Double Oven supply

[hightower]

32a dedicated rfc to grid switch. Rfc does nothing but this grid. Grid switch then has 4 x 20a DP switches and 4 x fuse modules. They supply a fridge, a freezer, a microwave (not in use), and an oven socket outlet respectively.

From the grid switch each run to their respective appliance is 2.5mm radial (or fused spur rather).

Single oven is shot and they now want double oven. Double oven is 4400w, so about 19A however taking in to account diversity it is 12.7A. In my mind this is fine swapping the socket outlet for a cooker connection plate and dropping it in, leaving it inline with the original DP switch and 13a fuse but would anyone have any objections to this?

 

[Rpa07]

I'm good!

 

[Wilko]

13A fuse might get toasted ?
Edit : just looked up fuse spec and seems ok. But I do worry about the ability of modern fittings to take rated current ...

 

[hightower]

13A fuse might get toasted ?

Well I've looked at charts and from what I understand it would sit there all day at 20a. Surely it's no difference than putting a 32a breaker in for a big cooker that pulls 40a at full load - only gonna pull full load for a few minutes.

 

[hightower]

By the way, MI say nothing of how to connect except for the cable type, so not imprisoned by their rules either.

 

[Richard Burns]

Operationally this would work but fuse modules are normally tightly designed to 13A loads so I would expect that this would melt the module over time, as per immersion heaters on FCUs.
The recommendation (Appendix 15) is that cookers rated over 2kW are supplied from their own circuit.
Professionally you cannot design a circuit where the design current exceeds that of the protective device, for a normal cooker circuit up to 15kW and applying diversity the load would be deemed 28A and so less than the 32A MCB. I get a diversity value for 4.4kW of 13.04A over the fuse rating (just).
Possibly you could rewire the grid switch to eliminate the fuse module and only retain the 20A DP switch as a cooker isolator and this would avoid the possibility of overloading the module and would then leave you with an unfused spur pulling 20A, you would still be pushing the recommendation of the 2kW loading but the design of the "ring" (as it is effectively a parallel radial with four branches) would mean there is just a concentrated load at a single point of the ring.
I would prefer a separate circuit, but it is just about OK in some ways.

 

[Rpa07]

I'm still good but I like the 20a DP in the grid without the fused protection, blank grid to take its place - good advice.

 

[hightower]

Thanks Lee, good post as always. I had considered removing the fuse module as you've covered. But some counters if I may - the ring is dedicated to this grid so there are no other points that could unbalance the ring. The grid is the only point and therefore the half way point. Also, using 230v as per the book, my diversity values are 12.7A so would be interested to know how you arrive at 13.04

 

[Richard Burns]

Not often called Lee, but anyway, the grid does not have to be at the half way point, but if it is wired as you described and as I assumed i.e. a ring where the cables run on the same route to the grid and is therefore a parallel wired radial then it is at the midpoint and there is no unbalancing.
I made a maths error on the diversity calculation; you are correct 12.7A so within the 13A fuse rating, however it would probably still suffer on being used at above 13A for a short time if both ovens are on, but it would be technically within the limits of design, apologies for the error.

 

[hightower]

Not often called Lee, but anyway, the grid does not have to be at the half way point, but if it is wired as you described and as I assumed i.e. a ring where the cables run on the same route to the grid and is therefore a parallel wired radial then it is at the midpoint and there is no unbalancing.
I made a maths error on the diversity calculation; you are correct 12.7A so within the 13A fuse rating, however it would probably still suffer on being used at above 13A for a short time if both ovens are on, but it would be technically within the limits of design, apologies for the error.

Thanks Richard, and apologies for mine - I'd had a few last night

 

[D00DY_T00DY]

Well I've looked at charts and from what I understand it would sit there all day at 20a. Surely it's no difference than putting a 32a breaker in for a big cooker that pulls 40a at full load - only gonna pull full load for a few minutes.

Can't see a problem like you said it's very rare everything's going to be on full whack.
However if they're cooking a crimbo dinner for all the family it may end up with the fuse blowing on Christmas day lol

 

[hightower]

However if they're cooking a crimbo dinner for all the family it may end up with the fuse blowing on Christmas day lol

How? A 13A fuse can take 20A continuous without even thinking about failing. A 4400W oven would pull ~19A on full load.

 

[D00DY_T00DY]

How? A 13A fuse can take 20A continuous without even thinking about failing. A 4400W oven would pull ~19A on full load.

Go easy on me haha
Am I right in thinking a fuse can take 1.5x it's rating for once hour before tripping?
I know it's 1.45x used in calculations for CBs so that would be 19a for one hour...

Just realised it's an double oven and not a stand alone oven and hob?
I was thinking of all the rings being on for their spuds and brousle sprouts as well as cooking everything else.

It's rare an oven is put on over half its power so taking diversity into account shouldn't be a problem

 

[Des 56]

The mention of the oven being on a dedicated circuit makes me think of the dedicated circuit where the cooker control already has a build in socket
Supplying a load from this dedicated circuit via a socket has never been a problem

The 5mm supply to the grid with its double pole for the oven and the switching for the 2 appliances seems to be little different in its design to the customary set up

 

[Resu]

Looking at it from a different angle you could split the ring into two 16/20A radials (depending on the installation method) 1st one to feed the other 3 appliances and the second would give you a dedicated supply for the double oven.