Cooker Circuit connundrum

[Stewart Bray]

I am re wiring the consumer unit in my daughters house, replacing fuses with a split load consumer unit. Replacing 5 amp lighting circuit with 6 amp mcb, and 32 amp rings with 32 amp mcbs is not a problem.
However, the cooker circuit is currently rated at 45 amps, which seems a hell of a lot of power for a 4 plate cooker with grill and oven. I know that unlike most other circuits cookers can be (easily) 90% loaded for Xmas dinner for example, but my question is "as there are no 45 amp mcbs, would it be better to fit a 32 amp mcb, risking the odd trip out under full load, or fit a 50 amp mcb". The cable from consumer unit to cooker is about 4 metres long and is 6mm x sectional area. What the max safe load for 6 mm - I thought about 8 kw?

 

[plugwash]

you can get 45A (and 40A i think) mcbs at least in the better brands like MK

going on the in the wasll figures as most cable has that installation method for at least part of the way 6mm only has a rating of 32A

we need to know the rating of the cooker before we can advise further but it may well be the cable is undersized

 

[BR]

Generally speaking the max loading for 6mmsq T&E is 40A about 9.5KW
10mmsq T&E is rated for 45A. Your cooker is as you say an appliance which potentially be loaded to 90% or more and so it is advisable to fit an MCB rated at the full load of the cooker.

MCB's ARE available in 45A, but you need to see do you really need this. If possible you should look at the rear of the cooker and see can you find a nameplate with it's max loading written on it.

As regards fitting a significantly lower rated MCB to the circuit, i wouldn’t advise it. MCB's are not designed for constant tripping like this and will likely 'burn out' relatively quickly.

40A MCB's are also available and my bet is that is what you will require. BUT check the rating on the back of the cooker.

Taken in consideration the short run of cable, it would be possible to use a 45A on the 6mmsq but only as a last resort. i.e. if the cooker is rated at 45A and it is not possible to replace the cable with 10mmsq.

Hope this is of help,
Any other questions regarding your new CU, feel free to ask.
Bryan.

 

[ban-all-sheds]

However, the cooker circuit is currently rated at 45 amps, which seems a hell of a lot of power for a 4 plate cooker with grill and oven.

Check the data plate on the cooker - see what it is. But in any event, the fuse is there to protect the cable - all you can tell from that is that the cooker takes less than 45A.

"as there are no 45 amp mcbs,

The MK range includes one.

would it be better to fit a 32 amp mcb, risking the odd trip out under full load, or fit a 50 amp mcb". The cable from consumer unit to cooker is about 4 metres long and is 6mm x sectional area. What the max safe load for 6 mm - I thought about 8 kw?

You can't put a 50A MCB on a 6mm cable. Even 45A might be pushing it - depending on the installation method, 6mm is rated at 32 - 47A.

You have to work out 2 currents:

1) Ib - how much current the cooker will draw

2) Iz - the current carrying capacity of the cable

If Ib>Iz, you're stuffed, but assuming it isn't, then you need to fit an MCB of rating In, such that

Ib <= In <= Iz

It is worth pointing out that you should ensure that the designed rating of a circuit is high enough to include all foreseeable peak loads of a protracted nature, and that is is never acceptable to use a fuse or circuit breaker as a load limiting device.

 

[BR]

going on the in the wasll figures as most cable has that installation method for at least part of the way 6mm only has a rating of 32A

We'll strictly speaking maybe, but it can generally be accepted that 6mmsq is rated at 40A in a domestic installation. Also taken into consideration the very short run in this case, 40A is quite safe in my opinion.

out of curiosity, what is the MAX rating for 6mmsq in other installation methods (e.g. clipped to the wall) . Just we have different standards here, that’s why I’m asking. About 47A??

 

[plugwash]

the figures i have for 6mm pvc twin and earth are

32 in a wall
38 in surface conduit
46 clipped to the surface
51 free (on cable tray or supported between points in free air)

 

[BR]

Thanks plugwash

 

[ban-all-sheds]

the figures i have for 6mm pvc twin and earth are

32 in a wall
38 in surface conduit
46 clipped to the surface
51 free (on cable tray or supported between points in free air)

Have those b*gg*rs changed their minds again?

Thought Table 4D5A said 47A for clipped......

(Not that it really matters in practice - if you're fretting about whether 46A will be enough, I submit you should be using 10mm.....)

 

[Stewart Bray]

I want to thank all contributors with regard to my post.
In fact the cooker cable is under floor, which is well ventilated, and so it looks as if its 45 amp wired fuse is justified. Fitting a replacement 50 amp mcb seems to be possible given that the cable is essentially in free air, and therefore able to dissipate heat.
What I had failed to mention is that my daughter rarely cooks anything more than simple dishes, and so the chances of the cooker on 100% load are as about as great as Mars as a Home Posting

 

[dingbat]

4D5A was added to the blue edition (we've all got brown ones now, haven't we? ) specifically to cover typical domestic installations. Plug's 32A rating is for cable "enclosed in conduit in an insulated wall", whereas Ref Method 1 (clipped direct) gives a ccc of 47A. However, if you consult Table 4A1, "Sheathed cables embedded directly in masonry, brickwork, concrete, plaster or the like (other than thermally insulating materials)" can use Ref Method 1.

This obviously leaves a few questions - what effect does capping have? If you have thermalite bock on one side and plaster on the other, should you use a correction factor? There's a hell of a difference between 32A and 47A. Does NICEIC have a definitive rule here?

 

[ban-all-sheds]

I want to thank all contributors with regard to my post.
In fact the cooker cable is under floor,

What - every inch of it? Are the CU and cooker under the floor as well? I can't help wondering if some of the cable must be in, or at the very least on, a wall....

which is well ventilated, and so it looks as if its 45 amp wired fuse is justified. Fitting a replacement 50 amp mcb seems to be possible given that the cable is essentially in free air, and therefore able to dissipate heat.

I would really advise against that - remember a 50A breaker will allow 72.5A to flow for an hour, and 56.5A to flow indefinitely.

What I had failed to mention is that my daughter rarely cooks anything more than simple dishes, and so the chances of the cooker on 100% load are as about as great as Mars as a Home Posting

The guideline for a cooker circuit is (10A) + (30% of the max load in excess of 10A) + (5A if there is a socket outlet in the control unit). If what you say is true, then you'd probably be justified in using that formula to verify that 6mm cable is OK, and to choose the MCB rating.

 

[plugwash]

for now sure but what will that flat be used for in future?

it is not that abnormal for a cooker to be fully loaded so imo cooker cuircuits should be rated to run the entire cooker

 

[ban-all-sheds]

I absolutely agree, but I was trying to steer Stewart away from putting a 50A breaker on his 6mm cable.

If in his situation he doesn't need to design for the full load of the cooker, he can put a more realistic breaker in place.