College Assignment - Large Design Loads

[hightower]

I've got a college assignment based around design. There's 4 DBs:

Main CCU
DB1 - Lighting
DB2 - Power
DB3 - Offices

I'm assuming that the Main CCU is the intake and that then has distribution circuits to the three DBs. Cool, so I started working through the lighting DB, got about 30A total demand across all lighting circuits. Nice and easy.

Begin on power DB and the total pull is over 300A. Even with diversity, the load is still up around 220A.

I can't find any DBs big enough to cope with this, let alone what protective device Main CCU would use to provide the circuit to it.

Should I make one of my assumptions that a second Power DB is required, and split the power circuits accordingly to get them under 125A each?

 

[telectrix]

is this 3 phase or 1 phase?

 

[hightower]

3-phase tel. Ah you beauty. I think you've just bloody answered my question based on that reply alone. When a 3-phase breaker is 125A, that means per phase. I've been thinking 125A max. So a 125A mcb will allow 125A per phase for a max of 375A? Is that right?

 

[Andy-1960]

If it is 3ph 375A you need a 3ph 400A MCCB

 

[Bobster]

Look for an MCCB board to feed the smaller DB's

 

[Bobster]

3-phase tel. Ah you beauty. I think you've just bloody answered my question based on that reply alone. When a 3-phase breaker is 125A, that means per phase. I've been thinking 125A max. So a 125A mcb will allow 125A per phase for a max of 375A? Is that right?

Is your design load 375A as a 3phase load? e.g is there any 3phase equipment?

Or are you splitting that loading across separate phases?

 

[hightower]

Okay, just to clarify further - my load is about 300A before diversity across all phases. So if I was able to balance those circuits perfectly that would be 100A per phase, could I then use a 125A three-phase MCB? Does the current rating apply as a maximum per phase or a maximum across all three phases?

@Rob - thanks, will do. However, DB2 isn't so small with 300A of gear in it. So the 125A main switch you get on 3-phase boards - is that 125A per phase, or 125A total?

 

[hightower]

Is your design load 375A as a 3phase load? e.g is there any 3phase equipment?

Or are you splitting that loading across separate phases?

It's a mixture - I have several circuits which are TP+N and then a few more which are single phase like RFCs etc.

 

[Bobster]

Ok this is off the top of my head as I don't know a lot of information here but I would suggest looking at 1 MCCB main DB

Then 3 x 3ph + N MCB DB's

One for lighting
One for Offices
One for Power.

Dependent on what loading is required on the sub circuits from the Power DB, this may need to be another MCCB board.

Be careful with discrimination. An if you're struggling the main MCCB board could be replaced with something like a BS88 Fuse board (Red spot for example) as discrimination between fuses and MCBs is a lot easier to achieve.

 

[Andy-1960]

Maybe a sketch of what you are trying to would help us advise you better.

 

[telectrix]

Okay, just to clarify further - my load is about 300A before diversity across all phases. So if I was able to balance those circuits perfectly that would be 100A per phase, could I then use a 125A three-phase MCB? Does the current rating apply as a maximum per phase or a maximum across all three phases?

@Rob - thanks, will do. However, DB2 isn't so small with 300A of gear in it. So the 125A main switch you get on 3-phase boards - is that 125A per phase, or 125A total?

125A per phase. and as won't be able to balance the phases perfectly, you will need to calculate the neutral current in order to size it.

 

[hightower]

Thanks guys, starting to make sense. @Rob - the boards are already on the drawings for me, and it is exactly as you've said.

Main CCU (MCCB Board)
DB1 - Lighting (About 30A load)
DB2 - Power (About 300A load before diversity)
DB3 - Offices (Not sure on load, not that far yet)

So DB2 is the one that I'm stuck on as 300A load seems a lot for a DB. From each DB there are no further DBs - it's final circuits from each DB.

So to go further down the rabbit hole, I have the following loads before any diversity is applied.

Single phase
3 x 32A RFC
1 x 20A Radial

Three phase
3 x 16A TP-N
4 x 32A TP-N
1 x 45A TP-N

So ignoring the SP stuff for now, across my TP-N stuff there is 221A...

 

[Bobster]

And do you expect these RFC's to be running at 32A?

Diversity....

 

[hightower]

And do you expect these RFC's to be running at 32A?

Diversity....

No, but even ignoring them for now, I have a 3-phase load of 221A. Most DBs I've seen have a 125A main switch.

 

[Bobster]

@hightower What are the 3 phase loads?

I assume this is a college question? If you've been given the fact that you're using a MCB board this suggest diversity can be applied to those.

An I've seen larger, from the schneider range. 125A is the common size though. Anything above gets real pricey