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12th Aug 2017, 07:42 PM #1Diamond Member
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Welding off an extension lead (or five)
A friend asked if I could weld some gate fittings on to a steel post he'd just installed. Only catch was that the post was some distance from a power point. About 150m.
So 5 x 30m extension leads later with the little BOC lunchbox inverter on the end, I had to wind the amps up a touch to 100A running 2.5mm 6012 rods. It was certainly harder to start and arc and took a moment to stabilise, and you could see it struggling to regulate the current, particularly on vertical down welds. During the weld, the fan speed dropped considerably, giving an audible indication of the lowering mains voltage, as you'd expect.
But got her done. That's one trick an AC buzz box could never do
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12th Aug 2017, 08:18 PM #2Tool addict
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Would've been fun to have pointed a thermal camera at the extension leads to see how warm they got
You're not wrong though, a tranny would've melted the cabling quicker than it would've made a spark!
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12th Aug 2017, 10:38 PM #3Member: Blue and white apron brigade
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I've done a few tests on 30m long 10A extension cords and see around a 1V drop over that length.
If there is 10A through the cord, that works out to 10W across 30m or 1/3rd watt/m which a standard 10A cord should easily cope with.
Provided the cords are are in good condition 150m would only drop 5V which is within standard supply tolerance.
However, if the cords and plugs are old and crappy their resistance can rise dramatically and that will result is a greater V drop and more problems.
[EDIT] Read post number 5 below
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12th Aug 2017, 11:52 PM #4Diamond Member
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I only did 1 inch or so welds in a few places on the fittings, so not long enough to really heat up anything. That said, even though they were short welds, the varying stability of the arc was a challenge.
As for actual voltage drop, an online calculator claims 10A over 150m of 1.5mm2 cable is 21V, which doesn't sound that bad - 220V is still a useable voltage, but I expect at 220V the welder would have been drawing more than 10A, driving the voltage lower and the current higher until an equilibrium was reached. The other thing I noticed was that the high impedance of the cable significantly affected the responsiveness of the current regulation.
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13th Aug 2017, 11:32 AM #5Member: Blue and white apron brigade
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I went back and checked my tests on the 10A extension cords and realised I was not operating at 10A but only at around 2A. This means multiplying my results by at least 5 for 10A equivalence.
Using basic resistivity calculations I get a V drop of 20V for 150m of 2.5mm^2 cable.
For 1.5mm^2 cable I get a V drop of 34V which agrees with a couple of other on-line calculators.
This means the resistance of the total 150m of 1.5mm^2 cable is 3.4Ω, but that is only for new cable in good nick.
Another significant thing to consider when using multiple extension cords is the "contact resistance" of the plugs.
Even a contact resistance of 0.2Ω for each plug will add an extra 1 Ω and now we're talking 44V total drop for the 1.5mm^2 cable.
If the plugs are in poor nick their contact resistance could be as much as 1Ω EACH and now have a real problem.
In comparison, 150m of 2.5mm^2 cable will have a total V drop of 20V.
The lesson here is if you have to use 5 extension cables then use
- new(ish) cables in good nick
- plugs in good nick
- short bursts only
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13th Aug 2017, 02:32 PM #6Diamond Member
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It was only afterwards that we remembered we had some old orange circ 3 phase bore cables that had been converted to 10A extension leads, doubling up the conductors where possible (I think one cable was 3C + E and the other 4C + E, both 2.5mm2). They would have run about 100m of the way, and I expect would have made a big difference to the voltage drop. Will try and remember them for next time
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13th Aug 2017, 11:53 PM #7
100m of 1.5mm2... and here I was worried about a 15m long by 4mm2 extension lead for my welder
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