SUB MAIN QUESTIONS. ADVICE NEEDED

[Nelly72]

Hi all.

I have been asked to take over a job an electrician has started, and has moved abroad.

The house is a big mansion, with a 3PH supply, with 100A fuse on each Phase.

The electrician has ran 2 25mm 3core armours to another 2 DB’s in the mansion, and there has to be a DB fitted where the incoming supply is, bringing it to a total of 3 DBs.

In the plant room, there is also a 3PH heat pump.

However, my question is how would you all terminate the sub mains. Would you bring each phase into a Henley block, and branch off into a 80A switched fuse for the armour sub mains, and take a tail off the Henley blocks for each phase into a separate enclosure with a protective device for the heat pump, or would you fit a TPN board, and bring all armours into it + the heat pump supply?

If it’s a TPN board would you fit MCBs or MCCBs for the 25mm armours? Is there even

I’m a new build house basher, so this is abit more complex to what I’m used to, even though it’s a new build, so please go easy on me I am simply just wanting advice to keep me right.

 

[westward10]

MCCB panel board. I hope they aren't three core SWAs.

 

[Nelly72]

MCCB panel board. I hope they aren't three core SWAs.

Why do you hope they aren’t 3core armours mate ?

 

[westward10]

Ignore me I assumed the DBs they were supplying were three phase but they are obviously consumer units.

 

[westward10]

You could also use a busbar enclosure and switch fuses.

 

[Nelly72]

You could also use a busbar enclosure and switch fuses.

I think the spark had went down the route of Henley blocks and splitting out to 2 80A switched fuses, and then splitting out to a 50A 3phase supply for the heat pump, for cost reasons to save money on the likes of an MCCB panel.

Now, I know it will look like spaghetti junction and what not with loads of 25mm tails, but is there any regs that state this isn’t a compliant method of supplying all the sub mains.

Would like to be clued up on this so I can give client all the options, and give me peace of mind that if they go with the cheap option of Henley blocks, then atleast it’s compliant.

Thanks so much for your time replying to the thread

 

[westward10]

It is all a bit ad hoc.

 

[buzzlightyear]

The electrician has ran 2 25mm 3core armours to another 2 DB’s in the mansion, and there has to be a DB fitted where the incoming supply is, bringing it to a total of 3 DBs.

Pic would be nice, and a diagram.

 

[Nelly72]

Pic would be nice, and a diagram.

Trying to attach a quick diagram sketch I just done. It says the file is too big

 

[DPG]

Trying to attach a quick diagram sketch I just done. It says the file is too big

Screenshot it and send the screenshot. Crop it if required.

 

[Nelly72]

This is what the electrician has intended to do it seems to save the client the cost of an MCCB panel.. would like to reiterate its not the way I would do it, but want to clear up if it’s compliant, regardless of how messy it may seem

 

[Nelly72]

View attachment 111394


This is what the electrician has intended to do it seems to save the client the cost of an MCCB panel.. would like to reiterate its not the way I would do it, but want to clear up if it’s compliant, regardless of how messy it may seem

I have omitted the neutrals to each point so save more lines to avoid confusion haha..

Just a quick sketch

 

[Nelly72]

This is what the client showed me that he had bought for the tails to enter from the Henley blocks, and then feed the armour to the other DBS, whilst protecting the armour with an 80A fuse

 

[westward10]

There is nothing wrong with this but it looks a bit "housebasher", apologies for this. When I do a job I like to look back and think that is a good job rather than not look back knowing there was a better more professional way. What about surge protection?

 

[oscar21]

Don't think you can run tails directly to a 3 phase MCB for the heat pump, would need some kind of main switch as well wouldn't it. Why not fit a small 3 phase board for DB3 and run the heat pump from there, maybe a type A board if it doesn't need to be too big.

Oh and BTW I've taken over the odd job over the years where the original electrician "has gone AWOL" one time he supposedly "went abroad to learn sky diving", another time the spark had "broke his leg". But those jobs had one common denominator, getting paid was a pain in the arse. Make of that what you will.

 

[plugsandsparks]

Reminds me of a large house i did, very similar. Used TPN board in plant room and fed all local loads and submains from that board. Submains were C63 MCBs, GSHP C50 from memory. As mentioned one isolator for whole property, easy to expand and does not take up huge space with fused submains.... who uses fuses nowadays, lol.

 

[Nelly72]

There is nothing wrong with this but it looks a bit "housebasher", apologies for this. When I do a job I like to look back and think that is a good job rather than not look back knowing there was a better more professional way. What about surge protection?

Can they not be fitted at each DB to protect all appliances ?

 

[westward10]

Personally I would run the heat pump from a switch fuse.

 

[Nelly72]

I had thought

Reminds me of a large house i did, very similar. Used TPN board in plant room and fed all local loads and submains from that board. Submains were C63 MCBs, GSHP C50 from memory. As mentioned one isolator for whole property, easy to expand and does not take up huge space with fused submains.... who uses fuses nowadays, lol

I had thought about a TPN board, but was wary of the fact one of the DBs the 25mm armour feeds has a 10mm shower circuit, 10mm induction hob circuit, 2 ovens, few rings circuits, few lightening circuits etc. So 63A on the armour may not be acceptable?

Do MCBs not go upto 63A max? Would the above load on the DB not potentially pop the 63A? MCCB panel sounds more appropriate as you can use the 80A MCCB to give it some more juice, but it must’ve been too costly for the client, and that’s why the spark had bought the 80A switched fuses

 

[Nelly72]

Don't think you can run tails directly to a 3 phase MCB for the heat pump, would need some kind of main switch as well wouldn't it. Why not fit a small 3 phase board for DB3 and run the heat pump from there, maybe a type A board if it doesn't need to be too big.

Oh and BTW I've taken over the odd job over the years where the original electrician "has gone AWOL" one time he supposedly "went abroad to learn sky diving", another time the spark had "broke his leg". But those jobs had one common denominator, getting paid was a pain in the arse. Make of that what you will.

Yeah that was my plan, a small board with a main switch/mcb, then out to a rotary isolator..

My sketch was a bit misleading

 

[plugsandsparks]

I had thought

I had thought about a TPN board, but was wary of the fact one of the DBs the 25mm armour feeds has a 10mm shower circuit, 10mm induction hob circuit, 2 ovens, few rings circuits, few lightening circuits etc. So 63A on the armour may not be acceptable?

Do MCBs not go upto 63A max? Would the above load on the DB not potentially pop the 63A? MCCB panel sounds more appropriate as you can use the 80A MCCB to give it some more juice, but it must’ve been too costly for the client, and that’s why the spark had bought the 80A switched fuses

Dunno, thats where design comes into play. To expand on the similar install. Although it was TPN in the plant room, because of the length of cable runs and TP loads, I had TPN board in plant room, submain TPN board on first floor, feeding 2 SPN submains also on first floor, TPN board in garage (submain off plant room) so for the house there were 5 DBs in total (and they were all stacked) - I think your concern about loading one phase up to 80A and they having a heat pump at 50A (bearing in mind how long there cycle is) maybe a bigger issue. Anyway its all about the design...

 

[westward10]

Cost! they own a mansion.

 

[Nelly72]

Cost! they own a mansion.

I know, you know how people behave sometimes tho lol

 

[pc1966]

You get some 250A TPN boards that take a mix of wide 80-125A MCBs, and normal (17.5mm width) <= 63A MCBs from Hager. But you won't get decent selectivity that way with downstream DBs.

Going for a MCCB board would be the best option as then a incoming switch for all, and easy choice for any out-going protection from tens of amps upwards, along with reasonable (typically kA-range) selectivity on faults. Ideally put your SPD in there.

Fuses are good in many cases, but a busbar chamber and switched-fuses coming off is a bit of work and not likely cheaper than MCCB board, and MCCB boards are far easier for anyone to reset or to isolate stuff as needed,.

 

[buzzlightyear]

have you taken any picks of the main board so we can establish what is needed ,

 

[buzzlightyear]

what is the DNO SET UP .is there a main DB or just feed from H blocks .

 

[Mike Johnson]

Use a main Three phase RCD CU to distribute a single phase to each floor/main area with a separate CU, at 100amps/phase it's not going to be a problem.

 

[westward10]

What is a main three phase RCD CU?

 

[nicebutdim]

All three lines protected by a single RCD? Seems less than ideal.

 

[Mike Johnson]

What is a main three phase RCD CU?

Just a CU with a three phase RCD to supply the three phase air pump and to distribute the single phase's to the other CU's.

All three lines protected by a single RCD? Seems less than ideal.

Each subsequent CU to have a single phase RCD or as many as you want.

 

[westward10]

Never seen a three phase CU and the pump is not likely to need an RCD.

 

[Rockingit]

Why not just put a 6 way Ryefield downstream of the DNO incomer? Gives you a box to gland into, choice of fuse sizes and a better disconnection time. 2 ways for the subs, 3 ways for the plant room (which I'm assuming will need a 3ph board in it regardless) plus a spare for the future.

 

[westward10]

Sounds like the client has already obtained the parts so other options are not likely to be favoured.

 

[westward10]

Just a CU with a three phase RCD to supply the three phase air pump and to distribute the single phase's to the other CU's.


Each subsequent CU to have a single phase RCD or as many as you want.

Mcbs are not likely to achieve selectivity. Using such devices for distribution is a poor design.

 

[Rockingit]

Sounds like the client has already obtained the parts so other options are not likely to be favoured.

Something about the blind leading the blind. At £90 a pop, plus all the other Henleys, Tails, time.... small MEM board would be cheaper!!

 

[plugsandsparks]

Came across a TP board with a RCD where one would normally find a 60947-3, thought that was odd. Then about a month later found another one, it must have been a thing 20-30 years ago. Think they were MEM boards but cannot be sure.

 

[Mike Johnson]

It's how I was instructed to do it by the French "Conusule" who are the regulating body in France, the incoming main is into a S type RCD at the meter this is EDF equipment you have no choice.

 

[DPG]

It's how I was instructed to do it by the French "Conusule" who are the regulating body in France, the incoming main is into a S type RCD at the meter this is EDF equipment you have no choice.

Luckily the OP is not in France.

 

[plugsandsparks]

Makes Sense, although these were in the UK and well downstream of the DNO head and pretty sure they were not S type but cannot be sure BUT there were no RCDs on the final ccts. It all looked strange to me

 

[plugsandsparks]

There is still one on a site i go to occassionally will try and get a photo.. as i would like to know if its was once a thing.....

 

[nicebutdim]

It's how I was instructed to do it by the French "Conusule" who are the regulating body in France, the incoming main is into a S type RCD at the meter this is EDF equipment you have no choice.

That would also be a TT supply?

 

[oscar21]

Mcbs are not likely to achieve selectivity. Using such devices for distribution is a poor design.

I had this argument with someone a while ago, he wanted to protect a sub main to a summer house with a 40A MCB in the consumer unit and I was saying that if the 6A lighting circuit has a problem it could well trip the 40A MCB before the 6A one, it just depends which is the quickest to go.

I was trying to explain that if you did a loop impedance test/IPF and it showed more than say 200A which it could well do, then if there was a dead short it could take out the 40A one instead, or maybe both but he was adamant thats what 6A MCBs are for and just couldn't understand it.

 

[pc1966]

I had this argument with someone a while ago, he wanted to protect a sub main to a summer house with a 40A MCB

Selectivity will be poor, but sometimes it comes down to how important it is. For an occasional summer house it is probably acceptable risk, but not for many circuits in a home where loss of stair lighting, etc, has other hazards.

Next time point them to the tables of selectivity as that will give figures for PFC/PSCC that are likely met, but in most cases you will find the end circuit exceed them so a real risk of supply loss.

That is why fuses or MCCB are the best choices, then the Ryefield or MCCB panel choice usually comes down to likelihood of a trip and need for non-skilled folks to reset.

 

[oscar21]

Are MCCB's less "sensitive" than an MCB then like fuses are, I'm no expert in commercial stuff.

 

[pc1966]

Are MCCB's less "sensitive" than an MCB then like fuses are, I'm no expert in commercial stuff.

Once you get pas the slow thermal trip side, many MCCB have a short-delay aspect (sometimes adjustable) of 20ms or so at modest overload currents, then once you hit really big faults in the kA range they go on an "instant" trip in the 10ms or less region to energy-limit the fault, just as MCB do.

It is this short-delay aspect that gives them decent selectivity with MCB that all go "instantly" as soon as you get past the thermal limit. That is one major advantage for applications like this where you are feeding MCB/RCBO boards.

The other aspect that help with MCCB is they often have adjustable settings, for the cheaper ones (and they are not cheap, this is a relative term) that is usually just the thermal trip point and you might get 80-100% adjustment on a given size, but that is enough so with a bit of choice you can design for any specific limit. For the fancy electronic ones you can dial in all sorts of trip points to get the sort of characteristics you need, and they can comes with RCCB aspects as well giving you a very fancy RCBO to practically any trip characteristic you want. At a price!

Fuses are old technology and cheap by comparison, but they naturally have a "delay" in how the wire heats up, and they offer exceptionally good energy-limiting. So if you really don't expect them to be blown/tripped, they are a very good alternative and will just sit there for decade after decade after decade, and still work if called upon.

 

[Rockingit]

Fuses are old technology and cheap by comparison, but they naturally have a "delay" in how the wire heats up, and they offer exceptionally good energy-limiting. So if you really don't expect them to be blown/tripped, they are a very good alternative and will just sit there for decade after decade after decade, and still work if called upon.

Hence my suggestion of a small Ryefield!

 

[pc1966]

Hence my suggestion of a small Ryefield!

If it is feeding sub-mains so 5s disconnect and all 63A or above it makes a lot of sense.

But I think there is at least on 3P load for a heat-pump, that would be better served by some 3P MCB or similar so on fault all phases go down. Still a DIN box and MCB from a set of fuses is still quite a viable option.

 

[Rockingit]

If it is feeding sub-mains so 5s disconnect and all 63A or above it makes a lot of sense.

But I think there is at least on 3P load for a heat-pump, that would be better served by some 3P MCB or similar so on fault all phases go down. Still a DIN box and MCB from a set of fuses is still quite a viable option.

Thing is, it's never just a heat pump, is it. It's some local sockets, a light or two...... so my choice would be to supply a small 4way 3ph board from said Ryefield. If you're building a plant room, build a plant room! Not some abomination of tails spaghetti.

 

[nicebutdim]

Not some abomination of tails spaghetti.

I think this part bothered me most about the proposed installation.

It's the sort of set up one might expect to be faced with in an old and regularly repurposed commercial property.

 

[Mark42]

Not sure if I’ve understood this properly, but it appears to have parallels with my own installation. I highly dislike the idea of feeding SP sub-mains from a TP board, unless for very small local loads - which are not likely to increase in the future - like, maybe, a garden shed. I would never even consider a single phase 80A sub main.
For anything else I run 4 (or 5) core SWA to additional TPN distribution boards. Then you have the ability to properly balance loads between the phases, and to run any future 3P loads.
And it looks a whole lot better.
I buy Hager 3P DBs when they come up on EBay, and populate them with Hager RCBOs from the same source. So it doesn’t even cost much.
OP: are these new 3-core runs long, buried, or otherwise non-negotiable?

Edit: Apologies -I just noticed this is a zombie thread. I came to it through a ‘recent posts’ link.