Post
by ericmark »
Fri May 19, 2023 10:46 am
There are two ways to protect cables from overload, one at origin and one at destination, so a 13 amp socket can only deliver 13 amp, (26 in case of a double) so having 2.5 mm² on a 32 amp MCB feeding a 13 amp socket even when he 2.5 mm² is only rated around 20 amp is OK as only a single socket supplied.
However if we have a row of 13 amp sockets, using 2.5 mm² either the overload needs to be smaller than 32 amp, or we use two cables, as with the ring final.
The ring final was introduced near end of world war 2 to reduce the amount of copper required with the rebuilding that was going to be required, it is a special situation, and not without problems, so we have some rules to help us ensure it is used safely.
"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.)"
This is taken from appendix 15 BS 7671:2008 and it goes on to give examples of how it can be used.
In real terms the kettle even when 3 kW is no problem as used for a short time, what does cause problems are items used for a long time, the immersion heater can take hours to heat the water so they are always on a dedicated supply, however tradition has resulted in problems arising with washing.
We would use the old twin tub in centre of kitchen for the whole day, but no cooking could really be done while the washing was being done, so no real problems with overload, and drying was done outside, not with a tumble drier, it is the tumble dryer or washer/dryer which is a real problem, as they can run for 90 minutes which is quite a long time, and can use in excess of 2 kW so really should have a dedicated circuit like the immersion heater, but this is rarely the case.
The washing machine without a built in dryer only uses over 2 kW for a short time, same with dish washer and oven, the chance of all switching the elements on at same time is remote.
As we started using more power things changed, we used a 100 amp supply instead of 60 amp, and we split the circuits with ring finals side to side of the home, as we tend not to use upstairs at same time as down stairs, so this split gave a more even load for 2 ring finals, it means if one fails on temporary extension leads are run up/down stairs, and it uses less cable so the loop impedance is better.
However when we first started using RCD's we did not want the lights to fail in same room as sockets if there was a fault, and the lights do not lend themselves to being split side to side, so the sockets were split same as lights so upstairs lights would be on same RCD as down stairs sockets and via versa, as RCBO's came in, then we can return to splitting side to side.
These changes has resulted in the wiring in many homes not being ideal, and clearly a re-wire is a big job, so we have to try and make the best of what we have.
The 40 amp MCB/RCBO is really only used for electric showers, most cookers have a 32 amp supply with 6 mm² ring finals also 32 amp with twin 2.5 mm², we get radials using 20 amp and 2.5 mm² or 32 amp with either 4 mm² or 6 mm² the problem with latter is getting the thicker cables into sockets which as said were designed for the ring final system.
We also have two types of spur, fused and unfused, the unfused is limited to one BS 1363 device (i.e. 13 amp socket) which always has a BS 1362 fuse no larger than 13 amp. The fused spur is really a radial, and you can have loads of sockets on that type of spur.
Then just as you think you know it all, we have the time it takes for a overload to work, a MCB is two devices in one, a magnetic and a thermal trip, so a B32 will in the fullness of time trip at 32 amp, but the magnetic part works in a fraction of a second, but needs 5 times the thermal value, so 160 amp to trip, for 160 amp to flow, the resistance or impedance as it is called with AC has to be below 1.44 Ω and to allow for volt drop reducing the current we give 5% safety margin so 1.38 Ω which needs a special meter called a loop impedance meter to test, which cost around £160 so not a DIY tool. Also we should ensure the volt drop is not over 5% for sockets, 3% lighting, so same tool used to check that.
So the ring final is limited to around 106 meters of 2.5 mm² cable, and radials a lot less.
The use of RCD protection and equipment with switch mode power supplies has resulted in the loop impedance not being quite so important, but the move from the fuse to the MCB made it a lot more important as with a fuse too much cable may mean 6 seconds instead of 5 seconds to disconnect, but with a MCB it can means 2 minutes instead of 0.01 seconds when the loop impedance is just a little too high.
Add to all this, cable comes in many types, two main ones are thermal plastic and thermal setting, so 70ºC or 90ºC running temperature, and any material the cable is buried in will mean it runs hotter or cooler, so 2.5 mm² cable has a large range of maximum current depending on how installed, but rule of thumb in the main we think of 20 amp, that being the lower limit for cable used with a ring final.
Although we can use 4 mm² with 32 amp and 2.5 mm² with 25 amp that is only when run so the cable can cool enough. In the main we use 6 mm² for 32 amp.
Batt cables do publish charts. Which are a copy of those found in BS 7671 which is the electrical bible.
As to way around your problem, likely the most cost effective is to get a heat pump tumble dryer, they use less power, so less likely to trip the MCB's and also cost you less in electric.
It may be you can use a socket from the cooker supply, or in my last house I used the supply that originally fed the immersion heater, which had been removed when the cistern was removed and I went to instant gas water heating.
But due to the cost of the test equipment, and the experience required, I have to agree with Mr White, really need an electrician to visit, maybe not a full electrical installation condition report, but those MCB's ring alarm bells.
I will guess some one at some point has changed the consumer unit, and have used a populated unit, rather than select what sizes are really required.
It does seem odd, fuses went 5, 15, 30, but the MCB went 6, 16, 32. Technically the ceiling rose were rated 5 amp and used as a junction box, so one should not use a 6 amp MCB, however we know the ceiling rose is unlikely to have a problem with one extra amp, however we also used RCD's rated 63 amp on 100 amp supplies with MCB's totalling over 63 amp, and it has been pointed out that is really not permitted. Clearly with a 60 amp supply it would be OK, but we can't draw the fuse, so how do we know if a 60 amp or 100 amp supply?
