# How much current safe for 30m extension?

Sammo wrote:

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.
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Jerry G. wrote:

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.
Ed

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Sammo wrote:

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
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Palindr☻me wrote:

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.
--
Sue
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3 core, actually. ;-)
--
*Happiness is seeing your mother-in-law on a milk carton

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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gives
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...

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Watson A.Name - "Watt Sun, the Dark Remover" wrote: <snip>

Kind of you to put it that way but it was a case of application of WNTL*. :). Otherwise known as SMAFN**
--

Sue
*Write Now Think Later
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"Sammo" wrote | 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
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<snip>
This is some 9% at rated load. (60m0

Short current is some 120A, plenty to blow a 10A fuse quite prombtly.
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Ian Stirling wrote:

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.

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On 12 Feb 2005, Owain wrote:

The cable reel has two ratings. I took the unwound rating to get 10A/2400W.

Ha! You have spotted my lack of knowledge about these things - like current carrying capacity. I wasn't too sure how these things are determined.
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.
Thanks, Sammo.
========MESSAGE ID = <
I am the original poster with two 30m extension cables. You refer to RCD protection.
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.
(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?
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"Sammo" wrote | 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
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On 14 Feb 2005, Owain wrote:

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?
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Sammo wrote:

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.
--

Sue

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"Sammo" wrote | > 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.
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
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On 15 Feb 2005, Owain wrote:

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! :-)
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Sammo wrote:

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
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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 };-)
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Roy Q.T. wrote:

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.
--

Sue

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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.