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.
A ~ 2 volt drop at 4 amps means ~ 1/2 ohms. You had 800 feet of wire - 400
out and 400 back. That means your extension cord wire had to be rated at
(1000/800) * 1/2 = .625 ohms per 1000 feet. Your extension cords had to
be made from #6 or #8, given those numbers. That is highly unlikely.
A typical heavy duty extension cord 100 feet long would most likely use
#14 wire. #14 wire is rated at 3.1 ohms per 1000 feet. At 800 feet, if
the 4 amp load was running, there would be a drop of about 9.9 volts.
If #12 wire (rated at 2 ohms per 1000 feet) were used in the cords,
the drop would be about 6.4 volts. The ohms per 1000 feet numbers
come from table 9 in the National Electrical Code.
There has to be an error in this, somewhere.
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.
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.
| 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
| 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?
The cable reel has two ratings. I took the unwound rating to get
Ha! You have spotted my lack of knowledge about these things - like
current carrying capacity. I wasn't too sure how these things are
However, if the cable has resistence then presumably current capacity
(ignoring voltage) gets reduced. I am thinking of W = I^2 * R.
I posted just now and asked about RCDs (see copy below). If you have
some info on using an RCD then I would be pleased to hear.
========MESSAGE ID = <
I am the original poster with two 30m extension cables. You refer to
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.
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
(2) In the middle of the two 30m extensions.
(3) At the far end of the two extensions where the appliciance I am
using is plugged in?
| 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.
| 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?
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?
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.
| > 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
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
| 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 :-)
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.
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
* Where few flat-dwellers even know the names of their
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 };-)
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
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.
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.
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