OK so I know I should have looked this up last night, but it's bugging me now! Running power and cooker cable around the kitchen loft and I would prefer to run most in flat (single cable size) trunking. The reason is that I will be running this across walls and along roof timbers (to keep it out of insulation) and this will give a degree of protection against knocks, or any critters that get in there. Also, I don't trust myself 100% when trying to clip cable to tough brick, not to welt the cable occasionally.
But what is the impact on the cable capacity when run in trunking against clipped? I would consider just using clips when running cable along roof timbers, as the hammer velocity doesn't need to be quite as serious. The drops down the walls will be in oval conduit chased and plastered.
On Wed, 16 Jun 2010 10:46:46 +0100, "TheScullster" wibbled:
Not enough to upset most assumptions most of the time (except one).
Specifically:
Enclosed in trunking or conduit *on* a masonry/wooden wall is Ref Method B Ditto, but buried in plaster is also Method B
Open cable clipped to surface is Method C unless it is clipped to the underside of a ceiling,then it is Method B
Ratings (Amps) for normal PVC T+E:
mm2 B C
1.0 13 15
1.5 16.5 19.5
2.5 23 27
4 30 36
6 38 46
That does make the difference between your (say 32A) cooker cable being run in 6mm2 rather than 4mm2 but 6mm2 is a common choice anyway.
Watch the size of the oval - 6mm2 is quite lumpy. Also, unless you can get the right choice of oval to go through the 25mm knockout in the box, don't forget to grommet the hole :)
Clip lid trunking is a good choice for fiddly bits in the roof space as you can lay the cable in - 6mm2 will not pull very easily through long bendy bits of conduit - though you'll be OK with the 1-2m drop down your wall if you make sure the cable is nice and flat before you stuff it down.
Thanks for comprehensive response! Can you clarify a couple of points please? For 2.5mm cable, each leg of the ring should only be pulling 16A presumably to cover the 32A total breaker capacity. ISTR that 2.5mm is nominally rated at 20A as a spur. So in both/all cases, the 23A rating will not be an issue - right?
The cooker cable is run in 6mm, currently serving a hob and double oven and run in clip-top trunking, so that should serve the future purpose.
Some time ago, another poster gave links to images of his first fix work. Can't remember the culprit, but he showed conduit being used between boxes, with the conduit passing through grommets into each box. From your suggestions above, it appears that as long as the conduit protrudes into the box to protect the cable from the sharp box hole edge, a grommet is not required - is this correct?
On Wed, 16 Jun 2010 13:52:32 +0100, "TheScullster" wibbled:
The regulations require each leg of the ring to be capable of handling
20A continuously. It's an arbitrary, round, made up number that has some leeway for current imbalance between the 2 legs. Meet 20A for any+every section of cable and you're OK.
Might I suggest procuring a copy of:
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's a condensed domestic summary of the regs with lots of diagrams, and the more pertinent tables (though they forgot to mention reference method B so you have to be a bit creative). You can do stuff that's not in there that is still regs compliant, but if you do what the OnSite says, you're guaranteed to be OK. Good section on bonding with minimum wire sizes too.
That is totally worth the 16 quid even if all you do is put in one circuit.
Yes. 32A is a pretty standard circuit. I'm running one, even though I will have a gas cooker. You are allowed to have a 32A radial socket circuit and that's what I will do - 13A socket on the bit behind the cooker. Right place and the isolator switch next to the cooker still does what is expected. If anyone wants an electric cooker, all they have to do is switch the socket for a cooker terminal plate.
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did it to stop the conduit sliding out.
One or the other is perfectly acceptable - as long as the cable is protected from the sharp edge of the hole. You could even use a conduit through-hole adaptor.
One other question - is your run of cable massively long? The other thing that can go wrong is too high a cable impedance (Live-Earth loop) and/or too much voltage drop.
"Normal" runs are Ok, but it's worth asking if you live in a 10 bedroom mansion and the main CU is in the garage beyond the stables(!!) You get the idea...
Wouldn't it be great if someone made a book or website showing good first fix practices? I'm working my way through ring mains and plumbing at the moment and having to figure out everything from scratch is rather tiresome.
The CU is at the other end of the building to the kitchen (not quite 10 bedroomed mansion though). The cable runs to get to the boundary of the kitchen loft space are about
12m. The kitchen is 5m x 3.6m and, at a rough estimate, this gives a total cable length of approx 50m.
I have managed to cadge a copy of Niceic's 17th Edition Toolbox Guide from our electrical department (don't think they knew they had it!). It seems pretty similar to the 17th Edition OSG, so that's a bonus. The problem with a lot of it though can be interpretation and application, which is difficult I find when you are DIYing rather than immersed in this stuff daily.
TheScullster wibbled on Thursday 17 June 2010 09:28
Hi,
OK - 50m...
OnSite Guide summarises your max cable lengths as follows:
6mm2 @ 32A = 49m
10mm2 @ 32A = 81m
That is with a BS88-2, BS88-6 fuse or Type B MCB *only*. What do you have?
For all earthing systems:
Your permitted L-E loop impedance including the supply for a Type B 32A breaker is 1.16 Ohms for a TN* earthing system
You must either assume for TN-S, the supply impedance (called Ze) is 0.8 Ohms or TN-C-S (PEN) 0.35 Ohms *unless* you measure it[1] and prove it is lower.
[1] With a proper tester not a multimeter(!)
So your max circuit loop resistance (it is a DC test) is TN-S = 1.16-0.8 =
0.36 Ohms, TN-C-S = 1.16-0.35= 0.81 Ohms
You could test that with a decent very low ranging multimeter (and allow the worst case stated meter error to be safe). Connect L-E at the CU of your cooker circuit (obviously connected to nothing else) with a chock block, then measure the resistance between L-E at the cooker connection point. If the resistance is lower than the stated values you are OK.
Note: for a TN-S earthing system, the L-E resistance limit (called R1+R2) coincides more of less with the voltage drop limit. If you have a TN-C-S system you might get away with a slightly longer cable run without danger[1] but your voltage drops will go outside of tolerance.
[1] The safety factory of R1+R2, combined with your Ze is making sure the MCB/fuse trips in
That maximum cable for a 32A RCD protected ring is given as 106metres of
2.5T&E in the 17th edition. That is for the maximum 5% voltage drop. For a 32A 6mm cooker supply the maximum length is 49m either with or without RCD protection using a type B MCB.
That maximum cable for a 32A RCD protected ring is given as 106metres of
2.5T&E in the 17th edition. That is for the maximum 5% voltage drop. For a 32A 6mm cooker supply the maximum length is 49m either with or without RCD protection using a type B MCB.
5% is 12v so the cable is dropping 12V which at 32A is 384 watts of power being wasted. That is a unit (~10p) every three hours. It won't take long to pay for the next size up or even thicker cable.
Just think of how many mobile phone chargers you have to leave plugged in to waste that much. I wonder if the "regs" take into account the carbon footprint?
Well in winter the heat would not be wasted and in reallity the circuits will not be running for long at full capacity. And all the appliances using the circuit are greater than 95% efficient of course:-)
Well in winter the heat would not be wasted and in reallity the circuits will not be running for long at full capacity. And all the appliances using the circuit are greater than 95% efficient of course:-)
Well in winter the heat would not be wasted and in reallity the circuits will not be running for long at full capacity. And all the appliances using the circuit are greater than 95% efficient of course:-)
I think my response to your last post was misleading... It's the power circuit that totals 50m - comprising 12m to the kitchen boundary from the CU and 12m return leg, with the remaining 26m used in the lap around and the drops down the kitchen walls.
The cooker cable will go to 2 points - length to point 1 approx 14m, additional length point 1 to point 2 approx 7m. The earth provision is TN-C-S and all ciruits have Type B MCBs - all power ciruits are currently protected by a single RCD (split box setup).
Yes I have access to a suitable meter. The resistance test you describe for the cooker cable will presumably also prove good connections at any junction boxes/face plate terminals along the route.
ARWadsworth wibbled on Thursday 17 June 2010 15:16
That's very good of you :) A picture is worth a thousand words (like that bit of advice you gave me the other day re: dressing in the CU).
Seriously though - there might be a market for a book of good practises with lots of photos of real work - which is something that the NIC type books (I have 2) seem to miss.
I'm sure lots of trainee pros would find it worth buying too - given some people don't apprentice in the way they used to.
Bit like a Reader's Digest approach but with higher standards(!)
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