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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. Sammo

    Sammo Guest

    I am in the UK (so mains voltage is about 230V or 240V).

    I have a reel of main extension cable made of 3-core 1.0 mm^2 wire
    rated at 10 Amps. So the nominal power rating would be about 2,400
    Watts. (Link to tech reference for the cable is below.)

    Presumably the current carrying capacity or power delivery
    capability of the unwound 30m length unwound is going to be a bit
    less than 10 Amps/2,400 Watts due to losses along the length of the
    cable itself.

    Is the reduction in current/power carrying capability significant?
    If so, then is there a rough guideline figure for available
    current/power which I can use?

    If I take *two* of these 30m reels then I can join them with the
    standard UK 13 Amp plug and socket supplied on the reels. This gives
    me an overall length of 60m. Taking into account losses, what would
    be the current carrying or power delivery carrying capability of the
    60m length if all the 60m cable is unwound?

    Thanks for any info.
    Sammo


    Technical reference for the cable is HAR type H05VV-F3 x 1.00 mm2 -
    (details at http://tinyurl.com/7y5xx)
     
  2. 30 metres at 10 amperes isn't going to be a problem - so I
    have considered just the longer lengths.

    Basically, you have two factors to consider:

    The first is how hot the wire will get. Now, provided that
    the cable is unwound and in the open air, then its length is
    immaterial. If you coil the cable up or cover it in
    something that restricts the flow of heat from it, that is a
    different matter. However, for an unwound cable, the rated
    current, say 10 amperes, is the maximum that is allowed due
    to heat constraints. Pass more current than this and the
    wire will get too hot. Pass a lot more current than this and
    the insulation could fail.

    The second is the volt drop. Its resistance is about 0.02
    ohms per metre - so a 60 m length will have a resistance of
    1.2 ohms and will drop 1.2 volts per ampere. A 120 m length
    would drop 2.4 volts per ampere.

    Now the equipment at the far end of the cable will have a
    specified range of input voltage for which it was designed.
    You must simply ensure that it gets the minimum rated
    voltage, or higher, at the current it draws. This minimum
    voltage will depend on the equipment type.

    As a rough guide, losing 12 volts in 240 is probably going
    to be fine. So, you could use a 60 metre cable at 10 amperes
    or 120 metre cable at 5 amperes.

    If your load is happier with a lower minimum voltage, say
    220 volts, then you could run the 120m cable at about 8
    amperes. However, you would still be limited to 10 amperes
    for the 60 metre cable, because 10 amperes is the most the
    cable should be used to carry, irrespective of length.

    Purely resistive loads, like heaters, aren't terribly fussed
    if their voltage is a bit low - so you could happily run a
    2kW heater at the end of 120 metres of your cable. If you
    have a lamp plugged in at the far end, then it will get
    noticeably dimmer when the heater is switched on. This isn't
    a safety problem, although it might appear so.

    Some loads are very fussy about their minimum voltage but
    even those should be happy with 10 amperes taken from your
    60 metre cable.

    Hope that helps.

    Sue
     
  3. Bob Eager

    Bob Eager Guest

    Plus the resistance on the two connections. Not sure whether the earth
    fault loop impedance is still OK...
     
  4. Owain

    Owain Guest

    | I am in the UK (so mains voltage is about 230V or 240V).
    | I have a reel of main extension cable made of 3-core 1.0 mm^2
    | wire rated at 10 Amps. So the nominal power rating would be
    | about 2,400 Watts. (Link to tech reference for the cable is below.)
    | Presumably the current carrying capacity or power delivery
    | capability of the unwound 30m length unwound is going to be
    | a bit less than 10 Amps/2,400 Watts due to losses along the length
    | of the cable itself.

    The 10A rating will be for the flex *fully* unwound. If you use the cable
    wound on the reel it will have a lower rating. All cables have resistance,
    and get warm as current passes through them. With the cable unwound, that
    warmth can dissipate safely. If the cable is wound up (or otherwise
    enclosed) that heat cannot dissipate and the cable will get warmer and
    warmer ... possibly to the point it melts and/or starts a fire.

    As an aside, using flex rated at 10A to wire extension leads with 13A
    sockets is unwise, as the cable probably insufficiently protected by a 13A
    fuse. A 1.25mm or, for longer lengths, 1.5mm, flex would be better.

    | Is the reduction in current/power carrying capability significant?

    There is no reduction in the current carrying capacity of the flex due to
    length. The resistance in the cable causes voltage drop, which varies with
    current drawn and length of cable. Voltage drop is a factor in determining
    whether a larger cable size is needed for a given load. Whether it is
    acceptable or not depends on your application.

    | If so, then is there a rough guideline figure for available
    | current/power which I can use?

    Voltage drop should normally not exceed 4--6% from the origin of the
    installation.

    | If I take *two* of these 30m reels then I can join them with the
    | standard UK 13 Amp plug and socket supplied on the reels. This
    | gives me an overall length of 60m. Taking into account losses,
    | what would be the current carrying or power delivery carrying
    | capability of the 60m length if all the 60m cable is unwound?

    This would be very unwise. A 60m extension lead strongly suggests there is
    a need for suitable fixed wiring to be provided. The earth fault loop
    impedance will be high and the circuit protective arrangements are likely to
    be insufficient. This is quite apart from issues such as physical protection
    and suitability of the flex.

    Is this some elaborate scheme to get round Part P?

    Owain
     
  5. OOPs! Yes, I forgot it was twin flex.

    Ignore my last post - I clearly shouldn't have got up so
    eary this morning...and will now correct that error. Well,
    it was a good party last night, from what I remember.
     
  6. IIRC, 1mm flex will just about make the test at 30 metres.

    We use a multi-core cable to feed a floor monitor on location filming
    which has mains, video and audio. And that's about it's length. Of course
    if you extend it everything still 'works'. And I don't have anything to do
    with the H&S regs testing. ;-)
     
  7. 3 core, actually. ;-)
     
  8. John Rumm

    John Rumm Guest

    You are assuming that the supply impeadance is zero however. If you
    factor that in, then you drop the PSC a bit...

    (Still ought to be plenty to blow a 13A fuse in under a second though)


    --
    Cheers,

    John.

    /=================================================================\
    | Internode Ltd - http://www.internode.co.uk |
    |-----------------------------------------------------------------|
    | John Rumm - john(at)internode(dot)co(dot)uk |
    \=================================================================/
     
  9. Others have discussed the current & voltage side of your post. However,
    something else to concider is where you are doing this. At 30-60m my
    guess is that you would be outdoors? If so, you should be using an
    earth leakage circuit breaker (RCD)to supply your extensions. This
    could be installed in your CU, built in to the supply socket or a
    plugtop type.
     
  10. Guest

    I suppose you mean the wire won't melt. Other safety issues
    come into play - the device at the far end may fail due to voltage
    drop. In some cases it could conceivably burn. Hardly safe if that
    happens. The job of the fuse is to protect the wire from
    overcurrent, but there is a lot more to safety. Psooible need for
    RCD, protection from damage, supporting the wire etc all may
    come into play.

    Ed
     
  11. Presumably, you did the calcs with it in mind that the current runs down
    the cable 30M and then back up 30M for a total of 60M of conductor...
     
  12. Yeah, true. You 220VAC and 240VAC guys think you got it bad, we have
    four times as much of a problem here in 120VAC land. ;-)

    Those poor souls that put a hundred feet or so of 18GA (about 1mm sq)
    extension cord on their weed wackers soon find that not only does it run
    slow, but the motor overheats. So we have extension cords that are 16
    or 14 gauge, and can handle the extra current. But people are too cheap
    to pay double for the heavy duty extension cord, so they end up eating
    their money up in burned out motors.

    And then when they get tired of doing that, they go out and buy a weed
    wacker with the gas engine. This is on the end of a long pole, so the
    engine is right up next to their face, so they go deaf from all the
    engine noise. And they put the weed wacker in the garage, where the gas
    from the tank runs out and catches on fire!

     
  13. I'm in the U.S. and this gives me a flashback on what I was thinking
    about trying a few months ago. We have a PBX at work that's on 48V
    batteries, but the batteries are 9 yrs old and need replacing. They
    cost a bundle so I thought it would be possible to run a power cable
    underground to the big UPS we have in our computer room. Problem is
    that the PBX's rectifier takes 30A max at 120VAC, or about 3.6kW. And
    the distance between is about 1300 feet or about 400m.

    I would guess that the UPS output should go into a transformer and come
    out 480VAC, so the cable losses would be minimized. Then another transf
    on the PBX end to bring it back to 120VAC. But should I expect to have
    a max loss of 5% at max current, or what? I think I came up with 4GA
    cable, but at 480VAC, I'm guessing that it would have to be special
    insulated underground cable.

    Someday I'll have to ask one of the electricians that work on our HV
    stuff. We have 4160VAC underground around campus but that's all
    specialized switchgear, etc.
     
  14.  
  15. Watson A.Name - "Watt Sun, the Dark Remover" wrote:
    Kind of you to put it that way but it was a case of application of
    WNTL*. :). Otherwise known as SMAFN**
     
  16. Guest

    Hmmm... if you do it, just connect the PBX directly to the
    480 volts. Just think how much faster the phone calls will
    be at 480 vs 48! :)

    I think installing a half mile of # 4 might be "politically" cost
    prohibitive, regardless of the technical merits. And I doubt
    it would be a good financial solution. How many times
    can you replace the batteries for the cost of digging a
    1/4 mile trench, and installing conduit, cable, fittings,
    transformers, etc ?

    Ed
     
  17. John Rumm

    John Rumm Guest

    For that money you coud buy the PBX a UPS all of its very own!

    --
    Cheers,

    John.

    /=================================================================\
    | Internode Ltd - http://www.internode.co.uk |
    |-----------------------------------------------------------------|
    | John Rumm - john(at)internode(dot)co(dot)uk |
    \=================================================================/
     
  18. Mike

    Mike Guest

    One question to somebody presumably residing in the western colonies.

    What's a "weed wacker" ?
     
  19. It's (I think) a trade name that has become a generic term for a gas
    or electric (with or without cord) powered lawn trimmer/edger:

    http://www.amazon.com/gp/product/im...0/103-1119717-3922242?_encoding=UTF8&n=228013

    They cut by flailing around the end bit of a spool of thick filament
    of extruded nylon, and are thus unlikely to cause serious injury. When
    the nylon wears down or breaks off you typically dispense a bit more
    by thumping the business end (as it spins) onto a spare bit of grass.

    We have one with a cute little gasoline engine. I assume it's a trade
    name because when I inquired about one in a B&D outlet store, the
    clerk pretended not to know what I was talking about.

    Is that what you'd call a "Strimmer"?


    Best regards,
    Spehro Pefhany
     
  20. Owain

    Owain Guest

    | >Those poor souls that put a hundred feet or so of 18GA (about 1mm sq)
    | >extension cord on their weed wackers

    It's a different language isn't it :)

    | >soon find that not only does it run slow, but the motor overheats.
    | >So we have extension cords that are 16 or 14 gauge, and can
    | > handle the extra current. But people are too cheap to pay double
    | > for the heavy duty extension cord, so they end up eating
    | >their money up in burned out motors.
    | Why use anything as small as 1mm sq cable???

    If it's in Wall-mart, they'll buy it.

    Owain
     
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