MCB tripped by dishwasher and washing machine

[fmackay]

I've just redone my kitchen, including the wiring. There was an existing radial circuit (labelled "wash boiler" at the CU) with a single socket on it which I have reused for an under-sink double socket for the dishwasher and washing machine. I have put in a 13A FCU above the worktop to isolate this socket.

This morning when both appliances were started at the same time (so both heating water) the MCB tripped and the fuse survived. Presumably the MCB is <13A. Question is, can/should I change the MCB for a 20A one (cable is 2.5mm) and see if the fuse can handle both appliances, or leave it as is and take care to avoid running both appliances' heaters simultaneously?

 

[ban-all-sheds]

Please get an electrician.

I've just redone my kitchen, including the wiring.

Why did you think that you were competent to do that?

If, in the context of this topic, I asked you what Ib &#8804; In &#8804; Iz meant, would you know?

There is a great deal more to doing wiring like that than just getting L/N/E connected properly.

I have put in a 13A FCU above the worktop to isolate this socket.

Why?

Presumably the MCB is <13A.

That presumption shows two things:

1) You don't understand discrimination, which is one of the things you need to, to be able to do circuit design.

2) You have used a circuit for something different without taking any notice of the rating of the MCB. Wrong, wrong, wrong.

Question is, can/should I change the MCB for a 20A one (cable is 2.5mm)

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://web.archive.org/web/20070327014113/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  

and see if the fuse can handle both appliances,

So this is the 'let's not actually understand things or do proper calculations, let's just try things and see what appears to work' school of electrical design, is it?

Please get an electrician, and use the time saved to learn more about electrics.

• http://www.google.co.uk/search?q=electrical+circuits+101
  
  
• http://kevinboone.net/electricity.html
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/domesticinstallations.html
  
  
• //www.diynot.com/wiki/electrics:books
  
  
• //www.diynot.com/wiki/electrics
  
  

 

[fmackay]

Please get an electrician.

I've just redone my kitchen, including the wiring.

Why did you think that you were competent to do that?

If I could have afforded to employ tradesmen on this job I would have done so. As I could not, I did not, and so here I am on a DIY forum... Apart from a few additional sockets on a ring this was all like-for-like replacement (except (oops) I have replaced the previous single socket on this radial circuit with a double). This is all pretty straightforward stuff and not notifiable in Scotland.

I have put in a 13A FCU above the worktop to isolate this socket.

Why?

So the supply can be switched off easily. I considered using a 20A DP switch, but I thought this would be a bad idea as the double socket could potentially draw 26A. In practice, with the appliances used, this wouldn't happen (dishwasher 1.9kW=8A, washing machine 2.3kW=10A). But I should have considered a 32A switch.

Presumably the MCB is <13A.

That presumption shows two things:

1) You don't understand discrimination, which is one of the things you need to, to be able to do circuit design.

2) You have used a circuit for something different without taking any notice of the rating of the MCB. Wrong, wrong, wrong.

Yes, ok.

Question is, can/should I change the MCB for a 20A one (cable is 2.5mm) and see if the fuse can handle both appliances,

So this is the 'let's not actually understand things or do proper calculations, let's just try things and see what appears to work' school of electrical design, is it?

Ok, that was a bit flippant. And the fuse probably would blow sooner or later. Would a better plan be: replace MCB with 20A one; replace FCU with 32A DP switch? The CU is probably 30 years old, will I be able to get an MCB which fits?

 

[mfarrow]

Presumably the MCB is <13A.

Either it will say on it __A or B__ or C__ or D__. That's an easy one, and for anyone to help you, that information will be required as a minimum.

 

[Lectrician]

Presumably the MCB is <13A.

Presumably you can read the rating of the MCB from the front of it?

Even 20amp can be problematic with two appliances such as these.....

VERY poor design with no thought.

 

[fmackay]

Presumably the MCB is <13A.

Either it will say on it __A or B__ or C__ or D__. That's an easy one, and for anyone to help you, that information will be required as a minimum.

It is marked "15" only, type is not specified (sorry, didn't notice this before, it's on the end of the switch so not very visible from below when the switch is on (up)) . Makes sense that this wpuld be overloaded by an 18A max load from the appliances. Looking at this CU again I am thinking it is very unlikely that a replacement will be available given its age - when a power shower has been installed the RCOB has been put in a separate box even though there are spare slots in the CU.

 

[mfarrow]

It is marked "15" only, type is not specified.

And I'm guessing it's black, and there's no RCD protection? It sounds like the board is quite old, which would mean the characteristics of it may not be easily obtainable, and two machines heating in parallel may be enough to trip it. Either way, you need a solution, which may involve running a new circuit, or trying to use what's there to best advantage.

It's not ideal, but a changeover switch would best, such as an MK grid switch and mounting plate in place of the FCU (assuming you've used 2.5mm² cable FCU to socket). Trying to incorporate that and RCD protection, which you need for all your kitchen wiring alterations buried <50mm into the wall with no earthed covering of live conductors or mechanical protection, may not be possible though without getting the consumer unit replaced.

 

[Adam_151]

Does your consumer unit / breakers within it look like any of these:

Crabtree C50

Dorman smith loadmaster (shown without enclosure)

Square D QO-X (breaker only)

Federal Stab-lok (breaker only)

Sorry I couldn't find any better pictures

 

[fmackay]

And I'm guessing it's black, and there's no RCD protection?

Yes and yes.

It sounds like the board is quite old, which would mean the characteristics of it may not be easily obtainable, and two machines heating in parallel may be enough to trip it. Either way, you need a solution, which may involve running a new circuit, or trying to use what's there to best advantage.

It would only be moderately inconvenient to have to avoid running both appliances simultaneously, so I can live with the situation for the time being. I believe that the circuit is safe (ie the cable could cope with the max load of both appliances), just over-protected at present.

It's not ideal, but a changeover switch would best, such as an MK grid switch and mounting plate in place of the FCU (assuming you've used 2.5mm² cable FCU to socket)0

.

What would the advantage of that be over eg one of these? Would there be any reason to replace the FCU unless I can also replace the MCB, since it is the MCB and not the FCU which is tripping on overload at the moment?

Trying to incorporate that and RCD protection, which you need for all your kitchen wiring alterations buried <50mm into the wall with no earthed covering of live conductors or mechanical protection, may not be possible though without getting the consumer unit replaced.

Yes, I think replacing the consumer unit will be required in due course. No, I will not be doing that myself.

 

[fmackay]

Federal Stab-lok (breaker only)s

It's possibly this one. I'll try to post a picture.