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Hello All

I'm an electrical engineering HNC student and I'm currently doing a research project about an office rewire. I have to price it up basically. I am not a DIYer looking for a quick way to burn my house down and I won't be doing any real electrical work off the back of anything that is said here.

First up whats the best way of calculating how many PC's I can put on a ring main (I cant just write that some electrician said.......)?

When wiring a ring main, with suspended ceilings and raised flooring, what kind of things would affect the price or make things difficult for a real electrician?

When taking the old wiring out what could affect the price of the job?

In order to price up the rewire what things should I remember to consider? I have 17 workstations each requiring 4 power sockets and a data feed. I have to look at the lighting also

Anything else that I could include in the project would be most appreciated

I'm sure that isn't enough info but its a conversation starter anyway. I have done a fair bit of research and looked a lot of info up but real knowledge and experience has always been better than books

I hope this isn't too much of a slog to answer

Thanks
 
Hi and welcome. In general we don't provide direct answers to assignment / research questions as it defeats their purpose. But if you would like to post some of yours as a basis for discussion, we can probably help you improve them.

As a starter, for the PCs, consider - running load, inrush, leakage current and maximum permissible number of units affected by a single fault?
 
Hello Lucien and thanks for the response

I'm only after a few pointers and I will have to back everything up with my own evidence and research.

17 PC's (roughly 300w) with LCD's monitors (Rounded up to 100w) look to use about 6800w. Divide that by 240V and you get 28.3A. With a 30A RCD, on the face of it and allowing for future expansion and the electrical equipment becoming more powerful as time goes by, I would be able to get away with 2 ring mains. But that is only considering 2 of the 4 sockets available to each workstation.

I'm a bit shaky with earth leakage. I think I should maybe allow 2mA per PC and 1mA per monitor, also the other 2 sockets. I could see an argument for only allowing 2 workstations per ring with a 30A/30mA RCD? 8 ring mains going into 1 room seems wild to an inexperienced guy like me
 
BS 7671 regulations543.7.1.201 and 202 cover protective conductor currents allied with multiple pieces of equipment with inherent earth leakage.
Also read7.5.1 in the Onsite Guide
 
17 PC's (roughly 300w) with LCD's monitors (Rounded up to 100w) look to use about 6800w.

This is highish for general purpose office PCs these days (although not for high-end graphics workstations with large monitors). For comparison I recently installed some video conferencing racks which included a bog-standard PC along with 11 other pieces of AV equipment. The total rack load peaked at around 190W, with the PC often making up only a third of that.

With a 30A RCD, on the face of it and allowing for future expansion and the electrical equipment becoming more powerful as time goes by, I would be able to get away with 2 ring mains
Are you required to use ring finals? (not 'mains', in today's terminology). How big is the building? Most large offices have track for the desk supplies, with pods run off to each desk. Is 30A a standard RCD rating?

Divide that by 240V and you get 28.3A
Only with a p.f. of unity. Is it, for PCs? Is 240V the best voltage to choose for the calc? What happens to the supply current of PCs as the voltage falls?

I think I should maybe allow 2mA per PC and 1mA per monitor,
Also generous, but don't guess, get typical manufacturers' figures.
 
The price could vary depending on what balance of quality vs price vs speed the client wants. It would be a lot cheaper and quicker to run T&E in basket under the floor or lashed to catenary wire above the ceiling than to run singles in galvanised trunking and conduit. Again it might be quicker to run dado or skirting trunking around the outside of an open plan office than to run individual drops in the internal walls of individual offices. It would be even quicker if you can reuse the old containment. If the sockets are in floorboxes in the raised floor it would obviously be cheaper to re-use these if you can.

So far you've mentioned sockets for the computers - is this the total extent of the rewire or are the lights to be rewired, even changed, as well? Are you rewiring the data? Is it just an office full of desks or is there a kitchen area, meeting room, manager's office, server room, toilets, reception etc? Will it just be computers and monitors plugged in or will they have printers, desk lamps, fans, a kettle, water cooler? In the winter is the heating system sufficient or is the woman who's always cold going to plug an electric heater in? Will the heating and/or HVAC system need any rewiring? Will the cleaner want to plug her vacuum cleaner in somewhere?
 
Cibse guide A gives guidance on maximum occupancy density for office environments if the room area is known. This if I remember correctly worst case is 5m2 for a office. The same guide also states a worse case of 300 watts small power usage per person. However is does make a note that in practice this figure is now alot less.

This should give a indication as to the loading of the final circuits.

Also are you sure on using rings. Maybe consider 4 mm radials

Hope this helps
 
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Thanks to everyone for their input. I have been engrossed in my project so sorry if I didn't respond.

I have taken all of your questions as direction and not really though to answer them directly, hope this is ok.

Well as for as a trainee is concerned, I am 35 and took this electrical engineering course out of boredom with my current job. I know I can't go onto apprentice wages so I'm looking for direction from that also. If anyone has any suggestions with what to do after my HNC/HND it would be good to hear from you
 
Thanks to everyone for their input. I have been engrossed in my project so sorry if I didn't respond.

I have taken all of your questions as direction and not really though to answer them directly, hope this is ok.

Well as for as a trainee is concerned, I am 35 and took this electrical engineering course out of boredom with my current job. I know I can't go onto apprentice wages so I'm looking for direction from that also. If anyone has any suggestions with what to do after my HNC/HND it would be good to hear from you

I think people would want a bit more feedback from you ideally. Makes it more worthwhile people making suggestions and helping. Daz
 
I think people would want a bit more feedback from you ideally. Makes it more worthwhile people making suggestions and helping. Daz

I see your point.

I have to narrow the scope of the questions and maybe deal with each point at a time.

So for now I would like to focus on the protective conductor currents, previously I called it earth leakage. Although I have been told it is quite generous I can't see a justification for not taking each desktop and monitor as adding 3.5mA to the conductor current. Using 30mA RCD's and not exceeding 15mA for each ring final, also allowing for a certain amount of redundancy due to each workstation having an extra couple of sockets. Is it feasible to say that realistically only 2 workstations can operate from 1 ring final? I can't get over how low this number appears to be.

I have read the regs and the on-site guide as advised and more to boot. I know more now about the protection of the circuit when the protective conductor current exceeds 10mA.
 

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