Is this correct for cooking appliances? What are the principles behind diversity calculations

[David Costelloe]

I'd be grateful for any comments on this proposed design. Is it OK from a regs viewpoint to have DP isolators downstream from the dual appliance connector?

As an alternative, could I just run a new cable back and power it from the same 45A MCB (no spares on the existing CU)?

I'm also curious about diversity. I'm guessing when we say a cable or MCB is rated at 45A, it is rated to be able to operate at that load continuously. But there is a different "unpublished" rating for peaky loads, which is what the diversity calculations is about. That is, for short periods the cable can carry in excess of that load. For example you put on a few rings on the hob and the two ovens, you are drawing well in excess of the "rated" load, but soon the cookers' thermostatic controls kick, turning the heating elements on and off. So 10A is the sustained load from the cookers, and up to 30% of the time, load can flex up to the peak load? Or something like that.

 

[David Costelloe]

 

[EFLImpudence]

As an alternative, could I just run a new cable back and power it from the same 45A MCB (no spares on the existing CU)?

If you want to.

Are the cooker and hob to be fitted near each other?
Just take the new cable from the oven outlet plate.

I'm also curious about diversity. I'm guessing when we say a cable or MCB is rated at 45A, it is rated to be able to operate at that load continuously.

Yes, 47A for 6mm² T&E, and a bit more but you should only use the 47A for design.

But there is a different "unpublished" rating for peaky loads, which is what the diversity calculations is about. That is, for short periods the cable can carry in excess of that load.

Yes, time is also a factor.
45A x 1.13 for a B type MCB, i.e. 51A will never trip.
45A x 1.45 for a B type MCB, i.e. 65A will take an hour to trip.
Plus other time/current combinations.

For example you put on a few rings on the hob and the two ovens, you are drawing well in excess of the "rated" load, but soon the cookers' thermostatic controls kick, turning the heating elements on and off. So 10A is the sustained load from the cookers, and up to 30% of the time, load can flex up to the peak load? Or something like that.

Nearly, but the 10A is nothing to do with 30% of the time.
Diversity is just a figure which has worked for decades.

 

[JohnW2]

Yes, time is also a factor.
47A x 1.13 for a B type MCB, i.e. 53A will never trip.
47A x 1.45 for a B type MCB, i.e. 68A will take an hour to trip.
Plus other time/current combinations.

Why are you quoting figures for a hypothetical B47 MCB? If such an animal existed, your figures would be correct, but I rather doubt that such an animal does exist.

The calculations for a B45 should obviously be (45A x 1.13) and (45A x 1.45), regardless of the CCC of any cable involved.

Kind Regards, John

 

[AndyPRK]

Was one of them meant to be type C ?

 

[EFLImpudence]

No sorry, just a typo thinking of the 47 for the cable. I will amend.

 

[JohnW2]

Was one of them meant to be type C ?

One of which?

As I said, EFLI presented correct calculations for a hypothetical "B47", but I don't think such things exist.

Kind Regards, John

 

[David Costelloe]

If you want to.

Are the cooker and hob to be fitted near each other?
Just take the new cable from the oven outlet plate.

It would take a 3m run to get from the CCU to the hob location - c 2m as the crow flies.... The cooker control unit is installed on a tiled wall and I am not sure if we made provision for running cable in conduit.

The cleanest approach by the sound of it would be to run a second radial c. 6m to the 45A MCB. I did not realize that you could run a number of radials from the same MCB - but thinking about it, so long as you have planned the loads properly, there is no reason why it should be a problem.

Many thanks.

[The 30% in the diversity calc reminded me of a factor I came across in telecoms that average demand (eg from phone calls) for a access to system was around 25-30% of peak demand - so you size for peak demand in the knowledge that on average capacity will only be 25% used.]

 

[flameport]

The cooker control unit is installed on a tiled wall

That is irrelevant.

New cable connects to the existing oven outlet, not the wall switch.

 

[PaddyLondon]

That is irrelevant.

New cable connects to the existing oven outlet, not the wall switch.

Thank you. That's feasible - just have to take the oven out to get access and navigate the cable safely behind a built in dishwasher.

 

[JohnW2]

... [The 30% in the diversity calc reminded me of a factor I came across in telecoms that average demand (eg from phone calls) for a access to system was around 25-30% of peak demand - so you size for peak demand in the knowledge that on average capacity will only be 25% used.]

In terms of electricity consumption (which is what we are discussing), that would be rather daft, and would negate the whole concept of diversity.

In an electrical installation/circuit, in situations where it was acceptable to invoke diversity, one would design ('size') the installation to cope with that 'average', assuming that excursions beyond that (up to peak 'demand') would be sufficiently rare and/or short-lived to not result in any problems/harm (given the 'safety margins' which exist in relation to most things).

In terms of telecoms, if one wanted to avoid a situation in which demand ever exceeded supply (i.e. some people could not make phone calls), one would have no choice but to design ('size') the system so as to be able to cope with the peak demand. In other words, the concept of diversity could not be invoked and one would have to accept that the price paid for having a system which never 'let customers down' would be that a high proportion of capacity would often be 'unused and unnecessary'.

The concept of diversity is crucial to the realistic design of the infrastructure for generation and distribution of electricity for domestic
consumption, since the average domestic demand is only around 5% of the theoretical peak demand (i.e. if every household 'switched on everything' simultaneously)!

Kind Regards, John