Main Fuse rating 60A/500V - enough?

[David WE Roberts]

My main fuse is rated at 60 Amp/500V.

Adding up all the circuits

5A Lighting 5A Lighting 16A Immersion 45A Shower 32A Ring 32A Ring 32A Cooker ___________ 167A Total

Now I don't expect to have all circuits maxed out at the same time but the full board is close to 3 times the main fuse rating. If (which I probably won't) I ever had the immersion heater and shower on at the same time I would be fairly near the loading of the fuse. If I had both ovens and the electric hob going as well to cook Christmas dinner then I might be drawing more than the rating on the fuse.

I do know that the lights dim slightly when the shower is switched on.

So how do you size the main fuse? Is it linked to the capacity of the mains supply? If so, when do you decide that youi have to uprate the mains supply?

Assuming 230V average supply I think I could draw up to 13.8Kw before exceeding the fuse rating.

Could alsways use the energy monitor to test how much I draw with the main appliances on :-)

Cheers

Dave R

[NT]

The keyword is diversity

NT

[David WE Roberts]

*The keyword is diversity

Really? Cool - both my kids went.

[David WE Roberts]

*The keyword is diversity

Having Googled

of the biggest break + 40% of the rest

45 + (122*.4) = 93.8

Which suggests that I should have a 100A fuse.

Next question, what diameter main cable do I need to support this? [a 60A fuse should not contribute to the lights dimming but insufficient power through the main feed might]

Which also leads me to question why an electrician would fit a consumer unit which had a diversity so far above the main fuse rating.

Obviously pre Part P, but is this normal practice?

Chers

Dave R

[John Rumm]

Indeed, and that is the fundamental point. Have a read of:

If (which I probably won't) I ever had the immersion heater and shower

Which in itself is not a problem - fuses blow on overload due to a combination of both severity of overload and time. A 60A incomer fuse will take 100A for a significant time without blowing. See the time Vs current curves here:

I do know that the lights dim slightly when the shower is switched on. Yup, quite common. You would be able to measure a voltage drop quite easily as a result of the load.

See above - however in reality, you normally get what you are given, unless you specifically ask for more.

Sometimes, also quite often the size of the tails between the fuse, meter and CU.

When it keeps popping the fuse during normal everyday use.

You could draw significantly more... see the comment about current and time above.

[js.b1]

There is no problem for 2 reasons...

#1 - Diversity. It is unlikely that all circuits will be drawing max current at the same time - and all the time.

#2 - Fuse does not blow at 60A. A 60A fuse will happily carry 100A for some time, by design. A 100A fuse requires over 1250A (0.3 megawatt) in order to blow within a second or so. A 13A plug fuse requires 19.7A to blow within a second or so.

Thus you can "overload" a 60A fuse to 90A for an hour or more, at 70A the fusing time is probably near a day. Sorry can not read my BS1361 fuse chart properly (scan of a photo of a photocopy).

The only exception to this is two 45A showers - but even then both are not likely to be operating at the same time AND a 90A load will not blow a 60A fuse that quickly. The fuse above 60A is 80A, some suppliers will upgrade for a fee. I recall they are downgrading 100A to 80A for network reasons during routine work.

[David WE Roberts]

Just read the second page and found a well known contributor to this newsgroup asking very similar questions. I blame the government.

[Andrew Gabriel]

I regard this recommendation as out of date now. It applied when everyone had 1 ring circuit, 1 lighting circuit, etc. I generally install several of each now, so that any problems with one are limited in scope, particularly when using RCBOs. That doesn't mean that because my house has 4 ring circuits, it will be drawing any more power than when it had just one.

That counts as "suppliers works" and it's up to them. It doesn't come under the wiring regs, and you will sometimes find it's thinner than the wiring regs would have permitted.

60A is a bit low for anyone with an electric shower, but maybe he also shares my views above, since you have 2 ring circuits. Nowadays, it would make more sense to base the diversity load from all socket outlet circuits on the total floor area supplied and if the house has central heating. He can't change the main fuse anyway - only the supplier can. Last time I had one uprated, it didn't cost anything, but that's a while back. It can only be done if the fuse box and associated wiring is in good condition and already rated for 100A, and the earth/service bonding is up to current standards.

Fairly.

[John Rumm]

Try that:

The only exception to this is two 45A showers - but even then both are

Indeed - they did that on ours when replacing all the local aerial LV wiring recently.

[John Rumm]

And a similar set of answers as well which should at least be reassuring. (aforementioned "forum" would appear to be little more than another of these usenet scraping web front ends - so basically just uk.d-i-y on the web)

[tony sayer]

Have you ever blown the 60 Amp fuse at all?..

If and when you do then perhaps is the time to have it changed, if its failure was from overload.

Also can you afford the power to blow said fuse;!.....

I was chatt>

Might be more to do with how and from where your fed...

[The Natural Philosopher]

Yup! :-)

But its not germane to this thread so I'll get my coat..

[PeterC]

Tried to do mine when the meter was changed but I persuaded the chap to leave in the 100A one.

[js.b1]

Looks like a 60A fuse will blow after 58min at 105A. Whilst an electric shower, cooker, couple of 2kW fan heaters, dryer, microwave, lights may be on all at once - not many people will shower for 58mins continuous.

Interestingly an 80A fuse will hold for 125mins at 130A. That is two electric showers and rather a lot more, for 125mins... would not be any skin left after a 125min shower :-)

I suspect that may be due to the cost of maintaining a sufficiently low Ze for a 100A fuse throughout the LV network, although it could simply be a very rational decision based on the real-world capacity of an 80A fuse. BS1361 has generous trip limits.

I think the supply cable is typically 35mm with typical 135A rating, but no idea if that is peak or what. I think the 135A figure is for PILC when it was used to supply two houses on a loop-in supply. Then I think one house can be 60A and the other 80A, so perhaps the max figure is basically just before it turns into molten copper marbles underground or steam comes up. Loses in LV are about 7%, a rather considerable amount of global warming... :-D

[John Rumm]

Ours gave a rather feeble reason of "he was not sure if the tails were large enough - they might only be 16mm^2). However we rarely pull more than 30A so it was a non issue really.

[David WE Roberts]

Isn't that a bit like asking "Have you ever had an accident? No? Then why do you need insurance?" Blowing the main fuse could be a real disaster in the winter and is certainly something I would prefer to avoid.

Also, I am planning to have power taken down to the Mother Of All Sheds when we have the building work done, so I would like to be sure that I have enough spare capacity.

Fortunately I should be able to trade this off against the electric shower which will be surplus to requirements once we get the new heating system in.

Cheers

Dave R

[John Rumm]

(which might be down to their choice of cable sizes - by keeping them small they transmute overload problems for their supply into undervolt problems for the consumer!)

[Andy Burns]

When I moved into this house, the cut-out was fused at 60A and all meter tails were 16mm^2, When I replaced the C/U I upgraded the tails to

25mm^2, the meter was recently replaced and the chap upgraded the suppliers cabling from cut-out to meter with 25mm^2 and upgraded the fuse to 80A without being asked.

[ARWadsworth]

You have to apply diversity to the diversity rules when there are 4 ring circits in a house:-)

I can count on one hand the number of times I have seen a blown main fuse (and I have a normal number of digits). Fuses do age with time and that probably accounted for most of the failures.

[John Rumm]

If you search the wiki for "fuse" and "mcb" you should be able to find decent scans of all the common trip curves. Just click on the graph to get a bigger version.

Yup, 125 mins in the shower does seem a little OTT (well on your own anyway! ;-)