Overload Protection - Major Renovation

[omega750]

Hi All,

First post on this forum, but I've been reading the great questions and answers for years, so I hope someone can help me out?

I've renovated a few houses in my time and done some of the basic wiring myself, but my latest 1940's house is a real project and needs a professional's attention. Unfortunately my usual electrician of choice is in hospital (no he didn't electrocute himself) so I have had to find another.

Overview
Old house, very poor 1940's wiring all needs replacing! From what I can tell there seem to be effectively 3 sub mains currently installed, all connected directly to DNO supply via a set of Henley Blocks.

• Main CU (old fuse based unit)
• Summer House 10mm2 SWA to 60A MCB CU
• Garage 10mm2 SWA to 80A MCB CU

Electricians Proposal
The electrician has pointed out that there is no overload protection on the circuit in the summer house and garage which are more than 3m away from the DNO supply and connected via Henley Block. He has proposed the following setup, again all connected to DNO supply via existing (probably replaced) Henley Blocks.

• Replace existing main fuse based CU with 100A MCB CU
• Install 1x Wylex 60A SF connect to 10mm2 SWA for Summer House, retain existing 60A MCB CU installed in Summer House
• Install 1x Wylex 80A SF connect to 10mm2 SWA for Garage, retain existing 80A MCB CU installed in Garage

Question:
Please bear in mind we are way past my level of expertise, but I am not sure how this is providing overload protection for the DNO supply as there is still only one set of tails to the meter and the DNO fuse behind that, presumably rated at 100A.

Whilst I see it protects the individual sub mains, what is preventing overload of the DNO supply other than its 100A fuse, given we have potential 100A + 80A + 60A draw via the Henley Blocks?

I was wondering if someone could just sanity check this proposal for me, hopefully it will just be a case of me not understanding overload protection otherwise I might need to consider finding myself another electrician?

Many thanks in advance.
Jason

EDIT: Garage has a car lift and some other machinery hence 80A CU

 

[ImpededLoop]

Your electricians proposals sound pretty fair. In reality it's unlikely that your circuits or submains will ever be fully loaded. For example what would use 60A in the summer house?

Basically, you can run tails up to 3m from the meter position without providing any circuit protection (fuses) so the tails to the house DB and to the switch fuses are only protected by the electricity boards fuses. The installation should be designed so that they are never overloaded.

 

[westward10]

I agree the loading of the supplies is probably well over specified. Not sure I would be protecting one with an 80A fuse though or 60A if it is buried in the ground. Don't know the garage demand but the summer house would suffice on a 45A fuse.

 

[SparkyChick]

Someone can correct me if I'm wrong, but the 80A fuse is too big to protect the SWA to the garage. In free air, assuming it's 2 core, 4D4A states 72A whilst buried in the ground it's down to 60A.

 

[Midwest]

Me thinks someone's just after a step by step guide. Might be wrong, lets see what other questions follow

OP whilst your electrician of choice is in hospital, and is unlikely to be able to assist in your project, why not ask him or her to recommend another?

 

[omega750]

Hi Guys,

Wow, thanks for your quick responses, here's a few updates.

• Both SWA are 3 core and buried in the ground, but have been conveniently put in plastic pipes so I could pull new if required
• I guess he is just trying to save me some money by reusing the existing 60A an 80A CU's in the Summer House and Garage

Regarding the load, I agree, seems a little excessive given my quick calcs don't add up to anywhere near that, here are the details for both.

Summer House

• 1x 32A ring for Kitchen (Dishwasher, 2 Ovens, 1 Hob, 5x DG Power Sockets)
• 1x 32A ring for rest (3 rooms 12x DG Power Sockets)
• 1x 6A ring all lights (4 rooms inc. Kitchen total 16x 5w LED spots - not very good diversity as all lights go out if you trip the circuit)
• 1z 16A radial for Boiler + Hot Tub seems to be using this circuit as well but that is probably going in the bin!

Garage

• 1x 32A ring (10x DG Power Sockets)
• 1x 16A radial (Compressor)
• 1x 16A radial (Car Lift)
• 1x 6A ring for lights (8x Striplights)

Hope that helps.

Cheers
Jason

 

[westward10]

Someone can correct me if I'm wrong, but the 80A fuse is too big to protect the SWA to the garage. In free air, assuming it's 2 core, 4D4A states 72A whilst buried in the ground it's down to 60A.

Indeedy I said the same. We need to know if it is two core or three core, thermoplastic or thermosetting.

 

[westward10]

Not sure I would call that a summer house, more like a small house.

 

[ImpededLoop]

There's a fair amount of work. If it were me I'd get a couple more quotes for comparison.

 

[telectrix]

Indeedy I said the same. We need to know if it is two core or three core, thermoplastic or thermosetting.

makes no odds. current rating must depend on the switchgear etc. that is connected to the cable. mostly 70ddeg.

 

[omega750]

Thanks again, you boys are on fire today ;-)

You're right about the summer house but I think that was for "Planning" purposes only ;-D

Did just edit the post, but all SWA are 3 core and buried in the ground, not sure if they are thermoplastic or thermosetting but could pull new if required, as they were conveniently laid in plastic pipe conduit.

As for the quotes, it might be a good idea, but it was actually quite reasonable as I will pull the new cables in the main property myself and he said the wiring in Garage and Summer House seems relatively new (its Blue/Brown not Red/Black).

Mainly I need him to do the design work and install all the big bits, I'll have a cable pulling party for the rest ;-)

Cheers
Jason

 

[westward10]

makes no odds. current rating must depend on the switchgear etc. that is connected to the cable. mostly 70ddeg.

Any idea what current these cables would have to draw to reach 70 degrees

 

[omega750]

@Midwest

If I was after a step-by-step guide I would have asked for one!

But hey, if you feel like you're being abused, I'm an IT geek so if you have any IT related questions you would like answering post them up and I will do my best to answer them, always happy to exchange knowledge.

As for asking for a recommendation, I would, but I've only spoken to his wife, he's in hospital for a heart bypass operation and its not like were best buddies so I kinda feel it would be overstepping the mark a little to pop in and ask him!

Jason

 

[Midwest]

@Midwest

If I was after a step-by-step guide I would have asked for one!

But hey, if you feel like you're being abused, I'm an IT geek so if you have any IT related questions you would like answering post them up and I will do my best to answer them, always happy to exchange knowledge.

As for asking for a recommendation, I would, but I've only spoken to his wife, he's in hospital for a heart bypass operation and its not like were best buddies so I kinda feel it would be overstepping the mark a little to pop in and ask him!

Jason

I don't understand why your asking such in depth questions, when as you've said yourself, your just an IT person, and obviously would not be doing this work yourself.

I don't feel abused but to quote you;

'I've renovated a few houses in my time and done some of the basic wiring myself, but my latest 1940's house is a real project and needs a professional's attention. Unfortunately my usual electrician of choice is in hospital (no he didn't electrocute himself) so I have had to find another'.

So that suggest to me you do intend or are thinking of doing this work yourself, and asking for design advice. I've seen similar statements in this & other forums before. Be honest. If I've completely read the situation incorrectly, then I apologies. However, the members here have a lot of knowledge & experience, I would not be impressed if someone was trying to hoodwink them.

Would it not be best you found that 'another electrician'. Of course, if you are feeling abused, you could just hit the report button

 

[davesparks]

The basic principle of feeding the submain via switchfuses is sound, and a better idea than feeding them from mcbs. But as mentioned the details seem a little wrong.
80A on a 10mm SWA doesn't sound like a good plan at first glance, especially when you consider volt drop