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How much current safe for 30m extension?

S

Sammo

Jan 1, 1970
0
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)
 
P

Palindr☻me

Jan 1, 1970
0
Sammo said:
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?

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
 
B

Bob Eager

Jan 1, 1970
0
Or for 60m, 8.4*10^-1 ohms, or .84 ohms.

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

Owain

Jan 1, 1970
0
| 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
 
P

Palindr☻me

Jan 1, 1970
0
Palindr☻me said:
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.

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.
 
D

Dave Plowman (News)

Jan 1, 1970
0
Plus the resistance on the two connections. Not sure whether the earth
fault loop impedance is still OK...

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. ;-)
 
J

John Rumm

Jan 1, 1970
0
Ian said:
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)


--
Cheers,

John.

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

Andrew Chesters

Jan 1, 1970
0
Sammo said:
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)

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.
 
Ian said:
It's always going to be safe, as it's got a fuse.
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
 
W

Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
Palindrâ~»me said:
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.

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...
 
W

Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
Bob Eager said:
Plus the resistance on the two connections. Not sure whether the earth
fault loop impedance is still OK...

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!

 
W

Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
Sammo said:
I am in the UK (so mains voltage is about 230V or 240V).

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.
 
P

Peter A Forbes

Jan 1, 1970
0
Bob Eager said:
Plus the resistance on the two connections. Not sure whether the earth
fault loop impedance is still OK...

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!


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
 
P

Palindr☻me

Jan 1, 1970
0
Watson A.Name - "Watt Sun, the Dark Remover" wrote:
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...

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

John Rumm

Jan 1, 1970
0
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 ?

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 |
\=================================================================/
 
M

Mike

Jan 1, 1970
0
Watson A.Name - "Watt Sun said:
And then when they get tired of doing that, they go out and buy a weed
wacker with the gas engine.

One question to somebody presumably residing in the western colonies.

What's a "weed wacker" ?
 
S

Spehro Pefhany

Jan 1, 1970
0
One question to somebody presumably residing in the western colonies.

What's a "weed wacker" ?

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
 
O

Owain

Jan 1, 1970
0
| >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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