Part P

[Big_Spark]

Once again I see people applying diversity on a circuit where it should not, and does not apply.

Diversity has a role to play in most electrical installations and certainly the majority of circuits, however circuits that supply fixed equipment should not have diversity applied.

A cooker may only be 80% used at any one time for the vast bulk of it's life, but there are occasions, especially in family homes, where the full potential load of the cooker may be used, such as at Christmas etc.

Please do not forget that cookers are NOT seperate circuits, they are a single piece of FIXED EQUIPMENT connected permanently to a single dedicated circuit.

This circuit should be capable of safely delivering the full potential load of that euipment should it be required, whether it is used as a rule or not.

6.1kW is 26.5A, and so the cooker in this case, should be connected to a Radial of not less than 4.0mm and protected by a 32A Type B MCB, preferably s 30mA 32A RCBO for added safety.

 

[Panjandrum]

This diversity business still has me puzzled. I had a quick search and didn't find anything that sorted it all out for me. I've seen the bit about the calculation for a cooker. But I know that in my kitchen on a Sunday it would be common for four rings to be glowing full-on with both ovens likewise. I don't suppose this is unusual.
And here, on this very thread, are what seem to me to be conflicting views:

Bas: "The diversity rules for a 6.1kW cooking appliance produce a load of 15A, so a 16A (or 20A, depending on length and how it's installed) on 2.5mm² cable would be OK." Which seems consistent with the rules.

FWL-E: "6.1kW is 26.5A, and so the cooker in this case, should be connected to a Radial of not less than 4.0mm and protected by a 32A Type B MCB, preferably s 30mA 32A RCBO for added safety." Which makes sense too.

I know this is wandering a little off the topic, but only a little.
Can anyone help explain?

 

[towman]

I guess it's because even if everything is "ON" it's unlikely that everything will be drawing power at the same time - ie the elements switch on and off as the various thermostats operate.

But I still reckion it would be better to allow for the worst case scenario and be guided by the rating plate on the appliance - after it it's put there for a reason!

 

[Damocles]

I find it rather strange that every sunday you have all rings glowing on your cooker. Don't you find all the water boiling over everywhere makes a right mess?

I quote you IEE on site guide, page 85, table 1A, 'current demand to be assumed...'

'Household cooking appliance....The first 10A of the rated current plus 30% of the remainder of the rated current plus 5A if a socket outlet is incorporated in the control unit.'

It specifically states that this formula should be applied to final circuits. There is a separate table of diversity allowances for the whole installation, but in the case of cookers it has the same rule.

 

[ban-all-sheds]

What FWL_Engineer is saying that there's a difference between current demand to be assumed for the purposes of calculating the installation MD, and that to be used when choosing the size of the cable.

And he's quite right, given a free hand, but in this case, where there is an existing circuit, I'd say try it and see - there is no danger.

IF you start to get the MCB tripping, then you will have to upgrade the circuit, as it is not acceptable to use a protective device to limit the load.

 

[Panjandrum]

I will say this only once.
Yep - know about the diversity sums.
Still doesn't make sense.
Still there are two divergent views on this forum.
And about my cooker......
Maybe your Sunday lunch is simpler than mine. But when I have all the tribe coming it would be common, at around half-an-hour before sitting down to eat, for my cooker to have four rings on full, glowing frantically, AND both ovens be working. Mind you, it hasn't blown the 30A fuse yet (been around since 1987).

 

[mapj1]

Perhaps as previously I can add water to troubled oil....
or salt to troubled veggies. The cable rating is based on the cable warming up, and is a steady state rating. However, that warm-up takes minutes, maybe tens of minutes to reach final equilibrium, so one can pas many times the rated current for a short time before anything nasty happens, so long as the cable is allowed to cool between bursts of heating. To match this, circuit breakers (and this is even more true of hot wire fuses) have a slow acting thermal element that will open the circuit eventually at about 120% or so of nominal rating, but this may take 20 or 30 minutes, but much less at greater overloads that would give more cable heating, and a fast acting part that fires in practiclly no time at say 5 times rated current. (the magnetic bit).
The clever part that makes it safe is to make the thermal time constant in the breaker or fuse slightly faster than the cable it protects, giveing the best balance between protecting the cable, and not trippping out on transient overloads that really posed no threat. Sand filled fuses and modern MCBs do this much better than the open frame fuses of yesteryear, and to find a burnt cable and an untripped trip is only seen if the wrong cable has been installed in the first place.
Coming to the cooker.
Because the oven thermostats are pinging on and off, and so are the ring controls, although it may well be that for short times the nominal cable rating is exceed the long term smoothed value over perhaps 15 minutes, which is what actually matters to the cable insulation heating up, need not actually be enough to be damaging to the cable.

This is the theory that underpins the diversity allowance. Of course if you bypass all the thermostats and turn all the rings to 10 out of 10 or whatever, and leave it like that for half an hour you will have a problem, that is the breaker will trip after some time and by it does the cable will feel warm, but it should not be melting in a life threataning way if the breaker is properly sized.
regards M.

 

[Big_Spark]

map, your simply confusing the issue. The thermal rating of any cable is an absolute, however the way it heats is far from simple as there are many factors to be taken into account.

When calculating circuits you do not assume it is safe to overload the circuit for short periods of time because "the cable can handle it", it is this dodgy slap dash working that causes fires and results in legislation like Part P.

Diversity, as I said, is used for good reason, and when properly applied is a functional tool that allows a practical sizing of cables for common usage.

HOWEVER, fixed equipment is different, especially when dealing with inductive and capacitive loads. Further, when it comes to cookers, we are also dealing with human beings..that rather unpredictable annoyance..If you assume a cooker rated at 6.1kW will never use more than the diversity calculation indicates, you are a fool and asking for trouble.

The diversity formula was created about 30 years ago and has not been updated since, yet in that time peoples eating habits have changed significantly, there are many migrant families in the UK now, and these tend to be larger than normal UK families, consequently more chance that all of a cooker will be used.

Further, some of the comments here regarding cookers certainly leaves me with the distinct impression that a few of you do not understand how they work in the real world, and supplying power to machines that you do not understand electrically is asking for trouble.

 

[Panjandrum]

Aha.
It is all clear to me now. Thanks to all of you.
PJ

 

[Scoby_Beasley]

You should be ok for the moment, the electricity police have all been drafted to help wire up cages for terrorist suspects.

I would say just about every house in the country has some unfinished wiring job left over from the last century.

I'm fitting a shower for the m-in-law. Test holes for cable routing were drilled and fishing line installed in late August 2002 when the water pipe was fitted and capped, Still no electric shower/dp switch/cable/mcb in sight but the job was started 2 1/2 years ago !!!

 

[ban-all-sheds]

it is this dodgy slap dash working that causes fires and results in legislation like Part P.

Could have sworn you said that it was the ODPM's desire to break NICEIC's monopoly that resulted in Part P.....

 

[Damocles]

So what you are saying FWL is that the on site guide is wrong and should be ignored?

 

[Big_Spark]

Could have sworn you said that it was the ODPM's desire to break NICEIC's monopoly that resulted in Part P.....

It was the excuse they used to introduce the legislation..they couldn't publicly say it was to break the NICEIC monopoly...the NIC would have had a fit!!

 

[Big_Spark]

So what you are saying FWL is that the on site guide is wrong and should be ignored?

That is not what I said, this is were experience of the qualified sparks comes into play. That part of the on-site guide is inaccurate as it has not been updated for many years, but inaccurate does not necessarilly mean wrong.

Diversity, as I have said in this thread, and in countless others in the past, is an IMPORTANT TOOL in CERTAIN situations but SHOULD NOT be applied to all circuits and situations. Experience of this is what seperates the DIYer form the professional (and no that is not a dig), but even some sparks make the mistake of sticking to the diversity track like glue.

 

[Damocles]

You mean the OSG is inaccurate in the sense that you might come across an installation where the published diversity rules suggest a supply requirement of 150A, yet it has been chugging along nicely on an 80A main fuse for the last 10 years?

I would not at all disagree with you about the importance of experience in judging a particular situation, but the published diversity calculations are fairly conservative. I was watching the meter the other day as I jacked up the settings on a cooker ring. There is a big difference between the current being used at normal simmer settings and at full power.

The big difference between a cooker and most other fixed equipment is that a cooker is usually already several different heaters arranged together in one unit. The maximum demand is way above what would normally be used at the same time.