Load calculations on three phase board

[Khanzada]

Costumer have 12 flats and need to feed for each flat a prepaid meter and consumer unit within the flat , the main supply to building is three phase , my question is how to distribute the load for 12 flats, each flat have a load of 63A.
Main supply is three phase .

 

[davesparks]

What exactly do you want help with here?

How you distribute the supply is via a suitable distribution board, I believe ryefield boards are the most commonly used for this application.

 

[Khanzada]

What exactly do you want help with here?

How you distribute the supply is via a suitable distribution board, I believe ryefield boards are the most commonly used for this application.

Thank you for reply.
I spoke to costumer to fit 12ryefield board with 63A fuses , but he bought it live three phase board with 125A main switch rating and 12 , 63A MCBs to make the cost cheaper.
My question is if I installed that it will be ok for load demand .

 

[nicebutdim]

What have the DNO specified?

 

[Khanzada]

What have the DNO specified?

He didn’t provide me with any info from DNO .

 

[westward10]

What does the design specification state.

 

[Khanzada]

What does the design specification state.

What does the design specification state.

each flat have a 8.5kw shower, 16A sockets, 32A cooker and 6A lights . I calculated it all load is 61.6A .

 

[nicebutdim]

As an apprentice my posts here should not be taken as advice, but as questions from someone who wishes to learn and what I'd really like to learn is how a 125A incomer might ever be considered suitable for a block of 12 flats?

Putting maximum demand to one side; if 4 residents happen to take a morning shower at the same time, that 125A incomer is going to be handling its full rated current before any other consumption is factored. If each flat were to draw an average 15A at any given time (not an unlikely situation), then the incomer is handling far in excess of its rated current.

 

[Khanzada]

As an apprentice my posts here should not be taken as advice, but as questions from someone who wishes to learn and what I'd really like to learn is how a 125A incomer might ever be considered suitable for a block of 12 flats?

Putting maximum demand to one side; if 4 residents happen to take a morning shower at the same time, that 125A incomer is going to be handling its full rated current before any other consumption is factored. If each flat were to draw an average 10A at any given time (not an unlikely situation), then the incomer is handling far in excess of its rated

As an apprentice my posts here should not be taken as advice, but as questions from someone who wishes to learn and what I'd really like to learn is how a 125A incomer might ever be considered suitable for a block of 12 flats?

Putting maximum demand to one side; if 4 residents happen to take a morning shower at the same time, that 125A incomer is going to be handling its full rated current before any other consumption is factored. If each flat were to draw an average 15A at any given time (not an unlikely situation), then the incomer is handling far in excess of its rated current.

As an apprentice my posts here should not be taken as advice, but as questions from someone who wishes to learn and what I'd really like to learn is how a 125A incomer might ever be considered suitable for a block of 12 flats?

Putting maximum demand to one side; if 4 residents happen to take a morning shower at the same time, that 125A incomer is going to be handling its full rated current before any other consumption is factored. If each flat were to draw an average 15A at any given time (not an unlikely situation), then the incomer is handling far in excess of its rated current.

Thank you for the information , because it’s three phase it will be 125 x 2 = 250A .

 

[nicebutdim]

Thank you for the information , because it’s three phase it will be 125 x 2 = 250A .

Would discrimination present an issue where 40A shower circuit is supplied from 63A MCB?

 

[Khanzada]

Would discrimination present an issue where 40A shower circuit is supplied from 63A MCB?

For the first item which is shower full 100% load 40A and the rest 56 x 0.4 = 21.6
40+21.6= 61.6 A

 

[nicebutdim]

For the first item which is shower full 100% load 40A and the rest 56 x 0.4 = 21.6
40+21.6= 61.6 A

I mean discrimination between supply MCB and shower MCB/RCBO. The reason for asking is that I believe the rule of thumb to achieve discrimination between MCBs is that upstream needs to be at least 2x the rating of that being supplied.

Happy to stand corrected.

 

[Khanzada]

I mean discrimination between supply MCB and shower MCB/RCBO. The reason for asking is that I believe the rule of thumb to achieve discrimination between MCBs is that upstream needs to be at least 2x the rating of that being supplied.

Happy to stand corrected.

I think as longer the load is not exceeding the main breaker load it’s ok , protective device must be equals or greater .

 

[Jim_e_Jib]

As an apprentice my posts here should not be taken as advice, but as questions from someone who wishes to learn and what I'd really like to learn is how a 125A incomer might ever be considered suitable for a block of 12 flats?

Putting maximum demand to one side; if 4 residents happen to take a morning shower at the same time, that 125A incomer is going to be handling its full rated current before any other consumption is factored. If each flat were to draw an average 15A at any given time (not an unlikely situation), then the incomer is handling far in excess of its rated current.

If four residents on the same phase took a shower at the same time, this would be the case on a 125A incomer. Are there diversity rules to be applied between dwellings on the same phase though?

I don't think the OP has yet supplied the specification of the incomer though, only that the main switch for the three phase board purchased is rated at 125A.

 

[James]

I think as longer the load is not exceeding the main breaker load it’s ok , protective device must be equals or greater .

It sounds like you are well out of your depth here.

a 3 phase board with 125A INCOMER can handle
3 x 125A
not
2 x 125 = 250A

your first action should be to meet the designer and decide on what the load per flat is going to be.

next decide if the existing supply is capable of providing this for 12 flats.
if not, apply for a suitable supply to be installed.

this all needs designing from the ground up, not lashed together with bits from the car boot and hope it works.

 

[davesparks]

I mean discrimination between supply MCB and shower MCB/RCBO. The reason for asking is that I believe the rule of thumb to achieve discrimination between MCBs is that upstream needs to be at least 2x the rating of that being supplied.

Happy to stand corrected.

Generally with MCB's in the 6A - 63A range full discrimination cannot be achieved.

You may be thinking of BS88 fuses where generally an upstream fuse 2 standard sizes bigger will discriminate with a downstream fuse.

 

[davesparks]

Thank you for reply.
I spoke to costumer to fit 12ryefield board with 63A fuses , but he bought it live three phase board with 125A main switch rating and 12 , 63A MCBs to make the cost cheaper.
My question is if I installed that it will be ok for load demand .

Why is the customer specifying and supplying the materials? I'd walk away from the job as that is a sure sign of trouble to come on a project of this size.

I would say that no, that DB will not be suitable for the installation. MCB's are not appropriate for selectivity (discrimination), fuses or maybe MCCBs would be.

What size is the incoming supply going to be? Or is it already installed?

Are the 12 meters going to be owned and operated by an energy supplier or are they privately owned and operated by the landlord?

 

[nicebutdim]

It's been a long day, but at least I've come home to learn something at the end of it.

I'd thought that 125A 3 phase incomer was rated for 125A total, rather than per phase and I've some reading to do about discrimination as I'd read elsewhere that it could be achieved if upstream rating was at least 2x downstream.

Sorry to the OP for asking so many questions, but they were piling up in my mind last night and it seemed prudent to ask them for more than one reason.

 

[westward10]

It's been a long day, but at least I've come home to learn something at the end of it.

I'd thought that 125A 3 phase incomer was rated for 125A total, rather than per phase and I've some reading to do about discrimination as I'd read elsewhere that it could be achieved if upstream rating was at least 2x downstream.

Sorry to the OP for asking so many questions, but they were piling up in my mind last night and it seemed prudent to ask them for more than one reason.

Each pole of the isolator is rated at 125A so you can have loading of 125A per phase a total of 375A but the isolator will never see this, each pole will only carry 125A.

 

[Khanzada]

It's been a long day, but at least I've come home to learn something at the end of it.

I'd thought that 125A 3 phase incomer was rated for 125A total, rather than per phase and I've some reading to do about discrimination as I'd read elsewhere that it could be achieved if upstream rating was at least 2x downstream.

Sorry to the OP for asking so many questions, but they were piling up in my mind last night and it seemed prudent to ask them for more than one reason.

It's been a long day, but at least I've come home to learn something at the end of it.

I'd thought that 125A 3 phase incomer was rated for 125A total, rather than per phase and I've some reading to do about discrimination as I'd read elsewhere that it could be achieved if upstream rating was at least 2x downstream.

Sorry to the OP for asking so many questions, but they were piling up in my mind last night and it seemed prudent to ask them for more than one reason.

Thank you for reply to my question, I calculated the load for all circuits after diversity 175A , in that case main incomer must be above the total load , I will change it to 250A .
That’s the benefit when you working with the team on site , so you will never make a mistake .