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AC power how low is too low?


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I'm involved in another "first event/ country music festival" for a local park NPO this coming August. I'll be supplying the lighting rig and my power distro stuff for the event. The sound and backline gear will be provided by another local sound contractor who's light on experience. I'm being pumped for advice and offering what I can to be helpful.

 

A question has come-up concerning AC power for the stage: The currently available & proposed power service for the stage is located approx. 300ft. from the stage. My preliminary plan/advice is to feed the stage with two runs of 6-4/ 220v, 50A feeder cable from the power service to the stage. One run for lighting (direct to my dimmer pack), and the other for the sound rig and backline gear off-a stage-set power distro.

 

I'm concerned about line-loss with that long of runs. I commonly my sound and lighting gear on up-to 150ft. of the 6-4 feeder cable with no issues, but this is twice the distance than I recall operating on.

 

Between now and the date of the event, I figure on setting up one of the power runs and load test with a dozen or so 500w par cans to see what the resulting stage voltage will likely be at various static loads. I'm not all that concerned about potential brown-out with my lighting rig... (I'll be running a NSI DS 12-24 dimmer and a Leprecon 612 board). I am more-so concerned about sufficient voltage for the sound person's rig, partially because his sound rig is a mixed assemblage of the who's who of contemporary "pro-sumer" sound gear.

 

I'm looking for advice on a general rule of thumb as-to how low is too low of voltage? I'm thinking if we're over 105v at 40A lighting draw, that should be fine. I suspect the sound and backline gear combined won't draw much more than 20A 220v... if that. Fortunately the voltage at the power service (and in that area) is a bit high... generally right-around 120v.

 

eh?

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I'm probably not giving you any information you don't know, but in case someone else needs it; First a handy calculator:

 

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

 

Second, I like to see voltage holding steadily around at least 115VAC. If I see it sagging much below that that or fluctuating a lot, I feel like I spend as much time looking/listening for issues, real or perceived, as I do mixing.

 

Third, again I'm sure you know this, but with at least a football fields distance between the stage and power, with who knows how many vendors between the two, the potential for coffee urns, whack a mole games and other potential power to end up on the distribution at the power end is always there.

 

If I were the sound providers, I would be considering a tow-able Whisper Watt if the power were questionable. A couple hundred bucks rental to ensure that the event goes off, seems like a pretty good insurance.

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Generally most manufactures allow for +/- 10% on the voltage (considering conventional power supplies). So that would be 108v. Most smps are rated down or 100v.

 

What does that mean? I would expect most analog gear to work down at 105v. Digital gear may or may not.

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I'm probably not giving you any information you don't know, but in case someone else needs it; First a handy calculator:

 

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

 

Second, I like to see voltage holding steadily around at least 115VAC. If I see it sagging much below that that or fluctuating a lot, I feel like I spend as much time looking/listening for issues, real or perceived, as I do mixing.

 

Third, again I'm sure you know this, but with at least a football fields distance between the stage and power, with who knows how many vendors between the two, the potential for coffee urns, whack a mole games and other potential power to end up on the distribution at the power end is always there.

 

If I were the sound providers, I would be considering a tow-able Whisper Watt if the power were questionable. A couple hundred bucks rental to ensure that the event goes off, seems like a pretty good insurance.

 

Thanks for the link to the handy voltage drop calculator.

 

And yes, I know about the caveats of stinging cables at a festival... and the further the run, the more likely the caveats. I agree considering renting a small festival generator for the event, or move the stage closer to the power, or move the power service closer to the stage, or move the venue to the other park in town that's much better set-up for the event. Fortunately where the power service pole is, and where the stage will likely be, and where the audience and vendor(s) will likely be... where the feeder cable would need to be run is "off to the side of the activities"... likely zero foot traffic in the feeder cable location. And this is likely to be a "small event"... 2 -3 bluegrass/country bands (local talent) and maybe a hundred or so attendance.

 

The festival committee, sound provider, and I are meeting this coming Wednesday to check it out and work-up some plans.

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Dumb question perhaps, but why don't we have small-value step-up transformers in our arsenals?

 

It seems to me that a transformer about the size of a wheelbarrow could be wheeled over to the power distro, then the tech selects the correct secondary winding tap to compensate for line-loss, and then we use less current through the long wire, and have the correct voltage at the other end? Even if the max step-up was only 10%, it would make a big difference in this use-case.

 

I guess cost is likely to be a factor. Anybody ever hear tell of such a thing? You'd think it would be more cost-effective than a generator.

 

Wes

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Dumb question perhaps, but why don't we have small-value step-up transformers in our arsenals?

 

If I understand correctly, the line loss is dependent on the amperage drawn (as well as the gauge and length of the conductors). The step-up transformer could likely function fine with a static load.

 

I recall when we lived in bush AK, we relied on ferroresonant transformers/ constant voltage transformers (CVT) to keep the line voltage "usable" for our fussy electronic gadgets... which worked well, but those CVT devices where big bucks per watt.

 

The best solution would be for me to be the one deciding where the stage should be set. smiley-wink

 

 

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You're right that the voltage loss varies with the load - can derive that from Ohm's Law - but you should still be able to get the correct secondary selected based on rough knowledge of voltage drop and load. Dips in voltage with fluctuating load wouldn't be too bad provided the loss wasn't extreme; well within the tolerances of the gear.

 

Re lost current - you make up for lost current by drawing more current. :) Your transmission losses would be lower with higher voltage anyway.

 

Those CVTs sound interesting. I wonder if they work by computer-selectable taps on the secondary. I contemplated them for my blue-sky proposal above, but decided that the cost of the super-heavy-duty relays needed would be better served by having a competent tech know his anticipated load ahead of time, and being able to do some math.

 

Getting to decide where the stage is would certainly make life easier. A friend had a show for Canada Day a year or two ago. He was told there was power on the stage, but he didn't make a site visit before taking the gig 'cause it was 40 miles away. He shows up ..... and has to make an emergency trip to the local hardware store for 300 feet of orange extension cords. I wonder how that resolved, because that would have chewed up all his profit if it came out of his pocket.

 

Wes

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If this was DC you'd be worried about losses. AC not nearly as much. AC is designed to travel long distances with minimal losses. The fact that it reverses polarity 120 times per second means the electrons don't have to travel the entire length or wire like DC. With DC, what goes in one end of the hose, comes out the other end.

 

In AC its more like a sealed hose filled with water. If you step on one end A it creates pressure on the opposite end B - then you quickly step on end B to create pressure on end A. The water in the hose oscillates back and forth vs traveling the entire length.

 

The other item is AC has high voltage low current till its converted by your gears power transformers. In solid state gear the voltage is stepped down and the current which does the work is stepped up. In tube gear you have both step up voltage which is used for the B+ tubes and step down for the filaments. The Filaments are heaters and need low voltage and high of current to heat them.

 

In all, all you have to worry about is your total current draw. Go read the tags on the back of all your music gear and add up the currents. I suspect for a typical band setup you'll need around 20 amps unless you're running a really big PA system with stacks of power amps.

 

Take the total current of your gear and add 50% for a safety margin, then check the extension cord specs.

 

Most bands are able to get by on a 20A breaker. The lighting is a big factor. If you're running modern LED lights the current draw isn't nearly as bad as the older Par cans using incandescent bulbs. Again, you do need to know the current draw of the light show and add a safety margin. Chances are you wont have all lights on for extended periods but you do have to worry about current spikes. Again knowing if your source is 15, 20A is important.

 

If you're running an LED light show I'd think you'd be safe using 20A extension cords. If its Incandescent, I'd run something heavier that can take the current. An inexpensive and heavy duty method of running power would be just to buy a roll of Romex house wire which can be bought in 20 and 30A ratings. You'd just need to buy some boxes, plugs and outlets to plug it in. (and someone competent to wire it safely) The stuff is stiff and difficult to lay out but its going to do a better job supplying current with less losses then a bunch of cheap extension cords strung together.

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WRGKMC, your analysis and suggestions raise more than an eyebrow with me in terms of safety and correctness. Your discussion of DC & AC losses are particularly inaccurate.

 

Mark, for the genre you are working with, you should be just fine. If you need more cable, I might have another 100' here looking for a home.

 

Regarding voltage regulators, there are CV transformers that are ferro-resonant and those that are commutating tap changers. The tap changers can suffer from instability problems under dynamic load be high source impedance conditions, I wouldn't use them. The F-R transformers have a little higher THD but don't suffer from instability, they are also heavy.

 

-10% is the testing standard for line voltage as Don stated, I wouldn't expect any issues for your application.

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Power is the product of Voltage times Current.

 

Power loss is calculated by multiplying the square of the Current in Amps by the Resistance in Ohms of the circuit. You can see how even a small Resistance can cause a large loss in high Current situations.

 

By stepping up the Voltage, power can be delivered with less Current resulting in significantly (because of the square) less loss.

 

Lower Current through a fixed Resistance (power cable) will result in less Voltage drop across the Resistance.

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Onelife ... I'm missing your point. You will never replace any current capability that you lose in transmission. So yes, you can raise the voltage back up but you will lose even more current in the process. The transformer is simply trading a voltage increase with a current decrease.

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The difference being that with a higher voltage and lower current there will be much leass voltage drop along the transmission line. At the destination the power can be transformed back into low voltage high current.

 

The loss due to transfomation will be less than what would have incurred in transmission.

 

Power is the element that remains constant during transformation - taking into account the losses, of course. One Hundred Watts may be twenty volts at five amps or fifty volts at two amps.

 

 

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So you are correct about high voltage transmission systems. Just take a look at the power lines outside. But I thought the subject was step up transformers. So I guess you could run 220v lines, but who's gonna allow something else to be run in a public place. Certainly not the AHJ.

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