| Author |
Message |
Graham W
Guest
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 Posted:
Mon Feb 14, 2005 4:30 am Post subject:
Re: How much current safe for 30m extension? |
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John Rumm wrote:
| Quote: | Jerry G. wrote:
As long as the maximum current load does not exceed the current
rating of any of the cables that are used in the extension, it will
be safe. There should not be very much loss over a few hundred feet
of extension. If there is a loss, you can then change the extensions
for ones that are of a larger gauge.
The other issue to beware of is as the length of wire increases, so
does
it fault loop resistance (i.e. path from line to earth / ground). This
will reduce the maximum current that can be passed to earth in the
event
of a fault, and hence extend the time between the fault occuring and
the circuit protective device (fuse, circuit breaker etc) opening to
disconnet the power. Obviously if *you* are forming part of this earth
fault loop, the sooner the power goes off the better!
|
I doubt that a *you* of any value would survive if it were to pass
the fusing current of the plug-top fuse!
--
Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.
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John Rumm
Guest
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| Posted:
Mon Feb 14, 2005 6:10 am Post subject:
Re: How much current safe for 30m extension? |
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Graham W wrote:
| Quote: | disconnet the power. Obviously if *you* are forming part of this earth
fault loop, the sooner the power goes off the better!
I doubt that a *you* of any value would survive if it were to pass
the fusing current of the plug-top fuse!
|
Depends on if you are in serise or parallel with it!
--
Cheers,
John.
/=================================================================\
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Guest
|
| Posted:
Mon Feb 14, 2005 6:10 am Post subject:
Re: How much current safe for 30m extension? |
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Jerry G. wrote:
| Quote: | As long as the maximum current load does not exceed the current rating of
any of the cables that are used in the extension, it will be safe. There
should not be very much loss over a few hundred feet of extension. If there
is a loss, you can then change the extensions for ones that are of a larger
gauge.
I have run extensions of up to 400 feet. These were 10 Amp rated. I had no
performance loss in the loads that I was using. The load was about 4 to 5
amps. I am working with 120 VAC at my location. I took a voltage reading, at
the outlet, and one across the load. There was a drop of about one or two
volts.
|
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
| Quote: | But, the total voltage at the extension output was still in specs
while under the load. The source voltage at the outlet was about 118 VAC.
Across the load, if I remember correctly, it was about approximately 116
VAC.
In areas that are using 220 VAC, the current is about 1/2 for the same
wattage, as when compared to 120 VAC. This would lead to less loss when
using an extension cord.
Most devices here in North America are rated at 105 to 125 VAC. Some are
rated from about 105 to 135 VAC.
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Darcy B
Guest
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| Posted:
Mon Feb 14, 2005 5:47 pm Post subject:
Re: How much current safe for 30m extension? |
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On 12 Feb 2005, Ian Stirling wrote:
| Quote: | On Sat, 12 Feb 2005 12:46:19 UTC, Ian Stirling
root@mauve.demon.co.uk> wrote:
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...
I took that into account. (but misremembered the resistivity of
copper) See other post for correction.
|
What is earth fault loop impedance?
(Yup, I have tried Google!) |
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Sammo
Guest
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| Posted:
Mon Feb 14, 2005 5:57 pm Post subject:
Re: How much current safe for 30m extension? - RCD location |
|
|
| Quote: | Jerry G. wrote:
As long as the maximum current load does not exceed the current
rating of any of the cables that are used in the extension, it
will be safe. There should not be very much loss over a few
hundred feet of extension. If there is a loss, you can then
change the extensions for ones that are of a larger gauge.
|
On 13 Feb 2005, John Rumm wrote:
| Quote: |
The other issue to beware of is as the length of wire increases,
so does it fault loop resistance (i.e. path from line to earth /
ground). This will reduce the maximum current that can be passed
to earth in the event of a fault, and hence extend the time
between the fault occuring and the circuit protective device
(fuse, circuit breaker etc) opening to disconnet the power.
Obviously if *you* are forming part of this earth fault loop,
the sooner the power goes off the better!
Out of interest, are US style plugs fitted with fuses?
(An extension lead running outside ought to have RCD protection
anyway of course, to negate the problem)
|
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
Guest
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| Posted:
Mon Feb 14, 2005 6:15 pm Post subject:
Re: How much current safe for 30m extension? |
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| Quote: | "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.
|
On 12 Feb 2005, Owain wrote:
| Quote: |
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.
|
The cable reel has two ratings. I took the unwound rating to get
10A/2400W.
| Quote: | 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.
|
| Quote: | | 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.
|
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.
| Quote: | | 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.
|
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 = <news:95FD799908DBB1A7D4@130.133.1.4>
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
Guest
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| Posted:
Mon Feb 14, 2005 6:17 pm Post subject:
Re: How much current safe for 30m extension? |
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|
On 12 Feb 2005, Interfacebus.com wrote:
Nice table.
What a pity they didn't list the cross-sectional area (calculated
from the diameter) as it seems that many people refer to cross
sectional area when determining current capacity. |
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Sammo
Guest
|
| Posted:
Mon Feb 14, 2005 6:18 pm Post subject:
Re: How much current safe for 30m extension? |
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|
On 12 Feb 2005, Andrew Chesters wrote:
| Quote: | 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.
|
CU? |
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Ian Stirling
Guest
|
| Posted:
Mon Feb 14, 2005 6:48 pm Post subject:
Re: How much current safe for 30m extension? |
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|
In uk.d-i-y Sammo <noone@no-where.com> wrote:
| Quote: | On 12 Feb 2005, Andrew Chesters wrote:
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.
CU?
|
Consumer Unit.
The box where the electricity comes into the house, which contains
RCD/breakers/fuses for the individual circuits. |
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Andrew Chesters
Guest
|
| Posted:
Mon Feb 14, 2005 7:02 pm Post subject:
Re: How much current safe for 30m extension? |
|
|
Sammo wrote:
| Quote: | On 12 Feb 2005, Andrew Chesters wrote:
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.
CU?
|
= Consumer unit. The RCD should protect the whole of the "outdoor" section.
Earth Loop Fault Current = the current that would flow in the event of a
live to earth fault. This current is calculated from the earth loop
impedance (measured or calculated) including suppliers cable. This
value is then compared with standard graphs for various protective
devices (fuse, circuit breakers) to ascertain the disconnect time. If
this does not meet the requirements, additional methods of protection
are required. This could be changing to a faster breaker, and/or adding
earth leakage protection. |
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John Rumm
Guest
|
| Posted:
Mon Feb 14, 2005 7:34 pm Post subject:
Re: How much current safe for 30m extension? |
|
|
Darcy B wrote:
| Quote: | What is earth fault loop impedance?
|
It is the sum total of the resistances af all the wiring in the path to
earth or ground. It is significant because should a major fault occur
(like a wire falls inside an appliance and shorts to the casework, or
you cut through an extension lead), the earth fault loop impedance will
place a limit on the maximum current that can flow to earth.
Ideally this fault current wants to be large, so that it causes the
protective device (fuse / breaker) to open quickly. We have regulations
in the UK that require a circuit with socket outlets to disconnect in
under 0.4 secs in these situations.
The fault loop impedance is also dictated by the impedance of the earth
provided at the supply where it comes into the house. In the UK there
are three common ways the power company can provide the supply - with
two of them (known as TN-C and TN-C-S (aka PME)) the supplier provides a
good earth (i.e. typically well under 1 ohm) which when used with
suitable fusing and circuit breakers should result in good disconnection
times in most cases. However if you add long circuits with undersized
earth conductors (i.e. big extension lead for example!), then the
impedance creeps up and lowers the fault current that could lead to much
greater electrocution risk. The third type of supply common over here
does not supply an earth at all (this is typical for power fed via
overhead wires - typically into rural locations), and relies on a local
earth rod that is staked into the ground. These tend to give much higher
resistance earths and hence you can no longer rely on them to allow
enough fault current to flow, so the whole installation must be
protected by additional Residual Current Device circuit breakers (RCDs)
to detect any leakage from the circuit and cut off the power that way.
(RCDs are also mandatory here for all socket circuits that may
potentialy feed portable equipment that could be used outside).
There is some more info here:
http://www.tlc-direct.co.uk/Book/5.1.1.htm
--
Cheers,
John.
/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/ |
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John Rumm
Guest
|
| Posted:
Mon Feb 14, 2005 7:40 pm Post subject:
Re: How much current safe for 30m extension? - RCD location |
|
|
Sammo wrote:
See my reply elsewhere in this thread for a fuller description of earth
fault loop impedance. Also see here for more background:
http://www.tlc-direct.co.uk/Book/5.1.1.htm
| Quote: | 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.
|
Yes
| Quote: | (2) In the middle of the two 30m extensions.
|
No - but better than 3
| Quote: | (3) At the far end of the two extensions where the appliciance I am
using is plugged in?
|
no - but better than not at all!
Having the RCD at the start of the cable will mean the whole cable is
protected by it. So should you accidentally do the "hedge trimmer
through the extension lead" exercise, the RCD hopefully will cut off
power to the lead, rather than leaving live exposed conductors flopping
about in the wet grass under your ladder!
--
Cheers,
John.
/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/ |
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Dimitrios Tzortzakakis
Guest
|
| Posted:
Mon Feb 14, 2005 8:15 pm Post subject:
Re: How much current safe for 30m extension? |
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|
cable is not that expensive, it's copper after all.I can see in an invoice
that I paid 28 cents a meter for a 3G1.5 mm^2 A05VV-U cable.A 2.5 mm^2
conductor, single core, which we generally use to connect washing machines,
costs 9 cents a meter .Or 6 mm^2, for ranges, 22 cents a meter.Why don't you
ask an electrician to construct you an extra heavy duty extension cable,
with industrial-grade plugs?I did in college, when doing my Practicum, for a
high-temperature oven, three-phase, 25A, which plug tended to overheat.Cost
was negligent.
--
Tzortzakakis Dimitri?s
major in electrical engineering, freelance electrician
FH von Iraklion-Kreta, freiberuflicher Elektriker
dimtzort AT otenet DOT gr
? "Peter A Forbes" <diesel@easynet.co.uk> ?????? ??? ??????
news:rpks01dusj3e57g4n976ii30l4ncc1stpj@4ax.com...
| Quote: | On Sat, 12 Feb 2005 10:51:00 -0800, "Watson A.Name - \"Watt Sun, the Dark
Remover\"" <NOSPAM@dslextreme.com> wrote:
"Bob Eager" <rde42@spamcop.net> wrote in message
news:176uZD2KcidF-pn2-pgQf8dbMMKeL@rikki.tavi.co.uk...
On Sat, 12 Feb 2005 12:46:19 UTC, Ian Stirling
root@mauve.demon.co.uk
wrote:
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...
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!
--
Bob Eager
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: diesel@easynet.co.uk
Web: http://www.oldengine.org/members/diesel |
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The Natural Philosopher
Guest
|
| Posted:
Mon Feb 14, 2005 11:58 pm Post subject:
Re: How much current safe for 30m extension? - RCD location |
|
|
Sammo wrote:
| Quote: | Jerry G. wrote:
As long as the maximum current load does not exceed the current
rating of any of the cables that are used in the extension, it
will be safe. There should not be very much loss over a few
hundred feet of extension. If there is a loss, you can then
change the extensions for ones that are of a larger gauge.
On 13 Feb 2005, John Rumm wrote:
The other issue to beware of is as the length of wire increases,
so does it fault loop resistance (i.e. path from line to earth /
ground). This will reduce the maximum current that can be passed
to earth in the event of a fault, and hence extend the time
between the fault occuring and the circuit protective device
(fuse, circuit breaker etc) opening to disconnet the power.
Obviously if *you* are forming part of this earth fault loop,
the sooner the power goes off the better!
Out of interest, are US style plugs fitted with fuses?
(An extension lead running outside ought to have RCD protection
anyway of course, to negate the problem)
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?
|
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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Owain
Guest
|
| Posted:
Tue Feb 15, 2005 4:50 am Post subject:
Re: How much current safe for 30m extension? |
|
|
"John Rumm" wrote
| Depends on if you are in serise or parallel with it!
I *still* get confused with blue being live because it's a 'brighter' colour
than brown in cables; WTF did cerise come into it? Will Channel 5 be giving
us Colin & Justin's How Not To Write Wiring Regulations in the near
future...
Owain
(This posting may contain UK-specific cultural references.) |
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