crispy_wafer Posted February 7, 2023 Share Posted February 7, 2023 Back story, Old bungalow had electric supply with 100A fuse in meter cupboard, I requested supply to be moved to the new build, meter box installed at a point that would allow easy access in future. I noted that I could install a switched fuse thingy and run my tails to the other side of the build for my CU approx 15m in total. So I was planning on an 80A switch to the plant room and CU. Last Week I've since discovered when WPD moved the supply they installed 80A fuses on the 3ph board thingy (only using single phase at the mo), Surely I don't really want to be stepping down from 80A just to get the tails to where I'd planned the CU... I've spoken to National grid/WPD to see if they'd upgrade the 80A fuse to 100A for me, they came out to have a look but cant. They can however do a few tweaks and unlock the 3ph for me for a few ££'s So, I think I'm deep in the do do here, and not sure what I can do. Less than 3m from the meter box leaves the consumer unit in the Kitchen, or a downstairs bedroom, and at 1.4m off the floor will look cack. I am a bit stuck for idea's, so is there way I can get the tails to the CU safely, and within regs. Or am I stuck trying to hide a CU in a bedroom or kitchen. Probably even worse if we have a 3ph unit put in, the ones at work are like cupboards! Link to comment Share on other sites More sharing options...
ProDave Posted February 7, 2023 Share Posted February 7, 2023 I would just go with the 80A switch fuse. How do you know it's an 80A fuse in the supply head you should not be pulling it to look? Link to comment Share on other sites More sharing options...
Kelvin Posted February 7, 2023 Share Posted February 7, 2023 The connection can be longer than 3m you just have to fit protection to the nearest point on the supply side of the house which could still be an 80A switched fuse. Link to comment Share on other sites More sharing options...
Jenki Posted February 7, 2023 Share Posted February 7, 2023 The fuse is to protect the cable. So why do you need to step down? If the cable is large enough for 80amp protection then off you go. Or are you saying that your worked out load is insufficient for 80A? I live rural. Three houses connected to the transformer. I've got 100A supply installed when they were installing they pulled the fuse at the post. I asked how big the fuse was.. 100A . Link to comment Share on other sites More sharing options...
ProDave Posted February 7, 2023 Share Posted February 7, 2023 4 minutes ago, Jenki said: The fuse is to protect the cable. So why do you need to step down? If the cable is large enough for 80amp protection then off you go. Or are you saying that your worked out load is insufficient for 80A? I live rural. Three houses connected to the transformer. I've got 100A supply installed when they were installing they pulled the fuse at the post. I asked how big the fuse was.. 100A . That's my view. the reason for your fuse, is the supplier says you can only rely on their fuse if the CU is within 3 metres of the supply head. So you fit your own fuse. Lets be honest, if your cable is designed and installed properly, it is not going to suffer damage that would blow the fuse. Link to comment Share on other sites More sharing options...
Kelvin Posted February 7, 2023 Share Posted February 7, 2023 It’s common on here for folk make their temporary supply their permanent supply and leave the meter and supply fuse there then run SWA to the house. A lot of these installs will mean it’s more than 3m away. Mine certainly will be. Link to comment Share on other sites More sharing options...
crispy_wafer Posted February 7, 2023 Author Share Posted February 7, 2023 39 minutes ago, ProDave said: I would just go with the 80A switch fuse. How do you know it's an 80A fuse in the supply head you should not be pulling it to look? I looked in the meter box and the cover says 80A on it, they then visited me to confirm. Link to comment Share on other sites More sharing options...
crispy_wafer Posted February 7, 2023 Author Share Posted February 7, 2023 Thanks. You guy's a more experienced in this than I, so I'm trying to piece it all together, maybe I got confused. 45 minutes ago, Kelvin said: The connection can by longer than 3m you just have to fit protection to the nearest point on the supply side of the house which could still be an 80A switched fuse. 39 minutes ago, ProDave said: That's my view. the reason for your fuse, is the supplier says you can only rely on their fuse if the CU is within 3 metres of the supply head. So you fit your own fuse. Lets be honest, if your cable is designed and installed properly, it is not going to suffer damage that would blow the fuse. so Normally it'd be DNO 80A----Meter---(<3m)----Consumer unit But I want DNO 80A---Meter---MySwitchFuse-(>3m)--Consumer unit. Can MySwitchFuse be the same rating as the DNO's then? I got the impression from a spark it needed to be less. Loads, Usual house stuff, Electric oven/hob, 9kW ASHP, 8kW A2A HP, Stables lighting and power, Garage Lighting and power, planning Car charger or 2. Link to comment Share on other sites More sharing options...
Nickfromwales Posted February 7, 2023 Share Posted February 7, 2023 27 minutes ago, crispy_wafer said: Can MySwitchFuse be the same rating as the DNO's then? I got the impression from a spark it needed to be less. If there's a 100a in the supply head, and there needs to be a 3rd party SF as the tails are >3m, then you're supposed to de-rate to 80a in the SF. 1 Link to comment Share on other sites More sharing options...
Kelvin Posted February 7, 2023 Share Posted February 7, 2023 6 minutes ago, Nickfromwales said: If there's a 100a in the supply head, and there needs to be a 3rd party SF as the tails are >3m, then you're supposed to de-rate to 80a in the SF. Supposed to or have to? Link to comment Share on other sites More sharing options...
Nickfromwales Posted February 7, 2023 Share Posted February 7, 2023 1 minute ago, Kelvin said: Supposed to or have to? AFAIK "have" to. It's to limit the current on the consumer tails. @ProDave? 1 Link to comment Share on other sites More sharing options...
crispy_wafer Posted February 7, 2023 Author Share Posted February 7, 2023 Any pitfalls in taking the option of having the 3 phase installed? Or is this a case of, if you have to ask then you don't need it... Link to comment Share on other sites More sharing options...
Thorfun Posted February 7, 2023 Share Posted February 7, 2023 I'm not an electrician by any shape or form but our head is on the boundary of our property and our CU is in the basement comms room. it's about 25m away iirc. we've got a 3-phase switched fuse next to the head in the meter cupboard and a 3-phase isolation switch in the comms room right next to the CU. Only a single phase is connected at the moment (only got a single phase meter) but I put the 3-phase switched fuse in as a potential to use at a later date. our head has 100A fuses and the switched fuses are 100A also. btw, the 3-phase switched fused unit was not cheap! Link to comment Share on other sites More sharing options...
Thorfun Posted February 7, 2023 Share Posted February 7, 2023 1 hour ago, crispy_wafer said: Any pitfalls in taking the option of having the 3 phase installed? Or is this a case of, if you have to ask then you don't need it... how much is 'a few ££s'? Link to comment Share on other sites More sharing options...
crispy_wafer Posted February 7, 2023 Author Share Posted February 7, 2023 5 minutes ago, Thorfun said: how much is 'a few ££s'? £125 So, beans on toast for me, and the cat can raid the neighbours bins for a week 🤣 Link to comment Share on other sites More sharing options...
Thorfun Posted February 7, 2023 Share Posted February 7, 2023 7 minutes ago, crispy_wafer said: £125 So, beans on toast for me, and the cat can raid the neighbours bins for a week 🤣 not much more than we paid to get 3ph over a new 1ph connection. even if we never use the 3ph i know it's there if we ever do want it. i can't say that your fuse size will go up to 100A with 3ph though, it might just be that you have 3 x phases on 80A fuses. Link to comment Share on other sites More sharing options...
crispy_wafer Posted February 7, 2023 Author Share Posted February 7, 2023 (edited) They wont upgrade the fuses for me, I did ask and then gave it the why not - stamp foot - bottom lip job but they were adamant. I bet there is loads of properties down my way with 100A fuses, just like my bungalow was before they moved the supply and nicked the fuse. Anyway, I appreciate your post and picture @Thorfun as that answers my question, in that I can have the 3ph installed to the head, and then continue to stay on 1ph in the house, until such time I can get a meter installed, then re evaluate. So, whilst thinking out loud on the internet. What's the next fuse size down to de rate the supply, and could I run the living space on it. Ring mains, lighting, Electric oven/hob, 9kW ASHP, 8kW A2A HP, Immersion. Edited February 7, 2023 by crispy_wafer Link to comment Share on other sites More sharing options...
Thorfun Posted February 7, 2023 Share Posted February 7, 2023 12 minutes ago, crispy_wafer said: So, whilst thinking out loud on the internet. What's the next fuse size down to de rate the supply, and could I run the living space on it. Ring mains, lighting, Electric oven/hob, 9kW ASHP, 8kW A2A HP, Immersion. again with the caveat that i'm not a sparky and don't really know what i'm talking about, but why do you need to de-rate the supply? i don't understand this question? your head has an 80A fuse so you can use up to 80A on your home supply, right? what's the de-rating all about? Link to comment Share on other sites More sharing options...
crispy_wafer Posted February 7, 2023 Author Share Posted February 7, 2023 Just taking what my spark said, and nick seemed to reiterate again earlier. that if I want to have the CU > 3m away, my tails need protecting with a lower rated fuse. Like all of us, just trying to get my head around lots of things, trying to cut through the grey stuff to the yes and no's. Link to comment Share on other sites More sharing options...
Thorfun Posted February 7, 2023 Share Posted February 7, 2023 understood. @ProDave is an electrician so he would know for sure! all i can say is i have 100A at the head and 100A switched fused isolator next to the head with 25m of SWA running to the house CU. so no de-rating here and it's been my electrician that has done all the work not me! i don't have any photos of the insides of the switched fuse isolator but i can go and take some if you want. Link to comment Share on other sites More sharing options...
crispy_wafer Posted February 7, 2023 Author Share Posted February 7, 2023 29 minutes ago, Thorfun said: understood. @ProDave is an electrician so he would know for sure! all i can say is i have 100A at the head and 100A switched fused isolator next to the head with 25m of SWA running to the house CU. so no de-rating here and it's been my electrician that has done all the work not me! i don't have any photos of the insides of the switched fuse isolator but i can go and take some if you want. That's fine, dont worry about photos. Spark at work threw a suggestion to use same rating but lower class fuse. So it would blow quicker than the DNO's... It'll be good to know for sure, and when all said and done I'll post my findings and doings. 1 Link to comment Share on other sites More sharing options...
ProDave Posted February 7, 2023 Share Posted February 7, 2023 I just take the view that the supply head normally says "100A" being the rating of the fuse carrier. It is unusual to have a label to say what actual fuse is in there and of course it is sealed. So I have no qualms fitting an 80A fuse after it. Lets be honest if something bad has happened to blow your 80A fuse it does not really matter if the DNO's fuse has gone as well, they will come and replace it and re seal. You are not going to overload anything or make a dangerous situation. 2 Link to comment Share on other sites More sharing options...
Carrerahill Posted February 7, 2023 Share Posted February 7, 2023 (edited) 1 hour ago, crispy_wafer said: Just taking what my spark said, and nick seemed to reiterate again earlier. that if I want to have the CU > 3m away, my tails need protecting with a lower rated fuse. Like all of us, just trying to get my head around lots of things, trying to cut through the grey stuff to the yes and no's. The DNO's, depending on which one, state 2m or 3m. The 3m probably stems from the positions of devices for protection against overload reg, 433.2.2 (ii) which states, and I paraphrase, that the protective device can be along the run of the conductor as long as it doesn't exceed 3m. In other words I can protect a 3m cable from overload at the 1.5m point if I want. If you wish to extend the supply from the DNO fuse/switchfuse some distance, i.e. greater than 3m, then you need to add a layer of protection before you head off into your property. This is often done with 25mm² SWA and a switchfuse or MCCB. The 25mm² SWA will carry a full 100A (probably pretty unlikely load requirement!) up to a max of 27m before voltage drop would be too great at max rated current, greater than 27m you would need to go 35mm². The 80A may be to discriminate, but, I'd just keep a spare fuse quietly somewhere in case! It's not that easy to blow a DNO fuse. Also, when you look at the breaking curve of a DNO BS88 80A and 100A fuse - there is not much in it... you could draw 100A through an 80A fuse for over an hour before it would blow. Edited February 7, 2023 by Carrerahill Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now