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Sizing a generator (ac mains)

Discussion in 'Miscellaneous [BG]' started by BillyB_from_LZ, Jun 11, 2005.

  1. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    A childhood friend of mine who lives on the West Coast is building a "cabin" somewhere out in the middle of nowhere. He asked me about sizing a generator so that he could have enough power to support a private-party size/volume rock band.

    Does anyone have any suggestions other than creating a hypothetical line up (two guitars, keyboards, bass, PA, monitors and lights), assigning current draws to each, adding them up and then doubling it (or some reasonable safety factor)?

    Since this will be a permanently installed generator and I have the feeling that my friend has plenty o' green, should I suggest anything additional equipment/features...regulators, filters, etc. to make the cleanest power that is reasonably possible?

  2. I thought about getting a portable generator myself, partly as a backup for my house, but also to power my PA for the occassional private party out in the boondocks.

    IMHO, the guitar and bass amps don't draw much, a 20 amp circuit could handle those (unless the BP runs a heckuva big amp). For the PA, another 20 amp circuit has been OK for my modest system.

    While I don't run a light system, I could easily see the need for lots of current for a light show. A dozen 300 watt fixtures would be 30 amps. Add a fog machine and that's another 10 amps.

    So that would be 80 amps, even with my modest PA. A bigger PA could easily be another 20 amps.

    That's 100 amps right there....normal house feeds are 200 to 250 amps, which would be a 25 to 30 kVA generator. As long as there isn't any other high-current loads (air conditioner, electric oven/range, water heater) I'd say something in the 25 kVA would be good, otherwise go up to 30 or more. (of course, this is way more than I would ever need or could afford, but for this guy's cabin in the woods...)

    I would *assume* the genset would already have regulation, dunno if much extra conditioning would be needed since this is an isolated power source...
  3. Steve


    Aug 10, 2001
    Figure the max connected load of the lighting by lamp wattage.

    Add the total connected load of the amplification using Peak Power Not RMS.

    add that together.

    Multiply that # by 1.25.

    THAT is the smallest generator you want. If you plan on using the generator for more than 4 hours contiuous....I'd go x1.5 but it's not really required.

    Most generators..properly set up and grounded provide power far cleaner and more stable than most utilities.

    I didn't stay at a holiday in last night. I'm a master electrician in Florida.

    It's worth repeating, calculate the amplification using PEAK power not RMS. The difference is significant.
  4. I'd maybe add a little more to allow you to handle some electrical needs of the cabin (refrig, lights, etc) at the same time the "concert" is going on. Unless that was what the "times 1.25 or 1.5" is supposed to cover. Otherwise, what Bikertrash said.

  5. Steve


    Aug 10, 2001
    Oop's that was strictly for the music.

    Calqulating a residential load can be a real pia.

    The stadandard method is a HUGE pia.

    The optiona approved method is a little easier. It also works out to be about half as much usually. Oddly...even though one is twice the other...both are aproved methods by the National Electrical Code and MOST but not all building dept's.

    Most building contractors use the optinal method because it WAY cheaper but....thats also why when you get into heavy usage periods like the height of summer...you get brown outs and transformers blowing off poles.

    There ya go. And you though you where never going to use that algebra you sat through in high school.... :D

    PM if you need a little more. The VA you see is volt-amps. A.K.A. Watts.

    I took a look at this...it seems pretty worthwhile for a down and dirty hip shot. I'd use it. The object of the game is to keep the maximum connected load under 80% of the generator's maximum output. You get there with the x1.25. Personally, I don't like my stuff running that hard all the time hence the x1.5.

    Head room is as good for generators as it is for bass amps.


    I just noticed you said "west coast". Some of the building codes out there are just crazy. If he's gonna have to deal with a west coast building inspector....he better find out what THEY want.
  6. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Thanks loads (no pun intended) for the great advice. It is my understanding that he wants the generator just for music and associated equipment (lights), etc.

    Algebra... I actually use that..it's the darn Calc and Diffy Q that I had to relearn for my Professional Engineers exam that I never, ever use... Maybe that's a good thing :confused:

    Thanks for the reminder about the inspector... As a UL safety engineer by day (with 23 years of experience in Industrial Controls and Power Distribution) I have the greatest admiration and respect for Tradespeople that have to deal with the varying personalities of AHJs (aka building inspectors). Some are great to work with, some less so :rolleyes:

    I'll take a look at those links and see what I come up with...
  7. Steve


    Aug 10, 2001
    I hope your friend isn't in California.
    He'll have to dig a moat around the generator for proper drainage...install a fire supression system tp prevent sparks from causing wilfires...place it on a concrete pad thick enough for NASA to use as a launch pad...shock mount it in case of earth quakes...
    then there are issues with the possible fuel spill contamination that you'll need to engineer to EPA requirements....get custom made earplugs for every animal living whithin half a mile....and then they will declare your new moat wetlands and tell you to remove everything.
  8. Damn.... How do you get the earplugs back from all the animals in the area? :meh:

  9. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    That he is!

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