Distance from Power source

Discussion in 'Amps and Cabs [BG]' started by Chris Fitzgerald, Sep 5, 2001.

  1. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator Gold Supporting Member

    Oct 19, 2000
    Louisville, KY
    My first time posting in this forum. Wow, it's cold up here... :cool:

    Anyway, I'm not sure if this is the right place for this question, but here goes: I have a gig in October where the people hiring the group want us to play outdoors. The problem is, the spot they have chosen is about 75 to 100 yards away from the house, which is the nearest power source. The last time I did a gig like that, the people just ran a long-ass extension chord out to where the band was, and it messed up my amp (luckily, I was using an old one).

    This guy says he's willing to consult an electrician to find out how to run power to the spot. I just heard from the guy today, and he says that the electrician wants to know how many Amps we'll pull total between the 3 of us. I have no idea where to find this information (it's not on the back of either of my amplifiers), or whether it's even relevant. Does anybody know what the issues invloved in this situation are, and what we would need to make sure he provides to pull this off safely (i.e. - with no equipment damage)?


  2. FalsehoodBass

    FalsehoodBass Guest

    Jul 22, 2001
    Denver, CO
    i think a wall outlet circuit breaker trips at ten amps, and the appliance outlets trip at 15.. i know that's not much help.. but its a start
  3. Tapp


    Aug 29, 2001
    USA, Mississippi
    Chris, Are you running everything through the PA; and do you have stage rigs? Depending on how big your PA is, I would say a 20 amp breaker for the PA and a separate 10-20A breaker for the onstage gear. We had a similar situation with a outdoor church function and the power company set it up right at the pole. Yeah, I wouldn't run long extension cords this time.

  4. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator Gold Supporting Member

    Oct 19, 2000
    Louisville, KY
    Thanks for the replies. This is a small group - a jazz trio, so we won't have much gear (just three small amps). But I don't want to f**k up my new Acoustic Image Clarus just because the client wants to throw a party by a lake! So if it helps, figure on 3 (three) 200 watt amps, max. The "electrician friend" claims that the guy can rent or buy some kind of SUPER HEAVY DUTY extension cable to run the power that far without losing too much, but I just want to be sure after last time.
  5. Matthias


    May 30, 2000
    Vienna, Austria
    My attempt: to find out how many amps your amp pulls, you have to find out what's max. power consumption (measured in VA or W) of the amp and divide this by line voltage (110V in the US?).
    E.g. my amp is rated 400W output power, max. power consumption is 850VA (it's higher due to heat losses)

    Three 200W amps would possibly have a max. power consumption of 1200VA, so you end up with 11A, but I believe that's only 'peak power' and your ok with a 10A fuse.
    (e.g. the internal fuse of my amp is 3.15A for 230V line voltage, that seems to confirm my maths...)
    I hope my calculations are ok, maybe someone can confirm that.

  6. It's USA, so the outlet's are protected by a 20Amp fuse or circuit breaker.

    Going with the same assumption of 400VA's per amp, gives 1200VA total, and 1200 / 120 = 10 Amps. Safety factor of 2 gives 20 Amps as a design goal.

    Size the extension cord so you have no more than a 5 Volt total drop at 20 Amps.

    Using a 2 Gauge cord, will result in a 6V round-trip drop at 20Amps. This would mean that your amp would see 114VAC when all 3 were pulling full load, and would see 120VAC when pulling no load. That seems fine to me. Check with the elecrician and see if he can get a 2 Gauge cord.

    Check the back of all the amps (and lights) you plan to use. It legally HAS to have the power consumption on the back near the cord. It will be in Watts, and you can divide by 120V to get amps. Add them all together, then double it for safety, and then run copper wire resistance drop calculations to find how big your wire should be, keeping in mind that you drop voltage running down one wire, and drop voltage again running back up the other wire.

  7. OOPs. I used 1000 feet as the wire length. For only 300 feet, you could use 4 Gauge and only have a 4 Volt round-trip drop at 20 Amps.

    Of course you need to verify this with your electrician.

    And just for you all to know how big a wire that is, 4 Gauge wire has a diameter of 5.2mm (not including the insulation) and 1000 feet would weigh 126 pounds.

  8. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator Gold Supporting Member

    Oct 19, 2000
    Louisville, KY
    Wow. that makes my head spin when I read it....could you write that in TAB? ;)

    Seriously, I THINK I might SORT of understand what you're getting at...Can anyone explain in what way an amplifier might get damaged if the voltage wasn't right? Last time I did one of these gigs, my amp kept cutting in and out, and eventually started distorting and then just cut out. The repair guy said that he had to replace the "Power_______" (i'm an idiot) to get the amp going again. Any ideas? Why is running too long of a too light gage extension chord dangerous to an amp?
  9. Phat Ham

    Phat Ham

    Feb 13, 2000
    I'm guessing he had to replace the power supply.
  10. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Amps using a switching supply (you did say you had Acoustic Image, right?) are very sensitive to low voltage. Many of these amps have safety circuits (I own a Spriit powered mixer ike this) that shut down the unit if voltage drops too low. The way they work is they "chop" the line voltage at a high rate (thousands of times a second) and use these "samples" of the 120V to feed storage capacitors which smooth it out into clean DC. If the incoming voltage is too low, lots of bad things can happen to the DC output of the power supply, it's not just lower voltage DC, it's "dirty" DC. Any power supply designers out there who want to explain this better (more correctly!)?

    I did a wedding last summer with generator power using a very long extension cord where the mixer kept shutting down every few minutes. Basically as the band got louder, the current draw would increase, the voltage would drop further and the amp's protection circuit would kick in. We also had trouble when the caterer plugged in a few huge coffee urns (BIG current draws).

    With traditional linear supplies, typically an amp will still operate but won't reach max rated power and can overheat quite a bit.

    The best protection (and worth the investment if you do lots of shows with questionable power) is a voltage regulator. Furman makes one that sells for about $450. It guarantees 117VAC out even with as little as 90VAC coming in, plus provides surge and RFI protection and gives you a half dozen outlets to plug into.
  11. MikeyD

    MikeyD Guest

    Sep 9, 2000
    Well, even a power conditioner can only do so much with a poor feed. The reason why getting power from a long extension can be problematic is because of the I*R losses. This is the current through the total resistance of the cord. R for a conductor follows the basic relation: R = rL/A = resistivity of the material x total length / cross-sectional area. r can be considered constant for typical copper wire. As L gets bigger, we also want A to get bigger to keep the R value the same. A is related to the wire gauge. The lower the gauge number, the bigger the A.

    As you get farther from a known source of voltage, the I*R voltage drop gets bigger, so you have to go to ever larger (lower gauge) wire to avoid excessive voltage drop. Eventually the wire becomes too big to be practical, which is why power companies transform to a higher voltage at lower current to transmit power over great distances in a wire that is noticeably smaller in diameter than the Chunnel.

    - Mike
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

    Jul 3, 2001
    Chester, Connecticut
    Former Technical Communications Developer, QSC Audio
    It's more dangerous to the extension cord and to bystanders (if the extension cord heats up enough to start burning) than to the amp. The amp itself shouldn't be damaged, though its ability to reach full power will probably be reduced, and if the voltage is low enough, it might not work properly.

  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

    Jul 3, 2001
    Chester, Connecticut
    Former Technical Communications Developer, QSC Audio
    Switching supplies don't chop the AC (except for in the early Carver amps, which turned out to be a problematic implementation, and in amps with power factor correction). Instead, they rectify the AC directly into a high-voltage DC reservoir and chop the DC to create high-voltage high-frequency AC to pass through a power transformer. At this point it works largely like a conventional supply does, only at a higher frequency.

    Some switching supplies are sensitive to line voltage fluctuations, some aren't, and most are in between; it all depends on the design and what the manufacturer accepts as a performance/cost ratio.

    Traditional or conventional supplies are not linear; they are very non-linear, as are most switching supplies. Because the rectifiers aren't forward biased until the instantaneous AC voltage exceeds the reservoir capacitor voltage by about 0.6 volt, they conduct only at the peaks of the AC waveform.

    Some amps have power factor correction, so they draw current throughout the AC waveform, but the circuitry involved is complex and relatively costly. But PFC puts a much more linear load on the AC line and reduces the peak current draw significantly, which means you'll get less voltage drop and heating in the AC line for the same amount of power drawn.

    Also, an irony of reduced AC voltage is that most amps will actually be more electrically efficient and will tend to run cooler, not overheat, because there's less excess power to be dissipated (instead, it's being dissipated in the AC wiring and extension cord). Performance-wise, the downside is that their maximum power capability will also shrink, so they'll clip at a lower output voltage. If the voltage drops low enough that any internal regulated supply rails lose regulation, you could get some audible artifacts and/or other abnormal operation.
  14. Now you guys know why i always bring my small UPS to gigs :D I have no problems with variations in Voltages.. the UPS makes sure i get 230V at all times ( until battery depleted ofcourse ) :)
  15. Allodox,

    yes, but we are lucky enough to have 230 volts around here. This reduces the problems with voltage drops drastically. Currents are only half that of a 115 volts mains system.

    My suggestion is to rent a generator. A 2000 Watts generator behind a sound barrier of some kind will allow you to use only 15 feet of household extension cords.

    This may be cheaper than a 100 lbs 100 yards cable. Even if you rent it.