Voltage Drop Over Distance

[Catwalker]

I am running a couple of events in facilities which have limited power right where we need it. That means we have to run power from all over the known universe (maybe even parts of the unknown). I am wondering, is there a magic formula that allows me to calculate voltage drop over distance? Or, is voltage drop not even an issue for AC power? If this information is helpful, we have either 12 gauge or 14 gauge cable.

Sorry if the answer should be obvious, or if it was referenced in another thread. I did use the search function, and found no direct answer to my question.

 

[sk8rsdad]

Voltage drop depends on the impedance of the conductor so the wire gauge is important as is the load. You may find this calculator helpful.

http://www.calculator.net/voltage-drop-calculator.html

 

[AlexDonkle]

Voltage drop depends on the impedance of the conductor so the wire gauge is important as is the load. You may find this calculator helpful.

http://www.calculator.net/voltage-drop-calculator.html

sk8rsdad is correct, and just to add that the max voltage drop should be 3% max.

 

[BillConnerFASTC]

If this is feeding incandescent or other resistance loads, you can probably stand more voltage drop as long as you can stand lower level of lighting. You should be concerned about the wires heating up, depending on where they are. I had a pair of number 12's running my house while I moved the panel and drop for several days but they were in free air and no problem. You run a bunch of SO in compact spaces and it could heat up a lot, even if you adhered to the other rules.

 

[robartsd]

Just to be clear, the length of the run has very little to do with the heating of the wire - in fact if you choose larger gauge wiring to reduce voltage drop, the heating of the wires should decrease (all else being equal). Of course when you make longer runs you're also more likely to be makeing them is places the don't allow the heat to disapate as freely, so that is something you do need to consider (as Bill pointed out).

 

[Jay Ashworth]

The issue, of course, is resistance, generally in ohms per foot.

At 60hz, the reactive component is /generally/ low enough not to factor into ampacity reduction AFAIK.

How much the voltage is going to drop, for a given conductor size and distance depends on the current draw; if you've ever learned Ohm's law, this is where you use it.

Note, too, that how much voltage you get out of one end depends a lot on how much voltage you put in the other end and this is often high or low from nominal 120VAC.

Finally, the lower the voltage at the load end of the wire, the more current the load will pull, and that way lies madness and sweaty palms.

 

[BillConnerFASTC]

Just to be clear, the length of the run has very little to do with the heating of the wire - in fact if you choose larger gauge wiring to reduce voltage drop, the heating of the wires should decrease (all else being equal). Of course when you make longer runs you're also more likely to be makeing them is places the don't allow the heat to disapate as freely, so that is something you do need to consider (as Bill pointed out).

True but these long runs tend to be maxed out.

 

[JD]

Just to be clear, the length of the run has very little to do with the heating of the wire - in fact if you choose larger gauge wiring to reduce voltage drop, the heating of the wires should decrease (all else being equal). Of course when you make longer runs you're also more likely to be makeing them is places the don't allow the heat to disapate as freely, so that is something you do need to consider (as Bill pointed out).

Generally true... But there is an old tech and new tech case where this is anything but true:
Old Tech- In running lines for our old carbon arc Super Troupers, we had to upgauge like you would not believe on long runs to get enough voltage and decrease heating! As the voltage drop got worse, you had to crank the transformers on those things way up, resulting in a much higher current draw, which of course caused more drop!
New Tech- If you use movers with electronic ballasts, as the voltage drops, the circuit compensates by drawing more current. Same basic concept as above but on a smaller scale.

 

[Robert]

Generally true... But there is an old tech and new tech case where this is anything but true:

Such as in figuring the voltage drop on 0 - 10 volts control voltage to dimmers from 250' in an outdoor coliseum!

 

[AlexDonkle]

Old Tech- In running lines for our old carbon arc Super Troupers, we had to upgauge like you would not believe on long runs to get enough voltage and decrease heating!

If you're sizing conductors per NEC, the the breaker should trip before the wires heat up past their jacket rating.

Generally true... But there is an old tech and new tech case where this is anything but true:
Such as in figuring the voltage drop on 0 - 10 volts control voltage to dimmers from 250' in an outdoor coliseum!

Likely part of the reason the latest MLB ballpark was designed with DMX controlled LED's instead of 0-10V.

 

[BillConnerFASTC]

If you're sizing conductors per NEC, the the breaker should trip before the wires heat up past their jacket rating.



Likely part of the reason the latest MLB ballpark was designed with DMX controlled LED's instead of 0-10V.

More than just sizing, and its those other things that come into play more for portable feeders, like air. All current carrying conductors heat up. Free air - such as conduit fill - is an essential part of it. Just leave a 100' piece of 12 SO in a tight coil on a 20 amp breaker with a 1.5 kW load and it will get warm or very warm.

 

[JD]

More than just sizing, and its those other things that come into play more for portable feeders, like air. All current carrying conductors heat up. Free air - such as conduit fill - is an essential part of it. Just leave a 100' piece of 12 SO in a tight coil on a 20 amp breaker with a 1.5 kW load and it will get warm or very warm.

The other consideration is, Just how much drop can you tolerate? In the case of the Super Troupres, there was a limit to those click settings! I can remember putting down tons of #6 extension, even though the old supers had a rated draw around 20 amps. We always spec'ed 30 amp circuits as they would often draw more than 20.

 

[BillConnerFASTC]

The other consideration is, Just how much drop can you tolerate? In the case of the Super Troupres, there was a limit to those click settings! I can remember putting down tons of #6 extension, even though the old supers had a rated draw around 20 amps. We always spec'ed 30 amp circuits as they would often draw more than 20.

No disagreement. I have lots of foh dimmed lighting circuits on #8; and that is with a buck and boost on the feed to the rack, all in an attempt to deliver 120 volts at the socket with a 2k load (1K units doubled up on a 2400 w circuit was pretty common, once upon a time. Always difficult to call out #6 because that made every plug box and strip custom. The 115 volt lamp, the 575 wattage, and the low cost of dimmers (about the same today as 20 years ago) greatly alleviate those issues.

Yes - always a 30 amp circuit per follow spot (of any significant size).

 

[JonCarter]

If you're designing a circuit, you can design for either conductor heating (and disregard voltage drop) which is usually done, or you can design for voltage drop, which will almost always result in very LARGE, cool wiring.

Or, you can get lamps suitable for the voltage available at the instrument. Some time ago at a large outdoor theatre in Cleveland, we had some circuits over 600' long from the source to the instrument. We had about 124v at the dimmers and ordered leko lamps (2k) from G.E. Large Lamp Dep't at Nela Park in Cleveland for the voltage we needed--95v, 100v, 105v, whatever. Don't know if that can be done nowadays though.

 

[AlexDonkle]

More than just sizing, and its those other things that come into play more for portable feeders, like air. All current carrying conductors heat up. Free air - such as conduit fill - is an essential part of it. Just leave a 100' piece of 12 SO in a tight coil on a 20 amp breaker with a 1.5 kW load and it will get warm or very warm.

True, and NEC isn't aiming to keep conductors cool to the touch. Just cool enough not to melt and/or start a fire.

 

[AlexDonkle]

We always spec'ed 30 amp circuits as they would often draw more than 20.

Just curious, with a 30A circuit / receptacle, does that mean there's no way to (legally) connect a smaller spotlight with a standard Edison 15/20A plug on it at those locations?

 

[derekleffew]

Remove the Edison male and install an L6-, L14-, or L21-30 male on the "smaller spotlight". (Although I don't know of any 15A/120V fixture that would be bright enough in a position designed for a carbon-arc or Xenon Super Trouper.) One can't use an adapter due to 2011 NEC 520.69 (A): No reduction in Current Rating.

 

[techieman33]

Remove the Edison male and install an L6-, L14-, or L21-30 male on the "smaller spotlight". (Although I don't know of any 15A/120V fixture that would be bright enough in a position designed for a carbon-arc or Xenon Super Trouper.) One can't use an adapter due to 2011 NEC 520.69 (A): No reduction in Current Rating.

May not be out there now, but an LED source spot could be just around the corner.

 

[AlexDonkle]

Remove the Edison male and install an L6-, L14-, or L21-30 male on the "smaller spotlight". (Although I don't know of any 15A/120V fixture that would be bright enough in a position designed for a carbon-arc or Xenon Super Trouper.) One can't use an adapter due to 2011 NEC 520.69 (A): No reduction in Current Rating.

Makes sense, but wouldn't that also void the UL listing for the equipment?

May not be out there now, but an LED source spot could be just around the corner.

Pretty much what I was thinking. I was looking at an LED light targeted at the film industry last week that claims to match a 6K HMI for brightness but only draws 800W.

 

[BillConnerFASTC]

Remove the Edison male and install an L6-, L14-, or L21-30 male on the "smaller spotlight". (Although I don't know of any 15A/120V fixture that would be bright enough in a position designed for a carbon-arc or Xenon Super Trouper.) One can't use an adapter due to 2011 NEC 520.69 (A): No reduction in Current Rating.

We've found that 1200 discharge lamps - very common for us - too often pops a 20 amp breaker on start up.