Current capacity of a "40A" electricity meter

[nomisr]

Hi,

Quick question. I have an old English Electric electricity meter (type C31B/M) which is stamped "40A MAX". I've read that this is just the limit of accuracy and the meter will handle much higher currents (well over 100A). Is this true?

I'm having 3 large extensions added to my house and as part of the work we're moving the electricity supply from overhead to underground and moving the meter to an external wall box. I contacted Scottish Power and they are looking for £50.80 to replace the meter with a reconditioned one. It's nothing in the scheme of things, but the guy I spoke to said that they are going to replace the meter with a new digital one next year. This will be a free upgrade if I can wait that long.

I was hoping that the DNO will replace the main fuse for 100A (it's 60A now). Her indoors has her heart set on a couple of Neff ovens which are rated at 3.7kW and 6.8kW, so that's already over 40A if the whole lot was on at once (unlikely, but you never know). The hob will be gas though. We tend to heat our water with the immersion heater, especially in the summer, so that's another 12A. Throw in a kettle, tumble drier, washing machine and dishwasher (which I have known to be all on at the same time) and you're looking at well over 60A in a perfect storm. Luckily the water supply is so poor that inline electric showers are out of the question.

Do I get the meter replaced now or wait a year?

Cheers,
Nomis

 

[ban-all-sheds]

I was hoping that the DNO will replace the main fuse for 100A (it's 60A now).

We must defer to Westie, but until then my 2p worth is that they won't upgrade it unless it keeps blowing.

Which it won't.

And as its purpose is to protect their cable, they may not be able to simply put a bigger fuse in.

Why do you want a 100A supply? That surely cannot be the revised maximum demand which your electrician has calculated?

Her indoors has her heart set on a couple of Neff ovens which are rated at 3.7kW and 6.8kW, so that's already over 40A if the whole lot was on at once

It's not, or at least not for long enough - ask your electrician to explain diversity to you.

Do I get the meter replaced now or wait a year?

You should find out more about the accuracy issue.

 

[JohnW2]

I was hoping that the DNO will replace the main fuse for 100A (it's 60A now). Her indoors has her heart set on a couple of Neff ovens which are rated at 3.7kW and 6.8kW, so that's already over 40A if the whole lot was on at once (unlikely, but you never know). The hob will be gas though. We tend to heat our water with the immersion heater, especially in the summer, so that's another 12A. Throw in a kettle, tumble drier, washing machine and dishwasher (which I have known to be all on at the same time) and you're looking at well over 60A in a perfect storm. Luckily the water supply is so poor that inline electric showers are out of the question.

As BAS has intimated, I think you are being unnecessarily concerned/ pessimistic. The sort of demand you describe is very typical of millions of houses (and many have a ~10kW shower on top of all you mention), and very many of them work happily with 60A or 80A DNO fuses which 'never' blow.

There are two main things you need to consider - firstly the concept of 'diversity' (you never get a current demand for a significant period of time anything like as high as you would think by adding ip the maximum demand of all loads) and, secondly, if the total load does rise above, say, 60A for a small period of time, it would take a lot more than 60A to cause it to blow.

If you added up all the theoretically possible 'maximum loads' for many houses, you could well get and answer of 200A, or more. You would probably be amazed how low is the average maximum current demand (averaged over, say, half-hour periods) for the average house.

Kind Regards, John

 

[westie101]

Yes the meter is good for over 100A so will never be an issue

It is likely as part of the work that the cut-out will be upgraded to 100a as that is the size of supply the regulator prefers to be installed

 

[Adam_151]

It is likely as part of the work that the cut-out will be upgraded to 100a as that is the size of supply the regulator prefers to be installed

I thought the current practice was 80A as the cutouts get a bit too warm on 100A constant demand?

 

[westie101]

I thought the current practice was 80A as the cutouts get a bit too warm on 100A constant demand?

We're still installing 100A, the heating only occurs very rarely and that in commercial premises.

Other DNOs may specify lower which is up to them.

(though in fact all cut-outs will be designed for 100A the 80A limitation is on the fuse size only. Though with the fuse characteristic they are good for 140% + continuously anyway.)

 

[ban-all-sheds]

I thought the current practice was 80A as the cutouts get a bit too warm on 100A constant demand?

Not as warm as an 80A gets when passing 100A....

 

[JohnW2]

It is likely as part of the work that the cut-out will be upgraded to 100a as that is the size of supply the regulator prefers to be installed

I thought the current practice was 80A as the cutouts get a bit too warm on 100A constant demand?

The magnitude of the demand is the magnitude of the demand, regardless of what size fuse the DNO choose to fit - and I would think it exceedingly unlikely that any normal domestic installation would produce anything even approaching a 100A (or even 80A, or 60A) 'constant demand' (which I'll take to mean 'for fairly long periods').

Furthermore, whatever the load actually is (within what the fuse would tolerate!), wouldn't the cutout tend to get less hot with a 100A fuse than with an 80A one?

Kind Regards, John

 

[EFLImpudence]

Yes the meter is good for over 100A

Then why is it stamped "40A max"?

 

[JohnW2]

Yes the meter is good for over 100A

Then why is it stamped "40A max"?

As I think someone said early in the thread (maybe even the OP), in the good-old days they all said that - and I believe it was the current above which the metering was not certified to be accurate (which presumably meant that they only tested accuracy up to 40A).

They could well have been (probably were) reasonably accurate way above 40A, but, in the absence of testing at such currents, could not be guaranteed to be. It's a bit like like those PVC signal/alarm/whatever cables said to be rated at only 50V - again, I think that means little more that that they haven't been tested beyond that voltage, since one would have to try quite hard to make a cable with PVC so thin that it couldn't stand a lot more than 50V (don't forget, around 50-60 kV/mm).

Kind Regards, John

 

[EFLImpudence]

Shouldn't they be replaced, then?

 

[JohnW2]

Shouldn't they be replaced, then?

I would imagine that the great majority of them have been replaced. However, I don't quite understand the 'meter replacement policies' of electricity suppliers - some seem to replace fairly modern ones pretty frequently (after only a few years - we've had 4 or 5 meters here in ~27 years), whereas others seem to leave ancient ones in service for many decades.

Kind Regards, John

 

[westie101]

In the back of my mind I think the 40A actually relates to the continuous current carrying capacity (continuous being day in day out for a very prolonged period)
In terms of shorter time periods they are good for the sort of normal domestic loads that are seen.

If they are still on the system it will be because they have been assessed as safe to continue in use

 

[JohnW2]

In the back of my mind I think the 40A actually relates to the continuous current carrying capacity (continuous being day in day out for a very prolonged period) In terms of shorter time periods they are good for the sort of normal domestic loads that are seen.

That may be true - but I've heard the story about 40A relating to the limited of the metering certification many times over the years - although that obviously doesn't prove that it's not an 'urban legend'!

If they are still on the system it will be because they have been assessed as safe to continue in use

That makes sense, but there's some serious inconsistency afoot. I cannot believe that any (other than perhaps the first, and that I doubt) of the several meters I've had replaced in the past 27 years have been replaced because they were not regarded as 'safe to continue in service' - they have just been 'routine meter changes', for whatever reason!

Kind Regards, John

 

[westie101]

'routine meter changes'

Given that there were well over 25 million households in the UK in 2008 changing meters to any programme is a huge task.
Often specific types are targeted, but records can be poor in some areas!

(though I'm not sure that the present system helps with the record keeping)

Often it relies on the meter readers being tasked to flag up certain types, but that depends on gaining access which can be difficult