Calc MCB needed?

[DamienGiles]

Hello,

I've been reading through this forum and found many reasons to ask a few questions before I plan on wiring this up.

Into this small place, I have the power source from an upstairs wall socket - which I presume is 32A.

I wish to add a MCB & RCD into this circuit of heaters, lights etc, thus if the circuit develops problems, it wouldn't affect my flat mates & freezer etc.

What calculations would I need to do to determine the amperage of MCD & RCD? I have access to the equipment that will run off this circuit so I can use this within the calculations or find this our from the manuals.

I understand that there are different types of MCB's (type B, C, D). I suppose I would need Type B for the reaction time.

Once I've determined the theoretical load on the circuit, do I have to multiply this by a certain factor and add a certain percent for future expansion?

Since this small area will also have several fire alarms which I presume shouldn't be run on the MCB/RCD side of the circuit... what should I look for when purchasing a CU?

All I need is 1 RCD & 1 MCB on the same side, with a master switch that controls the RCD & MCD side, with a separate side for the fire alarms which will be wired into a fused spur that's switchable.

When I'm armed with the total load on the circuit, how would I calculate the correct load wire? Do I just need to use 13A rated wire as this is the maximum that will be used from the wall socket?

Lots of questions, lots of answers. Thanks for reading. I'm trying to learn specifically to get this completed but will eventually get it looked over my a sparky when he's back off holiday.

edit: sp

[ricicle]

Not really practical.If your mates house is already protected by a 30mA RCD and you fit one also then the two would not discriminate and your mates could end up tripping before the one you fitted.

Also, as I read it, if you are intending to take this supply off an existing socket which is on a 32A ring or radial that powers sockets in your mates place also then all you can do with breakers to stop the above same problem is to fit smaller rated ones (20/16/10/6A).This limits your loading especially as a 3kW heater will take around 12A.

Ideal answer would be a seperate circuit back from the consumer unit.

__________________
Empty barrels make the most noise.

[DamienGiles]

Thanks for your quick reply.

I've just gone through all the labels of the items, and totted the wattage up:

200w, 600w, 20w, 25w, 125w, 125w, 25w, 200w, 450w, 8w, 8w, 13w, 40w

I make that 1839watts

Now, If I've done my calculations correct... this means that's 7.6625watts based on 240volts.

Does this sound correct?

If so, does this mean I can still use the wall socket rated at 12A as opposed to direct from the ring main at 32A?

[Spark123]

Doesn't really matter about the existing load, it matters more about what can be plugged in causing the circuit to be overloaded and what protection is in place to disconnect it if an overload/fault were to occur. Is the socket which you have access to on the ring? Is it possible to extend the ring? What else is on this circuit? What do you mean by fire alarms?

[DamienGiles]

Thanks for your reply.

There will not be anything added onto the current circuit within the small room that is mentioned above. It isn't possible to extend the ring as it's a rented property. The socket which I'm hopefully planning to plug the circuit into will be on the upstairs 32A ring. By Fire alarms, I mean just that. I want to have some main operated fire alarms within the small area to detect any smoke.

Does that help? I'm a bit new to this, and trying to look at it logically. Based on my calculation, I presume providing no further items are added on to the circuit to increase the wattage then running the circuit using the rated items mentioned would be fine through a 12amp socket in the upstairs main.

[Spark123]

If you are plugging it in then the plugtop fuse is a maximum of 13A which will means the breaker, unless it is lower, is a waste of money. One issue with double sockets is they are not rated the full 26A so if you were to plug 2x13A 4 way extensions in and cascade them out it can be a problem. Some info on sockets here: http://www.diynot.com/forums/viewtopic.php?t=8520.
If you have a lack of sockets can you not speak to your landlord? Are there no smoke alarms already fitted?

[DamienGiles]

Spark123, Thanks for your reply.

I honestly didn't know that a double socket wasn't able to take 2 x 13A, this shall be noted and I've since swapped a few appliances around to make sure this is well below.

I see your point about having an MCB rated at higher than 13A being a waste, it's feedback that you're giving that is opening my mind and answering the questions that I already knew deep down

I wasn't planning on taking a double feed from a double socket, but only a single feed from a double socket, ensuring that the other socket on the double plate wasn't being loaded with anything other than a 12w energy saving uplighter.

After working through the figs, I now see that taking a 13A supply from the plugtop on the double plate from the upstairs ring will be sufficient for my needs of running the total wattage of 1839w - providing I didn't go in excess of this (which isn't planned) then I see this as being perfectly safe now. Thanks.

Just to clarify finally, if I did add a 10amp MCB & RCD into a CU that sits between the 13A plugtop and the circuit running the 1839watts, could a fault on the Circuit Appliances still trigger the House CU MCB or RCD instead of just my Circuit CU MCB/RCD?

Happy easter all

[JohnD]

Another # plantation?

[DamienGiles]

JohnD, how dare you. Nothing of the sort, getting a small music recording room going on

[Spark123]

Having 2 RCDs of the same rating in line isn't the best thing in the world as they won't discriminate i.e. it is anyones guess which one trips first, if not both. However, saying that, 2 are better than none!! If you are wanting an additional RCD, why not just buy one of those plug in powerbreaker jobbies for about £10?