cable rating

[roughneck]

last summer i installed a compact oven in my kitchen. i used 2.5mm T/E to extend the cooker point about 15ft to the new location.
the wire runs behind the kitchen units and is wired directly into the oven.
the oven max load is 3.3kw for the grill and 1.04kw for the oven.
1. is this a legitimate way to extend the cooker point.
2. is 2.5mm T/E sufficient.
3. if i need to upgrade it to 4.0mm T/E what are the implications for part P

 

[Steve]

what was the original cable size? (before the extended bit)

Take a look at

http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

its a cable size selector. I typed in your oven load with 5m cable run clipped to a wall and it came out with 1.5mm cable. But this means 2.5mm is fine. Better to use higher rated cable anyway. 1.5mm would be bordering on its limit with this load.

 

[ban-all-sheds]

1. is this a legitimate way to extend the cooker point.

Depends on what you mean by "extend" - are you saying that you've got a 15' cable connecting the oven to the cooker outlet plate? I don't think there are any limits on how long this cable can be, other than volt-drop etc.. BUT - how far is the isolation switch from the appliance? Should be no more than 2m..

2. is 2.5mm T/E sufficient.

It's sufficient for the load you describe. It's almost certainly undersized given the rating of the protective device for that circuit.

3. if i need to upgrade it to 4.0mm T/E what are the implications for part P

The law says that you must either have the work done by a registered electrician who can self-certify compliance with the Building Regulations, or you must notify your LABC in advance, and pay their fees for inspection and testing. Or pay their fees and the cost of employing someone yourself to do the I&T.

You might think that you could replace the cable with 4mm² and claim that you put it in when you first installed the oven. I couldn't possibly comment on that.

 

[roughneck]

the distance from the oven to the switch is 2.2 metres. i moved it to use a bit of 'dead' space under a wall mounted central heating unit. if more than 2 metres is a definite no no, i can move it. but then part P rears its head again.

 

[felix]

It's the size of the fuse in the consumer unit that dictates the cable size, not the load you put on the end. The way to get your head around this is to think protection A fuse protects the cable that lies beyond it. You can use 2.5 mm cable if that fuse is small enough.

But here we have an anomaly. A ring main (2 lots of 2.5mm cable in parallel) goes on a 30 amp fuse but you can have a spur off that ring using 2.5 mm cable. This means that the 30 amp fuse is protecting a piece of 2.5mm cable. On the other hand, if you remove the ring and put a radial circuit onto your 30 amp fuse you have to use 4mm cable!

Is (was) your oven running off a spur on a ring main or does it have its own fuse - and what size is it?

 

[plugwash]

last summer i installed a compact oven in my kitchen. i used 2.5mm T/E to extend the cooker point about 15ft to the new location.
the wire runs behind the kitchen units and is wired directly into the oven.
the oven max load is 3.3kw for the grill and 1.04kw for the oven.
1. is this a legitimate way to extend the cooker point.
2. is 2.5mm T/E sufficient.
3. if i need to upgrade it to 4.0mm T/E what are the implications for part P

is there anything else connected to this cooker point?

 

[kendor]

But here we have an anomaly. A ring main (2 lots of 2.5mm cable in parallel) goes on a 30 amp fuse but you can have a spur off that ring using 2.5 mm cable. This means that the 30 amp fuse is protecting a piece of 2.5mm cable.

A question cropped up when i was at college along similar lines with the use of small cables alongside large cables in a busbar chamber. how when the backup fuses where rated in hundreds of amps could you get away with 35mm's alongside 16mm's and even 10mm's all bolted to the bars.
The get out was the fact that these cables where kept short not longer than a couple of metres if i remember well enough? these could then tap off to feed isolators close by.
Funnily enough i worked on a job where this situation happened and there was a fire, it was caused by a spark trying to be clever (and Lazy) by feeding a bare pyro through the Live Busbar chamber and unfortunately shorting out a phase not sure of the circumstances that caused what happened next but all cables 16mm and lower in size went up in smoke, turned out that the incomer fuses up in the intake room were not fuses but solid copper bars! the spark in question was found on the floor overcome by thick acrid fumes and with burns on his face. After a short time he recovered and was ok thank god. just goes to prove that even with all that training one moment of madness could cost you your life.

 

[Damocles]

Yes, but which was the moment of madness? It was the guy who put in thick copper instead of fuses, or the guy on his back who failed to check the fuses were actually fuses?

On a ring you can use a 2.5mm spur because the single socket you are allowed to connect to it will limit the total amount of current which can be drawn. Both absolutely because anything plugged in has a 13A fuse,and practically because you are very unlikely to plug in two things which would be pulling the full theoretical current at the same time. This limits the maximum operating current.

The cable also has to be able to withstand the full short circuit current. this is a much easier requirement to satisfy because the very large short circuit current gets disconnected quickly before it will damage the 2.5mm. Probably even a 1.5 would be safe from short circuit on a 30A fuse/mcb.

So in general you can use a smaller cable on a larger fuse which is protecting against short circuit, so long as the other end of the cable also has protection to limit the operating current within the cababilities of that cable. There are conceptually two different kinds of fault which you are protecting against and these can be protected against in different ways.

 

[kendor]

Yes, but which was the moment of madness? It was the guy who put in thick copper instead of fuses, or the guy on his back who failed to check the fuses were actually fuses?

On a ring you can use a 2.5mm spur because the single socket you are allowed to connect to it will limit the total amount of current which can be drawn. Both absolutely because anything plugged in has a 13A fuse,and practically because you are very unlikely to plug in two things which would be pulling the full theoretical current at the same time. This limits the maximum operating current.

The cable also has to be able to withstand the full short circuit current. this is a much easier requirement to satisfy because the very large short circuit current gets disconnected quickly before it will damage the 2.5mm. Probably even a 1.5 would be safe from short circuit on a 30A fuse/mcb.

So in general you can use a smaller cable on a larger fuse which is protecting against short circuit, so long as the other end of the cable also has protection to limit the operating current within the cababilities of that cable. There are conceptually two different kinds of fault which you are protecting against and these can be protected against in different ways.

although the switchgear was in the chamber room it was padlocked and it wasn't until the LEB(at that time) opened it up that it was found to be merely an isolator as opposed to a switch fuse.

 

[felix]

So in general you can use a smaller cable on a larger fuse which is protecting against short circuit, so long as the other end of the cable also has protection to limit the operating current within the cababilities of that cable. There are conceptually two different kinds of fault which you are protecting against and these can be protected against in different ways.

Thanks Damocles. That old anomaly makes sense now. I suppose the same rule applies to the armoured cable coming into the house. From what I've read elsewhere on this forum the substation fuses could never protect this against overload. That job is left to the 60 amp fuse at my end.

 

[roughneck]

to summarise: the cable is 2.5mm T/E. the oven is 2.2metres from the oven switch. the fuse in the consumer unit is 30A. the only other thing on the cooker outlet is the ingiter for the gas hob. the current drain for that i think is negligible. can i get away with it, or is the part P inquisition going to strike?

 

[felix]

If the original cable is 2.5mm then it was wrongly wired in the first place but you can remedy this by putting a 20 amp fuse in the consumer unit. This will give you 4.8 kW which is enough for your oven, grill and igniter - but don't go plugging a 3 kW kettle in there!

If the original cable is 4mm then you should extend with the same and that 30 amp fuse will be OK. You now have enough (just) for oven, grill and kettle. This is the better solution.

Just as a matter of interest, I think we're all assuming that the old cooker outlet was on the end of a single length of 2.5mm cable, ie a radial. Is this correct?

If you intend to renew the whole length why not go one better and use 6mm cable. The price difference is not great and you never know when you might want a bigger cooker.

 

[Damocles]

Part P will not be raising its head unless you actually do anything. Crazily, part P has nothing to say even if what you have got is dangerous. But if you change anything to make it safer, then Part p would strike.

I would guess the original cooker supply was wired in 4mm or above? if so you can get away with what you have as the 2.5mm is sufficient for the current drain of the cooker, but I would agree a 4mm would have been nicer.

The issue of the switch being too far from the cooker is not immediately dangerous, .2m is not really worth worrying about to correct something already installed. If you were expecting an inspector to admire your new work then it might be a different answer. Judge for yourself whether moving the switch closer would increase your safety if you had to turn off the cooker in an emergency.