Storage Heaters

[Oldgreybeard]

We are good, checked last year before upgrade and had it running at over 90A since. 21kW+ if memory serves.

I went through this at length with SSE, both when our new cable went in and again when the car charger was installed (the installer had to check with SSE to see if the supply was OK). I have the paperwork here in front of me, from 2013, when the cable went in. It's a pre-printed form with three supply options as tick boxes, we could have had either a 13.8kVA or 15kVA single phase supply, or a 45kVA single phase supply. There were no other supply options on the form and the paperwork isn't just a local thing, as the office that initially dealt with our new supply request was in Perth.

I also have the paperwork from when the car charger was installed, and that also confirms that our supply is the largest single phase supply available in the UK, 15kVA, safety fused at 100A, with a maximum continuous load of 65A.

My understanding is that the fuse will get hot if run close to its maximum rating, and present a potential fire hazard, as it is only there to protect in the event of a dead short. I also remember this being discussed at length on an electric car forum a few years ago and one or two knowledgeable people there agreeing that the maximum load on any UK single phase supply was either 13.8kVA or 15kVA.

I'm 100% sure that you cannot safely draw close to 100A through a 100A fuse without risking it getting too hot. I remember working on older wiring in houses back in the 1970s where people had plugged in several electric heaters and drawn so much current that the fuses would cook and damage the main fuse holder. Very common to find the fuse holders cracked from the heat.

[Fintray]

Check this out Iain!
100A 24kW at 240V

3.8kW battery charge
3kW Hot water tank
5kW EV

11.8kW used leaves 12.1kW

Main Lounge 850W
Front lounge 1700W
Hallway 2550W
Bedroom 1700W
Other bedroom 1700W
Dressing room 850W
Small bedroom 850W
Family room 850W

11,050 Watt used leaves 1,050W before hitting the fuse limit.
Fortunately about 1/3rd of the storage heaters are stand by units.

Your fuse will be fine, during one of the Agile negative price events (anyone remember those?) I had just under 20kW coming through my 60A fuse, admittedly not for long once I realised how much it was.

[Oldgreybeard]

Your fuse will be fine, during one of the Agile negative price events (anyone remember those?) I had just under 20kW coming through my 60A fuse, admittedly not for long once I realised how much it was.

The fuse holder will not be fine for long, though! The problem is that fuses get hot when run close to their rating and they overheat, not that they may blow. To blow a 100A fuse the current needs to be way over 100A, something like 150A or more I believe. Running the fuse close to 100A will make it very hot, though, and it is that heat build up that imposes the safe limit.

Also, the electricity supply in the street isn't capable of delivering the maximum to every house at the same time. The map I have from SSE showing our new supply cable has the cable ratings and the transformer rating on it. This shows that the 100kVA sub-station that supplies our end of the village is supplying 17 houses along our lane, with the cable running to our house being a 90mm² three phase underground cable (it's marked on the map as being Wavecon 90). 100kVA divided by 17 gives under 6kVA per house, yet each house probably has the same 15kVA rating as our supply.

When our supply was being put in I chatted at length with the guys that ran the big cable in and asked them a few questions about the supply and they told me that the fuses at the transformer were rated at 800A. A bit of basic arithmetic shows that each house cannot ever draw full power at the same time as if they all did the system would be badly overloaded.

I believe this is why the DNO needs to give consent for high powered loads, like a car charger or a storage heater installation, as they need to make sure that there is enough spare capacity in their cabling etc.

[Joeboy]

We will see.

[Fintray]

Your fuse will be fine, during one of the Agile negative price events (anyone remember those?) I had just under 20kW coming through my 60A fuse, admittedly not for long once I realised how much it was.

The fuse holder will not be fine for long, though! The problem is that fuses get hot when run close to their rating and they overheat, not that they may blow. To blow a 100A fuse the current needs to be way over 100A, something like 150A or more I believe. Running the fuse close to 100A will make it very hot, though, and it is that heat build up that imposes the safe limit.

Also, the electricity supply in the street isn't capable of delivering the maximum to every house at the same time. The map I have from SSE showing our new supply cable has the cable ratings and the transformer rating on it. This shows that the 100kVA sub-station that supplies our end of the village is supplying 17 houses along our lane, with the cable running to our house being a 90mm² three phase underground cable (it's marked on the map as being Wavecon 90). 100kVA divided by 17 gives under 6kVA per house, yet each house probably has the same 15kVA rating as our supply.

When our supply was being put in I chatted at length with the guys that ran the big cable in and asked them a few questions about the supply and they told me that the fuses at the transformer were rated at 800A. A bit of basic arithmetic shows that each house cannot ever draw full power at the same time as if they all did the system would be badly overloaded.

I believe this is why the DNO needs to give consent for high powered loads, like a car charger or a storage heater installation, as they need to make sure that there is enough spare capacity in their cabling etc.

I was simply stating that the main fuse wasn't going to blow at that load not that I was suggesting running at maximum fuse rating was ok. I'm sure most people on this forum would realise that running close to the main fuse limit is not a good thing.

Your post does show how little power there is available and the problems it would cause if everyone on your lane had EV's and heatpumps, not that they would all be on at the same time though. Wonder how things will work out when gas boilers stop being installed and heatpumps become more common and the DNO says there is not enough capacity in particular areas?

[Oldgreybeard]

We will see.

AS long as your DNO has approved the storage heater load and the inverter/charger load then you are at least the right side of the rules if anything does go haywire, although quite how they could approve drawing way over the safe maximum for the supply I'm not sure. IIRC any high load kit needs approval and the DNOs can get a bit awkward about it. Our nearest neighbour has been doing battle with them over his hot tub that was partly installed this summer. The DNO are refusing to allow it to be connected as, like us, he has a car charger and PV. Daft thing is that if he'd installed the hot tub before the car charger he'd have been fine, as the DNOs are obliged to do a free upgrade for car charger installations I believe. Sadly the same isn't true for his hot tub. Last I heard they were insisting he get three phase run in to cope with the load.

I was simply stating that the main fuse wasn't going to blow at that load not that I was suggesting running at maximum fuse rating was ok. I'm sure most people on this forum would realise that running close to the main fuse limit is not a good thing.

Your post does show how little power there is available and the problems it would cause if everyone on your lane had EV's and heatpumps, not that they would all be on at the same time though. Wonder how things will work out when gas boilers stop being installed and heatpumps become more common and the DNO says there is not enough capacity in particular areas?

That was the thing that struck me when SSE sent me a copy of the map of the area showing their cables etc. Not only did the transformer rating seem far too small (although it is the largest pole mounted transformer available, apparently) but also some of the cables aren't that big, either. The 1930's built house on the hill behind us has overhead cables directly from the transformer, around 100m long at a guess, that are marked on the SSE map as being two wires that are 7 x 0.064" each which is about 16mm² and a fair bit thinner than the 25mm² sub-main cable I had to run in here to take power from the meter to the house (the meter is about 10m from the house consumer unit). I was told that 25mm² was the minimum acceptable cable size for this short run, so goodness only knows how SSE can get away with running a much smaller cable for around 100m.

The whole electricity distribution thing seems to be a bit of a shambles, at least in areas like this, where new houses have been built and just sort of tacked on to the existing infrastructure. The guys that ran our supply in said they had had a hell of a problem getting access to the connections on the transformer, as it had been sat up the top of its pole since some time in the 1960s and all the bolts were very rusty. These guys also reckoned that the cables running overhead up to the big house are the ones that were put in when that house was built, around 80 or 90 years ago.

The feeling I got from all the lengthy discussions I had with SSE was that they didn't really know much about the true loads on their network, and the guys actually doing the work were quite candid about how they kept recommending upgrades, whenever they came across ancient and overloaded equipment and how their head office never agreed to do the upgrades until the equipment actually failed. Planned maintenance seemed non-existent. One example the guys quoted was of an underground cable running along a nearby lane. They reckoned that cable had more joints than original cable, as the gutta percha insulation was breaking down all along its length, but rather than replace it they were just told to keep on repairing it. As a consequence the road surface is a nightmare, full of patches where its been dug up many times over the years.

[Joeboy]

We will see.

AS long as your DNO has approved the storage heater load and the inverter/charger load then you are at least the right side of the rules if anything does go haywire, although quite how they could approve drawing way over the safe maximum for the supply I'm not sure. IIRC any high load kit needs approval and the DNOs can get a bit awkward about it. Our nearest neighbour has been doing battle with them over his hot tub that was partly installed this summer. The DNO are refusing to allow it to be connected as, like us, he has a car charger and PV. Daft thing is that if he'd installed the hot tub before the car charger he'd have been fine, as the DNOs are obliged to do a free upgrade for car charger installations I believe. Sadly the same isn't true for his hot tub. Last I heard they were insisting he get three phase run in to cope with the load.

I was simply stating that the main fuse wasn't going to blow at that load not that I was suggesting running at maximum fuse rating was ok. I'm sure most people on this forum would realise that running close to the main fuse limit is not a good thing.

Your post does show how little power there is available and the problems it would cause if everyone on your lane had EV's and heatpumps, not that they would all be on at the same time though. Wonder how things will work out when gas boilers stop being installed and heatpumps become more common and the DNO says there is not enough capacity in particular areas?

That was the thing that struck me when SSE sent me a copy of the map of the area showing their cables etc. Not only did the transformer rating seem far too small (although it is the largest pole mounted transformer available, apparently) but also some of the cables aren't that big, either. The 1930's built house on the hill behind us has overhead cables directly from the transformer, around 100m long at a guess, that are marked on the SSE map as being two wires that are 7 x 0.064" each which is about 16mm² and a fair bit thinner than the 25mm² sub-main cable I had to run in here to take power from the meter to the house (the meter is about 10m from the house consumer unit). I was told that 25mm² was the minimum acceptable cable size for this short run, so goodness only knows how SSE can get away with running a much smaller cable for around 100m.

The whole electricity distribution thing seems to be a bit of a shambles, at least in areas like this, where new houses have been built and just sort of tacked on to the existing infrastructure. The guys that ran our supply in said they had had a hell of a problem getting access to the connections on the transformer, as it had been sat up the top of its pole since some time in the 1960s and all the bolts were very rusty. These guys also reckoned that the cables running overhead up to the big house are the ones that were put in when that house was built, around 80 or 90 years ago.

The feeling I got from all the lengthy discussions I had with SSE was that they didn't really know much about the true loads on their network, and the guys actually doing the work were quite candid about how they kept recommending upgrades, whenever they came across ancient and overloaded equipment and how their head office never agreed to do the upgrades until the equipment actually failed. Planned maintenance seemed non-existent. One example the guys quoted was of an underground cable running along a nearby lane. They reckoned that cable had more joints than original cable, as the gutta percha insulation was breaking down all along its length, but rather than replace it they were just told to keep on repairing it. As a consequence the road surface is a nightmare, full of patches where its been dug up many times over the years.

I feel quite professional then! Nowty & I had a wee 'who can draw the most a year or so ago'. At that time I was keeping an eye on the tails temps and they were fine. The situation where everything is on is vanishingly small in likelihood. Fintray's comment on my fuse upgrade made me realise I was up there for potential load though. Hence the working out. I will be sitting down at the pc soon and setting up a schedule for all the storage heaters through Hive. The only heater that will be on for the full 5 hour window is the core heater at centre of the home. Amazing how it all adds up.

[nowty]

It turned out Nowty Towers only had a 60A main fuse, hence why my main fuse was getting so hot drawing 70A to 80A during previous winters. So, back in the spring, I applied for a fuse upgrade.

Obviously, I had to do a post upgrade test.



Now the main fuse only gets slightly warm at a constant 80A and I am looking forward to winter with confidence.

[Oldgreybeard]

Sorry, I just don't think that it is safe to run a supply with a maximum power rating of 15kVA at a load of over 18kVA, quite apart from the impact it has on the local network that simply isn't designed to allow this much to be drawn by each house for any length of time. I guess if people want to take the risk it's up to them, but there is absolutely no way I'm going to do it. Having heard about the pretty iffy state of the distribution network, first hand from the guys working on it every day, and having been told, in writing, that the maximum available on any domestic single phase supply is 15kVA, that is as much as I am every going to draw, even if it is a nuisance.

FWIW, I don't believe the wiring is a problem. If our supply is typical, then the 95mm² cable that comes in has an underground joint to a short length of 35mm² that comes up to the fuse with 25mm² cable from there to the consumer unit. The tables I've looked at give the current rating for 25mm² cable as 114A, so even the thinnest cable is rated at well over the 65A maximum that can be drawn. I was told, and I've first hand knowledge from years ago that supports this, that the fuse will run hot at high current and this will degrade the fuse holder and the terminals over time.

[Joeboy]

Sorry, I just don't think that it is safe to run a supply with a maximum power rating of 15kVA at a load of over 18kVA, quite apart from the impact it has on the local network that simply isn't designed to allow this much to be drawn by each house for any length of time. I guess if people want to take the risk it's up to them, but there is absolutely no way I'm going to do it. Having heard about the pretty iffy state of the distribution network, first hand from the guys working on it every day, and having been told, in writing, that the maximum available on any domestic single phase supply is 15kVA, that is as much as I am every going to draw, even if it is a nuisance.

FWIW, I don't believe the wiring is a problem. If our supply is typical, then the 95mm² cable that comes in has an underground joint to a short length of 35mm² that comes up to the fuse with 25mm² cable from there to the consumer unit. The tables I've looked at give the current rating for 25mm² cable as 114A, so even the thinnest cable is rated at well over the 65A maximum that can be drawn. I was told, and I've first hand knowledge from years ago that supports this, that the fuse will run hot at high current and this will degrade the fuse holder and the terminals over time.

Will keep you and anyone else interested updated. Great points you have made but i'm going for it regardless. Cheers!
Just away to set up start up times which will be staggered.

Family room 850W SH camouflaged, just went in today.