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Author Topic: Specifying Generators in Kva  (Read 13812 times)

Bennett Prescott

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Specifying Generators in Kva
« on: October 12, 2005, 12:16:43 am »

I'm trying to figure out what kind of power I can actually expect out of a 36Kva GE Show Power generator being rented for me by the venue for my show this coming Saturday. I used the same thing last time I did a show for these people, since there's no way I'm going to approach that kind of capacity.

This show, however, there's lighting, and the lighting guy wants to know what kind of cans to bring and how many dimmers he can run. I told him I could only give him a rough estimate, and to plan on having xKw available, where x is about what I think I'm safe pulling off the generator divided by three (one third for me, one third for him, one third for the blowers on some space heaters).

Rough calculations tell me that I should have about 140 amps available from this generator when it's running in single phase mode, 250v. Does that sound right? The same calculations (with an appropriation for the extra phase) tell me I should have 100 amps available in three phase mode, 208v.

So the question is, am I right? Do I really have all sorts of juice? Also, is the spec on this generator continuous or peak? If peak (or in any case) how much should I de-rate the generator in order to get a realistic continuous (i.e. resistive) power capacity so I can tell the lighting guy what he can get away with? Assume I'll be connected over 4/0, so minimal voltage drop. My system probably averages 20a, seeing as it's an audio load and there's no backline or huge amp racks.

Attached is the Excel spreadsheet I roughed this out on.
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-- Bennett Prescott
Director of North American Sales
ADRaudio d.o.o.
Cell: (518) 488-7190

"Give me 6dB and I shall move the world." -Archimedes

Bennett Prescott

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Re: Specifying Generators in Kva
« Reply #1 on: October 12, 2005, 12:18:41 am »

I would also be very appreciative if someone would explain to me what Kva is actually used for, and why they don't just spec the genny in, say, watts.
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-- Bennett Prescott
Director of North American Sales
ADRaudio d.o.o.
Cell: (518) 488-7190

"Give me 6dB and I shall move the world." -Archimedes

Mat Goebel

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Re: Specifying Generators in Kva
« Reply #2 on: October 12, 2005, 12:24:00 am »

AFAIK, 1KVA= 1,000w.  Recall that watts=volts x amps. Hence, Volt-Amps (VA).

I think VA is just the term more widely used by electricians and the like, for one reason or another.

Update:  I should have looked at the spreadsheet first. It seems that you're spot on.
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Mat Goebel

Rob Timmerman

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Re: Specifying Generators in Kva
« Reply #3 on: October 12, 2005, 12:36:37 am »

kVA is very similar to kW.  In fact they have the same units.  The difference between the two is something called power factor (pf).
kW = kVA * pf

Without going into too much detail, power factor is the ratio between the amount of power that is actually dissipated and the apparent amount of power dissipated.   A purely resistive load has a power factor of 1.  Because of how electronic dimmers and switching power supplies work, the power factor of your load will be significantly less than 1.  

A power factor of between .65 and .85 is probably appropriate for your application.  Personally, I'd err to the low side (.65) to be safe.  So this means that you have about 24kW available, total.
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Jason Ellis

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Re: Specifying Generators in Kva
« Reply #4 on: October 12, 2005, 12:55:31 am »

Since it appears that you and the squints are sharing the genny, and you believe that 20A of (120V?, 240V?) is all you require (I will assume 20A of 240V based off of your previous posts regarding power connectors). You should have about 90-100A of 240V to give to lights based on Rob's powerfactor estimation. Do be sure to find out if the genny is Split-phase or Three-phase, as several I've worked with in the 25KW and up can only provide total power under one or the other.

My office has a 25KW split-phase which is good for 100A or so of 240V (200A of 120V). Even with A/C on we barely break 25% of capacity (I like having a little extra room Smile )

Just for fun I'll estimate lighting cans...lets say Mr. Squint has 10 20A dimmer packs he's running 4 lamps apiece on (no movers and shakers) that would give him 40 lamps for the show...of course I may be way off in my math as well...don't the lights have a pretty good PF? Not that you two should be pushing it to the limit of the genny, but you should still have a good amount of genny headroom with the above layout...
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Ken Freeman

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Re: Specifying Generators in Kva
« Reply #5 on: October 12, 2005, 12:56:14 am »

Generators don't quite behave like the "Wall" does.  They are reactive and when pushed, will not behave the way you want them to.  They are more like children.  When you want a little "More" from them, they will give you less.  When you ask for a a little "Less", they will give you more.  Example: when you ask a generator for extra amps, lets say in a lighting chase where you go from 10 lights up to 20, the voltage will drop a bit while the generator spools up to supply the extra power.  The inverse will occur on the way down and you will get a short term overvoltage until the plant corrects itself.  Okay, why the story:  We always add up the maximum draw for any given system and then add 20% as headroom.  If you need a 100 KW gen set, order a 125 KW and play it safe.  In your case, plan on using no more that 28kVA and you should be okay...maybe...

...The other piece of this to keep in mind is that a generator will have a lot easier time going from 40% to 60% percent of capacity than from 5% to 25%.  If your lighting tech is fond of blackouts and fast changes, you may have a tough day.  Also beware that many dimmers kick a lot of noise back on to the A/C line which your audio system may well amplify thereby creating a nice buzz every time the lights fade up or down.  I suggest a second plant for the lighting guys unless they promise not to make any big changes until you make sure it will not mess with the audio system.

Have a good show!

Ken

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Philip Roberts

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Re: Specifying Generators in Kva
« Reply #6 on: October 12, 2005, 01:28:27 am »

Some of the other people around here know the practicalities better than I do but here's what I know
As Rob and Ken said is exactly correct W=VA*pf,
Wikipedia has a pretty good article on Power Factor.

Lights are almost the perfect example of resistive loads, however I know dimmer mess this up royally, though I don't know in which direction. My understanding is the DC power supplies in audio gear also pull the power factor away from 1.

We saw an example of this with some of QSC's big amps in the PL series if I remember right the PL x.x-PFC had what ever additional circuitry what was needed to get the power factor back up closer to 1, I believe that is was EU regs that required it for items with such a large current draw.

Hopefully there's more light that mud here.

[edit: trying to make the link work right]
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Philip Roberts
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Pioneer Memorial Seventh Day Adventist Church
Berrien Springs MI

Jim Brooks

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Re: Specifying Generators in Kva
« Reply #7 on: October 12, 2005, 09:52:03 am »

Real world experience.
I run a 13,000 watt sound system and a 20,000 watt light system on a 25KW (single phase) Wisperwatt generator on a regular basis.
Two 20 amp circuits for the amp rack, one 20 amp circuit for the remaining audio.
All other circuits are for lights.
I kick the crap out of the generator but it works.
I do have a voltage regulator on all of the sensitive equipment just in case I have a brown out.

If the light system was at full for any long length of time,
I'd be tripping breakers.
I know I'm pushing the envelope.
Just the way it it.

Jim Brooks
casperband.com
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Bob Healey

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Re: Specifying Generators in Kva
« Reply #8 on: October 12, 2005, 11:32:42 am »

I'm the lighting guy for this event with Bennett.  I plan to run 12 PAR 56 lamps at either 300W or 500W either off of a pair of 6 channel leprecon LD-360s with 15A Edison inputs or if i can get the needed cam gender benders off of a leprecon ld-2400 dimmer pack.  If I use the -360s, it will be 300W cans, the -2400 gives me the option of either.  The rack the -2400 is in is set to take 3-phase power, but the dimmer only needs 2 legs - am i asking for trouble running off only two legs?  
I'm going to need somewhere between 35A and 65A for the load based on past experience/breaker ratings on the gear, though I've never worked with a generator before.
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Bennett Prescott

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Re: Specifying Generators in Kva
« Reply #9 on: October 12, 2005, 12:24:53 pm »

I'm not sure I understand why I'm including a power factor in these calculations... wouldn't inefficiencies in equipment power supplies be taken into consideration at the power supply? Or does power factor represent the non-uniform current draw (i.e. at peaks) of switching power supplies?

In any case, it looks like if my equipment draws 20a at 240 and Bob's draws 40a or so (that lets him have a 10Kw rig, and he's only asking for 6), we should still have plenty of genny "headroom" left over... more than 50%, in fact, which allows for some slack in that I have no idea what those heaters are drawing, but it can't be enormous since they can't be purely resistive (well, in a world without crazy people, but I assume that's not the case here).

[edit: posted updated Excel spreadsheet, although there's something wrong with my three-phase amperes calculation]
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-- Bennett Prescott
Director of North American Sales
ADRaudio d.o.o.
Cell: (518) 488-7190

"Give me 6dB and I shall move the world." -Archimedes

ProSoundWeb Community

Re: Specifying Generators in Kva
« Reply #9 on: October 12, 2005, 12:24:53 pm »


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