Size of generator needed for outdoor gig?

[Chris]

Hi,

Not sure this is really the correct NG for this, but I figure there'll be people on here that know about this!

We are intending to put an outdoor gig on. I am guessing there will be:

- 1200w pa (I am guessing no more than around 10amps)

- 2xguitar amps I would guess the amps will draw max 5 amps. I haven't seen the gear yet, but I seem to remember my 50w valve head had a 5amp fuse(the max output of a valve head is lots more than the nominal rating)!

- miscellaneous other stuff (maybe power adapter for effects pedals, laptop, etc.)

Looking at something like a 10kva silenced diesel generator from HSS, this has 2x230v @16a and 1x230v @32a

According to their blurb, you add up the total watts needed (max about 3500 looking at it if I have my thoughts right) and this should be max 2/3rds the kva rating.

I can't suss out how one relates to the other though? Is this then effectively rated for about 6500watts or am I out by a factor?

Any help or further tips on this would be appreciated.

[cj]

Hi You will have to do the maths carefully bearing in mind that generators are rated as you say in KVA this is Peak output rating and needs to be converted to RMS values to get a better idea. Having done this in my sordid youth have falen foul of this before . To convert KVA to RMS multiply by .707 so 10kva give 7.07Kw This may seem plenty for a 1200w amp but what about the foldback amps,lighting,monitor amps and backstage mixers? Also the lister3 driving the 10kva can be a little sluggish under load if sudden power changes occur.(not to say smoky if an older model)

HTH and good luck CJ

[Chris]

Aha! That's exactly what I needed. We're not using lighting and doubt we will run even to monitors, although I think a desk will be vital. I remembered something about PMPO, RMS and the like but couldn't remember how it converted to real life.

I think I'll just get the biggest generator I can load into my trailer and turn it up until it trips! Rock and/or roll!

[Andy Champ]

Chris, I hope either you are nowhere near me, or you have good musicians.

1200W will carry a long way out of doors.

Do you have permissions in place? Otherwise Mr Plod may be calling...

Andy

[Chris]

There *is* a volume dial!

It's a while since I did any gigs etc. and all of those were indoors, but I remember that we needed a lot of "headroom" to ensure a decent depth to the sound and something along the lines of 1200w would be needed for a good, clean, vocal and leave space for the drums etc. I haven't ever done anything outdoors but remember a guy who had done a lot saying the sound just "dissapeared" with nothing to reverb off of.

Now it could well be the case that these days the amps and speakers are a lot better (it was 20 years ago) so I'm just going with what I've been told is available right now.

We'll have the permissions etc no problem but I am glad you said that about the 1200w as I was worried it might not be enough!

Oh, I've no idea how good the musicians are, so I'm just going to keep my car close by and make sure I hire anything in someone else's name just in case...

[Rob graham]

Something I've never really understood is the relationship of kva to kilowatts. Given that watts = volts x amps the two things would appear to be the same. But they're not, are they?

Rob Graham

[Martin]

IIRC (it was a very long time ago...) it's to do with power factor - a number between 0 and 1 which indicates what proportion of the "V x A" is actually useful power.

Both V & A are alternating (sinusoidally, with luck). But it they aren't in sync (i.e. both peaking at the same time time in their (50Hz) cycles) then the instantaneous V x A will be less than the "theoretical" max.

Generally, if the load is inductive (eg coils, motor windings etc) Volts will be a bit ahead of the Amps in the cycle. If the load is capacitive, then Volts will lag behind the Amps in the cycle.

Your meter reading (hence charges) is based on the "V x A", so you can pay for more than you actually get, if you see what I mean.

This "wasted" power is sometimes (often?) salvaged by using power correction circuits, to bring V & A close to synch.

Others will certainly know more - but I hope this is broadly correct.

[Rob graham]

I'm aware of power factors and believe that normal consumers are supposed to guarantee to their suppliers that they will not use equipment whose power factors are less than 1 (to avoid the supplier losing out). But does this explain the kva story? Also it conflicts with your explanation "Your meter reading (hence charges) is based on the "V x A", so you can pay for more than you actually get" in that you are getting more than you pay for. But I may have the wrong end of the stick. Rob

[Martin]

Sorry - I assumed that was the thing you askign about.

Sorry - I was simplifying a bit and/or was a bit misleading in my explanation. I've found this...

and the 4th para explains it better than I have.

I was recalling the old days, when working in a kW-hungry factory, that PF was constantly monitored and corrected, because the elec supplier measured it, and charged us a premium for poor PF.

So the typical domestic meter measures useful kW, but high-usage consumers effectively pay for kVA.

I've never heard of a domestic user (or his supplier) being troubled by PF

[Palindrome]

Afraid not, in a few key areas.

Domestic users pay for the kW used. Not the V x A.

However, as you say, many devices present a load to the supply such that the peak current drawn doesn't coincide with the peak voltage supplied. That means that, in order to supply the same amount of power, the current has to be higher than it would be if the peaks did coincide.

As a whole lot of things, like cable sizes, energy losses, fuses, etc all depend on the size of the current, not the power, this larger current makes for extra costs, inside the generator and out. So things like generators will have both a limit of how much power they can give out and also a limit of how much current that they can give out.

Generator designers quote the current that the unit will produce, in the form of the KVA rating. A 2KVA generator will be capable of outputting a maximum of 8A at a nominal 250v output. They will also tell you what the maximum power output will be, say 1.5kW. Your load mustn't demand >

1.5kW and mustn't draw > 8A.

Other things work the same way - eg carriers give both a limit of size and a limit of weight. The sender must ensure that neither is exceeded.

Of course what it really does is allow them to sell a 1.5kW generator as a 2KVA one. Which sounds as if you are getting a lot more for the money.

-- Sue

[Martin]

Well - one, at least... (confusing charging arrangements for domestic and industrial users, but corrected in following post, which prob crossed with yours.... says I, hoping not to get nought out of ten...!)

My real dilemma is fathoming whether the technology etc has changed (eg what is a domestic meter actually measuring these days) or whether it's my ageing memory playing tricks :-(((

[Rob graham]

I do remember some years ago, when I signed up for electricity, that one of the conditions was that you did not allow your equipment to operate with a power factor of less than one. I suspect this is still the case, but how many domestic consumers read that fine print, or even know the implications of it?

Rob