Testing Oven Wiring

[evenstephen]

Greetings, Everyone,

Does anyone know of a foolproof way to check whether an existing wiring run is unbroken by spars or junctions? My family currently has a single built-in oven terminating in a 13amp plug, and we are thinking about upgrading to something a little more substantial. I know that a cooker control unit would have to be installed, but I have a question about the wiring before I have this done.

Currently, our consumer unit has a 16amp MCB leading to a 6mm cable for the oven. From flipping the MCB and testing what's affected, I believe that the only outlet serviced by this cable is the existing oven point; however, I cannot be certain. The outlet certainly terminates in a 6mm cable, but I've found some creative wiring by previous owners of the house before, and I'm uncertain of whether this cable might pass through some junction box somewhere under the floorboards.

Is there any way to discern whether the cable forms an unbroken link to the consumer unit through use of a multimeter or some other equipment? If not, I suppose the only way to be absolutely certain is to tear up the floorboards and visually inspect the run. I'd like to avoild this scenario if at all possible, to the point that I would probably have a new run installed in a more accessible place rather than bothering with the flooroards. Before I do this, however, I thought I'd look in here for advice. Any help in this matter would be most appreciated!

 

[PrenticeBoyofDerry]

If upgrading cooker 6mm csa T+E cable usually good for a 32A breaker, so the power of your cooker can be increased but you would have to do away with the 13 amp sockect and have a cooker isolating switch installed.
If cooker above approx 3kw(that's without taking diversity in to consideration)
It would still be wise to install an islolation switch within 2 meters of the cooker.

As far as the unbroken link, it would be had to confirm as a JB will keep the continuity of the circuit and unless a break was at that point would be hard to tell even then could not confirm it was a loose connection in the JB or the circuit was broken through damage but that's a side issue as I don't beleive that's a problem.
What distance from the CU is the current cooker installed are you sure the circuit goes through floor or is it possible that it takes another route?

It would also be wise to make sure this circuit does not serve any other equipment.

 

[ericmark]

You can never be 100% sure there is nothing untoward on the cable run. However you can minimise the chances. If I look at my house for example pacing out the likely cable run I come to around 35 foot. With 6mm² cable at 7.3mV/A/m I would expect the Line – neutral resistance to be around 0.25 ohms add to that the Ze reading of around 0.35 I should measure around 0.6 ohms.

With a 2.5mm² earth cable at 18mV/A/m measured between line and earth if above is correct I should measure 1.24 ohms.

It is likely if the run has been tampered with the results will not match and it would then ring alarm bells.

However unless you lift the odd floor board you do not know if there is any insulation and if you are intending to draw near the limit then you would need to check.

If you consider MCB’s normally jump 16, 20, 25, 32 amp then 6mm² even with Reference Method 103# (in a stud wall with thermal insulation with cable not touching the inner wall surface) is still 23.5 amp and so it is unlikely to be down graded to 16A because of the route. If we consider a B16 then 16x5=80A and 230/80 = 2.875 ohms lift to a 20A MCB then we look at 2.3 ohms if Zs lies between the two we can assume that is why it only has a 16A MCB and by using a RCD we can up rate the cable.

But if we can’t see why a 16A MCB is fitted then we are taking a chance up rating it as maybe there is as you say a joint and 2.5mm² is used in part of the run.

Many of the alarm bells which ring are due to experience and what we have found in the past and I would not be happy changing a MCB for a larger one without some knowledge of what has been done. In theory we should all have the installation certificate but I was original owner of my house bought in 1980 and there was no installation certificate issued.

In around 1990’s we started to take a lot more notice of the regulations and started to issue installation certificates but it was not until 2004 when Part P came in that there issue became the norm.

With the use now of induction hobs if you think it may have been tampered with then I would fit some 10mm² cable to be future proof.

For Pirelli Earthshield and LSX cables I can’t find sizes available and for Flexshield 2.5mm² seems to be the limit but Afumex by Prysmian and Guardian by Batt cables both do a 6mm² version as their largest size. Since it is a 90ºC rather than 70ºC cable it is rated clipped direct at 58A and even when in a stud wall with thermal insulation is somewhere near 30A and it does not need RCD protection and RCD’s can be a problem with cookers so maybe some Ali-tube cable to BS 8436 is best option?

However since it would come under Part P as would any MCB change in the consumer unit other than like for like I think you would be better asking the electrician on site for his advice.

 

[evenstephen]

PrenticeBoy and Ericmark,

Many thanks for your replies, and apologies for taking so long with this reply (our infant boy was in the hospital with flu, but is all well now).

It looks as if there's no certain way to ensure that the 6mm cable is unbroken without full visual inspection. I should have done this while replacing floorboards last year but did not do so. After reading your messages and considering correcting numerous instances of 'creative wiring' by previous owners, I think I'll just have a new run installed to an isolation switch and cooker point. I'll then be able to rest assured knowing that firing up the cooker won't set the house on fire!

Ericmark, thanks particularly for the advice on cabling. I'll take your suggestion and check with an electrician for further advice.

Thanks much!