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| UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
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#1
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Wiring a hob and separate single oven..
We've got a caper on the go where we've got the above with a single switched 45 amp cooker outlet. Now I've been told that "as you can't get two bits of "cooker cable" into the outlet plate, the only way to do it is to run the oven off a 13 amp socket on the ring main, and wire the cooker onto the "cooker outlet" in "cooker cable", presumably the 6 mm stuff. What's wrong with running two lumps of 2.5 twin and earth, as the cooker hob is rated at 5.5 kW which is 23 amps and within the ratings of 2.5 cable in free space, the oven no more than around 3Kw IIRC. Two lumps of 2.5 will go into the cooker outlet, but it seems wrong. Comments anyone?..... -- Tony Sayer |
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#2
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The cooker may be rated at 5.5kW, but the principle is that every bit
of cable must be protected by a fuse or breaker that prevents it being overloaded (and that protection must be on the supply end!) If the cooker develops a fault that causes it to draw 44A the breaker on your 45A circuit will be perfectly happy - however the 2.5mm T&E will be no more than a memory and a paragraph in the fire brigade's incident report. |
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#3
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Matt Beard wrote:
The cooker may be rated at 5.5kW, but the principle is that every bit of cable must be protected by a fuse or breaker that prevents it being overloaded (and that protection must be on the supply end!) If the cooker develops a fault that causes it to draw 44A the breaker on your 45A circuit will be perfectly happy - however the 2.5mm T&E will be no more than a memory and a paragraph in the fire brigade's incident report. Not so really, a ring circuit is protected by a 32amp breaker but it's quite alloweable (by IEE regulations) to have a spur wired in 2.5sq mm cable. A fault on that spur could draw 32 amps which will overload the cable but not trip the breaker. The 2.5sqmm spur is protected by the fuse(s) in the socket or FCU at the end of it. The thinking is that a fault in the cable itself is *very* unlikely to be the sort of fault that will draw more than the cable can carry but less than will trip the upstream protection. The cable must be sized such that the earth loop impedance is low enough to trip the circuit protection (within 0.5 seconds?) if there's a short to earth but not such that it can carry the current allowed by the MCB. -- Chris Green |
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#4
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On Wed, 24 Nov 2004 11:11:02 +0000, tony sayer
strung together this: Comments anyone?..... There's loads on this issue over the past few months. Basically, technically, yes it will work. But.... it's not to BS7671 so you can't do it, no matter how many calculations you do. -- SJW A.C.S. Ltd Please reply to group or use 'usenet' in email subject |
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#6
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#7
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Lurch wrote:
On 24 Nov 2004 13:43:17 GMT, strung together this: Not so really, a ring circuit is protected by a 32amp breaker but it's quite alloweable (by IEE regulations) to have a spur wired in 2.5sq mm cable. But you're not allowed to run a 32A radial in 2.5mm cable. Yes, I know it's the same thing at the end of the day but one scenario complies with BS7671 and one doesn't. But a radial in 2.5sq mm could have multiple sockets, the spur on a ring circuit is explicitly only allowed to have one socket (or double socket) on the end of it. It's not perfect as, while a double socket is only 'officially' rated at 20 amps, in reality it could have 26 amps sucked out of it without the plug-top fuses going. -- Chris Green |
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#8
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Lurch wrote:
On 24 Nov 2004 13:43:17 GMT, strung together this: Not so really, a ring circuit is protected by a 32amp breaker but it's quite alloweable (by IEE regulations) to have a spur wired in 2.5sq mm cable. But you're not allowed to run a 32A radial in 2.5mm cable. Yes, I know it's the same thing at the end of the day but one scenario complies with BS7671 and one doesn't. It's precisely this reason that when I added a spur, I put the FCU directly into the ring so that the 2.5mm cable was protected by the FCU. That seems to make more sense to me. |
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#9
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#10
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tony sayer wrote:
What's wrong with running two lumps of 2.5 twin and earth, as the cooker hob is rated at 5.5 kW which is 23 amps and within the ratings of 2.5 cable in free space, the oven no more than around 3Kw IIRC. Two lumps of 2.5 will go into the cooker outlet, but it seems wrong. Comments anyone?..... It /might/ be compliant, since there is no risk of overload, but you need to consider a few things: 1. Ambient temperature (it can get warm near a cooker): the 2.5mm^2 cable "in free space" would be rated at 27A at 30deg. C. At 40 degrees Table 4C1 tells us that the ambient temperature factor (Ca) is 0.87 which brings the rating down to 23.5A which is on the limit (5.5kW at 230V is closer to 24A by the way, not 23A). 2. Grouping factors: there'd be a further derating if your two 2.5 cables were side by side. This factor (Cg) is 0.8 for two bunched cables (Table 4B1). Applying both the Ca and Cg factors the effective rating of the 2.5mm^2 cable is now down to under 19A... 3. Short circuit protection: the circuit is fused or MCB-protected at more than the cable rating. This may be OK since overload can't occur, but you do need to check that, in the event of a s/c or earth fault at the end of the longer of your two 2.5mm^2 legs, the fault would be cleared before the cable overheated [Reg. 434-03-03]. The following information is needed to do the necessary calculations: (a) type and rating of protective device, (b) circuit length, (c) circuit cable size(s) and type(s), including CPC sizes, (d) type of installation earthing - whether TN-S (cable sheath earth) or TN-C-S (PME). You're definitely into marginal territory here. Most cooker outlets have a terminal capacity of 10mm^2 or more. You might well be able to get two 4mm^2 cables in, which would put things on much safer ground, certainly so far as points 1 and 2 are concerned. You could also consider reducing the circuit protective device rating from 45A to 32A (YIK that 24A + 13A 32A, but diversity is allowable here (first 10A plus 30% of the remainder = 18A). Another alternative is to supply the oven from a ring circuit - it may well come with a 13A plug on in any case - leaving only one cable to go from the cooker outlet to the hob. -- Andy |
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#11
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Andy Wade wrote:
[lots of good sense] To presume to summarise: 2.5mmsq is *not* against regs as an absolute matter. But it *often* will be, and to establish exactly which side of the go/no-go line your particular situation lies needs detailed design calculations. Moving up to 4mmsq pushes you usefully further away from that dividing line - though with the UK cables the earth conductor's no bigger in "normal" 4mmsq than in 2.5mmsq, so the earth loop impedance isn't helped away from the margin as much as one might like. Getting a few metres of 4mmsq or 6mmsq would be the simplest way of staying with the current circuit arrangment safely; if the 4mmsq is chosen, Andy's further suggestion of changing the MCB for a 32A is a further Win. However, gory stories of "2.5mm T&E will be no more than a memory and a paragraph in the fire brigade's incident report" are just that, and fail to distinguish between the role of fuses/MCBs in overcurrent versus fault current protection. With split-appliance ovens and hobs being as common as they are, you'd'a thunk the IEE might get round to putting out a Note about wiring 'em to existing cooker circuits. Nor would MK, Contactum and friends do us a disservice by marketing a dual 45A switch (in both "landscape" and "portrait" orientations!) with neat little "hob" and "oven" labels, to fit existing deep double boxes, which would give us two sets of switch terminals to wire our 2.5/4/6/10mmsq final-leg cable to. Stefek |
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#12
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Stefek Zaba wrote
Lot's of good stuff on the subject over the months. My situation is that i've just bought a monster hob (7.5KW) and Oven (3.6KW) Both have weedy flexes attached (Miele). At the moment they are just wired in parallel to the cooker conection unit which is backed up by a 40A MCB. I am not overly keen on this, but not sure what else to do. |
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#13
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Zikki Malambo wrote:
My situation is that i've just bought a monster hob (7.5KW) and Oven (3.6KW) Both have weedy flexes attached (Miele). At the moment they are just wired in parallel to the cooker conection unit which is backed up by a 40A MCB. I am not overly keen on this, but not sure what else to do. Weedy they may seem, but "presumably" they're appropriate to the load, with diversity through operation of thermostats in mind - or the (responsible) mfr wouldn't have fitted them. Where you're on less firm ground is their suitability for short-circuit protection with only a 40A MCB - since in Miele-land they may well've been designed with separate connection to (separate) radial circuits in mind - a 16A one for the oven, maybe 32A for the 7.5kW hob. For one thing, you'll do no harm by replacing the 40A MCB with a 32A unit - as the estimable Mr Wade reminded us only yesterday, the 'usual' diversity for the combined load is to reckon on the first 10A + 30% of the rest: the combined hob+oven add up to a nominal 11.1kW which means 48A 'gross', so that'd be 10 + (38*0.3) = 10 + 11.4 or 22.4A Diversified. That way you'll get the possibly more appropriate short-circuit protection for the hob flex, while still providing for Christmas-Day everything-on-the-go demand in practice. This wouldn't be appropriate if you knew the hobnoven were going to be used more intensively than in a normal domestic kitchen, but you've not mentioned anything like that. And I'm guessing that you don't feel like replacing the "wimpy" cables until some years after the Miele guarantee period expires, right? Stefek |
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#14
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Stefek Zaba wrote:
Moving up to 4mmsq pushes you usefully further away from that dividing line - though with the UK cables the earth conductor's no bigger in "normal" 4mmsq than in 2.5mmsq, so the earth loop impedance isn't helped away from the margin as much as one might like. Good point, sir. Andy's further suggestion of changing the MCB for a 32A is a further Win. With 32A protection and a 1.5mm^2 CPC the situation is the same as the unfused ring spur, so there's not very likely to be a problem. With split-appliance ovens and hobs being as common as they are, you'd'a thunk the IEE might get round to putting out a Note about wiring 'em to existing cooker circuits. Funny, there's an article covering good practice in kitchens in the current edition of the IEE's "Wiring Matters" which completely fails to address this point. Nor would MK, Contactum and friends do us a disservice by marketing a dual 45A switch (in both "landscape" and "portrait" orientations!) with neat little "hob" and "oven" labels, to fit existing deep double boxes, which would give us two sets of switch terminals to wire our 2.5/4/6/10mmsq final-leg cable to. Something similar went through my mind while I was writing that piece yesterday. I'm not sure there's a need for a double-gang switch (Appendix 8 allows one switch to control two 'cooking appliances'), but ISTM that there is a clear need for a double-gang cooker outlet. -- Andy |
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#15
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In article , Andy Wade
writes Stefek Zaba wrote: Moving up to 4mmsq pushes you usefully further away from that dividing line - though with the UK cables the earth conductor's no bigger in "normal" 4mmsq than in 2.5mmsq, so the earth loop impedance isn't helped away from the margin as much as one might like. Good point, sir. Chopped a bit... Thanks to all those who responded and those who e-mailed individually. Seems quite a can of worms this one. I have been told two conflicting things by electrical firms as well  No wonder poor old me is a tad confused!. I'm going to wire both to the same outlet plate in 4 mill. Seems that as some have said this needs clearing up a tad!.. cheers -- Tony Sayer |
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#16
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Stafek wrote....
Where you're on less firm ground is their suitability for short-circuit protection with only a 40A MCB snip For one thing, you'll do no harm by replacing the 40A MCB with a 32A unit. Bah! When my friendly local Sparky was round a couple of nights ago I asked him if he thought the 30A MCB which had been feeding the previous setup was man enough for Christmas Day, and he said no and fitted a 40A. I suppose he works on the basis he doesn't want to get called away from his Christmas Lunch! |
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#17
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Zikki Malambo wrote:
When my friendly local Sparky was round a couple of nights ago I asked him if he thought the 30A MCB which had been feeding the previous setup was man enough for Christmas Day, and he said no and fitted a 40A. I suppose he works on the basis he doesn't want to get called away from his Christmas Lunch! I think your assessment of his motives is just about right ;-) The whole business we're arguing through here is a little bit marginal. On the one hand, you'd find it really quite hard in practice to pull enough current from the hobnoven to make the 30A breaker trip - the closest you could get would be by having the appliance isolator off, turn all the controls to demand maximum heat (i.e. turn on all rings, + heaviest-draw setting of oven - prolly 'oven+browning-elements' if your Miele does that), and then turn on the isolator. That gets you all elements drawing their cold inrush currents at once - but they very soon get warm, settle down to "merely" their rated draw, and a little while after that start dropping in and out of the load as their thermostats, simmerstats, or fancy PWM-based regulators - hey, you did say this was top-of-the-line Miele kit! - start to do their stuff. As shewn in previous response, that "maximum sustained" draw would be 48A, barely 1.6 times the 30A MCB's nominal current. Yer typical Type B/type " domestickle MCB will take 15-30 minutes to trip on so "small" an overload - and it's really, really, hard to believe that your hobnoven will keep drawing their full rated load for that long, even when you do the turkey, sprouts, parboiling-of-the-roasties, sosij-meat-balls, resteam-the-pud, and the nut roast in the second oven for the non-carnivores. On the other hand, though it's correct to worry about the thermal behaviour of the cables/flexes supplying your hobnoven in the case of a hefty short, the conditions needed in practice for the larger trip (your 40A) to pass fault current for long enough to cause irreversible damage - or even notably shorter cable/flex life - are also unlikely to occur. Kitchens are typically not a huge distance from the CU; with say 10m of 6mmsq and a couple of "wimpy" 2.5mmsq flex at the end, total loop resistance down the 'worst case' path of L-to-E (so as to put the fault current down the thinner protective conductor of the 6mmsq) will be in the range of 0.1 ohms. Let's quadruple that to allow for increased contact resistance in a couple of connections along the way: that's still a rather low 0.4ohms resisting the 240V, giving us a rather-MCB-persuasive current of 600A. That's 15 times the nominal rating of the 40A MCB, which takes you into the fast solenoid-based 'gross overload' part of the MCB's operating regime (rather than the slow thermal-based 'smaller overload' part), so you'll get disconnection within 0.1s or so - not nearly long enough for your 'wimpy' cable to heat up towards the 140 or so degrees which is the 'don't go there' temp for PVC. From which semi-quantitative argument, we conclude that your sparky's uprating of the MCB from 30 to 40A - while not best practice - doesn't cause a keep-you-awake-at-night level of risk. HTH - Stefek |
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#18
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On Wed, 24 Nov 2004 11:11:02 +0000, tony sayer wrote:
We've got a caper on the go where we've got the above with a single switched 45 amp cooker outlet. Now I've been told that "as you can't get two bits of "cooker cable" into the outlet plate, the only way to do it is to run the oven off a 13 amp socket on the ring main, and wire the cooker onto the "cooker outlet" in "cooker cable", presumably the 6 mm stuff. Are you talking about a cooker control point (the bit with the switch) or a cooker connection unit? eg http://www.mkelectric.co.uk/products...4&rangeid=1030 Which is basically a junction box with front cable outlet. I didn't have any difficulty getting three 6mm cables into a connection unit (one from the switch, one to the hob, one to a socket to plug the oven into. |
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#19
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In article , John Armstrong
writes On Wed, 24 Nov 2004 11:11:02 +0000, tony sayer wrote: We've got a caper on the go where we've got the above with a single switched 45 amp cooker outlet. Now I've been told that "as you can't get two bits of "cooker cable" into the outlet plate, the only way to do it is to run the oven off a 13 amp socket on the ring main, and wire the cooker onto the "cooker outlet" in "cooker cable", presumably the 6 mm stuff. Are you talking about a cooker control point (the bit with the switch) or a cooker connection unit? eg http://www.mkelectric.co.uk/products...4&rangeid=1030 Which is basically a junction box with front cable outlet. I didn't have any difficulty getting three 6mm cables into a connection unit (one from the switch, one to the hob, one to a socket to plug the oven into. Yes, but ours was LeGrand.. Anyway all done and dusted now thankx!... -- Tony Sayer |
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#20
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Stefek Zaba wrote:
[...] and then turn on the isolator. That gets you all elements drawing their cold inrush currents at once - but they very soon get warm, settle down to "merely" their rated draw, [...] Nichrome has a very low temperature coefficient of resistivity - there is no significant "cold inrush current" - unless you're talking about tungsten-halogen heat sources... [...] barely 1.6 times the 30A MCB's nominal current. Yer typical Type B/type " domestickle MCB will take 15-30 minutes to trip on so "small" an overload [...] .... Which is not long enough to count as a long-term overload in the context of regulation 433-01-01, especially considering that this overload scenario would not be repeated on a regular basis. [Fault current] Kitchens are typically not a huge distance from the CU; with say 10m of 6mmsq and a couple of "wimpy" 2.5mmsq flex at the end, total loop resistance down the 'worst case' path of L-to-E (so as to put the fault current down the thinner protective conductor of the 6mmsq) will be in the range of 0.1 ohms. 0.15 ohm is nearer the mark for those lengths - Table 9A in the OSG gives a handy reference for 'R1 + R2' (phase + CPC) resistances. Let's quadruple that to allow for increased contact resistance in a couple of connections along the way: Eeeek - if you had 0.3 ohm contact resistance in a couple of connections you'd have over 300 W being dissipated in inappropriate places (at 32 A). The short-circuit then would be the one that occurred as a result of the fire... that's still a rather low 0.4ohms resisting the 240V, giving us a rather-MCB-persuasive current of 600A. You've forgotten the source impedance at the point of supply. The worst case condition here is 0.8 ohm external earth fault loop impedance (Ze) for TN-S earthing, so the total earth fault loop impedance (Zs) becomes 0.95 ohm, making the fault current only about 240 A. That's 15 times the nominal rating of the 40A MCB, which takes you into the fast solenoid-based 'gross overload' part of the MCB's operating regime (rather than the slow thermal-based 'smaller overload' part), so you'll get disconnection within 0.1s or so - not nearly long enough for your 'wimpy' cable to heat up towards the 140 or so degrees which is the 'don't go there' temp for PVC. From which semi-quantitative argument, we conclude that your sparky's uprating of the MCB from 30 to 40A - while not best practice - doesn't cause a keep-you-awake-at-night level of risk. It doesn't take you into risk at all if you do the sums, provided that the MCB is Type B. For Type B 'instant tripping' (i.e. 0.1 s) will take place at = 5 * In, which is 200 A for the 40 A device. Since our fault current is more than that, even with the worst case Ze value, we know that the fault will clear within 0.1 s and thus that the disconnection time requirement of BS 7671 is met. It now just remains to check that the 1.5mm^2 CPC in the bit of 2.5 T&E won't fry. The relevant limiting temperature for a PVC cable, by the way, is 160 deg. 140 deg. applies for cables over 300 mm^2, which would normally be thought of as beyond DIY territory :-). The simplest way to tackle this is to use the adiabatic equation backwards to determine for how long a 1.5 mm^2 conductor will stand 240 A. The equation is given in BS 7671 as S = sqrt(I^2 * t)/k. Turning it round to find t gives t = (k * S / I )^2. (k is the constant from Table 54C, S is the CSA of the conductor and I is the fault current.) Hence t = (115 * 1.5 / 240) which works out at a tad over half a second. With the B 40 A MCB we've already determined that the fault will clear in 0.1 s, so all is well. If you work through the same process using the same cable lengths, but for different devices you'll find as follows: - 30 A BS 1361 (cartridge) or BS 3036 (rewireable) fuse - OK - 45 A fuse of either kind - not OK - Type C MCB - not OK. If the supply is PME we can take the max. Ze value to be 0.35 ohm rather than 0.8. Zs now becomes 0.5 ohm and the fault current is 460 A. The 1.5 mm^2 CPC will stand this for 0.14 s. This doesn't change much though. The 45 A fuses would still fail to protect the CPC, but a Type C MCB would be OK (which is a bit academic because there's no reason whatever to need to use a Type C device in this application). There is now a clear answer to Tony's question (as modified): - provided that the original cooker circuit is wired in 6 (or 10) mm^2 T&E, and - the circuit length from the CU is not vastly in excess of 10 m, and - the circuit is protected by a 30 A fuse or 32 A or 40 A Type B MCB, - then the proposal to use short 4 mm^2 T&E cable 'tails' to the appliances is perfectly OK. (I've said 4 mm^2 here because of the concerns over the current rating of 2.5 when ambient temperature and grouping factors are applied.) QEF -- Andy |
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#21
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Andy Wade wrote:
Eeeek - if you had 0.3 ohm contact resistance in a couple of connections you'd have over 300 W being dissipated in inappropriate places (at 32 A). The short-circuit then would be the one that occurred as a result of the fire... You're right, of course - I was trying to handwave an "even if there's notable contact resistance along the way" without thinking at all seriously about probable values for same. You've forgotten the source impedance at the point of supply. The worst case condition here is 0.8 ohm external earth fault loop impedance (Ze) for TN-S earthing, so the total earth fault loop impedance (Zs) becomes 0.95 ohm, making the fault current only about 240 A. Again "d'oh" applies - thanks for picking it up. It doesn't take you into risk at all if you do the sums, provided that the MCB is Type B. For Type B 'instant tripping' (i.e. 0.1 s) will take place at = 5 * In, which is 200 A for the 40 A device. Since our fault current is more than that, even with the worst case Ze value, we know that the fault will clear within 0.1 s and thus that the disconnection time requirement of BS 7671 is met. It now just remains to check that the 1.5mm^2 CPC in the bit of 2.5 T&E won't fry. The relevant limiting temperature for a PVC cable, by the way, is 160 deg. 140 deg. applies for cables over 300 mm^2, which would normally be thought of as beyond DIY territory :-). The simplest way to tackle this is to use the adiabatic equation backwards to determine for how long a 1.5 mm^2 conductor will stand 240 A. The equation is given in BS 7671 as S = sqrt(I^2 * t)/k. Turning it round to find t gives t = (k * S / I )^2. (k is the constant from Table 54C, S is the CSA of the conductor and I is the fault current.) Hence t = (115 * 1.5 / 240) which works out at a tad over half a second. With the B 40 A MCB we've already determined that the fault will clear in 0.1 s, so all is well. If you work through the same process using the same cable lengths, but for different devices you'll find as follows: - 30 A BS 1361 (cartridge) or BS 3036 (rewireable) fuse - OK - 45 A fuse of either kind - not OK - Type C MCB - not OK. If the supply is PME we can take the max. Ze value to be 0.35 ohm rather than 0.8. Zs now becomes 0.5 ohm and the fault current is 460 A. The 1.5 mm^2 CPC will stand this for 0.14 s. This doesn't change much though. The 45 A fuses would still fail to protect the CPC, but a Type C MCB would be OK (which is a bit academic because there's no reason whatever to need to use a Type C device in this application). There is now a clear answer to Tony's question (as modified): - provided that the original cooker circuit is wired in 6 (or 10) mm^2 T&E, and - the circuit length from the CU is not vastly in excess of 10 m, and - the circuit is protected by a 30 A fuse or 32 A or 40 A Type B MCB, - then the proposal to use short 4 mm^2 T&E cable 'tails' to the appliances is perfectly OK. (I've said 4 mm^2 here because of the concerns over the current rating of 2.5 when ambient temperature and grouping factors are applied.) QEF Paul Cook couldn't've put it better ;-) Now, whether it's got too many Numbers, Equazhuns, and Square Roots to be circulated as-is in NICEIC helpful-hints is another question. Stefek |
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#22
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Stefek Zaba wrote:
Now, whether it's got too many Numbers, Equazhuns, and Square Roots to be circulated as-is in NICEIC helpful-hints is another question. What have they got to complain about: no complex numbers, no hyperbolic functions, no integral signs, no weird functions to have you reaching for Abramowitz & Stegun. Easy... -- Andy |
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