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Calculating Generator Requirements

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Calculating Generator Requirements

Postby Stef Andrews » Wed Feb 01, 2017 12:04 pm

Hi All,

I've been asked to take my rig to a good few outdoor events this year, and as such need to know what I'll be pulling off a generator.

I've been meaning to get a proper idea of what power I'm using anyway, as although I've never run into any problems there have been a few occasions where I've been a bit wary.

My PA is QSC KW122 tops, RCF905 subs, QSC K10/Yamaha DBR10 wedges/fills, so all fairly new and efficient, but I'd just like to know roughly what info I need from each unit, and then the calculations to find out exactly what I need to power everything happily.

I've done a few searches but not come across anything that's seemed particularly helpful, so any pointers to reading material would be great.

Thanks in advance,
Stefan
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Re: Calculating Generator Requirements

Postby Mike Stranks » Wed Feb 01, 2017 12:56 pm

Hi Stef

Hope all is well with you and yours...

A few thoughts...

Get yourself something like this https://www.amazon.co.uk/dp/B01N4BP23P?psc=1 - there are loads available from all the usual suspects.

Connect all your gear to one plug connected to this device. Don't worry; you won't be pulling anywhere near 13 amps. :)

Switch on the gear in the usual staggered way and play some music at typical levels.

Set the meter to read amps.

From the reading you can calculate what power requirement you need.

The formula is Watts = Volts x Amps

So assuming you're pulling 3 amps...

Watts = 230 x 3 = 690 Watts - 0.7 KW

So you'll see that you'll only need a modest generator to provide that... 1 KvA (not quite KW) would be 'adequate' in my example - but risky.

I'd go for a minimum of 5 kvA - assuming no-one else is drawing power from the same generator. If they are - especially if it's non LED lights and the infamous tea-urns - then the power requirement increases dramatically and you need to factor that into the calculations.

Some folks will tell you that you shouldn't get an ordinary commercial generator. All I'd say is that I've used commercial generators from hire companies with no problems at all.

You'll need a good earth - metal stake deep into the group with a cable well-bonded to it and a bucket of water around the base of the stake.

Some sites now insist on diesel generators for safety/fire reasons. Make sure you know of such restrictions before the day. If there's paying public then there will be a site inspection. Wrong generator = event cancelled! :blush:
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Re: Calculating Generator Requirements

Postby Wonks » Wed Feb 01, 2017 1:13 pm

Mike's suggested power meter gives the instantaneous power in watts, so you won't need to calculate those. It also gives the power factor value, so divide the watts (or kilowatt value) by the power factor value to get the kVA figure for your equipment (it will be larger than the kW value). Ignore the power factor sign for now - positive simply indicates it's a leading (capacitive) load, negative a lagging (inductive) load.

There's no accuracy figure given for the meter, so I'd add on 5% just to be on the safe side.
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Re: Calculating Generator Requirements

Postby ore_terra » Wed Feb 01, 2017 1:56 pm

There are also regulations on your earth stake and cables. Have a look in advance


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Re: Calculating Generator Requirements

Postby Stef Andrews » Wed Feb 01, 2017 2:41 pm

Brilliant, thanks gents.

I've picked up that unit on Amazon, been meaning to grab one for a while. I'll be leaving the actual arranging and powering of the genny to the events companies I'm working with (certainly the 2/3 I've been dealing with so far) and just connecting up to them on my arrival, so I shouldn't need to worry about the earthing etc. Although, thanks for the notes.

I knew it was simpler than I'd been thinking, but just powering it up and reading a few numbers is *definitely* something I can do :headbang:
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Re: Calculating Generator Requirements

Postby Sam Spoons » Wed Feb 01, 2017 3:23 pm

All good advice. I have some firsthand experience of running from a generator and my rig is similar but a little smaller than yours (1 x EV sBa750 sub, 2 x QSC K12, 3 x Yamaha DXR10 and 2 Alto TS110) using a similar meter (Maplin rather than Amazon) I measured a max draw of less than 2.5A. There is a thread I archived here https://www.dropbox.com/s/9wzxi7evnjxsu2p/Generator%20for%20small%20PA%20system%20%7C%20Sound%20On%20Sound.webarchive?dl=0 which my interest you.

However, while I would be happy to run my 2KVA genny for my own little informal gigs I always specify a bigger genset for 'proper' paid gigs. Min of 5KVA diesel (which must have AVR or some other high quality form of control, especially if you run a digital desk, and a UPS just for the desk is not a bad idea too) and get the genny and distro installed and signed off by a qualified sparky. It probably goes without saying that RCCD protection is an absolute must too.

I'm just about retired now but intend to keep on playing and doing the odd small PA job as long as I can lift the gear so my little ebay genny will still get some use :)
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Re: Calculating Generator Requirements

Postby Hugh Robjohns » Thu Feb 02, 2017 10:51 am

Stef Andrews wrote:...so I shouldn't need to worry about the earthing etc.

You ALWAYS need to worry about earthing! It's the only way to make sure people don't die!

If you are connecting your equipment to the supply, and letting other people use your equipment, it is your legal responsibility to make sure it is in a safe operating condition, whether or not you installed the generator/mains supply. You may not be providing the power, but you need to make absolutely sure it is connected safely, in full compliance with all the appropriate regulations, and that it is operating in a safe condition -- just as you would plugging into a wall socket in a village hall! Hopefully, the installation will be signed off by a qualified electrician, as Sam says, but accidents, cockups, and backhanders happen...

That means you need to have a reasonable knowledge of what is required, technically and legally, and you need to look carefully at the supplied system and make sure that it is configured correctly before you plug into it. I've seen earth spikes pushed under the genny trailer tyres before now ("because the ground was too hard")... People get killed that way, so please don't abdicate your responsibility...

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Re: Calculating Generator Requirements

Postby Mike Stranks » Thu Feb 02, 2017 11:24 am

Hugh Robjohns wrote:Hopefully, the installation will be signed off by a qualified electrician, as Sam says, but accidents, cockups and backhanders happen...
H

I haven't highlighted Hugh's full post to keep this short(ish), but I agree with what he's said above about earthing.

People can be casual in the extreme about electricity... they assume that because everything's alright with it 'at home' it'll be fine in a field.

I have horror-stories a-plenty of organisers of events just not being bothered... and the quote above reminds me of one event in a public park, 3500 people in attendance with 'open' access virtually everywhere. Thus, all the requirements of a public, licensed event were in place. As expected the electrician turned up to check all was well. I'd sourced the genny and overseen it's set-up so wasn't particularly concerned when he turned up. I WAS concerned when he said to me, "So, what am I s'posed to do then?" :o
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Re: Calculating Generator Requirements

Postby ore_terra » Thu Feb 02, 2017 12:10 pm

To add more to the scary mood, I used to work in oil and gas industry on the field... and electrical accidents due to poor earthing do happen


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Re: Calculating Generator Requirements

Postby Dramatic Hammer » Thu Feb 02, 2017 12:28 pm

It seemed rather large at the time but reading this I reckon the 40kva generator I hired for my wedding might have been more than a little overkill, even running toilets, bouncy castle and fridge trailer off it as well as sound! :bouncy:
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Re: Calculating Generator Requirements

Postby Stef Andrews » Thu Feb 02, 2017 12:51 pm

Hugh Robjohns wrote:
Stef Andrews wrote:...so I shouldn't need to worry about the earthing etc.

You ALWAYS need to worry about earthing! It's the only way to make sure people don't die!

... so please don't abdicate your responsibility...

H

Hugh, having re-read my last post - definitely a bad worded reply. My point was more than I shall be checking that those installing have done their job, rather than being left to do it myself and potentially buggering it up. I know enough about electricity to know that not respecting it will probably mean killing myself at some point, or worse - someone else!

Once again, thanks for the words of advice AND warning.
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Re: Calculating Generator Requirements

Postby Hugh Robjohns » Thu Feb 02, 2017 1:08 pm

Stef Andrews wrote:My point was more than I shall be checking that those installing have done their job, rather than being left to do it myself and potentially buggering it up.

:-) Good stuff.

I tend to get a bit heavy when it comes to this kind of thing as I've seen too often people approaching it with a very cavalier attitude. Glad to hear you're not going to be one of them!

Hope it all goes well!

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Re: Calculating Generator Requirements

Postby MarkPAman » Thu Feb 02, 2017 1:15 pm

Try to avoid running a small generator with only sound stuff on it. The load required for a PA will vary very quickly, and in between songs can drop to almost nothing. Most small generators don't like this and are unable to keep the voltage stable when this happens as they can't react fast enough.

Factor in a couple of old school (not LED)1kW PAR cans or even an electric heater, to run all the time, and keep the load from dropping too low, if the PA's the only thing being supplied from a generator.
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Re: Calculating Generator Requirements

Postby Brian M Rose » Sun Jun 10, 2018 4:57 pm

OK, I know this is an old thread, but....
At our Hospital/Community station, we are often asked to supply the PA for outside events. These can sometimes be away from conventional mains (ie is a field etc). In these cases, the 'client' usually supplies a generator, hired in from an ordinary tool hire shop.
1) These always seem to be petrol driven but I understand that these can be potentially banned.
2) I am still confused about earthing. Max power is a couple of Kw at most. Yesterday, my Martindale tester indicated Missing Earth. Worrying but advice 'SEEMS' to be that under 10KvA you don't need an earth (??). We use a camping type RCB sisatro for power.
So where do we go in future? In general, the genny is supplying us ONLY (ie no tea urns). The Genny supplied yesterday has a rather rusty earthing bond, but as we were on concrete,.... My test wand (light up stick) didn't indicate any voltage - I assume because everything was floating, rather like an isolating transformer. As I said, WE don't supply the Genny, it is always supplied by the event organisers themselves.
Any definitive advice would really be appreciated.
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Re: Calculating Generator Requirements

Postby Hugh Robjohns » Sun Jun 10, 2018 5:34 pm

Brian M Rose wrote:In these cases, the 'client' usually supplies a generator, hired in from an ordinary tool hire shop.

This is actually quite a dodgy approach, since the client doesn't know what you need in terms of a stable, clean mains supply and it's earthing/floating requirements, and may not pass on safety-critical information about the wiring and use of the generator.

Inevitably, the client will hire on the basis of price and convenience, not quality and safety! So I would either insist on hiring a suitable genny yourself, or have a carefully written requirement sheet in your rider to ensure they hire something adequate and safe!

1) These always seem to be petrol driven but I understand that these can be potentially banned.

I think the issue here is about refuelling while the generator is running, because of the fire risk with static/sparks/heat igniting petrol vapours -- and so some councils (etc) don't permit the use of small petrol generators at public events. Storing spare petrol on site is also inherently dangerous because of the risk of ignition of fumes. There are strict rules about fuel storage...

Diesel generators are far more fuel efficient, so are less likely to need refuelling, and the fuel is much harder to ignite anyway so inherently safer. Diesel generators are also quieter and often provide a more stable mains supply, but tend to be more powerful and may therefore be too big for a small PA event.

2) I am still confused about earthing.

Some small gennys are designed and configured to provide a 'floating-earth' supply. In these, there is no internal connection to the power outlet earth terminal at all, and the genny should not be earthed. Importantly, it should also only be used to power multiple class-II (double-insulated) items and/or a single class-1 (grounded) item.... Which may not be practical in a PA situation. If more than one class-1 item is attached, and one becomes faulty, lethal voltages could appear between their exposed metalwork so it is not safe to connect more than one class-1 device to this kind of floating-earth generator.

In my limited experience, most small generators have internal bonding between the supply outlet earth and neutral terminals AND they have MCB/RCD breakers on the output(s). This arrangement renders them safe to use with multiple class-1 devices because there's a secure path for ground currents to ensure the trips activate in a fault situation. However, the earth/neutral from the generator is still 'floating' in the sense that there is no connection to the actual physical ground under your feet!

And then there are generators which are designed to be properly grounded to physical earth in much the same way as the national grid supply. In all cases, the generator should only be used to supply a small amount of equipment in a compact area -- not a stage in one corner of a field and a catering tent in the other corner!

Different situations call for different solutions, of course. It depends a lot on what the generator is supplying. The relevant HSE guidance is here: http://www.hse.gov.uk/foi/internalops/o ... c482_2.htm As always, you need to know what you're doing, though, because if the worst happens ignorance is no defence in the eyes of the law! If in any doubt at all, consult someone properly qualified and get the installation signed off in writing before using it!

...my Martindale tester indicated Missing Earth. Worrying but advice 'SEEMS' to be that under 10KvA you don't need an earth (??).

That HSE guidance above only applies to generators under 10kW... Which is probably where that 'advice' comes from.

But the absence of an connection to the earth under your feet should not necessarily be considered 'worrying' -- it actually makes perfect sense in many situations. If a fault occurs where one side of the mains supply comes into contact with exposed metalwork -- because of an equipment fault or cable damage, say -- no one can be electrocuted because there is no return current path to the generator via the body and 'the earth' that person is standing on. The whole system works on the same principle as a mains safety isolation transformer.... but that safety factor can be negated if using more than one item of class-1 equipment in some circumstances, as mentioned above, so you need to know what will be plugged in, and how the generator is configured!

We use a camping type RCB distro for power.

Seems a good idea... But this may not provide the safety you think it does! RCBs are generally designed to work on mains supplies where the neutral is bonded to the earth -- which obviously isn't the case with a floating-earth type of generator. They should work by detecting an imbalance in the line and neutral currents, but some older types detect the current in the ground connection. Also, it is essential that any breakers, MCBs and RCDs cut both line and neutral connections, not just the line. So check that your distro is safe for use with a floating supply.

It is possible to ground a generator, of course, but only if suitably configured may require internally (with a bond between the neutral and ground terminals), and you will need a ground spike which penetrates more than 1m into the ground... Plus the ability to test the ground impedance regularly... This is usually a lot harder and far more hassle than it is worth!

And even then, there's no guaratee that exposed metalwork on/around the stage are properly grounded too, which would defeat the intended safety benefits anyway. In such situations, it becomes imperative that all metalwork is properly bonded together and connected to the generator's reference ground... Which is a lot more hassle. So a floating generator supply looks a lot more attractive...

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