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,
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
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.
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
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!
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.
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.
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<1 - it would be pretty hard to reduce it much below unity, in relation to
total demand on the local sub-station.
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
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.
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 :-(((
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
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