Author Topic: Junctioning 32A cooker circuit with a 13A spur.  (Read 10997 times)

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Online paulcaTopic starter

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Junctioning 32A cooker circuit with a 13A spur.
« on: February 09, 2022, 11:53:49 am »
I bought a new cooker hob and oven.  The new cooker hob is gas, but has a 13Amp fused plug on it for the ignitors.  The new oven is 16A has a plastic box with screw terminals inside for 6mm2 cable.

The incoming power is via a T&E 6mm2 coming from behind a cabinet.  This incoming line is isolated with a dual pole 40A isolator in the wall... as it should. And runs to a single 32A OCB (and the shared 100mA general trip).

The "correct", by the book, method is to replace the 6mm2 cable and it's outlet with a dual outlet plate (ideally independently fused).  Run 6mm2 to the 16A oven and 13Amp round flex to a trailing socket for the hob... or mount it in a cut out of box.

Of course a spark will charge me a small fortune for that, given the feed is located behind a kitchen corner unit where the "old" pre-kitchen refit cooker used to be and would need to be pulled to get behind it.

So, my "not always to code" Spark who I get to odd jobs and network cabling, advised...

A 60A rugby-ball style junction box.  6mm2 in, 6mm2 + stranded flex out.  Those boxes have really beefy through tube terminals with 4 screws per tube.  He suggests if I strip the 13A flex really far back, fold it over, so it's at least half the length of the terminal, then slide it "underneath" the 6mm and clamp the 6mm2 down on top of it with the terminal screws. 

My only concern with this is the trailing socket flex is 13Amp.  The circuit feeding it is 32Amp.  It's not a problem of load, the gas hob pulls less than a watt and very rarely when the ignitors are used.  The issue is the 13 amp cable in a 32Amp circuit.  Sure the hob is fused, as long as I actually use a socket and retain the plug's fuse.  But if something bad where to happen to that socket, like water ingress (it IS a kitchen), or someone put a drill through it, the 32A could melt and set fire to stuff running down that 13Amp cable.

I think his response was that this is not worth worrying about and I'd be fine with the flex as long as the plug connected to it is fused.

The even lazier approach is to connect the 6mm2 to the new oven and shove the bare tails from the hob in there too.  How much damage would 32Amp fault current do to a 13Amp flex before the 32OCB goes?  Worth the risk?  I mean it's not "inaccessible" and smoke would be noticed.  The oven can be pulled in a minutes.
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Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #1 on: February 09, 2022, 11:56:59 am »
Is it feasible to consider running 6mm2 through a standard UK wall socket?  Would 2x6mm2 fit into the terminals?  That way I could stay 6mm2 through out and use the plug socket as the junction box.  I know they get used with 30Amp mains rings all the time... but would it be wise to attempt this?
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Offline david77

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #2 on: February 09, 2022, 12:12:02 pm »
I know nothing about UK wiring rules but I'd do it this way.

Put a small ditribution panel behind the kitchen like this https://www.spelsberg.co.uk/small-distribution-boards/ak-compact-3-24te/73540301/ run the 6mm² feed in there and junction it off to an outgoing 6mm² to the oven. Additionally put a B13 MCB in there that goes out to the hob.

If there's a fault with the hob the B13 will trip before the 32Amp breaker in the CU.
 
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Online Zero999

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #3 on: February 09, 2022, 01:17:43 pm »
The 13A cable should be fused at 13A. You could connect wire it to the same feed as the oven, via a 13A fused spur.
 
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Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #4 on: February 09, 2022, 01:18:31 pm »
Interesting ideal.  Assuming I can find room for it.

Something like this:
https://www.screwfix.com/p/chint-nx3-5-module-2-way-populated-garage-consumer-unit/9864v

I can even just use those modules.  The gas hob will not use 6A and the cooker is supposed to be a 16A fused appliance anyway.

The cavity is pretty tight though.
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Offline DenzilPenberthy

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #5 on: February 09, 2022, 01:27:32 pm »
Just use a cooker isolator that has a built in socket. Like this one.

Feed the oven from the isolator and plug the hob into the 13A socket, then its flex is protected by the fuse in the plug.

https://www.tlc-direct.co.uk/Products/AA45CCU.html

 

Offline themadhippy

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #6 on: February 09, 2022, 02:47:39 pm »
use the existing 6mm and 40A  isolator for the oven,making sure to use 6mm from the  isolator to the oven,and  loop another 6mm (you can use 4mm if its a 32A breaker)from the supply side of the isolator  into  either switched a fused spur or single socket  for the hob.No regs will be broken by this method,but getting 2 6mms into an isolators terminals can be fun
Quote
his response was that this is not worth worrying about and I'd be fine
my response would be time to find another tame sparky who knows what there  doing
« Last Edit: February 09, 2022, 02:59:14 pm by themadhippy »
 

Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #7 on: February 09, 2022, 03:29:54 pm »
Just use a cooker isolator that has a built in socket. Like this one.

That would work, but... the hob flex is underneath the worktop.  So would need drilled up through and replugged.  That's fine.

Second problem is I would need to cut the single tall vertical box out and replace it with a double box, replaster and retile.  If I was doing a full refit of the cabinets/tiles the I would consider this.
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Offline themadhippy

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #8 on: February 09, 2022, 03:46:36 pm »
Quote
Second problem is I would need to cut the single tall vertical box out and replace it with a double box,
or use a contactum X1457
 

Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #9 on: February 09, 2022, 04:27:48 pm »
I just checked, it's a single box, not a double height, so screwed there too
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Offline themadhippy

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #10 on: February 09, 2022, 04:34:49 pm »
so its a standard single box? is is buried in the wall or surface?
Either way it dont make any difference,if its a surface pattress whip it of and replace with a 47mm double pattress and fit a new cooker control unit,if its flush and you dont mid the new box being surface, whip of the switch,fit a 47mm double pattress and new ccu
« Last Edit: February 09, 2022, 04:42:29 pm by themadhippy »
 

Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #11 on: February 10, 2022, 09:37:11 am »
so its a standard single box? is is buried in the wall or surface?
Either way it dont make any difference,if its a surface pattress whip it of and replace with a 47mm double pattress and fit a new cooker control unit,if its flush and you dont mid the new box being surface, whip of the switch,fit a 47mm double pattress and new ccu

Buried in a tiled wall.

Interesting.  If I take the existing switch off, I should expect to find at least a plastic or metal back box...  I should be able to find a way to fix the new surface mount box onto the metal box's lugs or through it into the wall if I have to and as long as the new box is held on tight it should sit fine on the surface allowing the double CCU with the plug to go there.

I still have options and time, oven is delivered Tuesday and as I accidentally paid £44 to have it "installed" by their experts, I might actually leave the work until after that, so they will say, "Sorry our experts could not fit your cooker due to inadequate electrical installation... we are refunding the install fee."... you can hope, right?
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Offline MadScientist

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #12 on: February 10, 2022, 09:52:50 am »
I don’t see the problem , spur off to a  13A fused outlet . Fuse the spur as well
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Offline themadhippy

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #13 on: February 10, 2022, 11:39:47 am »
Quote
should expect to find at least a plastic or metal back box...
my moneys on metal,unless its a very old installation
Quote
  I should be able to find a way to fix the new surface mount box onto the metal box's lugs
yes,theres even boxes designed for just this,for example
https://www.toolstation.com/axiom-pattress-box-conversion/p80152
 but  32mm is a a bit tight for cooker control units,you really want 45mm for an easy life,just make sure its got fixing holes in the back of the box that will line up with lugs on a single box,not all do,this looks like it might do the job
https://www.screwfix.com/p/british-general-2-gang-surface-pattress-surface-box-47mm/714ht#product_additional_details_container
Quote
Sorry our experts could not fit your cooker due to inadequate electrical installation... we are refunding the install fee."...
What will really happen is they will bodge it in any way they can
« Last Edit: February 10, 2022, 11:41:47 am by themadhippy »
 

Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #14 on: February 10, 2022, 11:56:30 am »
I don’t see the problem , spur off to a  13A fused outlet . Fuse the spur as well

Yes.  That is one option.  However it requires a junction at some point.  At the point is drops to 13A the fuse needs to be present between the 32A circuit and the 13A one.  I don't find junction boxes with fuses, so that only leaves the 2 way consumer unit with 2 fused outputs the "best" option.

The cowboy approach is to accept the risk of the 32A circuit spuring to a 13Amp outlet with no fuse.  Knowing the only thing plugged in there has a 13Amp fuse in it's plug.  The risk is the 13A flex could technically overload between the 6mm connection and the fuse in the appliance plug.  That is extremely remote risk, the result would potentially be a hidden smoldering cable and or connections, which may persist to fire in the unlikely event the heat melts some flammable plastic, paint or varnish.  My tame spark says that is done all the time.  That is obviously tame spark code for, it's not how it's meant to be done.

Finally I have the easiest option of all, but it's a bit janky.  I can just run the hob off the plug socket for the dishwasher and a double extension.  The janky bit is, when someone flips the isolator for the cooker, it will NOT isolate the hob.  I can put a sticky label on the isolator that says "OVEN ONLY" and on the dishwasher spur:  DISHWASHER + HOB.  Would that cut it?  Seems the easiest option.
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Offline themadhippy

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #15 on: February 10, 2022, 12:09:01 pm »
Quote
At the point is drops to 13A the fuse needs to be present between the 32A circuit and the 13A one.  I don't find junction boxes with fuses, so that only leaves the 2 way consumer unit with 2 fused outputs the "best" option
The only time you need to fuse down is when the current carrying capacity of the circuit  is reduced,and thats exactly what the fused spur is doing,the point were the circuit splits into 2 doesn't need protecting if the junction box and cables are rated for whatever size fuse if feeding them
Quote
I can just run the hob off the plug socket for the dishwasher and a double extension
best stock up on double extension leads and 13A fuses
 

Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #16 on: February 10, 2022, 12:41:14 pm »
Quote
At the point is drops to 13A the fuse needs to be present between the 32A circuit and the 13A one.  I don't find junction boxes with fuses, so that only leaves the 2 way consumer unit with 2 fused outputs the "best" option
The only time you need to fuse down is when the current carrying capacity of the circuit  is reduced,and thats exactly what the fused spur is doing,the point were the circuit splits into 2 doesn't need protecting if the junction box and cables are rated for whatever size fuse if feeding them
Quote
I can just run the hob off the plug socket for the dishwasher and a double extension
best stock up on double extension leads and 13A fuses

Yes, the feeding circuit is 32A on 6mm cable.  You mention that the point at which the cable splits does not need protecting as long as the cables it splits into being capable of carrying the full feed circuit current. A fused spur would need to be fed with the 6mm cable then.  That still involves trying to spur, split or junction the 6mm cable. 

The old hob had a junction box on it, using brass bus clips to join the incoming 6mm to an outgoing 13Amp flex to the over, ironically the previous OVEN was 13A.  No fuse.  In fact their bus contacts where so cheap and flimsy there was evidence of burning and high heat around the live connections.

On the dishwasher extension... the new hob is gas.  It only has a mains plug for purposes of running the spark ignitors.  That' can't be more than 1 watt instantaneous load?
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Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #17 on: February 10, 2022, 12:43:13 pm »
So we have established various options with different amounts of "correctness".

My plan now is to wait on the oven arriving and if it happens to have a junction box for the 6mm and implies I can link the hob to it, then that is what I will do.

If the oven has no provision to share it's junction box with the hob, I will wire the hob to the dishwasher spur "for now"... "permanently temporarily" with the aim to fix it up properly later.
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Offline themadhippy

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #18 on: February 10, 2022, 01:18:20 pm »
Quote
junction the 6mm cable
yes
Quote
I will wire the hob to the dishwasher spur "for now"
whip the spur off  and fit a double socket,job done, or  fit another spur beside the existing,if the existing is flush you could use a surface dual accessory box fit the existing spur oneside and the new spur/single socket on the other
 

Offline dmills

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #19 on: February 10, 2022, 01:46:05 pm »
I think you are over thinking this.

A standard UK ring circuit (It is a weird UK thing for the non UK crowd) is typically fed by a 32A breaker, and nobody worries about a 2.5mm spur to a single socket.
Seems to me that the 6mm 32A line to the oven switch with a short 2.5mm spur to a socket (or more usefully, a switched fused spur for the igniters) is very much equivalent and should be just fine.

Or just change the breaker to a 16A one at the DB and call it good, I don't see the issue. 

 
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Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #20 on: February 10, 2022, 02:01:00 pm »
A standard UK ring circuit (It is a weird UK thing for the non UK crowd) is typically fed by a 32A breaker, and nobody worries about a 2.5mm spur to a single socket.
Seems to me that the 6mm 32A line to the oven switch with a short 2.5mm spur to a socket (or more usefully, a switched fused spur for the igniters) is very much equivalent and should be just fine.

Or just change the breaker to a 16A one at the DB and call it good, I don't see the issue.

You have a point.  I know single spurs off the ring on 2.5mm are common and as you point out it's basically the same thing.  If someone puts a drill through that 2.5mm cable it will get give 32Amps of current from the ring until its breaker pops.

Also, good point about the 16A breaker.  The oven only needs a 16A fuse, the ignitors are negligible and I can more easily use 16A flex for the socket spur and let the hob fuse it's self at 13Amp.

The 60A junction box should be grand for creating the spur.
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Offline themadhippy

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #21 on: February 10, 2022, 03:50:54 pm »
Quote
Seems to me that the 6mm 32A line to the oven switch with a short 2.5mm spur to a socket (or more usefully, a switched fused spur for the igniters) is very much equivalent and should be just fine
Indeed,but come the time you want to sell the place and mr cartel member turns up to do your electrical safety certificate the potential buyer is paying for he's going to say no because it isnt in the big expensive book of rules,and quote a rip off price to "make it safe"
 

Online paulcaTopic starter

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #22 on: February 10, 2022, 04:11:29 pm »
Quote
Seems to me that the 6mm 32A line to the oven switch with a short 2.5mm spur to a socket (or more usefully, a switched fused spur for the igniters) is very much equivalent and should be just fine
Indeed,but come the time you want to sell the place and mr cartel member turns up to do your electrical safety certificate the potential buyer is paying for he's going to say no because it isnt in the big expensive book of rules,and quote a rip off price to "make it safe"

That's easy to flip the other way.  (1) it's on the end of a cable, it's consumer equipment and not installation work.  (2) "Fine, I'll remove the cooker/appliances from the sale and remove them on my departure"

It's sort of why I like the more cowboy approach as it's IS just consumer side add ons.  A 13A extension lead for the hob and the 6mm tail to the cooker.  I can switch back to all electric or leave it as it was later.  If I start installing actual spurs and fused/switched spur boxes, THEN it WILL have to be to code as it IS installation work.... and if I change my mind later and go back to electric, or switch to induction, I don't need to rip it out and resinstall another setup.

EDIT:  I think the terms "TO CODE" and "SAFE" and both usable.  But many SAFE installations would not be "TO CODE".  All I really want is SAFE.  SAFE enough that an actual spark would frown, but be happy enough, rather than backing away and refusing to work on it. 

And I don't want to set my kitchen on fire either.
« Last Edit: February 10, 2022, 04:16:10 pm by paulca »
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Offline mikeselectricstuff

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #23 on: February 10, 2022, 04:46:57 pm »
UK Regs do allow cable to be protected by fusing at the far end under certain circumstances - I'm not sufficiently familiar with the details to know if this would apply here.

Why does the cooker socket need to be on flex ? 4mm cable to that socket would be fine, assuming you can get a 6 and a 4 into the terminals of the isolator.

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Offline richard.cs

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Re: Junctioning 32A cooker circuit with a 13A spur.
« Reply #24 on: February 10, 2022, 05:10:13 pm »
UK Regs do allow cable to be protected by fusing at the far end under certain circumstances - I'm not sufficiently familiar with the details to know if this would apply here.

This is exactly what applies here, and why 2.5 mm2 spurs are allowed on a 32 A circuit. You have to separately meet these two criteria:
1. Protection against overload unless the load by its nature can't fail so as to produce overload. This can be at any point, so it is met by a downstream 13 A or smaller fuse.
2. Protection against short circuit, assumed to be a zero-Ohm short at the most distant point.

There are some assumptions baked into this, but the end result is you can take a 13 A rated flex* off a 32 A circuit to feed the hob, and connect the hob via a FCU** or a 13A plug, (probably with a 3 A fuse, that's plenty***, but you may be able to get away with higher). That fuse protects the cable against overload, because if some weird fault were to make the hob igniter draw say 20 A the fuse would blow before the cable is damaged. Separately, if a short circuit were to occur at the last point in that cable before the fuse, the total loop resistance (external supply impedance + 6mm cable + flex) needs to be low enough that the 32 A MCB would trip quickly enough to protect the cable. You check this by looking at the expected fault current, and corresponding max tripping time, and then you can work out the let through energy and the maximum temperature the cable gets to. You or your electrician should do this calculation, but in practice you'll find it is fine for sensible lengths of flex (too long and the MCB trips too slowly and the let though energy and final temperature is higher).

UK wiring regs are like this, you're expected to do actual engineering calculations rather than the US "rigidly follow code" approach. There are upsides and downsides to this. :)

Also, why not reduce the MCB feeding the 6mm cable to 16 A anyway? You may find the manufacturer's instructions (which you're supposed to "take account of") require that anyway, and that is a cheap and straightforward job.

*Why flex here though? A bit of 1.5 or 2.5 mm2 T&E to a FCU would seem more conventional. If you do use flex you should really use ferrules.
**FCU = fused connection unit. It's a thing we have in the UK that takes the same type of fuses as the plugs but is for permanently installed things.
*** These ignition things take a couple of milliamps.
 


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