Cable Size

Spazz · Nov 10, 2012

Hi all - can someone confirm my thoughts please

Here is the situation:

6 floor hotel
45 bedrooms over 4 floors
bottom 2 floors are reception and kitchen - not worried about these at present
3 phase supply coming into building - 1 phase per 2 floors - current setup
Each floor has 2 DBs
The manager now wants to put separate DBs in each room (I am pushing for RCBOs in main floor DB and no DB in each room)
12 rooms per floor (on average) (6 rooms per DB)
The supply cables from main supply DB to floor DBs are traveling up an old shaft over 100mm from surface - no need for RCD protection
Between floor DB and Room is less than 50mm from surface - would need 30mA RCD protection via RCBOs

The manager wants separate DBs in each room so if a room develops a fault it wont effect anything else - RCBOs in main floor DB would do this anyway - separate circuit per room - (1 for lighting and 1 for sockets) - also thinking about discrimination here! - with 30mA RCD in each room.

I need to work out:

cable size to the main floor DB (from the basement 3-phase supply)
cable size to the individual room DBs

Each room has:

1 kettle
2 bedside lamps
main lamp on ceiling
4 sockets in total
shower is NOT electric

If im not mistaken each room would draw 25A max first thing when someone boils the kettle and using their hair dryer/straightener
Was thinking about a 4mm radial to the 4 sockets and 1.5mm to lighting

Was thinking a 10mm cable from main floor DB to individual room DB (could get away with 6mm as max current would be less than 36A - RCBO in main floor DB would be a 40A)

Not sure about the cable from floor DB to main supply DB - it currently has 16mm T&E installed - not sure if this is enough or would have to be 25mm?????? - thought please

Plonker 3 · Nov 11, 2012

Nic, if you can't work out what is needed on this install, why are you taking it on?

Engineer54 · Nov 11, 2012

Not counting for the kitchen and office reception floors, going by your calculations this hotel will need it's own 1MVA distribution transformer. Your already nearly up to 1200A just for the rooms!!! lol!!

I think your a bit outside your comfort zone on this one. So if it's going to go past installing RCBO's in the letting room floors two DB's, pass this one on to an electrician that can work out the design side of things!!!

Knobhead · Nov 11, 2012

I’m intrigued, just how do you get 25A per room? Most people (my ex excluded) leave the tumble drier and washing machine at home!

timbobelfast · Nov 11, 2012

I smell something fishy......

snowhead · Nov 11, 2012

Isn't this the same O.P that was putting hundreds of E.M lights in national supermarket that had never had any a while a go, and asking for advice?

Bit of a giveaway here, not used to Commercial Instal " Between floor DB and Room is less than 50mm from surface - would need 30mA RCD protection via RCBOs"

MDJ · Nov 11, 2012

Nicholas Owen said:
Hi all - can someone confirm my thoughts please

Here is the situation:

6 floor hotel

45 bedrooms over 4 floors

bottom 2 floors are reception and kitchen - not worried about these at present

3 phase supply coming into building - 1 phase per 2 floors - current setup

Each floor has 2 DBs

The manager now wants to put separate DBs in each room (I am pushing for RCBOs in main floor DB and no DB in each room)

12 rooms per floor (on average) (6 rooms per DB)

The supply cables from main supply DB to floor DBs are traveling up an old shaft over 100mm from surface - no need for RCD protection

Between floor DB and Room is less than 50mm from surface - would need 30mA RCD protection via RCBOs

The manager wants separate DBs in each room so if a room develops a fault it wont effect anything else - RCBOs in main floor DB would do this anyway - separate circuit per room - (1 for lighting and 1 for sockets) - also thinking about discrimination here! - with 30mA RCD in each room.

I need to work out:

cable size to the main floor DB (from the basement 3-phase supply)

cable size to the individual room DBs

Each room has:

1 kettle

2 bedside lamps

main lamp on ceiling

4 sockets in total

shower is NOT electric

If im not mistaken each room would draw 25A max first thing when someone boils the kettle and using their hair dryer/straightener
Was thinking about a 4mm radial to the 4 sockets and 1.5mm to lighting

Was thinking a 10mm cable from main floor DB to individual room DB (could get away with 6mm as max current would be less than 36A - RCBO in main floor DB would be a 40A)

Not sure about the cable from floor DB to main supply DB - it currently has 16mm T&E installed - not sure if this is enough or would have to be 25mm?????? - thought please

On a project this size I would want an engineer to do the specification, firstly my Indemnity insurers would wet their pants If I gambled on it and second let someone else who is used to these schemes take the responsibility, I would have thought a Hotellier with 45 beds would look to an architectial consultancy to over see the project, still your gamble, I wouldn't take the Job on myself, too risky for a spark to be making these decisions.

richy3333 · Nov 11, 2012

timbobelfast said:
I smell something fishy......

Is it a fish?

snowhead · Nov 11, 2012

richy3333 said:
Is it a fish?

No it's something from Whales

Knobhead · Nov 11, 2012

snowhead said:
Isn't this the same O.P that was putting hundreds of E.M lights in national supermarket that had never had any a while a go, and asking for advice?

Bit of a giveaway here, not used to Commercial Instal " Between floor DB and Room is less than 50mm from surface - would need 30mA RCD protection via RCBOs"