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Power draw on electrical equipment


bitofagiggle

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Morning all.

 

My question is (hopefully) simple. How do I calculate the power draw of various bits of kit? I have a small PA system (2x Yamaha DXR112 tops 2x DXS18 subs) with various bits of analogue outboard, active monitors, some small LED pars etc... But how do I safely calculate how much I can plug into one 13amp domestic socket? I've been taking the RMS output of the FOH mains (by far the biggest draw) and not exceeding 3120watts, but I know this is not entirely accurate. I've also no idea how to spec the power draw from my bits of outboard (although I know that with the amount of gear I'm using I'm more than safe on a single 13amp socket.

 

I'm starting to land marginally bigger gigs and my bargain basement domestic extension cables are starting to give me the jitters! Any guidance or links to things I can read up on are vastly appreciated.

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The short answer is you can't really calculate it accurately. You can add up the power on the ratings labels of all the devices (which everything is legally required to have) but these are often a lot more than the actual power used.

 

Get yourself one of these plug in meters from Maplin for a tenner (other suppliers are available)

 

http://www.maplin.co.uk/p/13a-plug-in-energy-saving-monitor-n67fu

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Good advice above...

 

Just to add to the 'confusion' surrounding power draw...

there is a modern trend to report amplifier power as what the amp can produce with typical "peak to mean" ratio music signals. The speakers you have are rated:

 

DXR12 - Dynamic:1100W (LF: 950W HF: 150W) / Continuous: 700W (LF: 600W HF: 100W)

 

DXS18 - Dynamic:1020W / Continuous: 800W

 

...but the "power consumption" figure is given as 110W and 100W respectively, so you are only looking at "420W" for the main PA.

Having said all of that, it's good to think through power requirements and the suitability of your mains cabling and distribution. Whilst ordinary domestic extensions can be OK, it's often a good idea to use well specified mains plugs and extensions (e.g. MK or Duraplug) and where possible to use heavy duty cable. Normally 1.5mm^2 H07 rubber cable is considered the best starting point. You do need an RCD at the start of your mains distribution, so if you end using two 13A wall sockets, you'd want an RCD on each. If you have a known stage layout, it can be better to have (say) a couple of long extension leads to put power where it's needed rather than a large number of daisy chained short extension leads.

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Good advice above...

 

Just to add to the 'confusion' surrounding power draw...

there is a modern trend to report amplifier power as what the amp can produce with typical "peak to mean" ratio music signals. The speakers you have are rated:

 

DXR12 - Dynamic:1100W (LF: 950W HF: 150W) / Continuous: 700W (LF: 600W HF: 100W)

 

DXS18 - Dynamic:1020W / Continuous: 800W

 

...but the "power consumption" figure is given as 110W and 100W respectively, so you are only looking at "420W" for the main PA.

 

Having said all of that, it's good to think through power requirements and the suitability of your mains cabling and distribution. Whilst ordinary domestic extensions can be OK, it's often a good idea to use well specified mains plugs and extensions (e.g. MK or Duraplug) and where possible to use heavy duty cable. Normally 1.5mm^2 H07 rubber cable is considered the best starting point. You do need an RCD at the start of your mains distribution, so if you end using two 13A wall sockets, you'd want an RCD on each. If you have a known stage layout, it can be better to have (say) a couple of long extension leads to put power where it's needed rather than a large number of daisy chained short extension leads.

 

 

 

Thanks for the info. So what's the sort of calculation they use to get that "power consumption" figure? I was aware that real world power consumption is less than RMS, but how can it be that much of a difference between continuous output and real world consumption unless someone is just trying to wind up Einstein's ghost? Where in the numbers is that difference in power hiding?

 

Also, what purpose does the 'wall plug' RCD have? I thought all circuits were protected by RCD anyway? Or have I got that wrong?

 

Sorry for the uninformed questions! I'm certainly no electrician but I'm also not a complete idiot so I'll get there eventually. Haha.

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A lot of power rating figures for speakers and amps are generated by marketing departments - bigger numbers = better init :rolleyes:

 

If you're playing sine waves or white noise right up to the point of clipping, you'll be utilising just about the maximum power that an amp can generate. But in the real world, music (for the most part) doesn't sound like that. There are greater dynamics so the average power used is much lower.

 

I thought all circuits were protected by RCD anyway? Or have I got that wrong?

 

In a modern installation, they probably will be. But you might also just have a fuse board with wire fuses. There are still some pretty hideous installations out there. There's never any harm in using your own RCDs.

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Also, what purpose does the 'wall plug' RCD have? I thought all circuits were protected by RCD anyway? Or have I got that wrong?

 

As Shez says, there's no guarantee that any particular venue has RCDs fitted or that they actually work. Of course, there are venues which have electrics to the latest standard and are tested periodically. Equally, there are venues where the wiring is dodgy or they've even avoided using RCDs "because they might trip and mess up the performance".

However, having your own RCDs protects stuff downstream. If your performer knocks over a drink or places a chair leg over a mains cable and cuts it in two, you'll be grateful for having got protection in place.

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1/ measure what you use, use one of the meters linked above or one of several like that.

 

2/ DON'T hope or assume that you are the only user of a 13a ring main. With a 16a socket, you should be able to draw a full 16a.

 

Fair point. What should I consider as far as other loads on the same circuit goes? And how might I be able to tell the total capactiy of a ring main? I'm assuming they're fairly standardized capacities?

 

Thanks for the help so far guys! Very useful thread.

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Fair point. What should I consider as far as other loads on the same circuit goes? And how might I be able to tell the total capactiy of a ring main? I'm assuming they're fairly standardized capacities?

 

Other loads.... depends on how much control you have over the venue. You should try to identify all sockets fed from the circuit so you can spot other things plugged in. Beware of people plugging in kettles for interval refreshments etc.

You can tell the capacity from the MCB fitted on the distribution board. They are often 32A for a ring and 20A for a radial but not always.

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"other loads" includes bar fridges and coolers, caterers' urns and kettles, other entertainers and lots of other small but cumulative small items. The 13a on the stage could be yours alone or might be shared with several others.

 

A 13a socket may be part of a ring main or not, a 16a socket should be a fused radial. What they actually will support depends on the building and it's test and maintenance regime.

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  • 2 weeks later...

Lots of good advice above, I agree that the plug in energy and power monitors are very useful.

 

Power ratings of audio equipment should be regarded as marketing claims and not as actual measured electrical watts. An audio amplifier is nothing like 100% efficient, so IN THEORY an amplifier with a claimed output of say 1000 watts will need a lot more than 1000 watts input. In practice though remembering that the "1000 watts output" is a marketing claim, the actual output might be perhaps 100 watts, and the actual measured electrical input might be 200 watts at full volume.

 

Most other equipment uses what it says. A "3000 watt" boiling water urn will almost certainly actually use about 3000 watts.

 

 

 

 

 

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