name doesn't go on the design and I am merely the installer.