Main Fuse rating 60A/500V - enough?

Discussion in 'UK DIY' started by David WE Roberts, Dec 22, 2011.

  1. 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
    --
    No plan survives contact with the enemy.
    [Not even bunny]

    Helmuth von Moltke the Elder

    (\__/)
    (='.'=)
    (")_(")
    David WE Roberts, Dec 22, 2011
    #1
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  2. David WE Roberts

    NT Guest

    On Dec 22, 11:43 am, "David WE Roberts" <> wrote:
    > 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 onat
    > 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


    The keyword is diversity


    NT
    NT, Dec 22, 2011
    #2
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  3. "NT" <> wrote in message
    news:...
    On Dec 22, 11:43 am, "David WE Roberts" <> wrote:
    > 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


    *The keyword is diversity



    Really?
    Cool - both my kids went.

    --
    No plan survives contact with the enemy.
    [Not even bunny]

    Helmuth von Moltke the Elder

    (\__/)
    (='.'=)
    (")_(")
    David WE Roberts, Dec 22, 2011
    #3
  4. "NT" <> wrote in message
    news:...
    On Dec 22, 11:43 am, "David WE Roberts" <> wrote:
    > 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


    *The keyword is diversity

    Having Googled http://www.diy-forum.net/rating-main-fuse-t50249.html

    100% 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

    --
    No plan survives contact with the enemy.
    [Not even bunny]

    Helmuth von Moltke the Elder

    (\__/)
    (='.'=)
    (")_(")
    David WE Roberts, Dec 22, 2011
    #4
  5. David WE Roberts

    js.b1 Guest

    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.
    js.b1, Dec 22, 2011
    #5
  6. "David WE Roberts" <> wrote in message
    news:...
    <snip>
    > *The keyword is diversity
    >
    > Having Googled http://www.diy-forum.net/rating-main-fuse-t50249.html

    <snip>

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

    --
    No plan survives contact with the enemy.
    [Not even bunny]

    Helmuth von Moltke the Elder

    (\__/)
    (='.'=)
    (")_(")
    David WE Roberts, Dec 22, 2011
    #6
  7. In article <>,
    "David WE Roberts" <> writes:
    > *The keyword is diversity
    >
    > Having Googled http://www.diy-forum.net/rating-main-fuse-t50249.html
    >
    > 100% of the biggest break + 40% of the rest
    > 45 + (122*.4) = 93.8


    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.

    > Which suggests that I should have a 100A fuse.
    >
    > Next question, what diameter main cable do I need to support this?


    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.

    > [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.


    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.

    > Obviously pre Part P, but is this normal practice?


    Fairly.

    --
    Andrew Gabriel
    [email address is not usable -- followup in the newsgroup]
    Andrew Gabriel, Dec 22, 2011
    #7
  8. David WE Roberts

    tony sayer Guest

    >*The keyword is diversity
    >
    >Having Googled http://www.diy-forum.net/rating-main-fuse-t50249.html
    >
    >100% of the biggest break + 40% of the rest
    >45 + (122*.4) = 93.8
    >
    >Which suggests that I should have a 100A fuse.


    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 chatting to a UK Power networks bloke at a roadside distribution
    unit the other month seemed each phase had just a 300 amp fuse on for
    supplying the estate, some 100 odd houses so he said...


    >
    >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]


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

    --
    Tony Sayer
    tony sayer, Dec 22, 2011
    #8
  9. tony sayer wrote:
    >> *The keyword is diversity
    >>
    >> Having Googled http://www.diy-forum.net/rating-main-fuse-t50249.html
    >>
    >> 100% of the biggest break + 40% of the rest
    >> 45 + (122*.4) = 93.8
    >>
    >> Which suggests that I should have a 100A fuse.

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

    Yup! :)

    But its not germane to this thread so I'll get my coat..
    The Natural Philosopher, Dec 22, 2011
    #9
  10. David WE Roberts

    PeterC Guest

    On Thu, 22 Dec 2011 14:51:04 +0000, John Rumm wrote:

    > On 22/12/2011 13:12, js.b1 wrote:
    >
    >> 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).

    >
    > Try that:
    >
    > http://wiki.diyfaq.org.uk/images/0/06/Curve-BS1361.png
    >
    >> 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.

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


    Tried to do mine when the meter was changed but I persuaded the chap to
    leave in the 100A one.
    --
    Peter.
    The gods will stay away
    whilst religions hold sway
    PeterC, Dec 22, 2011
    #10
  11. David WE Roberts

    js.b1 Guest

    On Dec 22, 2:51 pm, John Rumm <> wrote:
    > Try that:
    > http://wiki.diyfaq.org.uk/images/0/06/Curve-BS1361.png


    Brilliant, thanks.

    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 :)

    > > 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.

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


    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
    js.b1, Dec 22, 2011
    #11
  12. "tony sayer" <> wrote in message
    news:...
    > >*The keyword is diversity
    >>
    >>Having Googled http://www.diy-forum.net/rating-main-fuse-t50249.html
    >>
    >>100% of the biggest break + 40% of the rest
    >>45 + (122*.4) = 93.8
    >>
    >>Which suggests that I should have a 100A fuse.

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


    <snip>

    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
    --
    No plan survives contact with the enemy.
    [Not even bunny]

    Helmuth von Moltke the Elder

    (\__/)
    (='.'=)
    (")_(")
    David WE Roberts, Dec 22, 2011
    #12
  13. David WE Roberts

    ARWadsworth Guest

    Andrew Gabriel wrote:
    > In article <>,
    > "David WE Roberts" <> writes:
    >> *The keyword is diversity
    >>
    >> Having Googled http://www.diy-forum.net/rating-main-fuse-t50249.html
    >>
    >> 100% of the biggest break + 40% of the rest
    >> 45 + (122*.4) = 93.8

    >
    > 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.


    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.


    --
    Adam
    ARWadsworth, Dec 22, 2011
    #13
  14. David WE Roberts

    Toby Guest

    On 22/12/2011 11:43, David WE Roberts wrote:
    > My main fuse is rated at 60 Amp/500V.
    >


    Excuse my ignorance, but if it is rated at 60A at 500V, then that is 30
    kW, therefore at 230V it is therefore 130A isn't it?

    --
    Toby...
    Remove pants to reply
    Toby, Dec 23, 2011
    #14
  15. David WE Roberts

    Tim Watts Guest

    Toby wrote:

    > On 22/12/2011 11:43, David WE Roberts wrote:
    >> My main fuse is rated at 60 Amp/500V.
    >>

    >
    > Excuse my ignorance, but if it is rated at 60A at 500V, then that is 30
    > kW, therefore at 230V it is therefore 130A isn't it?
    >


    No.

    it means it is designed for 60A continuous max current of 60A and designed
    to break a supply of upto 500V, which is related to both the insulation of
    the fuse body and the ability to quench the arc caused by the fuse blowing
    under load.

    eg if that fuse blew while carrying 10kV, it would do sod all as the arc
    would just carry on burning across the fuse ends. Hence higher voltage fuses
    tend to be longer.

    Power has no meaning with respect to fuses.

    Cheers

    Tim
    --
    Tim Watts
    Tim Watts, Dec 23, 2011
    #15
  16. David WE Roberts

    charles Guest

    In article <jd1i4u$r9$>,
    Toby <> wrote:
    > On 22/12/2011 11:43, David WE Roberts wrote:
    > > My main fuse is rated at 60 Amp/500V.
    > >


    > Excuse my ignorance, but if it is rated at 60A at 500V, then that is 30
    > kW, therefore at 230V it is therefore 130A isn't it?


    no - those are simply maximum for each unit.

    --
    From KT24

    Using a RISC OS computer running v5.16
    charles, Dec 23, 2011
    #16
  17. David WE Roberts

    Toby Guest

    On 23/12/2011 09:44, Tim Watts wrote:
    > Toby wrote:
    >
    >> On 22/12/2011 11:43, David WE Roberts wrote:
    >>> My main fuse is rated at 60 Amp/500V.
    >>>

    >>
    >> Excuse my ignorance, but if it is rated at 60A at 500V, then that is 30
    >> kW, therefore at 230V it is therefore 130A isn't it?
    >>

    >
    > No.
    >
    > it means it is designed for 60A continuous max current of 60A and designed
    > to break a supply of upto 500V, which is related to both the insulation of
    > the fuse body and the ability to quench the arc caused by the fuse blowing
    > under load.
    >
    > eg if that fuse blew while carrying 10kV, it would do sod all as the arc
    > would just carry on burning across the fuse ends. Hence higher voltage fuses
    > tend to be longer.
    >
    > Power has no meaning with respect to fuses.
    >
    > Cheers
    >
    > Tim


    Ahh yes, I see, thanks :)

    --
    Toby...
    Remove pants to reply
    Toby, Dec 23, 2011
    #17
  18. David WE Roberts

    Tim Watts Guest

    Toby wrote:

    > On 23/12/2011 09:44, Tim Watts wrote:
    >> Toby wrote:
    >>
    >>> On 22/12/2011 11:43, David WE Roberts wrote:
    >>>> My main fuse is rated at 60 Amp/500V.
    >>>>
    >>>
    >>> Excuse my ignorance, but if it is rated at 60A at 500V, then that is 30
    >>> kW, therefore at 230V it is therefore 130A isn't it?
    >>>

    >>
    >> No.
    >>
    >> it means it is designed for 60A continuous max current of 60A and
    >> designed to break a supply of upto 500V, which is related to both the
    >> insulation of the fuse body and the ability to quench the arc caused by
    >> the fuse blowing under load.
    >>
    >> eg if that fuse blew while carrying 10kV, it would do sod all as the arc
    >> would just carry on burning across the fuse ends. Hence higher voltage
    >> fuses tend to be longer.
    >>
    >> Power has no meaning with respect to fuses.
    >>
    >> Cheers
    >>
    >> Tim

    >
    > Ahh yes, I see, thanks :)
    >


    There is another element too.

    While you have an arc (which is why all but the tiniest of fuses are filled
    with powder, and massive substation breakers are filled with oil), you have
    a voltage drop along with a flowing current through the arc. This lasts for
    a certain amount of time, which is a funtion of the fuse characteristic and
    the amount of current flowing.

    The powder (or oil) helps quench the arc, but it has another role too.

    Simple application of E=VIt integrated means that to blow under a fault,
    there will be some energy imparted in the fuse body which ultimately = heat.

    This is why higher voltage and higher current fuses tend to be fatter (100A
    fuse wire is still pretty thin, no need for a 10mm dia fuse body to contain
    it at first sight). The more energy given up means the more thermal-mass
    buffer you need to avoid bad things like the fuse exploding.

    There's another parameter that is related to this too - the "rupture
    capacity". A tiny fuse in a small transformer powered alarm clock can never
    see much short circuit current.

    A dead short on a pair of meter tails with the substation in your back
    garden means a failt current of several thousand amps burning in the arc in
    the fuse. Even if the fuse blows in a few tens of milliseconds, that still
    equates to quite a large number of joules.

    :)

    --
    Tim Watts
    Tim Watts, Dec 23, 2011
    #18
  19. David WE Roberts

    tony sayer Guest

    >> Also can you afford the power to blow said fuse;!.....
    >>
    >>
    >> I was chatting to a UK Power networks bloke at a roadside distribution
    >> unit the other month seemed each phase had just a 300 amp fuse on for
    >> supplying the estate, some 100 odd houses so he said...

    >
    >The diversity applied by the suppliers is actually quite surprising...
    >IIRC they allocate only something like 2kW per household in their
    >calculations. Still, empirical evidence would suggest they get it right
    >more often than not, since supply failure at that level seems very rare.



    Matey from UK power networks told me that fuses mainly fail due to olde
    age rather than overload!...

    --
    Tony Sayer
    tony sayer, Dec 24, 2011
    #19
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