How much current safe for 30m extension?

It's always going to be safe, as it's got a fuse.

Ok... Copper has a resistivity of 0.7*10^-8 ohms/meter.

Or, for a 1mm^2 wire, 0.7*10^-2 ohms/meter. Or for 2 wires, 1.4*10^-2 ohms. Or for 60m, 8.4*10^-1 ohms, or .84 ohms. At 10A, 8.4V, or heating by 84W. If at the plug end is 240V, at the socket end will be 232V, which is (232^2/240^2)= .93444444444444444444

So, you lose 7% of the power for a heater, for example.

Doh, 1.7.

1.7*10^-2 3.4*10^-2

2 ohms.

At 10A, 20V, or the cable heating by 200W. If at the plug end is 240V, at the socket end will be 220V, which is (220^2/240^2)= .84027777777777777777

So, you lose 16% of the power for a heater, for example.

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

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

"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

OOPs! Yes, I forgot it was twin flex.

Ignore my last post - I clearly shouldn't have got up so=20 eary this morning...and will now correct that error. Well,=20 it was a good party last night, from what I remember.

--=20

Sue

I took that into account. (but misremembered the resistivity of copper) See other post for correction.

The other consideration would be the length of time it would take to disconnect from the power source. I don't know the requirement ~ time required to disconnect from the source (60m) or disconnect from the extention (30m).

Additional link:

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. ;-)

3 core, actually. ;-)

This is some 9% at rated load. (60m0

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

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)

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.

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

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...

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!

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.

Why use anything as small as 1mm sq cable???

For 30metres I'd want to see 2.5mm sq at least, if not 4mm sq.

If not for the volt drop or lack of, for the mechanical strength and resistance to damage.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

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