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rtbcomp
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The current rating of a cable defines a safe current to avoid overheating. As mentioned earlier you may have to move a size up on a long run to avoid an unacceptable voltage drop. This is one reason for using a ring main to connect sockets.
If you know the ohms per metre of your cable and the acceptable voltage drop you can calculate the maximum length for a given current. Under no circumstances use a cable rated less than the current you will draw.
If you know the ohms per metre of your cable and the acceptable voltage drop you can calculate the maximum length for a given current. Under no circumstances use a cable rated less than the current you will draw.
irondarren
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2.5mm cable will run 25amps but only if its on a ring and not a spur. On a ring the distribution, will be more even acrossed the ring. Im sure one of the electricians on the forum will confirm this. Thats the trouble with a little knowledge, its dangerous, and in effect B&Q are giving dangerous information, but technically it isn't wrong.
found some good info here
http://www.kevinboone.com/cableselection_web.html
if you scroll down far enough it gives cable ratings correctly, that is for cable in plaster, wall clipped, and free etc.
6mm wall clipped is 46amp max current which is 50% over rated beyond fuse capacity, factor in correction factor and length/voltage drop etc etc etc, and 6mm will be fine.
4mm is only 36amps wall clipped, factor in various bits and its max will be 30amps, meaning effectively you could be one of the 50,000 a year fires caused by electrical faults. Use 2.5mm George and it will be 50,001 fires next year.
found some good info here
http://www.kevinboone.com/cableselection_web.html
if you scroll down far enough it gives cable ratings correctly, that is for cable in plaster, wall clipped, and free etc.
6mm wall clipped is 46amp max current which is 50% over rated beyond fuse capacity, factor in correction factor and length/voltage drop etc etc etc, and 6mm will be fine.
4mm is only 36amps wall clipped, factor in various bits and its max will be 30amps, meaning effectively you could be one of the 50,000 a year fires caused by electrical faults. Use 2.5mm George and it will be 50,001 fires next year.
chriskelland
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Quote - For anyone who really knows their stuff, it should be quite possible to demonstrate some of these principles with calculations based on ohms law, etc. but perhaps that's too much to expect.
Ohms Law only works for DC, it fails to take into account power factor and frequency.
I fail to understand how we are talking about an extention lead. Most cooker boxes have a 13 amp socket, so where is the advantage in just plugging in a long lead? If it is your intention to connect inside the cooker outlet as these guys have been trying to tell you your houshold wiring will no longer be fit for purpose. What is so wrong with consulting an electrician?
As a result of flooding I have just had my kitchen re wired and a new consumer unit fitted, it certainly did not cost an arm and a leg, plus I had the entire system certificated. In the event of any problems the insurance company will need to see that certificate. Is it really worth the risk?
Chris.
Ohms Law only works for DC, it fails to take into account power factor and frequency.
I fail to understand how we are talking about an extention lead. Most cooker boxes have a 13 amp socket, so where is the advantage in just plugging in a long lead? If it is your intention to connect inside the cooker outlet as these guys have been trying to tell you your houshold wiring will no longer be fit for purpose. What is so wrong with consulting an electrician?
As a result of flooding I have just had my kitchen re wired and a new consumer unit fitted, it certainly did not cost an arm and a leg, plus I had the entire system certificated. In the event of any problems the insurance company will need to see that certificate. Is it really worth the risk?
Chris.
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although I agree with the sentiments, haven't we all gone a tad OTT, quoting Regs??, on the same vain I could easily say, you should be all tested ott, before touching a welder, this in hand would eliminate the need for anyone to run a cable to there garage to dabble
the most important factor to work from is working temperature, then calc back to it, this is what is going to determine the working load of a specific cable, ambient temperature is used as a mean, voltage+current then resistance can be used in a formula to calculate voltage drop, resistance is calculated from ohms to nutral........
most domestic sparks don't bother its looked up in a book and over sized for the app, however large industrial apps, where there difference between one size of a cable to another could save 10s of thousands £££, full calcs are essential.....
BTW splicing a cable of course has affects on Vd and overall performance on a given cable and is another calc that should be used......
the most important factor to work from is working temperature, then calc back to it, this is what is going to determine the working load of a specific cable, ambient temperature is used as a mean, voltage+current then resistance can be used in a formula to calculate voltage drop, resistance is calculated from ohms to nutral........
most domestic sparks don't bother its looked up in a book and over sized for the app, however large industrial apps, where there difference between one size of a cable to another could save 10s of thousands £££, full calcs are essential.....
BTW splicing a cable of course has affects on Vd and overall performance on a given cable and is another calc that should be used......
+1 for snowy, the voice of reason. I think ive pointed out before that we'd all be really unhappy if the gov made us weld our car bonnets shut because we couldnt be trusted to do things under it. And there have been calls for "professional only" working on motor vehicles. Stop evolution, some people want to get off 
I suspect the real reason your not getting a straight answer with ohms law as applied to ac and some example calculations is if you for eg stick a inductor on the end of the extention (ie a transformer welder), then this introduces some lag while it builds up the field strength, which changes the phase angle, which in turn changes the power factor which changes the average power used. If that all sounds like a b*gger to work out, thats because it can be, and you'd have to know what inductance the welder etc was to get a real answer. Which is why all the domestic sparks look it up in a book and apply a safety factor. Like darren has already done for you to come up with 6mm
I suspect the real reason your not getting a straight answer with ohms law as applied to ac and some example calculations is if you for eg stick a inductor on the end of the extention (ie a transformer welder), then this introduces some lag while it builds up the field strength, which changes the phase angle, which in turn changes the power factor which changes the average power used. If that all sounds like a b*gger to work out, thats because it can be, and you'd have to know what inductance the welder etc was to get a real answer. Which is why all the domestic sparks look it up in a book and apply a safety factor. Like darren has already done for you to come up with 6mm
Thanks everyone for the latest batch of posts. There are some very good links and suggestions there and your comments are most helpful. I'm now sure that 2.5mm cable is out and I need to determine whether it's safe to use 4mm cable, given I'm highly unlikley to plug in anything drawing above 20A. That's similar to using a 6A extension lead on a 13A socket: you just make sure not to overload it. 6mm would undoubtedly be best but may be too expensive and a bit OTT for my needs, below 20A.
There's a table here which gives details of volts, amps, voltage loss, etc:
http://www.alertelectrical.com/upload/pdf/32962423Y.pdf
There's a table here which gives details of volts, amps, voltage loss, etc:
http://www.alertelectrical.com/upload/pdf/32962423Y.pdf
madkayaker
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volt drop is cable lengthxdesign currentx the value of the table / 1000. you need to take into account the drop to the point you are running from from the mains and the max allowed is 3% of nominal voltage (230V)
Hi, just adding my thoughts on this!
just purchased a Cobel welder, sold ILLEGALLY ( BRAND NEW ! )WITH A FACTORY MOULDED ON 13amp PLUG, WITH THE FUSE BRIDGED OUT !!!!!I will go cee-form.
I too have decided to go the cooker circuit route. True, its always best to play on the safe side & show respect, but dont let the professionls frighten you with 'burn your house down' stories, as I have known pro sparks do dodgey sustandard work! Any way, what I wanted to point out here, is that if you use 30 /32 amp line, the 32amp TRIP will protect that cable, & if you overlaod it due to length drops, the trip will go before the wire! A super safe way is to always use a cable higher load rating than the trip that protects it, that way it can NEVER overload the cable !!!
As for part p........! Im saying nothing!
just purchased a Cobel welder, sold ILLEGALLY ( BRAND NEW ! )WITH A FACTORY MOULDED ON 13amp PLUG, WITH THE FUSE BRIDGED OUT !!!!!I will go cee-form.
I too have decided to go the cooker circuit route. True, its always best to play on the safe side & show respect, but dont let the professionls frighten you with 'burn your house down' stories, as I have known pro sparks do dodgey sustandard work! Any way, what I wanted to point out here, is that if you use 30 /32 amp line, the 32amp TRIP will protect that cable, & if you overlaod it due to length drops, the trip will go before the wire! A super safe way is to always use a cable higher load rating than the trip that protects it, that way it can NEVER overload the cable !!!
As for part p........! Im saying nothing!
Why the discussion on 2.5 cable if we are talking cooker circuits ???
Thats unsafe even for the cooker !!!
Cooker circuits in uk should have multistrand copper cables, & if used with an integrated 13amp socket on the switch, would have a 40 amp trip 32 amp trip is for cooker circuits with NO 13 amp outlet on the switchplate ). Subject to the above being observed, a 30 amp cee-form on the end if no other item is connected ( DONT DO IT ON THE CIRCUIT WHERE THE COOKER STILL IS!!!) should be ok. ( disclaimers on not being a sparkie applies ).
brightspark
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ok so u have your 30 amp suply all well and good and the welder with bypassed 13 amp fuse. but what is the rating of the welder 30 amp?? i probably think not. its probably about 16 amp or no more than 20 with a 2.5 flex on it. so therefore the fuse rating is over rated for the welder components unles u downrate the suply mcb to suit welder draw current
I wasn't talking about a cooker, actually. I was talking about plugging a new extension lead into the socket intended for an electric cooker. I remain confident that 2.5mm cable is adequate for the 16A I seek to draw but, in the year since we last discussed this topic, I've been given some 4mm cable and so I shall now be using that.
