Load calculations on three phase board

[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 .

 

[westward10]

Using 63A mcbs is a bad idea as others have said you will not achieve selectivity. Any fault in a dwelling may well disconnect at the main board you need to insist on a Ryfield.

 

[Khanzada]

Using 63A mcbs is a bad idea as others have said you will not achieve selectivity. Any fault in a dwelling may well disconnect at the main board you need to insist on a Ryfield.

If I change MCB to RCBO that will be ok ?

 

[westward10]

If I change MCB to RCBO that will be ok ?

That will make it worse. Do the circuits need additional rcd protection. Is there any design spec for the job it can't be an ad hoc design and from what you are saying it is outside your comfort zone.

 

[Khanzada]

That will make it worse. Do the circuits need additional rcd protection. Is there any design spec for the job it can't be an ad hoc design and from what you are saying it is outside your comfort zone.

That will make it worse. Do the circuits need additional rcd protection. Is there any design spec for the job it can't be an ad hoc design and from what you are saying it is outside your comfort zone.

That will make it worse. Do the circuits need additional rcd protection. Is there any design spec for the job it can't be an ad hoc design and from what you are saying it is outside your comfort zone.

Thank you for your help and suggestion , Monday or Tuesday I will see the landlord and will update it .