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How much current safe for 30m extension?

Discussion in 'Electrical Engineering' started by Sammo, Feb 12, 2005.

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  1. You can draw the same current through any length of wire, asd long as it
    isn't coiled up, because the heat per unit length does not vary with the
    length, and that is what causes temp rise.

    Which is why cable is rated in amps, not watts. Nor yet amp-meters.

    The v drop on even quite extended lengths of extsnion cable is not
    likely to be an issue.
     
  2. Owain

    Owain Guest

    | Depends on if you are in serise or parallel with it!

    I *still* get confused with blue being live because it's a 'brighter' colour
    than brown in cables; WTF did cerise come into it? Will Channel 5 be giving
    us Colin & Justin's How Not To Write Wiring Regulations in the near
    future...

    Owain

    (This posting may contain UK-specific cultural references.)
     
  3. Owain

    Owain Guest

    | However, if the cable has resistence then presumably current capacity
    | (ignoring voltage) gets reduced. I am thinking of W = I^2 * R.

    No, because that resistance - and hence the heating effect - is spread along
    the length of the cable. If 1m of cable is rated for 10A then 100m of cable
    will also be rated for 10A. You will probably need a thicker cable to reduce
    the voltage drop, but that is not the same as the current-carrying capacity
    of the cable.

    | I have got a plug-in RCD device but I know nothing about earth fault
    | loops. It is similar to the one in this illustration.
    | http://www.argos.co.uk/wcsstore/argos/images/9828331A59IFN104773M.JPG
    | Where should my RCD device be plugged in to best avoid the problem of
    | earth fault loops:
    | (1) At the mains supply end where the first extension goes into the
    | mains socket.

    Here, because the RCD protection must be applied to the whole extension
    flex, because it is in effect a portable appliance.

    | (2) In the middle of the two 30m extensions.

    No, because that would leave the first flex (or more specifically - someone
    handling it) unprotected.

    | (3) At the far end of the two extensions where the appliciance I am
    | using is plugged in?

    No, same reason.

    If you were wiring a *permanent circuit* then RCD protection could usually
    be applied at the load end, because fixed wiring does not need such a high
    standard of protection.

    Note, however, that using an RCD plug on only one extension lead raises the
    possibility that someone might plug the extension leads togther the wrong
    way round, with the result that the first flex would be unprotected. The way
    round this is to have an RCD plug on both leads -- this will do no harm, but
    either or both may trip in the event of a fault -- or use an RCD socket.

    The point remains -- why are you contemplating using 60m extension cables?

    Owain
     
  4. Negligent! Hah-hah-hah-hah!
     
  5. Sammo

    Sammo Guest


    I live in a flat and my car is kept in a garage in a block which is
    in the yard. The garages do not have power and sometimes I might
    need to use a power drill, electric light, soldering iron, charge a
    run-down battery, etc.

    This is the layout:

    I'm on the third storey and my cable flex would be slung along the
    building for about 30 metres and be supported once where it goes
    out of my flat and supported again (at about the same height) where
    it leaves the building 30m later.

    The next 15 metres of cable would be a sort of descent to my garage
    roof. The last 5 metres or so takes the power to where I want it.
    (I am using 30m + 30m because those are the lengths on my
    two extension reels).

    In this scenario, I don't really need to protect the cable from
    accidental cutting or damage anywhere along its length as it runs
    along the building or in the air away from any likely harm.

    For my *own* protection as a user of an applicance at the far end of
    the cable it seems that it might be better to put an RCD close to
    whatever appliance I am using. Is this correct?

    OTOH maybe a domestic RCD is so sensitive that it is likely to work
    perfectly well at the mains end even when I am chiefly looking to
    protect me at the far end?
     
  6. Basically, the gizmo senses when the current being shoved up
    the live wire is greater than what comes back down the
    neutral wire. If it is greater, then the difference, instead
    of coming back down the wire, is going somewhere else - like
    through /you/. If it finds a difference it cuts off the
    power, very quickly.

    So its sensitivitity will be the same no matter where you
    put it - as the same current should be going up the live and
    back the neutral - where ever you measure it.

    Putting it in the wall socket ensures that the insulation of
    the whole length of leads as well as the load is being
    monitored. which give the most protection. Plugging it in
    after the leads means that the unit doesn't monitor for
    faults in the leads themselves, only in the load.

    The only thing you gain from plugging it in at the load
    end is if you have a piece of hardware that is prone to
    tripping rcds on switch-on. Having it near the load makes it
    easier to reset the thing if it trips and saves the hike
    back upstairs. But you would only plug it in there for that
    sort of reason.

    Always use the test button each time you set this up. They
    are remarkably sensitive devices that have to sense a
    difference of a few hundredth's of an ampere in a load of
    many amperes. For the cost, as I think another op may have
    said, why not fit two - one at each end? Particularly if you
    are lying on fairly damp ground under a car, clutching a
    mains lamp in your hand.. I have a main one protecting the
    sockets for the whole house (excepting a ring main that only
    feeds my fridges and freezers - where each socket has an
    rcd), but still use an additional plug-in one when doing
    anything that makes me nervous.
     
  7. Owain

    Owain Guest

    | > The point remains -- why are you contemplating using 60m
    | > extension cables?
    | I live in a flat and my car is kept in a garage in a block which is
    | in the yard. The garages do not have power and sometimes I might
    | need to use a power drill, electric light, soldering iron, charge a
    | run-down battery, etc.

    [f/u set to uk.d-i-y as getting off-topic for other groups and definately
    on-topic for this group; subject line changed]

    Ah-ha! "How do I get electricity to my garage" is the real question you
    should have asked.

    This has been discussed before -- google for threads about garages without
    mains power. Suggestions usually include a battery (recharged in the house,
    possibly on a trolley for portability) with an inverter, or a small
    generator. Photovoltaic (solar) panels are sometimes viable for keeping a
    car battery topped up.

    Especially as you have a 'block' of garages, it might be financially viable
    to have a new public supply laid on to the block, as the cost can be divided
    between all the garages, if other owners agree. Power to a garage is useful,
    especially if remote from the house, and the cost of the supply would
    probably be recouped in sale value. That might not be the case for a new
    supply for one garage. A proper fixed supply will also allow garages to be
    fitted with mains-powered alarms, which might help persuade your neighbours
    to agree with the scheme.

    | This is the layout:
    | I'm on the third storey and my cable flex would be slung along the
    | building for about 30 metres and be supported once where it goes
    | out of my flat and supported again (at about the same height) where
    | it leaves the building 30m later.

    PLEASE STOP RIGHT NOW.

    What you are proposing is wholly unacceptable. Ordinary flex and cable is
    not designed for being self-supporting over this distance. The flex will be
    under considerable mechanical strain at the supports. Have you considered
    what happens when 30m of cable comes loose and whips through the air?
    Extension lead flex is also not designed for permanent exterior installation
    and is not completely waterproof or resistant to abrasion and uv light.

    What you are proposing doing is fixed wiring and should be done according to
    the regs for a permanent installation -- designed, installed, and inspected
    and tested in full compliance with the IEE Wiring Regulations.

    As I surmise you will also be running this supply across other people's
    property -- even if only the freeholder's -- you will need legal permission
    from them to do this.

    | The next 15 metres of cable would be a sort of descent to my garage
    | roof.

    There are strict regulations over the height of suspended cables -- they
    have recently been increased following, I think, the death of a BT engineer.

    | The last 5 metres or so takes the power to where I want it.
    | (I am using 30m + 30m because those are the lengths on my
    | two extension reels).

    And you are going to waterproof the connection between the two extension
    reels how?

    | In this scenario, I don't really need to protect the cable from
    | accidental cutting or damage anywhere along its length as it runs
    | along the building or in the air away from any likely harm.

    On a domestic installation "protection by placing out of reach" is not
    allowed as a means of protecting either cables or humans.

    There are serious public liability issues with what you are proposing. If
    anyone is hurt or killed -- even as a result of their own actions eg
    vandalising the flex -- you will have to justify your actions in a coroner's
    court, and possibly face a charge of manslaughter.

    | For my *own* protection as a user of an applicance at the far end of
    | the cable it seems that it might be better to put an RCD close to
    | whatever appliance I am using. Is this correct?
    | OTOH maybe a domestic RCD is so sensitive that it is likely to work
    | perfectly well at the mains end even when I am chiefly looking to
    | protect me at the far end?

    I'm not going to say *anything* which could possibly encourage you to
    consider what you are proposing any further. It frightens me :)

    Owain
     
  8. Sammo

    Sammo Guest


    Yes I want power in my garage but only on a temporary basis such as
    those occassions when I ned to charge my car battery or use power
    tools, etc as I described.

    You talk a lot of sense but my neighbours do not. They are not DIY
    or car enthusiasts and have no ineterst in laying on power to the
    garage block. :-(

    I was thinking of this arrangement purely on a temporary basis. So
    far I have laid the wire on the ground but as it can get damp and/or
    dirty then I figured it might be better to suspend it in the air.

    Furthermore other might trip on it (even though it is a red colour)
    and it seesm better not to inconvenience them.

    This is unlikely to be an issue as we have a 999 year lease and the
    freeholder is an overseas trust fund. This means that we maintain
    the flats through our own management company and it holds a lease for
    the common parts.
    No need as I will be using the two extensions in much the same way as
    someone might use them for powering a mains hedge trimmer.
    Heh! :)
     
  9. I must admit to doing much the same when I was living in a
    London flat - for very similar sorts of reasons and only to
    rig up and put away each time I used it. It obviously
    depends on your physical arrangements as to whether it is
    totally reckless or not quite totally so.

    What I did, however, was to take some polypropylene cord and
    tied figure of eight knots on the bight every foot or so,
    with the loop big enough to pass the plug through. You have
    guessed the rest - one end of the cord was tied to the
    window frame and the other to the garage and the power cable
    just ran down through the loops. That way, the weight of the
    cable was taken by loops in the tensioned cord -
    particularly important at the ends, where otherwise the
    cable would have had to be tied around something. Where I
    had to do a change in direction to avoid the cable rubbing
    against anything, I just took a piece of cord from the main
    cord and tied it off, to a tree branch, IIRC - and then
    tensioned the cords to get the run I wanted..

    I don't know your layout and can only advise that doing
    things like this is inherently high risk - but, if you are
    going to do it anyway, this might help reduce that risk a
    little. If it goes wrong, as you have been clearly told -
    you will be wishing that you just bought an inverter or a
    small genny (IIRC, B&Q do a perfectly good one for under
    100GBP) - I have one and it has paid for itself time and
    time again. They weren't available at anything like that
    price when I played silly games with cables.


    If you do think about leaving the cord(s) (but not the
    cable) in place - with a pull through extra cord so that you
    can pull the cable down as and when you need it, be aware
    that man-made cords can be very affected by sunlight, by
    relatively modest sudden shock loads, by abrasion, by
    pressure at points of sharp change in direction, by the
    knots and by a whole lot more. So you need to select the
    cord carefully with a very high factor of safety and inspect
    it often. Also note that birds will sit on it and do what
    birds do to whatever is underneath - so routing it over a
    neighbour's patio is not a good idea - unless you live in
    London*.

    --

    Sue
    * Where few flat-dwellers even know the names of their
    neighbours and then only because of arguments about noise,
    parking spaces, children, curtains, etc - you might as well
    add the birds' efforts...
     
  10. Roy Q.T.

    Roy Q.T. Guest

    SHEESH ! all that fuss over this ?

    I dug out a small 12-18 inch trench for a landlady upstate and
    eliminated a Fine Mess of Cords just like the ones portrayed here, over
    10ft high the finshed grade It had a Guy Wire and was Semi-Permanent she
    just got tired of the eyesore it was.

    Any how; I used some liguid tight rigid conduit with three no. 10 awg to
    a New doube GFCI Breaker straight into the Garage Siding and even laid
    out some swithced Lighting and convenience Outlets for her....

    If you're going to live there for 999 years, I Mean };-)

    Why can't you do the same there ?

    Send a Bill to the Holding Company, they just might pay for it after the
    fact, or a court would probably award the deduction from the rent :-o
    given the necesary & permanent nature of the upgrade to the garage.

    No Breaker Panel ???., Sheesh ! Move outa there };-)
     
  11. Roy Q.T.

    Roy Q.T. Guest

    If a wood chuck, could chuck wood ?
     
  12. Lots of reasons, I'm sorry to say.

    1) This would count as domestic wiring and come under Part
    P. Which means a qualified /company/ would have to do the
    work or it would have to be inspected by the local authority
    building inspectorate, as it is exterior to the flat.
    Either of which would really cost.

    2) Flats in the UK tend to be built of either a lot of brick
    or concrete and getting a cable to (under) ground in the
    right place, from an upper flat, would be both very
    difficult and expensive. Plus there could be big issues
    about things like breaching fire barriers. Not to mention
    getting all the way leaves. Also, many people are scared
    stiff of the mention of the words "power lines" and would
    probably refuse a wayleave simply because /they think/ it
    might lower the value of the property or give their kids
    cancer.

    3) Depending on the location, the changes could either be
    considered "change of use" or could come under the umbrella
    of a National Park, Area of Outstanding Natural Beauty,
    changes to a listed building, National Rivers Authority,
    Department of the Environment and a whole host of other
    bodies that delight in making anything simple into a
    nightmare of red-tape and expense. I come under several of
    the above and to put a 4'x4' tool store in my garden took
    over a year and cost, well, you wouldn't believe what the
    total came to.

    4) At this time of the year, large parts of the UK are cold,
    damp and miserable and an garage in a block of garages is
    not a place you want to be. The roof and door are probably
    single ply steel sheeting with no heat insulation at all.
    The floor is probably concrete with no underlying
    insulation. The walls are the thinnest, single leaf,
    concrete block. If anyone noticed that you had left tools
    and things of value in it, they would be gone very quickly.
    So the temporary rigging of a power line for the odd times
    that you might want to use one makes sense.
     
  13. Al

    Al Guest

  14. Roy Q.T.

    Roy Q.T. Guest

    I understand the business of red tape and diversity and that cutting
    corners is not the best approach in all circumstances., specially if you
    have national or park real estate or nothing but concrete in between.


    Point Taken: why can't he get one of those 50-100 Ft retractable cord
    systems and attach it to his flat, this way all he needs to do is pull
    it out to the garage and pull to retract it back to his window or
    location when he's done., I've seen some with brackets that makes the
    cord assembly easy to unhook & put away safe from adversely fast hands.


    all he'll ever need is to put a large hook on top side the garage or
    something to temporarily hold the cord up out of the way.
     
  15. Roy Q.T.

    Roy Q.T. Guest

    Finally ! A Real Answer to this Question ? hahaha


    Thanks !
     
  16. Roy Q.T.

    Roy Q.T. Guest

    are woddchucks chucked yuck !!!

    Palindra ???????? hmmmm you married or otherwise tarried ?

    it kind of prjingles off the tongue };-)

    ®oy
     
  17. Sammo

    Sammo Guest


    I am the OP and in effect this sounds like I wanted to do (unless I
    have misunderstood you). However instead of owning a 50 to 100 ft
    length of cable, I own two 30 metre lengths.
    Yup that's what I was going to do but the power losses and safety
    worried me a bit.
     
  18. Bill Woods

    Bill Woods Guest


    This sounded interesting so I checked out B&Q's website by searching
    for "generator".

    I found this http://tinyurl.com/4bo8k They cost from £250 to £430!

    What was the £100 one you are refiing to?
     
  19. Rob Morley

    Rob Morley Guest

  20. Bill Woods

    Bill Woods Guest

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