Maximum Demand for a church

[dunny123]

A builder has asked me to look at a kitchen refurb he is doing at a church. In the new kitchen there will be a 6KW water heater for the sink, a 6KW tea making machine, 3 x 3KW plinth heaters, and a 6KW water boiler/cofee machine. Along with the usual lighting and sockets there is a 4.7Kw commercial dishwasher.

The existing supply is 100A 3phase CU.

There is another existing small kitchen with an electric cooker/hob, and a 2KW water heater.

Elsewhere i nthe church there are ring final circuits for sockets and obviouly a fair amount of lighting circuits.

My question is around whether the existing supply is adequate for the additional load as there is no diversity allowed for the water heaters, they alone would add up to 20KW = 86A.

There are 3 spare circuits in the existing board, I was prosing to supply three sub boards from 40A breakers (one on each phase)and spread the load over them.

So in summary, is the supply adequate, and do you agree with the design ?

 

[securespark]

You have 300A to play with, yes?

You have to think about loading up the phases evenly.

Check what circuits are on which phases and try to design the loading such that it is as evenly loaded as you can get it.

I take it the three spare ways are across RYB?

Are you saying put one 6kW and one 3kW appliance on each phase?

That would indeed be just under 40A, but I would factor in some room for expansion in the circuit: I would beef up the MCB and the cable supplying the DB's.

Have you checked the current loading on the phases?

With diversity, what is the total loading of the current circuits plus the proposed circuits?

 

[Lectrician]

I would be looking to install a three phase sub-main, not three singles. What's the point in that?

Then spread the circuits with a little thought. I would also encourage the use of TP water boilers/dish washers. Usually actually cheaper than the single phase ones (manufacturer dependent).

 

[dunny123]

Thanks for response and help..replies below.

You have 300A to play with, yes?

Yes but the CU will not be rated at 300A..the main switch is 100A for example, and although this could be changed the rating of the existing TPN board would not be 300A ?

You have to think about loading up the phases evenly.

Check what circuits are on which phases and try to design the loading such that it is as evenly loaded as you can get it.

I take it the three spare ways are across RYB?

Yes they are

Are you saying put one 6kW and one 3kW appliance on each phase?

That would indeed be just under 40A, but I would factor in some room for expansion in the circuit: I would beef up the MCB and the cable supplying the DB's.

Yes, sort of..,I meant supply 3 small DB's from the 3 x 40A (or 50A if proteus do them) breakers and split the load evenly accross the 3 new boards such that the breaker rating is not exceeded under normal use.

Have you checked the current loading on the phases?

With diversity, what is the total loading of the current circuits plus the proposed circuits?

I havent done that yet but will do

Cheers

 

[securespark]

Leccy's made a very good point, one that I missed.

Why not a TP+N board?

 

[JohnW2]

Yes but the CU will not be rated at 300A..the main switch is 100A for example, and although this could be changed the rating of the existing TPN board would not be 300A ?

I'm a bit confused and/or ignorant. Is the main switch not a 3-pole (i.e. 3 x 100A) one, and is the '100A TPN Board' not rated at 100A per phase?

Kind Regards, John.

 

[dunny123]

Ah I see if its a 100A Triple pole then thats rated at 100A per phase ?

Reason didnt intend to install a TP&N sub board was that it would need to be something like an 80A braker from the mainboard, which I dont think are available ?!

 

[Lectrician]

If it is a TPN board at the moment, you are BOUND to be able to get upto a 63amp breaker.

A 63amp TPN submain to a new 100amp TPN board would be just the job. To be honest, it is the most obvious, convenient and pretty much expected way to go.

Prating around with three 40amp sub-mains is really pointless. Why three 40amps anyway? Why not 50's, 63's?

What length run is the sub-main? Is the existing TPN board a sub-main in itself too?

 

[flameport]

This is a commercial installation, far outside the scope of this website and any DIY.
If you need to ask questions about how to design it, maximum loads etc. you shouldn't be doing it.

As for the questions - there no way to know if the supply is adequate without details of all the other circuits already installed.
Putting in three separate SP boards is ridiculous, and exactly what you would expect from someone who has no idea what they are doing.
80A MCBs are available, but you don't need one.

Then there are all the other things you don't know about, and therefore won't ask questions about, but are essential.

 

[westie101]

For that sort of additional load from fixed appliances you need to be getting approval from the DNO to connect it, that should be one of the first steps before any work commences

 

[dunny123]

Then there are all the other things you don't know about, and therefore won't ask questions about, but are essential.

So out of interest, what are some examples of such things that are essential , that I dont know or havent asked ?

I have advised the builder to ask EON for advice, clearly I have not enough experience of commercial installations, however, it appears from the helpful comments that a 63A TP OCD supplying a DB would suffice.

I remain to be convinced that a 100A rated main switch is still compliant for the additional load though.

 

[RF Lighting]

I remain to be convinced that a 100A rated main switch is still compliant for the additional load though.

That right there is one of the reasons why you shouldn't be getting involved with this job.

Just because it's stamped at 100A, it doesn't necessarily mean it can only switch 100A. A commercial electrician would know this.

I know you don't want to hear this, but you are out of your depth. Stick to you SP domestic work until you can gain some experience of working along side an electrician and learning about commercial installation work. You'll need to do a LOT of reading too.

There are far too many things to even begin to list them that differ between domestic and commercial installations. I gather you've never worked on three phase before?

I know it's still electricity, but that's where the similarity ends. You working with twich the voltage, and 3 times the current. You need to start considering things like increased fault currents and using devices with higher kA ratings, load balancing across the phases, keeping the neutral current down, TP and SP devices, and SP fuses on TP supplies, you need to consider prevention of eddy currents, and EMI interference, especially in a church where there will most likely be an induction loop.

That's just the first few bits that come into the top of my head.

I've seen so many commercial installations botched by DIs who thought they could have a go. Please let someone who knows what they are doing take this job on.

 

[dunny123]

Then there are all the other things you don't know about, and therefore won't ask questions about, but are essential.

So out of interest, what are some examples of such things that are essential , that I dont know or havent asked ?

I have advised the builder to ask EON for advice, clearly I have not enough experience of commercial installations, however, it appears from the helpful comments that a 63A TP OCD supplying a DB would suffice.

I remain to be convinced that a 100A rated main switch is still compliant for the additional load though.

 

[RF Lighting]

Did you miss my post above?

 

[dunny123]

Did you miss my post above?

Yes sorry missed that, what is the rated switching current of a TP&N 100A switch ? Is it ascertained from the BS number ?