Hard wiring American style dryer

[Rhys91]

Hi
I'm about to purchase an American style tumble dryer - 4750 Watts.
I know this needs hard wired.
My question is, can this be hardwired into the ring final in the utility room? If replace the 20amp dryer switch with a cooker style switch.
The kitchen and utility room have their own circuit, plus oven on separate circuit.
So the only other appliance on the same circuit would be the fridge freezer, kettle and microwave. The washer is a top loader which does not have its own heater, so low power draw there.
I realise it should ideally have it's own circuit but it looks like a difficult install with concrete floors.

I dont see how any of the circuit would he overloaded on the 32 amp breaker so I assume this would be a safe set up?

 

[James]

Short answer is no, it can’t be added to a ring.
it needs it’s own dedicated supply.
also, have you checked that this will work on 230v 50hz?

 

[Rhys91]

Short answer is no, it can’t be added to a ring.
it needs it’s own dedicated supply.
also, have you checked that this will work on 230v 50hz?

Thank you for the reply.

It is a British appliance, just American style.

Could you give a bit more detail on the no if you can please?

 

[James]

The only approved accessories for a ring final are
13A socket
13A fused spur

note the Can be fused down but not increased.

 

[brianmoooore]

I have a similar drier, and it needs its own dedicated RCD protected 32A supply direct from the consumer unit. It can be hard wired, or connected, like mine, via a blue 32A plug and socket.
The ring may be protected by a 32A MCB, but fuses/MCBs don't suddenly trip when their rated current is reached, and at 50A, a 32A MCB will take around 15 minutes to trip. Regular, designed in overloads like this will gradually cause deterioration of the wiring.

 

[Lucien Nunes]

I dont see how any of the circuit would he overloaded on the 32 amp breaker

A ring final circuit consists of two legs of cable from the supply to any point, neither of which are rated at 32A, sometimes as little as 20A depending on installation method. It is acceptable because the load is assumed to be spread around the ring, in which case it is shared sufficiently well between the two legs that neither is overloaded.

But a large single load at one point, such as the dryer, might overload one leg because the degree to which the current is shared depends on the resistances of the two legs. Near one end, for example, most of the current will flow through one 20A cable and very little through the other, therefore any load over 20A will cause overload of one cable. If the heavy load is at the middle of the ring the current will in theory divide equally, but it is vulnerable to open-circuit faults and therefore the two legs are not permitted to be considered as parallel cables because adequate precautions cannot be taken to prevent the full load being transferred to one in the event of a high-resistance connection in the other.

For this reason, the 'exemption' that allows the ring circuit to be protected at 32A specifically limits what may be connected and how, to avoid these problems. Any single point load must be limited by a fuse no greater that 13A or MCB no greater than 16A, therefore your dryer cannot be connected in a compliant manner, no matter how little load is present elsewhere on the circuit.

 

[Rhys91]

Thanks everyone for your responses.
It does all make sense and it is the same conclusion I came to when I was looking into it.

So my next question is more of a passing comment and in no way arguing with with you are saying - I do understand that a high powered tumble dryer may cause one leg of the ring to be overloaded, however...

For context, this will be in a new house I move into in a few weeks time where the utility room has separate counter top switches for the washer and dryer feeding unswitched sockets.
If I don't buy the new tumble dryer and use my old appliances, I have a 2600 watt dryer and a 2200 watt washing machine.
As the sockets are next to each other in the ring, if using both at the same time then there would be 4800 Watts at the same point in the ring final if both appliances are on at the same time. Incoming mains at 5'c heating 15 litres water to 40'c would take at least 20 minutes, 30 for a 60'c wash.
Again, not arguing at all, it's just to get more understanding, but how is this different from the 4750 watt being wired in taking 30/40 minutes for a dry cycle?
Does that make sense?
It's quite interesting learing about electrics in more detail.

 

[pc1966]

As the sockets are next to each other in the ring, if using both at the same time then there would be 4800 Watts at the same point in the ring final if both appliances are on at the same time. Incoming mains at 5'c heating 15 litres water to 40'c would take at least 20 minutes, 30 for a 60'c wash.
Again, not arguing at all, it's just to get more understanding, but how is this different from the 4750 watt being wired in taking 30/40 minutes for a dry cycle?

There is a big difference between "Would I get away with it?" and "Is this acceptable within the regulations?"

@Lucien Nunes has covered the assumptions for the RFC in detail, and when it comes to the case of multiple high power loads then it is of course possible they end up being close together, but the more they are split between appliances the less likely they are all to be on simultaneously.

What you also have to consider is the cable lifetime, as for modest overloads it will not burst in to flames (hopefully!) but you get a rapid reduction in lifetime. Cable heating is an I2R factor, so doubling the current leads to a 4 time increase in temperature rise, and for many ageing process you see life being reduce by roughly half for every 10-20C rise in temperature.

So while a kitchen RFC might be quite well balanced due to similar leg-lengths to the room (compared to the length around the room) and you could easily have more than your dryer's load in use at one time, it would still not make such a load acceptable as far as the regulations are concerned.

 

[Lucien Nunes]

@pc1966 hits the nail on the head. Electrically, there is no difference in the demand placed on the cables, whether the load is one 20A appliance or two 10A appliances plugged into a double socket. What the regs are doing in this case is implementing a management strategy based on probability. The larger the maximum permitted load at any one point, the more likely there will be times when the cable heating exceeds desirable levels if the symmetry is not ideal. So a maximum was set that is shown by tests and practical experience to work well, which is 13A; in effect a 'unit load' on the BS1363 system.

As a concept, the RFC was specifically designed to serve a large number of socket outlets with a small number of circuits, using a convenient size of cable. If you want 20 sockets in the living room, put a ring round and plug in all your electronic gadgets, no extra circuits needed. It replaced a system where every 15A socket had to have its own separately fused circuit. 20 single sockets meant 20 circuits. Separate circuits are however still the correct approach for large single loads, and mandated by the regs. Horses for courses.