New Wiring for separate Hob & Double Oven

[ban-all-sheds]

for every circuit the following MUST be true..

433.1.1 The operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions:

(i) The rated current or current setting of the protective device (In) is not less than the design current (Ib) of the circuit, and

(ii) the rated current or current setting of the protective device (In) does not exceed the lowest of the current carrying capacities (Iz) of any of the conductors of the circuit, and

(iii) the current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current carrying capacities (Iz) of any of the conductors of the circuit.

or in simpler terms Ib ≤ In ≤ Iz..

Ah - but how do you know that Ib can't be calculated with the 10+30% formula?

 

[ColJack]

so you jump straight to the tables?
Page 95 of the OSG..

This appendix provides information on the determination of the maximum demand for an installation and includes the current demand to be assumed for commonly used equipment.

I'd still rather not put a 10.4KW cooker on 1.5mm cable if that's alright with you..

 

[ColJack]

Diversity for cookers has been allowed since 14th ed. at least, slight variations but always there.

didn't say it's not allowed, just not in the context of designing the circuit for it..

even at main festive seasons thermostats still work to ensure individual rings etc. not all on at same time.

so you know for a fact that the thermostats are interlinked to prevent them EVER all being on at the same time? or do you just you utterly dismiss the possibility that they can all be on at the same time?

 

[davelx]

I'm sure i've read that some radial circuits for showers use 50A MCB's with 10mm cable, if this is correct I don't understand why this could be dangerous for other high wattage appliances?

That is a potentially dangerous generalisation. The maximum capacity of the capable is only usable under certain limited conditions.

The capacity of the cable doesn't just depend on its size, it also depends on the length of the run and how and where the cable is physically installed - for example, is it clipped on the wall, run in conduit, buried under insulation, grouped together with other cables?

All these factors can reduce the current capacity of the cable due to their impact on heat dissipation from the cable. So unless you understand all these factors, and how to apply them, you cannot know if it is safe to uprate the MCB.

 

[mikhailfaradayski]

Diversity for cookers has been allowed since 14th ed. at least, slight variations but always there.

didn't say it's not allowed, just not in the context of designing the circuit for it..

even at main festive seasons thermostats still work to ensure individual rings etc. not all on at same time.

so you know for a fact that the thermostats are interlinked to prevent them EVER all being on at the same time? or do you just you utterly dismiss the possibility that they can all be on at the same time?

Hi Col,

Don't you remember when we discussed this a few weeks ago?

//www.diynot.com/forums/viewtopic.php?t=193504

I have to take exception to your repeated 'YOU CANNOT USE DIVERSITY' claim, when it is clearly stated that you can.
How far you go in applying the rules is a choice for someone experienced in installations to make.
I agree that creative use of maths to try to use 1.5mm for a cooker supply is dangerous and negligent, but i have to ask why you feel that
'max. instanteous load' = 'design current'.

If that were the case, in the lovely book of non-mandatory guidance, why does it refer to design current and not simply to max load?

 

[ColJack]

Hi Col,

Don't you remember when we discussed this a few weeks ago?

//www.diynot.com/forums/viewtopic.php?t=193504

I do, and it appears that BAS was on my side then.. can't tell if he is this time though..

I have to take exception to your repeated 'YOU CANNOT USE DIVERSITY' claim, when it is clearly stated that you can.

again, I never said that you cannot use diversity, just what you use it for..
I'm all for diversity for a house overall, just not for determining breaker and fuse size for an individual circuit within that house, after all not everything will be on all the time....

How far you go in applying the rules is a choice for someone experienced in installations to make.

exactly, and I will continue to ignore diversity for designing individual circuits as it's my name on the cert to say it's safe and I'd rather have too big a cable in than one that's too small just because I can tweak the figures to make it say it's safe..

I agree that creative use of maths to try to use 1.5mm for a cooker supply is dangerous and negligent, but i have to ask why you feel that
'max. instanteous load' = 'design current'.

at any given point and for an unknown duration, there is the possibility of all of the elements being on.. if you've designed the circuit for a lower rating than the maximum of a fixed appliance then there's the possibility of a sustained overload on the cable and breaker due to the operating characteristics of the OCPD.... .. I just think it's bad design practice..

I mean would you install brakes that just about stop your car on a 10° hill on the knowledge that hills over 10° are few and far between and that you'll probably never have to park on one?

If that were the case, in the lovely book of non-mandatory guidance, why does it refer to design current and not simply to max load?

and on the flip side of that argument, why doesn't the reg for diversity mention design current rather than "maximum demand"..?

it's ambiguous wording again..

if they mean to say that..

to determine the maximum current demand of an installation, calculate the design current of each individual circuit based on the following table and add them together

then they should have said that and included the "industry standard" diversity tables..

 

[mikhailfaradayski]

diversity doesn't apply to individual circuits..
it applies to the overall demand of the installation, in this case the house..

Maybe its just the way i read this, but you seem to say that diversity cannot be applied, i disagree with that.

However, i do agree that it is the judgement of the person responsible for the design of a circuit to apply the guidance sensibly.

WRT to brakes on a car, thats a different scenario altogether, after all, the spec brakes on a smart car would be low, but a bugatti Veyron would be much higher. Horses for courses i think

 

[properleckie]

so "I think its bad design practice..." so the IET OSG is not right then coljack?
That's why they give tables and 'standard circuits' so we don't have to keep using oversized cables and redesigning the wheel.
Diversity not included in a domestic ring circuit would need at least 6mm2 conductors to allow for a 3kW heater in every room and as you say can't count on stats not all on at same long time?.
As for rating for max possible load what about motor starting loads?
standard is to cable for full load current of motor, starting will be 6+ times that for several seconds, surely you wouldn't put in cable 6X size for that so as not to overheat cable?

 

[Stu1H]

OK guys, on to the cable run, it's well under 10m, in the wall its in plastic conduit and through the joists there's no insulation threfore in theory I should b able to up the breaker to 50A, would this be correct?

Anyhow, i think for the time being I'm going to leave as is apart from run new 10mm cable from the CCU (as it's currently running across the wall diagonally!!!) and swap the outlet for a double as suggested by Col and see if there's any issues with the 40A MCB once I get the oven and hob.

Cheers all.

 

[mikhailfaradayski]

HI Properleckie,

.... it appears that BAS was on my side then.. can't tell if he is this time though....

I don't think Col will be able to respond to your post until he finds out what BAS's thought are on the subject.

Shame that, i thought Col was a human, independent thinking Electrician. Not just an echo of BAS.

 

[mikhailfaradayski]

Hi Stu,

Would this be considered safe / in compliance with regs etc.??
Cheers.

The answer is most probably NO.

Reason?
Coljack has already told you that this is notifiable work.
It seems as though you overlooked this and have so far not replied as to how you intend to deal with notification.
It seems that this work is already in progress.
It seems that you are doing the work.
It seems that you are not a self-certifying Electrician

Do you have £5000 spare that you dont mind losing?

 

[flameport]

Diversity is not used for single circuits, it is for estimating the total load of an installation.

However - the duty cycle of certain items is important and should be used when designing circuits. Such as this:

As for rating for max possible load what about motor starting loads?
standard is to cable for full load current of motor, starting will be 6+ times that for several seconds, surely you wouldn't put in cable 6X size for that so as not to overheat cable?

otherwise cables everywhere would be grossly oversized. While duty cycle won't be relevant to most items in a domestic situation, a cooking appliance is one example where it is.

For a motor which is started infrequently, the starting current is not relevant when considering the size of the cables. The cable will heat up when the starting current is applied, but this won't last for long, so the cable would be sized to the running current only.
If the motor was started often, such as every few minutes, the start current would be significant, as the cables would heat up, but probably wouldn't cool completely before the next start cycle. The cables would be sized on the equivalent load current, which is calculated using the starting and running currents, how long the start current is applied and how often the motor is started.

For the hob, a similar situation applies. If all the rings were on at the same time, the initial current would cause the cable to heat up. However after a short time, the load will drop significantly as the various thermostats operate. The equivalent load current is much less than the maximum of all the elements combined, even though for short periods it is possible that all elements are on at the same time.
For the hob, one method would be a fairly complex analysis of the cycle times of the various elements both individually and when used in various combinations. Or, see page 160 in the OSG, which although refers to the 10A+30%, doesn't mention diversity on that page at all.

Note that this doesn't mean the 10A+30% is correct for all types of cooking appliances, as ratings do vary considerably and a powerful hob and large double oven is probably not equivalent to a single freestanding appliance with 4 open coil rings, a grill and an oven.

 

[mikhailfaradayski]

Diversity is not used for single circuits, it is for estimating the total load of an installation.

Might i humbly guide you toward OSG pg95 para. 3

However - the duty cycle of certain items is important and should be used when designing circuits. Such as this:

As for rating for max possible load what about motor starting loads?
standard is to cable for full load current of motor, starting will be 6+ times that for several seconds, surely you wouldn't put in cable 6X size for that so as not to overheat cable?

otherwise cables everywhere would be grossly oversized. While duty cycle won't be relevant to most items in a domestic situation, a cooking appliance is one example where it is.

I think that is exactly what Properleckie is trying to illustrate to Coljack

For a motor which is started infrequently, the starting current is not relevant when considering the size of the cables. The cable will heat up when the starting current is applied, but this won't last for long, so the cable would be sized to the running current only.
If the motor was started often, such as every few minutes, the start current would be significant, as the cables would heat up, but probably wouldn't cool completely before the next start cycle. The cables would be sized on the equivalent load current, which is calculated using the starting and running currents, how long the start current is applied and how often the motor is started.

For the hob, a similar situation applies. If all the rings were on at the same time, the initial current would cause the cable to heat up. However after a short time, the load will drop significantly as the various thermostats operate. The equivalent load current is much less than the maximum of all the elements combined, even though for short periods it is possible that all elements are on at the same time.
For the hob, one method would be a fairly complex analysis of the cycle times of the various elements both individually and when used in various combinations. Or, see page 160 in the OSG, which although refers to the 10A+30%, doesn't mention diversity on that page at all.

Might i humbly guide you toward OSG pg160, 8.3.1.
Also, might i draw your attention to 8.4, second paragraph '...assessment of current demand....in accordance with table 1A of app.1....' = Diversity

Note that this doesn't mean the 10A+30% is correct for all types of cooking appliances, as ratings do vary considerably and a powerful hob and large double oven is probably not equivalent to a single freestanding appliance with 4 open coil rings, a grill and an oven.

agreed, Diversity is a hard one to paint in black and white. The designer of a circuit is the person responsible for a sensible, considered application of diversity

 

[ColJack]

HI Properleckie,

I don't think Col will be able to respond to your post until he finds out what BAS's thought are on the subject.

Shame that, i thought Col was a human, independent thinking Electrician. Not just an echo of BAS.

my choice not to reply was entirely my own..

I wrote "i think..." = personal opinion, to which I am entitled, not "it is.." which would be a statement of fact..

we could go deeper into the debate whith me pointing out that the OSG is just that, a guide, and that "standard circuits" does not detract from the responsibility of a person designing an installation to check that it's all suitable for the intended use and so on, but I think it's safe to say that you probably won't see it from my point of view, and I probably won't see it from yours, so we'll just have to agree to differ on the subject.

I see no fault on my part for wanting to over engineer an isntallation rather than install what has been deemed by the masses as the normal way.

 

[mikhailfaradayski]

HI Properleckie,

I don't think Col will be able to respond to your post until he finds out what BAS's thought are on the subject.

Shame that, i thought Col was a human, independent thinking Electrician. Not just an echo of BAS.

my choice not to reply was entirely my own..

I wrote "i think..." = personal opinion, to which I am entitled, not "it is.." which would be a statement of fact..

Thats fine, everybody is entitled to their opinion. It just that on a number of occasions in the past you have been more direct in telling people that you CANNOT use diversity for a final circuit ( i could find a number of quotes by you to illustrate this) I just disagree with that declaration.

we could go deeper into the debate whith me pointing out that the OSG is just that, a guide, and that "standard circuits" does not detract from the responsibility of a person designing an installation to check that it's all suitable for the intended use and so on,

I think that means you would then be pointing out to me what i've been pointing out to you. Have a scan through this post to see what i mean

but I think it's safe to say that you probably won't see it from my point of view, and I probably won't see it from yours, so we'll just have to agree to differ on the subject.

I do see your point of view, the only thing i take exception to is the 'cannot use diversity' approach when the truth of the matter is more 'you can use diversity, but beware of how you do it'

I see no fault on my part for wanting to over engineer an isntallation rather than install what has been deemed by the masses as the normal way.

I see no fault either, in terms of safety and future proofing an install. In a domestic setting, there will be little difference to the cost. However, as i've mentioned before, in an industrial setting, that may put your costs up un-necessarily.