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.
Qu 1: What is the best way to extend the existing double socket to add 3 extra sockets.
You want to increase the total load from 15A to >27A.
You probably don't want the breaker tripping if you, for example, run the washer and dryer at the same time.
If the existing circuit is really a radial circuit with a dedicated breaker in the CU you will need to check
- The breaker rating
- The wire diameter.
If the wire diameter is small, you cannot increase the breaker rating without first replacing the wiring with a suitably sized new circuit. Under UK regs I believe you should get an electrician to do this work.
I can't use a spur from existing socket as this will pull too much current.
It depends what you plug in. The breaker should protect the circuit.
It sounds better to continue this as a ring main rather than a radial circuit.
Ring mains are typically 32A but I don't see creating a ring main as easier than upgrading a radial circuit.
Using a double socket for washer/dryer isn't great as they don't deal well with 20A+.
So long as the circuit is properly constructed, you should be fine. I certainly run those two appliances from a double socket.
Qu 2: Can I just connect the 3 x 13A switches in parallel from the existing wires coming from the consumer unit?
You should draw a diagram of your intended circuit. You don't wire switches in parallel. I guess you mean something different but you need to be clear.
You may be over-complicating things here. You can use normal switched sockets (even though inaccessible with appliances in place) and when necessary, just isolate the whole circuit using the switch on the breaker.
Qu 3: I presume if I want a normal double plug/switch (above the worktop), I can add this in parallel as well.
Yes.
Qu 4: Presume makes these changes will need me to get an electrician to certify it (even if I do the work myself)
As I understand part P, You can extend an existing circuit but you need an electrician to certify it if you create a new circuit. It might be no cheaper than paying an electrician to do the work themselves.
Best Answer
The answer to that question in Electrical standards is NO!
The problem is that both these machines use heating elements that can cause surge power to heat water. Now if you use(not you but anybody unknowingly) use them at the same time it could cause an overload. Fine- you are protected by 1-The wall plug, 2-The DB Breaker.
Causing such an overload during peak operation of machines can cause unwanted deterioration of equipment and unnecessary load on electrical wires.
There do exist double isolators(the one on the wall with the red switch/light/fuse)(but they are rare) that can be rated separately by Amperage and should not exceed the amperage of A) The circuit breaker in the DB (taking into account that the proper mm2 wire is used) B) The wattage rating of the actual device
I am completely talking theory here based on what I think it should be(but does not mean that is your case)It could be likely that using the single isolator you could use two sockets-but they have to be certified in order to count towards a propel installed wiring system- and only a certified electrician(qualified does not mean certified) can help you out.
BUT
So the forumula is simple Ohm's Law
Washing machine = 1000Watt How much is a 1000watts? 1000(Watt) / 240(Volt) = 4.2AMP MAX
If I were a gambling man i would bet a 100 bucks your wall isolator is up to 15AMP and your DB breaker is 5AMP using a 1.5mm2 cable. So how could you possibly accommodate another device of a similar wattage.
Another way. How much is 5AMPs? = 5(Amps) * 240(Volt) = 1200Watt
When we used to install electric water boilers we were required to use 5mm2 solid copper (3500Watt) and 15AMP Breakers and nothing else is allowed to leach of that circuit.The Units had dedicated isolators in the loft near the unit and internal safety breakers.
To elaborate on your question and demonstrate the difference.
What you are replacing is a switch system that "Isolates" electricity in the event of electricity leakage or short circuit. Isolates means it creates a gap in the Neutral and Live at the same time Isolating the device from all electrical currents(Earth should also be isolated but that is another story-and usually is not)
What you want to install is a normal switch.When you turn it on-it turns on weather there is a short or leakage and will not turn off in the case of fault- causes backwards stress to the DB to trip the circuit. This does not Isolate the circuit. It just cuts out the Live, meaning that if for some unknown reason there was electricity pumping back into the neutral.. it could cause a fire or other damage.
The AMP rating is just what it could handle at MAX peak current.
SO..
The simple answers by many would be- Yea use a splitter- it will be OK (and most likely if you call Dave your neighbourhood white man van- he will install for you for a few bob and nothing will ever happen.. but if he guarantees it he is lying.. so don't be fooled.)
But a professional answer will be NO- It wont be OK- especially if your home insurance finds out after it ....
It may sound stupid, and could cost a bit- but be safe- Rent a certified electrician and demand a certificate of testing once complete and Tell your insurance.
There can be no maybes in human life - these things cannot be taken lightly- You feeling lucky? Go play euro millions.