Replacing Gas Hob With Induction Hob.

[mikhailfaradayski]

Hi Col,

I think the bumper book of fun is so vague in this respect, they might aswell not have bothered putting it in.
WRT to cable sizes, 'Economic' = dont go too big, 'reliable' = dont go too small.

I think the OSG app.1 serves two things,
1, assesses max demand for a designer/builder to give info to the DNO for required feed size for new service connections.
2, assesses realistic design currents to be used for individual circuit cables within any given installation, taking into account duty cycle and likely operating temperatures in practice.

I agree that the cable/cpd must be rated to the maximum 'design' current, but design current is derived from the appliances rated current with diversity applied.

Having said all that, i think anyone who tries to employ every twist and turn to reduce cable size is not really foreseeing a reasonable change in likely loadings throughout the expected life span of the installation.

sorry if it seems like im teaching granny to suck eggs, i don't mean to sound that way, but diversity is often misunderstood by even highly experienced sparks and is worthy of clarification.

 

[ColJack]

design current is what the power rating says divided by the voltage..

a 10.4KW cooker..

10400W/240V = 43.3A
43.3A-10A = 33.3A
33.3A x 0.3 = 10A
10A + 10A = 20A..
1.5mm² T+E will take 20A clipped direct.. so I'll install 1.5mm² T+E and put a 20A breaker in the board for a 43.3A cooker.... ?
according to diversity it's ok....

 

[mikhailfaradayski]

I think we have a subtle disagreement.
My opinion is that the maximum instantaneous demand of the device is as you suggest, but the design current of the circuit supplying that device can have diversity applied to it.
BTW, socket included or not, i always add the 5A because its just too easy to change the CCO afterwards.

Given your example of 10.4kW oven, using Uo=230V, i arrive at a design current of 26.1A (as mentioned, socket outlet include in calcs).

Clipped direct would call for 4mm.
ref 100 or 102 would call for 6mm
ref 101 or 103 would call for 10mm.

60898 B32 as CPD, IMO this would hold 42A for 16mins without tripping.

Although personally i dont like electric hobs, if you were to install one and go from cold to full blast, i would rather hope that the time taken to get to temp would be far less than this. Then the cyclic nature of the load throughout the duration of cooking would most probably be less than 26A on average.

Looking at it from a different perspective, I would hate to think that if i was reducing my braised lamb shanks in red wine and rosemary jus for 2 hours, at a mid temperature (say 50%), i've just used 10.4kW/h. That could be awfully expensive in the long run

Diversity being the black art that it is, its always open to differing opinions. I guess thats why we have jobs as opposed to the spotty teenager at our much maligned DIY stores just doling the info out willy and indeed nilly

 

[mikhailfaradayski]

actually, i think my calcs might be slightly out ( should be 25.5A? ) would someone else care to verify?

Also, thinking on a bit about whether 230V or 240V is more appropriate for calculations, i understand about how ohms law affects currnt over a fixed resistance, but how is the currnt affected given that we are most probably talking about an induction hob?
Does this confrom to the same rules as a purely resistive load?

 

[ban-all-sheds]

BTW, socket included or not, i always add the 5A because its just too easy to change the CCO afterwards.

So yet more diversity - when in use that socket could have a 2-3kW load plugged in, so add that to your 10.4kW cooker and you're looking at a total of around 13kW....

 

[mikhailfaradayski]

possibly, but like i say, if the socket is there or not, i always alloew for it in the calc.

Whats your opinion on this subject as a whole?

 

[ban-all-sheds]

I vote for ColJack.

 

[mikhailfaradayski]

no problem, would you care to expand on that?

What are your thoughts on the way the OSG illustrates application of diversity?

 

[ban-all-sheds]

I don't think that using the OSG formula for diversity on a cooker final circuit is a good idea - it's too extreme.

Ideally I'd say don't use it at all at final circuit level unless you really have to, and then back off as little as possible. The economic aspect of the design is not significantly changed by using 10mm² instead of 6mm², and a B40 instead of a B32.

There's a useful chapter on diversity in this:



which also (to the chagrin of some on here, I'm sure) says that it might often be better to use 240V in your calculations, not 230V....

 

[ban-all-sheds]

Electric induction hobs are faster, and put less heat into the kitchen and have no indecent parts so can't cause a fire.

I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.
I will not do a Google image search for indecent parts in the kitchen.

 

[mikhailfaradayski]

I tentatively think we agree

I dont have that book myself, could you give a brief synopsis of it's guidance on diversity? taking the exaple as given above.

I also agree, as mentioned, that someone who tries to apply diversity to really wittle a few quid out of a domestic job is not really doing a great job. (larger industrial stuff is a different ball game altogether, un-necessarily oversizing cables could cost thousands and then some)

I understand that Uo=230V, but i also undertsand that we still actually have 240V here in the UK.
So, what do we do? Use the regs that we are all generally working to, or make a reasonable departure from the regs based on what we know?

I am still considering the effects of this difference on the design of a supply circuit to a device which may not be wholly resistive. What are your thoughts on this?


edited for spelling

 

[Mark Elvin]

Well I've decided against adding an extra circuit as the cost would be prohibitive & I've bought a gas through glass hob insted.

 

[ColJack]

gas through glass? must google that one..

the OSG even says that the figures given are guidance only and that it's down to the individual designer to decide what figures to use..

 

[mikhailfaradayski]

HI Mark,

Do you mean something like this?

http://www.kitchenappliancecentre.co.uk/cda-four-burner-bevelled-glass-gas-hob.html

HI Col,

Agreed

 

[Mark Elvin]

Nope, this - http://www.tomorrowskitchen.co.nz/#izona-products/cook-surface

That's if I can get a supplier, no one, not even the manufacturer saem bothered about selling me one.