Washing machine fused spur... can I also run a microwave from the same socket?

[cartoongeorge96]

Hi all,

Basically I'm looking to wall mount a microwave which happens to be above the worktop that has a washing machine underneath. The washing machine runs from a socket behind the machine with a standard fused switch above the work surface.

The easiest route for me to power the microwave would be to go down and make use of the socket.

It feels like it would be against some kind of regulation but worth an ask. From my limited Googling, I understand a fused spur has around 3kw to play with. We have a standard washing machine and a 800w microwave.

The socket is a single one... is it a big no no to plug a double adapter in?

Appreciate any help, and apologies if this was a ridiculous question to ask!

 

[chivers67]

If you have a Standard FCU then I'd say you're ok to power the Microwave off it but I wouldn't use the Washing machine at the same time. It's unlikely to overload the 13A fuse unless you're using it to defrost something for a length of time and the Washing machine is on a Spin cycle.

Though to answer this fully is the FCU served off a Ring?

 

[sparkwright]

You may be able to replace the switched fused spur for a 20amp double pole switch, and replace the single socket for a double socket.

 

[LondonLad21]

Fused spurs from ring finals are ok, subject to diversity calculations. The fuse should be off the ring, and likely onto 1.5mm cable. You could technically have a few sockets if the diversity calcs provided that you were not going to over rate the fuse. Having seen a current draw on a washing machine, and good microwave, then there's a chance you will blow the fuse. It's not like you are having low current items. Hence given that you have no idea what will go on that spur in future, it is probably more prudent, and safer to extend the ring (though they are less well favoured these days as you will never know if you simply have two radials on 2.5mm cable).

Best bet is to get a qualified electrician in, or, change your layout to be prudent and just take one item off the spur. If you were talking about broadband routers, tv's, a light supply etc, then its all low current and low risk.

 

[sparkwright]

Fused spurs from ring finals are ok, subject to diversity calculations. The fuse should be off the ring, and likely onto 1.5mm cable. You could technically have a few sockets if the diversity calcs provided that you were not going to over rate the fuse. Having seen a current draw on a washing machine, and good microwave, then there's a chance you will blow the fuse. It's not like you are having low current items. Hence given that you have no idea what will go on that spur in future, it is probably more prudent, and safer to extend the ring (though they are less well favoured these days as you will never know if you simply have two radials on 2.5mm cable).

Best bet is to get a qualified electrician in, or, change your layout to be prudent and just take one item off the spur. If you were talking about broadband routers, tv's, a light supply etc, then its all low current and low risk.

I would say the cable from the switched fused spur unit to the single socket would 'typically' be 2.5mm2 (oversized), rather than 1.5mm2 - seems to be the norm.

But the op must check this cable size first.

 

[JohnW2]

Fused spurs from ring finals are ok, subject to diversity calculations. The fuse should be off the ring, and likely onto 1.5mm cable. You could technically have a few sockets if the diversity calcs provided that you were not going to over rate the fuse.

That really is not a very helpful statement. Just one double socket is theoretically enough to 'overload' a 13A fuse. As for "diversity calcs", they are essentially impossible for a sockets circuit, unless the designer has a crystal ball.

The whole point of allowing fused spurs (with an 'unlimited' number of sockets) is that the 2.5mm², or maybe 1.5mm², cable will then be adequately protected. Those who cannot think beyond what examples are shown in Appendices to the regs don't seem to understand that, from the point-of-view of cable protection, there's nothing wrong with an unfused spur (from a ring final) supplying multiple sockets if it's wired with 4mm² cable.

Kind Regards, John

 

[sparkwright]

Is the microwave lead really going to reach where the washing machine socket is??

 

[danechip]

Is it really a Fused or just isolator from the socket below?

 

[plugwash]

The whole point of allowing fused spurs (with an 'unlimited' number of sockets) is that the 2.5mm², or maybe 1.5mm², cable will then be adequately protected.

We can't predict with certainty how sockets will be used in the future. Electrical installations often live through many changes of use of the space in which they are installed. The sockets may be used in ways the installer never anticipated. The guidelines for ring circuits and their non-fused spurs in the appendix of the regulations provide a design that if installed in a non-degenerate manner is unlikely to result in overloading problems, even if the use of the space changes.

Overload protection devices are not supposed to be used as load limiting devices, so if the intended use of an installation is likely to lead to overloading then the design is bad. Where fused spurs come in is situations where 13A is more than adequate for the intended use of the sockets, but use of the sockets in ways not intended by the designer could lead to overloading.

Those who cannot think beyond what examples are shown in Appendices to the regs don't seem to understand that, from the point-of-view of cable protection, there's nothing wrong with an unfused spur (from a ring final) supplying multiple sockets if it's wired with 4mm² cable.

I suspect the reason the IET don't endorse such a construction (though they don't prohibit it either) is that it would increase the chance of degeneracies that would overload the main ring cable.

 

[cartoongeorge96]

Thanks all. Attempting to answer all the queries in one reply...

The fused switch seems to be part of the kitchen ring as there is no separated label on the fuse box unless I'm misunderstanding what this means. I've unfortunately got no idea about the cable sizing that's been used or the fuse that's in the switch, but I could check.

The microwave cable probably isn't long enough on its own but I imagined using some kind of extension and keeping the microwave cable and plug tidied behind it.

The mixed replies probably suggest I should avoid doing this, I'm not sure I like the idea of not being able to use both appliances at the same time should I need to.

Is there anything else I can check to come to a clearer conclusion?

Thanks for the help so far!

 

[sparkwright]

Some pictures would help - there may be somewhere else you could get a feed from - because it's not 'ideal' coming off the washing machine socket (though not necessarily impossible).

 

[EFLImpudence]

We can't predict with certainty how sockets will be used in the future. Electrical installations often live through many changes of use of the space in which they are installed. The sockets may be used in ways the installer never anticipated. The guidelines for ring circuits and their non-fused spurs in the appendix of the regulations provide a design that if installed in a non-degenerate manner is unlikely to result in overloading problems, even if the use of the space changes.

So would cable of the appropriate rating. As you say, knowledge of future use is unknown.

Overload protection devices are not supposed to be used as load limiting devices, so if the intended use of an installation is likely to lead to overloading then the design is bad. Where fused spurs come in is situations where 13A is more than adequate for the intended use of the sockets, but use of the sockets in ways not intended by the designer could lead to overloading.

I don't think that is the reason for the 13A fuse.
It is simply because, as I implied above, that there is no higher rating of BS1362 fuse.

As for not being a load limiting device, what, if not that, is the 32A MCB on socket circuits?

I suspect the reason the IET don't endorse such a construction (though they don't prohibit it either) is that it would increase the chance of degeneracies that would overload the main ring cable.

They do endorse it - in Appendix 15B unless you can think of a difference between a spur on a ring or a radial.

 

[JohnW2]

We can't predict with certainty how sockets will be used in the future. Electrical installations often live through many changes of use of the space in which they are installed. The sockets may be used in ways the installer never anticipated.

Indeed. As I wrote:

.... As for "diversity calcs", they are essentially impossible for a sockets circuit, unless the designer has a crystal ball.

The guidelines for ring circuits and their non-fused spurs in the appendix of the regulations provide a design that if installed in a non-degenerate manner is unlikely to result in overloading problems, even if the use of the space changes.

As above and below, the designer can usually only guess that a user will not load a ring final (with or without fused or unfused spurs) such as to overload some of the cable - they could only be sure of achieving that if they ensured that there were no sockets closer than a calculated distance (proportion of the ring length) from an end of the ring.

Overload protection devices are not supposed to be used as load limiting devices, so if the intended use of an installation is likely to lead to overloading then the design is bad.

Indeed, and that's as true of a radial circuit as it is of a ring, or spurs from that ring.

However, unless all loads are hard-wired, or the number of sockets (on a radial or ring circuit) is strictly limited (to no more than one double socket on a "20A" circuit, or no more than two double sockets on a "32A" circuit), the designed can only guess as to whether loads in excess of the OPD rating will ever be 'plugged in' - so, in that situation, all the designer can do is to ensure that the cables are adequately protected even if loads greater than the OPD rating are plugged in.

Where fused spurs come in is situations where 13A is more than adequate for the intended use of the sockets, but use of the sockets in ways not intended by the designer could lead to overloading.

That still requires 'guesswork' on the part of the designer. As I said, and as you imply, the loads plugged in to even one double socket can 'overload' a 13A fuse. Again, all the designer can do with certainty is to make sure that the cable is always adequately protected - either by having a 13A fuse (given that higher ratings of BS1362 fuses are not available) or (assuming a "32A circuit") using 4mm² cable for the spur.

I suspect the reason the IET don't endorse such a construction ...

As EFLI has said, they effectively do - since an 4mm² 'unfused spur' from a 32A ring final is no different from a branching 4mm² radial circuit.

Kind Regards, John

 

[plugwash]

That still requires 'guesswork' on the part of the designer.

To an extent yes.

Personally I don't think a fused spur feeding multiple sockets should be installed without knowing the intended use of the sockets.

As EFLI has said, they effectively do - since an 4mm² 'unfused spur' from a 32A ring final is no different from a branching 4mm² radial circuit.

It is different though, because a ring relies on the assumption that the load will be "spread around", such that despite the lack of complete over-current protection, the ring cable is unlikely to have overloading problems. Concentrating load at a point on a ring can therefore be a problem if that point is not close to the center of the ring.

 

[JohnW2]

Personally I don't think a fused spur feeding multiple sockets should be installed without knowing the intended use of the sockets.

I agree, but it's still 'guesswork' as to whether the currently 'intended' use of the sockets will persist.

In one part of my workshop, I have a long string of sockets on a fused spur which, as far as I am concerned, is only 'intended' for very small loads - chargers, soldering irons, test equipment, small bench PSUs etc. However, there can be no guarantee that a subsequent user will not 're-purpose' that part of the workshop and, say, use several of those sockets for relatively high-powered tools etc. (maybe even a fan heater or two!!). However, the fact that it is a fused spur at least means that no cable will come to any harm should that happen.

A many-socket fused spur from a ring final is, after all, no different (functionally) from a plug-in 'extension lead' (which may have 6 or 8 'socket outlets'). Again, although certainly not 'intended' for such use, there's nothing to stop someone plugging in 6 or 8 'fan heaters' - but, again, the 13A fuse should ensure that nothing comes to harm if that happens!

It is different though, because a ring relies on the assumption that the load will be "spread around", such that despite the lack of complete over-current protection, the ring cable is unlikely to have overloading problems. Concentrating load at a point on a ring can therefore be a problem if that point is not close to the center of the ring.

Conceptually true, but I'd say that that's a little over-stating the potential problem in practice. With the common situation of Method C 2.5mm² cable (CCC 27A) in a 32A ring, an entire 32A load has to be applied all on one side of the ring, and all no less than about 70% of the cable distance from the midpoint of ring to the origin (OPD in CU) for the CCC of any part of the cable to be exceeded - which I wouldn't necessarily call 'close to the centre'

However, I agree, for that reason a multi-socket unfused 4mm² spur from a 32A ring final should not be connected to the ring closer than about 30% of the cable distance from origin to the midpoint of the ring. Provided it wasn't connected any closer than that, there would be no risk of any cable being 'overloaded', even if the circuit's full 32A load were going through that spur.

Kind Regards, John