Diversity and the resilience of circuit protection.

That sounds like a kitchen which is designed for showing off rather than cookery.

If the householders did the use full 32 A x 2, how would they deal with the 15 kW of heating?

The 14th Edition (Regulations for the Electrical Equipment of Buildings by the I.E.E.) dated 1966 and including amendments up until 1976 in Reg. A.27 had the old cooker final sub-circuit rating calculated from:

 The first 10 Amps of total rated current.

Plus 30 per cent of the remainder,

Plus 5 Amps for a socket outlet incorporated into the cooker control unit.

That may have been o.k. for free standing cookers of the time which had four rings, a grill and an oven, with perhaps a kettle plugged into the socket outlet.

But today?

Z.

Diversity is reliant on a certain amount of estimating and engineering judgement, but the installation as described does sound a bit much on a single 32 amp circuit.

Probably fine most of the time, but still a bit much if cooking an unusually large meal.

The standard diversity is primarily intended for a single cooker that incorporates several boiling rings, a grill and at least one oven. It has a good record in practice.

The standard diversity allowance could reasonably be applied to several appliances that in total are similar in design and application to a standard cooker. For example, two twin boiling rings, an oven and a table top grill, all fitted with 13 amp plugs could in my view be supplied from the same 32 amp circuit despite  the total loading being in excess of 32.

It is however IMHO debatable if other cooking appliances that would not normally be part of a  standard cooker should be considered in the same way.

I would normally expect a microwave oven to be plugged in to a 13 amp socket independent of the cooker circuit. And the steamer to have its own circuit, NOT shared with the cooker, or other appliances that approximate in function to a cooker.

 How big is the rest of the house ? this is IMHO indirectly relevant since it partly determines the chances of preparing an elaborate meal for a dozen guests. Or even for a large family.

 

If the householders did the use full 32 A x 2, how would they deal with the 15 kW of heating?

Turn off the central heating and run around in shorts and tee shirts obviously!

Z.

As I understand it, deliberation is currently underway with respect to diversity in domestic installations, primarily as a result of adding in EVSE. However, many manufacturers of EVSE are including current monitoring to ensure that the load on the dwelling does not exceed 60A at any one time. This allows the EVSE installer to notify their DNO up to 28 days after installation, rather than by prior permission. I mentioned before that our DNO,  raised the issue of whether these devices fail to safety. 

Zoomup: 
 

The 14th Edition (Regulations for the Electrical Equipment of Buildings by the I.E.E.) dated 1966 and including amendments up until 1976 in Reg. A.27 had the old cooker final sub-circuit rating calculated from:

 The first 10 Amps of total rated current.

Plus 30 per cent of the remainder,

Plus 5 Amps for a socket outlet incorporated into the cooker control unit.

That may have been o.k. for free standing cookers of the time which had four rings, a grill and an oven, with perhaps a kettle plugged into the socket outlet.

But today?

Z.

Fine “back in the day” and still fine IMHO in the modern age for a cooker as you describe.

But should not mean that multiple high loading other cooking appliances can be prudently connected to the "cooker" circuit.

Steamers and microwave ovens were not normally part of a conventional electric cooker, and should not in my view be added to an existing or proposed cooker circuit.

(a microwave oven built in to a cooker instead of a conventional oven is arguably OK as the loading is not thereby increased over a conventional oven. But a microwave oven IN ADDITION is different as both might be used together.)

I always install multiple 4.00mm or 6.00mm supplies for instances such as those you describe. Think of better functionality if a single supply fails.

Why did you change the breaker to 32A? There was no sign of a problem, but you tried to decide that the circuit design is unsatisfactory. Why is it unsatisfactory? It works, shows no sign of damage, and is satisfactory to the customer. Did you tell him he needed more circuit(s), because if so I find your opinion to be both groundless and fraudulent? You may feel I am being unfair, but I do not. The commercial microwave (or perhaps microwave / conventional) oven is very unlikely to take more than 5A ON AVERAGE, and the other heating appliances all have thermostats so take around the ½ power rating even if all on at once ON AVERAGE. Average in this case is over a suitable period, typically an hour, so the cable rating and breaker size (45A) are both perfectly satisfactory. I assume you think that the cable may overheat, but it will not at around half the rating, although the peak current may be more than its continuous rating. Another case where the Electrician does not understand diversity! By the way, the total load is NOT 92kW as you say. An EV charger is completely different from any other domestic load except off-peak heating. Nothing else runs for long periods at full power.

I would be uneasy at allowing 6mm cable passing through thermal insulation to remain in use on a 45 amp circuit breaker, even without any evidence of damage so far.

davezawadi (David Stone): 
 

Why did you change the breaker to 32A? There was no sign of a problem, but you tried to decide that the circuit design is unsatisfactory. Why is it unsatisfactory? It works, shows no sign of damage, and is satisfactory to the customer. Did you tell him he needed more circuit(s), because if so I find your opinion to be both groundless and fraudulent? 

I think you will have the minority view then. The cable is running in insulation, albeit not continuous. It was originally on a 45 Amp breaker. The cable is rated at 34 amps when downrated for its installation method.

The amount of appliances connected is not a standard cooker load, and it is easily possible to exceed the 34 amp cable rating with only 3 of the appliances on. Yes, I realise diversity can be taken into consideration, and the load may not be on for long, but, really, would you leave 6 (largish) appliances on a  45 amp breaker with a cable capacity of 34 amps?

If it was a new circuit, no-one would install such an arrangement, so , if inspected, and given a C2, then what would you do? (I didn't inspect it BTW) Tell the customer that there isn't much chance of a fire, but, don't turn everything on at once? Of course you wouldn't, you would suggest a better alternative would be to split the appliances up, so they aren't all on one circuit, and you would rate the circuit protection for the cable installation.

As it happens, the cost wasn't much more to fit a new cable and the correct breakers, as it was only a small part of the repairs/replacements required, a consumer unit change was needed to give it RCD protection, along with a list of round 20 other C2's. even so, If I was doing the inspection myself, I would recommend the downrating of the cooker circuit CB, there is no way I could give a ‘Satisfactory’ knowing that cable wasn't big enough for the potential current.