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I've split my question up into two parts:

Part 1

After investigating my consumer unit (after I first started to wonder about things in part 2 below) I'm confused regarding the AMPS my RCDs are rated to take.

My home (which is a new build from 2010) has the consumer unit split in two, with two RCD units (I think they are) splitting up the trip switches. The split is as follows

Left most trip switches

6A trip - Down stairs lights

20A trip - Kitchen Grid switches for fridge, washing machine, dish washer

32A trip - Ring main wall sockets

32A trip - cooker

90A total - These are connected to an 80A RCD

Right most trip switches

6A trip - Up stairs lights

6A trip - Smoke Alarms (up/down stairs)

16A trip - Central Heating

32A trip - Ring main wall sockets

40A trip - Electric Shower

100A total - These are connected to an 80A RCD

Lastly

100A - Main Switch/Trip for consumer unit

As you can see the RCDs are rated at 80A but the combined amps of the trips going into them both, exceed the rated amperage and not only that the combined amps of all trips/RCDs far exceed that of the main consumer unit trip. What am I missing here, im sure it's right, I just don't understand it.

As I understand things, we're always told to not exceed 13A on a wall socket if we use a strip plug. Hence I'm guessing all plugs connected to either ring main shouldn't exceed 32Amps. Is this correct? It just seems low to me. Run the Kettle-13A, Microwave-13A and TV-10A and I'm above 32A there...I don't get it.

If I apply the same logic to the consumer unit, why is there potential for all trip switches combined to exceed the RCDs they are connected to and further to this the main consumer unit switch? I must be missing something fundamental here and would greatly appreciate some clarity on the matter.

Part 2

I'm sure an explanation of the above will also explain my confusion here to. Basically I confused regarding the max AMPs a UK ring main will accept (given a strip plug shouldn't exceed 13A) and was seeking some advice and information to clear up the confusion.

I'm setting up a new marine aquarium which has a lot of equipment. There are at least 15 plugs going to be used. At minimum thats a potential 45A right there if all plugs were 3A (which they are not, some are 5 &13). If the potential amps is above 32 does this mean I can't connect them to my ring main via multiple strip sockets plugged into several wall sockets. If so then how do I get around this? Install a new Trip switch in the Consumer Unit and run a cable specifically for this project?

People regularly do what I'm doing, use multiple strip sockets plug everything in, everything works and they don't worry. But I'm baffled by it as I'm not sure thats the safest thing to be doing plus have spare Amps for all the other normal house hold electricals. I'm so confused with AMPS because of what people say in overloading those strip sockets, surely the same logic applies to the trip switches themselves?

Thanks.

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  • Nothing to do with electronic design. Question should be closed.
    – Leon Heller
    Jun 20, 2015 at 23:48
  • This is not really on topic for this site (which is for electronic engineering, not electrical engineering as such, and certainly not DIY). In any case, what you need to do is call an accredited electrician, because whatever you do by yourself may burn your house down and at the very least will invalidate your insurance and make the house unsaleable.
    – Oleksandr R.
    Jun 20, 2015 at 23:48
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    It says electrical engineering at the top of the page? Maybe if I drew a schematic I'd get a better reply. Honestly, just ridiculous. I haven't said I'm dong anything, I said I want to do and theres a difference. I'm first trying to find out before taking action. @Leon thats just a douche and unhelpful comment. You could probably formulate an answer given your reputation but wont reply because you're being pedantic. You answered a question before about solder fumes being harmful. Thats a health and safety issue "Nothing to do with electronic design"
    – Dan
    Jun 21, 2015 at 0:36
  • Well, the breakers are probably sized this way because it is improbable that the full capacity could be used simultaneously. For example, a microwave surely does not use 13 A (3 kW), nor a TV 10 A (2.3 kW). These are just the currents above which you can say for sure they have a fault. But here you are asking essentially about considerations for installing a new, high current circuit. (Can an aquarium really use 10 kW?) It is not necessarily difficult electrically, but you need someone who knows the relevant regulations, otherwise it is a non-starter. DIY.SE is a better place to ask than here.
    – Oleksandr R.
    Jun 21, 2015 at 2:58
  • You're not intended to use the maximum capacity of each & every breaker in your panel, just as you're not intended to try to draw 13A off each and every socket on one ring circuit. You can pull 32A off a your ring of sockets (split into multiple <13A loads) if you're also not simultaneously running the dishwasher, washing machine, and cooker at their maximum load.
    – brhans
    Jun 24, 2015 at 14:31

1 Answer 1

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Breakers and fuses are sized to limit the fault current that can flow. Each is sized to limit the current to that which the cable connected to it can safely carry. For example a 32A ring main circuit will protect a 2x 2.5mm2 cable ring circuit. Another example will be the 6A lighting circuits. These could be a single 1mm2 cable (though 1.5mm2 seems to be more commonly used).

You aren't expected to be running everything at capacity at the same time. Just because a circuit can supply a given amount of current doesn't mean that it will be doing. As long as the current draw at any point doesn't exceed the capacity of the fuse or breaker protecting it, it won't trip.

You mention 3A and 5A "plugs". This is the current above which the fuse in the plug should blow (at some point - fuses have different "trip times" for different overcurrent ratings). The appliance supplied by these won't normally draw the current marked on the fuse. For example if you had a 1W appliance, it would be likely to have a 3A fuse (the smallest common fuse for a UK 3 pin plug), but would only actually draw around 0.004A. You could technically therefore connect around 3000 of these 1W appliances through one 13A fused plug without a problem (practically however, you might find it more of a challenge...).

The important number to be considering is the total power draw (wattage) of the appliances in use at any one time, not the total of the fuse ratings.

In relation to you aquarium, as long as you don't overload the ring main, you'll not have a problem. If you total up the wattage of all your equipment, you can run approximately 3000W through one 3 pin plug. (Note though that it's common for double sockets to be rated for less than 26A, but it's not often marked on them). Your 32A ring main is therefore capable of supplying a bit over 7000W.

In the worst case, you could always have an electrician install a dedicated circuit to supply the aquarium equipment.

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