Could I have some advice please. I need to get a twelve volt, 1/2 amp supply five hundred metres up a hill. I do have a fence a metre high made from wooden posts and seven runs of Num eight (4 mil) galvanised steel wire. Is it feasible to use two of these wires to carry twelve volts that distance ? Thank you.
----------------------- If you use # 8 copper, the voltage drop at 1/2 A will be about 1V. However the resistivity of iron which would be approximately that of steel would be about 5 times as much giving a 5V drop so for 12V at the sending end, and
1/2 A load, the load voltage would be about 6-7V. Chances are highly likely that this would not be satisfactory.4 mil wire is pretty flimsy stuff even if made from steel. With a breaking strength of 100,000 psi,it would take only a bit more than one pound of force to break.
Aside from the low electrical conductivity compared to pure annealed copper, the conductivity of alloys like steel is greatly reduced. by impurity (non-ferric atoms) scatter electrons as they flow. Dislocations from drawing the wire also decreases conductivity. Rule of thumb: The harder the metal alloy, the lower the conductivity.
Bill
I suspect he means number 8 steel wire, which has a diameter of about 4 millimeters (not 4 circ mills area). That gives it a cross-section of about
0.02 in^2 and if your 100,000 psi is right for his stuff, a breaking strength of about 1900 lbf.What good would a fence be if the wire broke with just one lbf ?
Steel wire is usually drawn 'hot' and kept flexible for obvious reasons. It isn't all that 'hard'. But excessive working of the wire can end up 'work hardening' it and make it harder and subject to cracking. But if he ran his own fence, I suspect he knew not to keep bending and kinking it until it started cracking :-)
He doesn't mention much about the application, but I wonder if he might use a somewhat higher AC voltage and a step-down / rectifier at the far end. Not so high a voltage that he has to worry about insulation, safety, insulators and all, maybe just 30 VAC or so. Would reduces his losses.
daestrom
Feasible means capable of being used or dealt with successfully. Successful surely means legal. So no, you cannot use the fence wire as electrical conductors because it is not legal by the NEC. You shall use approved wiring methods or be subjected to a curse from the electrical devil who appears as the authority having jurisdiction.
try it and see, might work.
You have some options open to you, which could be used in combination.
1) Use all 7 wires - one group of 3 in parallel for 1 side of the supply, and the remaing 4 in parallel for the other side of the supply. That will reduce losses in the wire. 2) Reduce the current drawn by the device(s) at the top of the hill, if possible. 3) Use higher voltage AC as the source and step it down, rectify and regulate at the top of the hill. 4) Use batteries at the top of the hill and charge them from the source at the end at the bottom. #4 is viable only if the load at the top of the hill draws current intermittently. Even then, you would need to determine if on time versus off time allows the battery to recharge at whatever charge rate the setup would allow.Ed
What about adding a solar panel to option 4? Solar powered fencers are getting to be fairly common. Even if the panel can't keep up the battery wouldn't have to be exchanged as often for a fully charged one.
Dean
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Why not just crank up the voltage at the bottom of the hill until you get 12 at the top, using all the wires like EHS suggested
No. 8 has a diameter of 0.1285 inches. 0.1285 inches is 128.5 thousandths of an inch. CMA is 128.5 squared or 16512 circualr mils. The distance of 500 meters =3D 500 x 3.28 feet =3D 1640 feet Load =3D 0.5 amperes VD=3D2KLI/CMA K or circular mil ohms per foot for galvanized soft steel wire is not easy to find, but 95.8 at 20 degrees C is given by one standard. This is about 9 times that of copper that is 10.371 for 20 degrees C. VD=3D2(95.8)(1640)(0.5)/16512 VD=3D9.5 volts This is a voltge drop of 9.5/12 x 100 or 79 percent.
To achieve a voltage drop of 5 percent maximum:
0.05 x 12 =3D 0.6 volts CMA=3D 2KLI/VD CMA =3D 2(95.8)(1640)(0.5)/0.6 CMA=3D261,853 which is about the size of a 250 Kcmil conductor. This assumes a low temperature of 20 degrees C. which is 68 degrees F. Voltage drop at room temperature will be more. Therefore, No 8 the steel wire cannot be used without having too high of a voltage drop."Feasible means capable of being used or dealt with successfully. Successful surely means legal. So no, you cannot use the fence wire as electrical conductors because it is not legal by the NEC.
Good point.
What's the highest AC voltage that can be used without getting into "code problems?" Is it 48 VAC? No question that 24 VAC is OK.
It might make good sense to ship up 24/48 volt AC and transform it to DC at the load.
You shall use approved wiring methods or be subjected to a curse from the electrical devil who appears as the authority having jurisdiction.
** Posted from
If he isn't using the Code, he could try jacking the voltage up and down and use the steel wire. Maybe, the electric fence idea would work. It sure would beat buying enough copper wire to go 3200 feet with the price of copper being so high. Cheap fence wire insulators and transformers can be purchased at a food lot store, the kind farmers use. An ignition transformer might also work. A used oil burner ignition transformer could raise the voltage to
10,000 volts then at the other end back feed another transformer to get 120 volts. Then feed the 120 volts into a common Class 2 50 va bell transformer to get 12 volts. I have never seen this done, and do not know if it would work. At 10,000 volts: Power at 12 volts: P=3D EI =3D12 x 0.5 =3D 6 watts At 10,000 volts: I =3D P/E =3D 6 / 10,000 =3D 0.0006 amperesVD =3D VD=3D2(95.8)(1640)(0.0006)/16512 VD =3D 0.01 volts Percent voltage drop =3D insignificant
I would place signs around the fence warning people, and tell the inspector that the fence is electrified to keep the wildlife out. Electric Fences are not covered by the NEC for obvious reasons. The NEC has a purpose that is the practical safeguarding of persons and property from the hazards arising from the use of electricity while an electric fence has the prupose of shocking. When I was an inspector, the foreman at the Atigun camp for the Trans Alaska Pipeline repair job in about 1990 asked me about this. They installed an electric fence around the camp to keep the grizzlies out and he wanted to know where the code rules were. There are none! Atigun Pass is in the Brooks Range in Alaska and is a very beautiful place. It is also the highest point on the pipeline. I flew in there from Fairbanks in a Cessna 150 with a bush pilot. He couldn't make it over the pass because of fog so we landed at Chandalar field and I hitch hiked over to Atigun. Those were the days, my friend, those were the days.
If he isn't using the Code, he could try jacking the voltage up and down and use the steel wire. Maybe, the electric fence idea would work. It sure would beat buying enough copper wire to go 3200 feet with the price of copper being so high. Cheap fence wire insulators and transformers can be purchased at a food lot store, the kind farmers use. An ignition transformer might also work. A used oil burner ignition transformer could raise the voltage to
10,000 volts then at the other end back feed another transformer to get 120 volts. Then feed the 120 volts into a common Class 2 50 va bell transformer to get 12 volts. I have never seen this done, and do not know if it would work. At 10,000 volts: Power at 12 volts: P= EI =12 x 0.5 = 6 watts At 10,000 volts: I = P/E = 6 / 10,000 = 0.0006 amperesVD = VD=2(95.8)(1640)(0.0006)/16512 VD = 0.01 volts Percent voltage drop = insignificant
I would place signs around the fence warning people, and tell the inspector that the fence is electrified to keep the wildlife out. Electric Fences are not covered by the NEC for obvious reasons. The NEC has a purpose that is the practical safeguarding of persons and property from the hazards arising from the use of electricity while an electric fence has the prupose of shocking. When I was an inspector, the foreman at the Atigun camp for the Trans Alaska Pipeline repair job in about 1990 asked me about this. They installed an electric fence around the camp to keep the grizzlies out and he wanted to know where the code rules were. There are none! Atigun Pass is in the Brooks Range in Alaska and is a very beautiful place. It is also the highest point on the pipeline. I flew in there from Fairbanks in a Cessna 150 with a bush pilot. He couldn't make it over the pass because of fog so we landed at Chandalar field and I hitch hiked over to Atigun. Those were the days, my friend, those were the days.
And what are the voltage regulation and exciting current requirements of the two (or 3) (high impedance) transformers needed as well as leakage on the crappy little fence insulators? They might be such that the scheme still wouldn't work. In addition, aren't most electric fences pulsed? At least 24VAC makes sense from a safety point of view.
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24 vac does not make sense because the voltage drop will be too high. Do the math. 0.5 amperes at 12 volts equals 6 watts. I for 24 volts is 6/24 =3D 0.25 amperes VD=3D2(95.8)(1640)(0.25)/16512 VD =3D 4.75 volts 4.75/24 x 100 =3D 19.8 percent If an igniton transformer is used the output is not pulsed and losses are minimum. I think some electric fence power supplies are electronic and would not work.Additonally, 24 volts is not safe in a wet environment. That is why
12 volts is used for bell transformers. Read note 2 to Table 11A and B in Chapter 9 of the NEC quoted below. This note is based on finding from the original work by Charles Dalziel, who, by the way, invented the GFCI."Gerald Newton" wrote
Why would a square wave be allowed to run at a higher voltage than a sinusoidal wave?
It seems to me that a true sine wave would have more energy and potential to do damage than a sin wave.
-- Stephen B. just a (possibly dumb) ME heading to bed
The peak of a square wave is the same as the RMS, the peak of a sine wave is higher than the RMS.
"James Sweet" wrote
Thanks, I forgot about RMS, since I tend to play with nominal 1/4" rods not electrons.
-- Stephen B.
24 vac does not make sense because the voltage drop will be too high. Do the math. 0.5 amperes at 12 volts equals 6 watts. I for 24 volts is 6/24 = 0.25 amperes VD=2(95.8)(1640)(0.25)/16512 VD = 4.75 volts 4.75/24 x 100 = 19.8 percent If an igniton transformer is used the output is not pulsed and losses are minimum. I think some electric fence power supplies are electronic and would not work.
Additonally, 24 volts is not safe in a wet environment. That is why
12 volts is used for bell transformers. Read note 2 to Table 11A and B in Chapter 9 of the NEC quoted below. This note is based on finding from the original work by Charles Dalziel, who, by the way, invented the GFCI.------------------------- Some time ago, I did the math at 12V and I'll generously take your math as correct even though the numbers that you have used are undefined: 95.8 what? 1640 what? 16512 what? You appear to be using a cookbook expression that I am not familiar with -without giving the units so I have no way to check your data/calculation except that the result appears to be reasonable. Yes the voltage drop is high but I wasn't considering DC and neither were you. Now using a 24/12V transformer with a 19.2 V input at the receiving end and converting to DC gives a peak voltage of 13.5V and a fat capacitor will leave you close enough to 12VDC average. There will be some voltage drop in the transformer impedances but this should be fairly small as the smallest
12/24 or 120/24 transformer that one can get will be rated at a fair amount more than 6 Watts. In any case dropping from 19+VAC to 12VDC at 0.5A for DC is not a big deal, even using a resistor, is actually cheaper than the warning signs .If 24V (42.4V peak) is not safe in wet environments and the "safe" limit is
21.2 peak( the peak voltage of a 15V sinusoid)_- then what about 10KV? Sure- we both know fencing units are can be safe ( not according to the code that you quote) but that is due to their high impedance and that impedance along with wet condition leakage can have a considerable effect on your scenario-which is not according to any code. There is no use in considering code in one situation and ignoring it in another. If code is to be ignored- then I would rather go with the 24V setup.In particular, the 24V can be floating with respect to ground so contact between both wires is needed for a hazard and this can be limited by some planning as to which wires are "hot" and which are grounded "shield" wires.
It isn't damnfoolproof but neither are signs- in either case, the hope is that there are no damnfools around .
I note that you did not consider the impedance of the transformers in your proposal. It will be high- by design.
Actually, there are other alternatives and these are based on location, purpose, etc. Paralleling conductors in a 3/4 grouping as suggested by others will bring the voltage drop down to less than 6% so the problem, after a 24/12V transformer is too high a voltage at the load.
My point is that 12V wont do the job so what is the lowest standard voltage that will do the job. 24V appears to be OK.
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K=3D95.8 circular mil ohms per foot. It is the resistance of a conductor 0.001 inch in diameter one foot long at 20 degrees C in this case. L=3D1640 is one way circuit length in feet CMA=3D16512 is the circular mil area of the No. 8 steel wire that is the diameter in thousandths of an inch squared I is amperes VD is voltage drop VD=3D2KLI/CMA is a standard formual for finding voltage drop for 60 hertz single phase or Direct Current This is the standard formula used by electricians to find voltage drop for the last 50 years or so.. We multiply VD by 0.866 for three phase three wire and by 0.5 for three phase four wire If you use 24 volts the NEC rules apply and bare conductors cannot be used, but by using the electric fence model the NEC rules do not apply.
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