Maximum demand exceeding main fuse. | on ElectriciansForums

Maximum demand exceeding main fuse.

Discuss Maximum demand exceeding main fuse. in the UK Electrical Forum area at ElectriciansForums.net

Bobble23

-
I've got a customer who currently has a 60A main fuse running in 10mm tails to a board with breakers 32/32/20/6/6A which, doing the diversity calc at biggest breaker + 40% of the rest comes in at 57.6A, just about OK.

She wants a new 32A cooker circuit, so I said she should get the tails upgraded and the main fuse increased, but this would involve pulling up her newly-varnished floor so she's said no.

If I was to down-rate the breakers to 32 (for the new circuit) then 20/20/16/6/6 the calc. still comes in at less than 60A (59.2). Can you see any problems with this? Or do I this time have to forget the easy calculation and go round counting sockets and appliances instead?

why do you need to pull up the floor?

Do the tails run under the floor?

The tails should be 16mm at least anyway, so if thats the case and she refuses to upgrade them then theres not much you can do is there

Doing load calculations purely based on breaker size is not how i would determine load, its likely massively misleading, think loads regarding heating, cooking and hot water, these are domestically where your main loads will come from, what are the ratings of these units? electric showers, electric heating, electric cookers/ovens, hot tubs, car chargers etc

got any pictures

Last edited:
I've got a customer who currently has a 60A main fuse running in 10mm tails to a board with breakers 32/32/20/6/6A which, doing the diversity calc at biggest breaker + 40% of the rest comes in at 57.6A, just about OK.

She wants a new 32A cooker circuit, so I said she should get the tails upgraded and the main fuse increased, but this would involve pulling up her newly-varnished floor so she's said no.

If I was to down-rate the breakers to 32 (for the new circuit) then 20/20/16/6/6 the calc. still comes in at less than 60A (59.2). Can you see any problems with this? Or do I this time have to forget the easy calculation and go round counting sockets and appliances instead?

I don't see the sense in dropping the socket circuit breakers from 32A to 20A.

A) You’ll be fine
B) You don’t work out diversity that way - refer to the OSG
C) I’d be looking to upgrade the tails and check the Earthing and Bonding conductor sizes

Thanks folks. I think you're right that I'm trying to find a way to keep the customer happy without doing it the right way which is to upgrade the tails and the main fuse.

I don't have any photos but it's a big old Victorian terrace with the consumer unit above head height half-way down the hall. I've guessed that the cable run (up the side of the house, under the bedroom floors then down into the board) is about 15m. I've done the volt drop calculation which is fine, but as things stand the existing tails are 10mm SY cable, which I've just been told aren't ideal for tails anyway because of the lack of BS certification. Added to this they run straight into the meter without a switch-fuse, which I was going to put in with my new tails because they're more than 3m long.

I suppose the question is, is there a way I can use the existing tails if I can get around the max demand/diversity issue, or should I just walk away if the customer refuses to upgrade?

Is the installation safe for continued use, mmm.

Those 10mm tails are under sized anyway, 16mm for a 60A main fuse minimum.

Thanks folks. I think you're right that I'm trying to find a way to keep the customer happy without doing it the right way which is to upgrade the tails and the main fuse.

I don't have any photos but it's a big old Victorian terrace with the consumer unit above head height half-way down the hall. I've guessed that the cable run (up the side of the house, under the bedroom floors then down into the board) is about 15m. I've done the volt drop calculation which is fine, but as things stand the existing tails are 10mm SY cable, which I've just been told aren't ideal for tails anyway because of the lack of BS certification. Added to this they run straight into the meter without a switch-fuse, which I was going to put in with my new tails because they're more than 3m long.

I suppose the question is, is there a way I can use the existing tails if I can get around the max demand/diversity issue, or should I just walk away if the customer refuses to upgrade?
I think I would be walking away from this job.

SY cable for connecting meter to cut out fuse would have me a bit concerned to begin with.
I assume it is correctly glanded and bonded to the shield?

It is unlikely to be UV protected so will not last well outdoors.

I would be looking to upgrade to 16mm swa as a minimum before adding any load to the board.

Thanks all. I'm tending to agree that if she won't upgrade the tails/fuse I'm better walking away - if she's paying thousands for a new kitchen a couple of hundred quid extra for a safe supply shouldn't be an issue for her, so that's an alarm bell there.

Just for my interest what's the problem with the 10mm SY on a 60A fuse. As far as I've seen it's rated at 61A.

Good point about the UV. It's glanded properly, but I'm assuming white masonry paint isn't best practice for UV protection?!

Just for my interest what's the problem with the 10mm SY on a 60A fuse. As far as I've seen it's rated at 61A.
61A at best, assuming no derating factors.

Personally I would like a little more headroom as the fuse will allow in the region of 90A for 4 hours and by that time things might be getting a little warm.

61A at best, assuming no derating factors.

Personally I would like a little more headroom as the fuse will allow in the region of 90A for 4 hours and by that time things might be getting a little warm.
Can’t say that I necessarily agree with you on this one in terms of logic… we all know that cartridge and rewireable style fuses (of all the various BS standards) are capable of withstanding significantly higher overload currents than the cables commonly attached to them for hours on end, yet nobody is suggesting swapping out 2.5mm 6242Y to 6mm ‘just because’.

Can’t say that I necessarily agree with you on this one in terms of logic… we all know that cartridge and rewireable style fuses (of all the various BS standards) are capable of withstanding significantly higher overload currents than the cables commonly attached to them for hours on end, yet nobody is suggesting swapping out 2.5mm 6242Y to 6mm ‘just because’.
You are right, a couple of beers and an irrational hatred for SY cable installed for no good reason probably had something to do with it.

sounds like one of those jobs to swerve and let it be someone else's problem

Reply to Maximum demand exceeding main fuse. in the UK Electrical Forum area at ElectriciansForums.net

The norm is that it goes from the DSO (ESB) fuse to a DSO isolator with a contractor's side, and then to the main overcurrent device. The ESB...
Replies
9
Views
3K
D
The revised arrangement - please check that I have fully understood the points made in the above threads. Obviously a qualified electrician will...
Replies
5
Views
3K
Deleted member 155212
D
Replies
17
Views
3K
In years gone by many of us would feed an additional small CU [say for a shower or outbuilding for example] with 6mm tails, often directly...
Replies
6
Views
2K
Therein lies the problem. I see no merit in further circular debate, so will bow out of this discussion.
Replies
55
Views
7K
nicebutdim
N