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[Robert Piascik]

This is along the same line as another thread regarding extension cords and voltage drop.

Tonight I played a wedding ceremony at a family farm. The site for the ceremony was in a field located approx 500 yds from the main house. Since I told them I would need electricity at the ceremony site they ran a line from the house with an outlet on it (seen in the picture). The cable looked like typical home installation wire and everything seemed to work fine (although my power draw was minimal). If I had tried to run extension cords to accomplish the same task certainly I would have experienced significant voltage drop. My questions are:

1) why was it better to do it the way they did? (or was it?)
2) what's the difference (if any) if the cable was buried? In other words, if they had wanted to make the outlet a permanent installation at the site, how would you do it so there would be 120 volts/20 amps at the outlet?
3) let's say you wanted to build a shed or gazebo or some other structure and have a few circuits at the site, how would you get power to it from the house without experiencing significant voltage loss over the long cable run?

Thanks in advance!

[Mark Cadwallader]

The minimal load on 12 AWG wire is what kept voltage drop to a (presumably) reasonable level. The same would have been the case with 12/3 SO cable. The electrons don't care, as someone else here is fond of saying. Mark C.

[Cailen Waddell]

Although I believe direct burial cable might be able to dissipate heat better than typical SO - is this correct?


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[Jonathan Johnson]

The cable in the picture appears to be 12/2 with ground or 10/2 with ground direct burial (type UF) wire, but the markings aren't visible so I'm only guessing. With minimal load on 12 AWG wire, you will not notice significant voltage drop, whether the cable is a continuous strand of solid wire or a series of extension cords. If the female receptacles on the extension cords are worn and the male plugs are corroded, or the cords have been subject to repeated flexing resulting in frayed strands, then you almost certainly would have more voltage drop.

Direct burial cable has thermoplastic insulation on the individual conductors, and a solid thermoplastic (comolded) outer jacket. SO cable typically has some paper or fiber fillers and wraps. I don't know how the properties of the thermoplastic vs. rubber-like jackets are, but I imagine that the paper or fiber wrap adds some thermal insulation to the SO cable. Also, direct burial wire, if buried, is in direct contact with the soil so there is a better thermally conductive path for heat dissipation into the surrounding environment. Even so, conductors are given a higher ampacity "in free air" -- think overhead power lines -- because they generally have pretty good heat dissipation and if they get hot they are unlikely to ignite surrounding materials. However, heat dissipation is only one factor in determining ampacity, because the ability of the conductors to remain cool or prevent ignition has little to do with voltage drop.

If you were to provide permanent power (or build a shed) at that location, you would want to determine the maximum load and the minimum acceptable voltage. Knowing those two figures will be necessary in order to determine the gauge of wire you will use.

For a 20A circuit with a 500 foot run of direct burial cable, if the voltage at the panel is 123V (typical voltage at the service entrance) and you want to ensure that the voltage at the end of the run when loaded to 16A is no less than 118V (~4% voltage drop), you will want to use 4AWG wire. That's significantly larger than 12AWG! (It appears that an 8AWG wire would provide 112V at 16A.)

(I used 16A in my figuring since the NEC limits "continuous loads" to 80% of a circuit's capacity.)

I used this site for the above recommendation: http://www.southwire.com/support/voltage-drop-calculator.htm

EDIT: This calculator is easier to use: http://www.calculator.net/voltage-drop-calculator.html

[David Sturzenbecher]

For a 20A circuit with a 500 foot run of direct burial cable, if the voltage at the panel is 123V (typical voltage at the service entrance) and you want to ensure that the voltage at the end of the run when loaded to 16A is no less than 118V (~4% voltage drop), you will want to use 4AWG wire. That's significantly larger than 12AWG! (It appears that an 8AWG wire would provide 112V at 16A.)

(I used 16A in my figuring since the NEC limits "continuous loads" to 80% of a circuit's capacity.)

I used this site for the above recommendation: http://www.southwire.com/support/voltage-drop-calculator.htm

Make sure to adjust as the OPs situation is 500yds not ft.  Or 500OMGs should be more like it.


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[Tom Bourke]

For that kind of run I would probably do a step up and step down transformer combination or generator.  For short term use battery power is also viable.

[jasonfinnigan]

That looks like UF cable It certainly isn't rated for the usage in that picture. It needs to be buried.

[Mike Sokol]

For that kind of run I would probably do a step up and step down transformer combination or generator.  For short term use battery power is also viable.

There are a lot of battery-powered stick speakers available that would work great for this sort of gig. They're going to be WAY cheaper than that much copper, not to mention digging the trench.

[Stephen Swaffer]

(I used 16A in my figuring since the NEC limits "continuous loads" to 80% of a circuit's capacity.)

One thing to keep in mind on long runs is that voltage drop gets even worse with peak loads-like amps running heavy bass or motors starting.  While wire heating and breakers are designed to limit continuous loads-breakers will allow a slight overload for quite some time (the inverse time property)-but voltage drop is determined by ohms law and instantaneous-so if you have a large demand for a short time your power system will fail you when it is needed the most.  Motors are really bad-low voltage causes them to start slower and draw more amps which causes even lower voltage-an ugly cycle that can shorten life considerably.

[Frank DeWitt]

Doing some guessing,  assume that the cable is #10 and 500 yds long 1500 ft.  120 volts on the input and a modest 5 amps for your equipment.
In the above setup you would have 96.2 volts at your equipment.  Not good but you got away with it.

If the wire is #10 and you drew 5 amps you had 105 volts at your rig.

About the shed.  I have a shed 150 feet from my house and I feed it with #12 wire.  It is protected with a 15 amp breaker followed by a 7 amp fuse.  (I couldn't buy a 7 amp breaker to fit my panel.

with 120 volts in I would see 112.65 volts at the end.  All I use it for are lights and a small battery charger so I am OK.  If I needed real power out there I would have run #10 with single phase 220 on it and put a small breaker panel in the shed.   

[Robert Piascik]

I doubt that I even drew 2-3 amps with my one mic and amplified acoustic guitar playing at soft volume. The gig is done and maybe there was very low voltage at the site but my X32R and powered speaker seemed to power up with no issues. There was no thought of making the power permanent at the site so it seems odd that they would have 1000-1500 ft spool of cable to be used once and then coiled back up (maybe they borrowed the spool from an electrician?) but I guess this was better than trying to run twenty or thirty 50' extension cords.

Yes, gig could have been done with a generator or battery powered equipment but this is what they provided and my questions were more theoretical as to why does a permanent installation not suffer (seemingly) the same voltage loss as running extension cords.

Thanks for all the contributions!

[Mike Sokol]

About the shed.  I have a shed 150 feet from my house and I feed it with #12 wire.  It is protected with a 15 amp breaker followed by a 7 amp fuse.  (I couldn't buy a 7 amp breaker to fit my panel.  With 120 volts in I would see 112.65 volts at the end.  All I use it for are lights and a small battery charger so I am OK.  If I needed real power out there I would have run #10 with single phase 220 on it and put a small breaker panel in the shed.

I had a similar situation perhaps 30 years ago. I had a spare 240/120 volt 2 KVA transformer in my junk box (yes, it's a big junk box), so I sent both hot legs down the 12 gauge wiring to the 240-volt side of the transformer mounted in the garage, then re-established the G-N bond on the secondary 120-volt side of the transformer along with a ground rod and fed it to a sub-panel with a 15-amp breaker.  Minimized the voltage drop on my 120-volt gear, and I didn't have to re-run the wires. Of course, that's not a cheap transformer nowadays, but it would reduce the amount of copper needed for any long run like the OP asked about. 

[Jonathan Johnson]

I had a spare 240/120 volt 2 KVA transformer in my junk box...

I have a spare 14,400/120 volt transformer, but it's only 400 VA. So far all I've managed to use it for is as a Jacob's Ladder.

[Peter Morris]

It’s also important to understand the problem is more than just voltage drop.  Under fault conditions the resistance of the cable has to be low enough to reliably trip the circuit protection.
 
e.g. 20 amps @ 110 volts = 5.5 ohms.
 
For 500 yards of extension cable there is 3000 feet of wire in the circuit. The cable has to have a resistance (much) less 1.83 ohms per 1000 foot for the circuit breaker to operate reliably.

http://www.bulkwire.com/wireresistance.asp

[Craig Hauber]

3) let's say you wanted to build a shed or gazebo or some other structure and have a few circuits at the site, how would you get power to it from the house without experiencing significant voltage loss over the long cable run?

Thanks in advance!

The only way to do that would be having the utility co provide another service.  That's 3000' and will require a few poles and another transformer, meter and service entrance.
We have many farm and oilfield clients right now and that's all we've been doing lately.  If it's one farmstead with outlying service they usually just extend utility wires from the existing centrally mounted pole and transformer.  If it's a mile or more down the road they will just pull from whatever the nearest distribution branch is.
(We are in an area where the farmers don't refer to their land in "acres" but use "square miles" instead -and it's all cropland!)

[Tommy Peel]

(We are in an area where the farmers don't refer to their land in "acres" but use "square miles" instead -and it's all cropland!)

That's a lot of land....

[ProSoundWeb Community]