Many computers in office. How many circuit breakers?

[ashirusnw]

We're wiring up a new large office but I don't trust the electricians calculations.

We allowing for about 21 computers (mixture of laptop and desktops) and at least 30 double sockets (for phone chargers and assorted electrical equipment like routers and printers).

There is a standard consumer unit with place for only 3 more circuit breakers.

I'm assuming that one ring circuit on a single circuit breaker (trip switch / fuse) won't be enough and may easily get overloaded.

Each circuit breaker essentially covers one ring and all the sockets on that ring, right?

If so will 3 circuit breakers (i.e. three ring circuits, say 10 double sockets non each) support all the computers, screens etc., printers that we expect?

 

[Electrifying]

We're wiring up a new large office but I don't trust the electricians calculations.

We allowing for about 21 computers (mixture of laptop and desktops) and at least 30 double sockets (for phone chargers and assorted electrical equipment like routers and printers).

There is a standard consumer unit with place for only 3 more circuit breakers.

I'm assuming that one ring circuit on a single circuit breaker (trip switch / fuse) won't be enough and may easily get overloaded.

Each circuit breaker essentially covers one ring and all the sockets on that ring, right?

If so will 3 circuit breakers (i.e. three ring circuits, say 10 double sockets non each) support all the computers, screens etc., printers that we expect?

You don't trust the electrician - so you thought you'ld double check on a DIY forum?? Ha Ha

 

[ashirusnw]

We're wiring up a new large office but I don't trust the electricians calculations.

We allowing for about 21 computers (mixture of laptop and desktops) and at least 30 double sockets (for phone chargers and assorted electrical equipment like routers and printers).

There is a standard consumer unit with place for only 3 more circuit breakers.

I'm assuming that one ring circuit on a single circuit breaker (trip switch / fuse) won't be enough and may easily get overloaded.

Each circuit breaker essentially covers one ring and all the sockets on that ring, right?

If so will 3 circuit breakers (i.e. three ring circuits, say 10 double sockets non each) support all the computers, screens etc., printers that we expect?

You don't trust the electrician - so you thought you'ld double check on a DIY forum?? Ha Ha

Well, basically he hasn't calculated it and could justify at all whether it would be enough. I don't know how to calculate this but it appears neither does he. I'm sure many people here know how much load 30 computers use.

(You might ask why I'm using him ... lonh story but he came with the job)

 

[Lectrician]

Not quite the same story from the other sites you posted this on.....

Read section 543 for a start.

 

[Taylortwocities]

You'd better make sure that he does not provide any RCD protection on that/those circuits and that it is wired with high-integrity earthing.

See http://www.tlc-direct.co.uk/Book/7.8.2.htm

He'll need specific socket outlets for this.

 

[plugwash]

Loading wise assuming these are fairly normal computers and not insane gamer rigs i'd think 1 32A ring would be tight but probablly ok and 2 32A rings would be more than enough.

Agree that RCDs may be troublesome if the PSUs are old and leaky and high integrity earthing should be considered.

I'd say two rings with special sockets, high integrety earthing and no RCD protection for the computers/monitors and a third ring with normal sockets and RCD protection for other loads is the way to go.

 

[sparkiemike]

Before dismissing RCD protection I would advise that the electrician discusses the use of RCDs with the business owner, and the implications of having or not having them.

We don't know if there are skilled or instructed persons on site or if the business owner would be prepared accept that risk or responsibility

 

[Lectrician]

Before dismissing RCD protection I would advise that the electrician discusses the use of RCDs with the business owner, and the implications of having or not having them.

We don't know if there are skilled or instructed persons on site or if the business owner would be prepared accept that risk or responsibility

Yes, thats a given, but you certainly would not put in high integrity earthing (which is done because you know their is likely to be leakage), and then go and put an RCD on the same circuit - That would just be stuuupid!

 

[sparkiemike]

Stupid would be not designing the circuit(s) propery and not considering the effect high protective conductor currents on RCDs. That does not mean you cannot have RCDs on those circuits.

You could have high protective conductor currents and still have RCDs - but limit the number of socket outlets for example. (see reg 531.2.4)

Guidenace from the IET
"Wherever a designer so chooses to omit RCD protection, traceable confirmation must be obtained from the client to identify the reason for the omission and such confirmation shall be included within the documentation handed over to the client upon completion of the work. Where no such confirmation can be obtained, RCD protection should not be omitted."

The client needs to be aware of the risk that has been transferred to them, and that it is their responsibility to ensure that adequate precautions are taken by employees and contractors, to prevent danger.

 

[bernardgreen]

The leakage current from Live to Earth via the mains input filters in the power supplies of the computors is significant. There is a capacitor between Live and Earth which allows this leakage current to flow. This means a 30 mA RCD will trip when too many computors are supplied by it. It depends on the make and model of computors how many can be supplied by a single RCD as the leakage varies enormously form model to model. In one office set up where RCDs were necessary it only needed 8 computors to trip a 30 mA RCD

 

[JohnW2]

The leakage current from Live to Earth via the mains input filters in the power supplies of the computors is significant. There is a capacitor between Live and Earth which allows this leakage current to flow. This means a 30 mA RCD will trip when too many computors are supplied by it. It depends on the make and model of computors how many can be supplied by a single RCD as the leakage varies enormously form model to model. In one office set up where RCDs were necessary it only needed 8 computors to trip a 30 mA RCD

Indeed. I would have thought that what we probably really need for such situations is an RCD which will operate in response to a sudden 30 mA increase in L/N imbalance (probably also with an overriding 'must trip' absolute imbalance level). Provided one was careful not to put a lot of computers (or whatever) on-line simultaneously, that would avoid unwanted trips, but would provide the desired RCD functionality, no matter how high up to the 'must trip' limit) was the background level of earth leakage. Do devices anything like that exist, I wonder?

Kind Regards, John.

 

[Electrifying]

Fit 3 x 32A, 30mA RCBOs and run two 4mm radials off each RCBO.

Each radial will feed 5 socket outlets - giving 30 outlets in total.

Spreading the computers over these six radials - giving 3 or 4 computers per radial - should minimise any leakage problem.

 

[Taylortwocities]

Brilliant,
except for this wee limitation:

There is a standard consumer unit with place for only 3 more circuit breakers.

 

[JohnW2]

Fit 3 x 32A, 30mA RCBOs and run two 4mm radials off each RCBO.

What is the perceived advantage of running two radials off each RCBO? Since each RCBO would then 'suffer from' the combined earth leakage of the two radial circuits, you might as well just have one radial for each RCBO (and save a bit of cable), mightn't you?

Kind Regards, John.

 

[riveralt]

I haven't got my BGB - its away on holiday at the moment but I have read in the Competent Person rag that Section 534 includes requirements for the installation of Surge Protection Devices and filters to limit transcient overvoltages and divert damaging surge current away from sensitive equipment such as computers and electronic circuits.
Would this installation require SPD's?