Spur off dishwasher socket

[Ohmslaw]

Have a socket on a ring main for the dishwasher - okay to spur off this for a 13amp fcu to supply socket for oven?

 

[JohnD]

if you can verify that the dishwasher circuit is on the ring (turn off all power and test for continuity between the two L, the two N and the two E) then yes, you can

There must be two, and only, cables currently going to it, each with the three cores I mention.

 

[Ohmslaw]

Cheers, yes it is on a ring.

 

[JohnW2]

Cheers, yes it is on a ring.

Just to be clear, I presume that what John was asking is whether you are sure that the socket is part of the ring, and not already a spur from a socket on the ring.

Kind Regards, John

 

[Ohmslaw]

Yes, it's on the ring circuit with no cables spurring off dishwasher socket. Cheers

 

[flameport]

Yes, it's on the ring circuit with no cables spurring off dishwasher socket

The presence of 2 cables on it's own means nothing.

You need to do:

turn off all power and test for continuity between the two L, the two N and the two E)

to eliminate the possibility of it being a spur with another spur connected to it.

 

[securespark]

if you can verify that the dishwasher circuit is on the ring (turn off all power and test for continuity between the two L, the two N and the two E) then yes, you can

Actually, to be certain, an end-to-end test does not guarantee a correctly installed ring final.
There could be a ring within a ring or other anomalies.
To check for a true, properly installed ring final, you'd need to do cross connections at the board and check the readings at each outlet.

 

[ban-all-sheds]

There must be two, and only, cables currently going to it, each with the three cores I mention.

That doesn't guarantee it.

Actually, to be certain, an end-to-end test does not guarantee a correctly installed ring final.
There could be a ring within a ring or other anomalies.
To check for a true, properly installed ring final, you'd need to do cross connections at the board and check the readings at each outlet.

 

[EFLImpudence]

This is all getting rather complicated.

Assuming there is continuity between all three pairs of conductors, which anomaly would preclude a spur being added - and how would the OP tell?


Should it turn out not to be a ring then the OP has to determine if it is a spur off a spur or a satisfactory radial.

 

[JohnW2]

Actually, to be certain, an end-to-end test does not guarantee a correctly installed ring final.

I don't think JohnD was talking about 'end-to-end' continuity, since that woul tell one nothing about how an individual socket had been wired. I think he was talking about disconnecting the socket in question and testing continuity (of L, N & E) between the two cables that had been disconnected.

There could be a ring within a ring or other anomalies. To check for a true, properly installed ring final, you'd need to do cross connections at the board and check the readings at each outlet.

For a start, I don't think one would be able to detect a short spur by doing that - low resistance measurements are simply not accurate enough. Furthermore, as EFLI has asked, which of the 'anomalies' of circuit arrangement do you think would preclude taking a spur from the socket in question (if it satisfied JohnD's test)?

Kind Regards, John

 

[East01]

Hi, if I have a dishwasher on a circuit of its own in the kitchen there's a fused spur for dishwasher above work but I'm looking a double socket next to it, can I spur of the fused spur for dishwasher into the new socket

 

[EFLImpudence]

Yes, if you just had a spur straight from a 32A ring circuit without a fused spur, you could only have one socket (single or double) on that spur as it is not fused but as you have a fused spur on the ring making your spur a fused spur you may have as many sockets on that fused spur as you wish - although, obviously, because of the fused spur's fuse a fused spur is limited to 13A so it depends what else you want to run..

 

[ericmark]

I see the problem, we should not take an unfused spur from an unfused spur, so two cables should mean ring final or radial and a fused spur is really a radial. But the tests for a figure of 8 are complex, and not really some thing one can expect a DIY guy to do, and even if figure of 8 is that really a problem?

As an electrician with a loop impedance tester reasonable easy to work out order of sockets, the same with a low ohm ohmmeter using the required 250 mA, but a DIY guy is hardly likely to have them. So power off, end to end continuity is in the main going to show it is a ring final, there are exceptions, but unlikely to find them with most installations. For the exceptions some one in the past has not been following the rules.

And today the dish washer heats up very little water, yes technically over 2 kW and not portable so it should have a dedicated supply, but in the real world we are looking at immersion heaters and tumble driers, the latter both stand alone and washer/drier, but not so much of a problem with a heat pump drier, as the heat pump does not run all the time.

The other one is the range cooker, as often 13 amp but for an extended time. The normally cooker is over 13 amp so must have a dedicated supply, same with most hot plates, although there are some which work on 13 amp, and as for oven if under 3 kW then the time it is on for is short, once temperature is reached, it uses a mark/space ratio to maintain it.

So yes appendix 15 of BS 7671 does suggest one method to ensure no overload for long periods is

connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit

this is rarely complied with, and one is being a bit pedantic to suggest such measures. i.e. behaving like me!

 

[JohnW2]

And today the dish washer heats up very little water, yes technically over 2 kW and not portable so it should have a dedicated supply, ...

What aspect of dish washers (or, indeed, washing machines) do you think qualifies them as "cookers, ovens and hobs with a rated power exceeding 2 kW" - which, as you go on to quote, are the appliances for which the guidance of App 15 of BSA7671 suggests that they should ideally be on dedicated radial circuits ?

Kind Regards, John

 

[ericmark]

It says

The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). This can generally be achieved by:
(i) locating socket-outlets to provide reasonable sharing of the load around the ring
(ii) not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile from the ring circuit
(iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit
(iv) taking account of the total floor area being served. (Historically, limit of 100 m² has been adopted.)

It is not a hard and fixed rule, and the load from an oven under 3 kW will be a lot less than a washer/drier or drier on the drying cycle. Due to the time it draws the high current for.

The Oven, Dish Washer, Washing machine (without drier) are all around the same load and time, they may use 3 kW to get up to temperature, but not for long, the exception is the drier and the range cooker.

It does seem strange as the electric space heating was the whole reason why the ring final came into existence, but 7/0.029 was bigger than 2.5 mm² and the fuse was 30 amp not 32 amp plus the pins of the plug were solid brass and conducted heat better, no insulated bit of the pins.

Conductor outside diameter 2.5 mm² = 0.075 inches, 7/0.029 = 0.087 inches.