Newbie power distro questions.

[Spettitt3]

Hi everyone,

I've been doing audio for a while in the studio but have just started looking at live sound stuff. I've done a couple of things, and have started teaching myself basic electrical principles, but have become a little stumpted on some distro stuff. I thought the Remote forum would be the best place to ask.

A festival I've just finished helping at had 3 phase power from the gennie split into seperate phase outlets by a first distro box, two of which went into further distro which gave 63A and 16A outlets. As I understand, if the voltage (240V here in U.K.) and amperage were fixed, this would imply a specific load resistance/impedance. Talking about the amperage ratings, I understand that this specifies that the socket is fused at, for example, 16 amps, and doesn't carry near 16 amps unless the resistance of the load is low enough, and that the connectors and cable are rated to carry that much current. Am I correct in thinking this?

Next, related to my last question I read the manual for a power adaptor I had lying around and it said that the device supplies high current...wouldn't the current be determined by the load impedance? Or is the current fixed at a certain amperage and the voltage fluctuates with resistance?

Thanks so much for any help you can be guys, I'm entirely self taught, and I just need to get a whole bunch of newbie questions off my chest!

Thanks alot, Simon

[Jim vanBergen]

Simon,

Your thinking is correct. All the parts must be rated to work together in a PD.

The power distro box has breakered outlets. Each outlet has a specific connector designed for a rated current and a maximum amperage draw, and that is protected by the breaker for the circuit. The cables and connectors should all be rated for the given circuit.

Anyway...The current stays the same, current multiplied by resistance provides the actual amperage being drawn (and what we eventually calculate into watts); as electrical power is the result of BOTH voltage and current. This is the first equation you'll want to master E=I*R.

Instead of being vague, here's some light reading that will help you out

http://www.ibiblio.org/obp/electricC.../DC/index.html

Here's an example of a PD from a popular provider in the US for power distribution. I'm sure there are many options close to home for you, this is just what I am used to:

http://www.motionlabs.com/c-15-typic...o-distros.aspx

I hope these resources are useful to you!

JvB

[Spettitt3]

Jim,

Thanks so much! Things are very clear now. I actually quite like this power distro lark...

Cheers!

Simon

[LX3]

Electrical current, voltage, power, etc is pretty straightforward at a basic level. And yes, you're most of the way there by the sounds of things. Uh-oh, I can feel an electricity 101 coming on... Here comes one of my patented, thread-killing long posts!


An (ideal) electrical power outlet has a fixed voltage, say, 240v. (Let's not worry about AC and DC right now... but the rules are largely the same for both). That outlet also has a maximum amount of current it can supply. Say 13A, or 16A, or 32A. You get different kinds of connector for each to avoid confusion.

But the actual amount of current that's flooding out of that outlet depends on the "load"... i.e. the equipment that's connected to that outlet. Each piece of equipment draws a certain amount of current (in amps), and you can normally find out how much by looking on the device somewhere, or checking the manual.

Add together the current drawn by all the devices connected to the outlet. It's important that the total doesn't exceed the rating of that outlet. In fact, it's best to stay well within the rating. If you exceed the rating, something should blow... a fuse, a circuit breaker... and suddenly the gig grinds to a halt. You don't want that.

(Watch out, some devices, power amps in particular, will draw varying amounts of current, depending on how hard they're working. You don't get owt for nowt. When things get loud, power amps start drawing much more current.)

The "power" (in watts) consumed by a device is the current drawn by a device multiplied by the supply voltage. So an 8-channel mic preamp that draws half an amp from your 240v power outlet is using 120W of power.

"E" (power in watts) = I (current in amps) x V (voltage in volts)

Another way of looking at it is, if you plug in a table lamp with a 100W lightbulb in it, it should be drawing just under half an amp... rearrange E = I x V to get:

I = E / V: 100W / 240v = 0.4-ish amps.

A common analogy people use to visualise current and voltage is water in a pipe. Voltage is like the water pressure, while current is how fast the water is flowing through the pipe. You can have water at a really high pressure (voltage) without it actually going anywhere (zero current... tap turned off).

So that's current, voltage, and power. The other thing worth mentioning is resistance. Resistance is a measure of how much a component impedes the flow of electricity (a bit like a narrow water pipe versus a big pipe). A big resistance means less current flows in the circuit. A low resistance means lots of current flows. Resistance is measured in ohms. The relationship between voltage, current and resistance is called Ohm's law. Look it up. Very simple.

The extremes of resistance are "open circuit" (infinite resistance... no connection at all, no current flows) and "short circuit" (zero resistance... like a piece of plain wire, and an infinite amount of current flows! Or tries to.).

The problem with short circuits (particularly in power systems) is that the heat generated in a wire depends largely on the amount of current flowing through it. When you get a short circuit, you get lots of current, and any wire... or transistor, or transformer, or anything... with lots of current flowing in it has the potential to get really hot. I mean sparks flying, things catching fire hot... which is why we have fuses and circuit breakers.

The heating effect in a wire actually depends on the current flowing thru the wire and the resistance of the wire itself. High currents need extra low resistance cables to avoid them heating up. That's why it's fine to have a thin mains cable on a table lamp (remember, about 0.5 amp flowing in that) but you need big fat cable on your 16A Ceeform connector... just in case someone needs to draw all 16 amps that the outlet is capable of.

You could use a thin table lamp cable on a 16A ceeform plug and it'll work fine... until someone starts connecting lots of devices to the other end of that cable. The circuit breaker won't blow until 16A or more, but by the time there's 10 amps in that cable, it's going to be red hot.

This was proven quite dramatically when the wimpy original 1902 supply wiring to my victorian london flat literally burned out after I installed an electric shower. The fuse didn't blow, but the wires did!

I'll stop there. Hope that helps you with this stuff. It sounds like you were getting to grips with it anyway.

Paul

PS Beware, no such word as "amperage". The word you're after is "current".

[Spettitt3]

Hi Paul

Again, thanks for the reply. Very interesting what you said about power calculations. Thankfully, your response just clarified the theories I had formulated in my head for what must be happening.

Regarding the 'amperage' thing, I read an article somewhere on the net that dropped in the point that amperage was actually the full correct term for a measure of current, and will become more widely used in the future. I'll let it slip out of my vocabulary...

Cheers,

Simon

[LX3]

Yeah, this "amperage" thing.

Maybe things have changed, but I was always taught that the word "amperage" was rubbish, invented by people who reasoned "If volts give you voltage, watts give you wattage, surely amps give you amperage?".

Not in my book. Ohms law is about voltage, current and resistance, not voltage, "amperage" and resistance (yuck).

(The unit of current is the Ampere, letter A. No-one ever says ampere of course, abbreivating it to "amp".)

Maybe people talk about "amperage" (ugh) to refer to the current drawn by a device, the same way you use wattage to refer to the power consumed by a device. But power in watts is a more useful indicator than the current draw, because the wattage of a device is pretty much a constant (you can say something uses 35W, regardless of whether you're using it in the US on 110v or in Europe on 240v), whereas the current drawn by the device depends on the supply voltage.

Maybe I'm old fashioned, but whenever I hear "amperage" I shudder a bit. Mind you, when I draw schematics, I also can't help putting in those little "hops" when I need one line to cross another

Paul

[LX3]

Oh, one last thing, I need to correct Jim a little. Power = I x R is a bit out...

Power consumption of a device in watts (let's call it P) is the supply Voltage (V) x Current (I) drawn by the device.

P = VI

Ohm's law says that the voltage drop across a resistor (or a wire) = current flowing in the resistor x it's resistance in ohms.

V = IR

So, how much power is dissipated in that resistor? (good to know, because that power is going to come out as heat). You can combine the two formulae above:

P = V x I and V = I x R

To give you P = I x I x R ... or I-squared-R.

So the power dissipated in a resistor is it's resistance in ohms, times the SQUARE of the current flowing through it. So, if you have a wire with a resistance of 1 ohm (not very much - measure the resistance of 50m of mic cable for instance), and you put one amp through it, there'll be 1W of power dissipated as heat.

But, try to put 10 amps through it, and the power dissipated in that wire will be 10 x 10 x 1 = 100 Watts! That's going to get hot. (think how hot a 100W lightbulb is).

That's why you use fat cable with really low resistance for high power feeds. It's also why you have to make sure all power connections are really tight, because any point where there's even a small amount of resistance is going to really heat up when high currents start flowing.

Paul

[Jim vanBergen]

Paul, nicely done.

I accept that perhaps my explanation was not spot on, but WHEN did E=IR become an untruth? If you have an online resource for electricity that I'd be very happy to peruse it and update my knowledge base so my answer will be more correct and more timely.

[LX3]

Hey Jim, no offence meant. I just thought it was important that we get facts right for the guy.

it was this line that had me scratching...

"Current multiplied by resistance provides the actual amperage being drawn (and what we eventually calculate into watts); as electrical power is the result of BOTH voltage and current. This is the first equation you'll want to master E=I*R."

Current x resistance gives you the voltage drop across that resistance. That's Ohm's Law. I don't know what "amperage" is... surely the same thing as current.

Yes, electrical power is the product of voltage and current. Where I got confused is when you then followed that with E=I*R... I thought you were still talking about power, because (at least in the UK) a capital E has been used to denote power in Watts. In which case E=I-squared-R.

Yes, V=IR is and always has been Ohm's Law. No change there. thumbsup

Not sure when a capital E started getting used for voltage. Maybe it's a transatlantic thing (I'm in the UK). Or, as I said before, maybe I'm a bit old-school and I've lost touch!

Paul

[Jim vanBergen]

No offense was taken, Paul and I hope none was given by me.

I know that the nomenclature Across the Broad Atlantic (to quote a live album I'm quite proud of) is different as are our approaches to dealing with power, and I am simply looking for a BETTER reference than I pointed out. The first link I sent was to a website whose description of Ohm's law comes down to E=IR before getting into specifics. I only want to improve my communication skills regarding electronics and electricity, not to argue the point that everything you said was absolutley accurate.

if we can find a better web resource I'd like to quote it as well as refer to it. Just last week the road electrician with Deep Purple was screaming at a group of stagehands about how American electricity was all wrong... because he wanted L6 Hubbel connectors when he had a PD that was all L5/20. I find it funny now, but not if somone gets hurt.

Hope that will clarify, and anyone with a better set of web links, please provide!

Cheers--

JvB (who right now, is only interested in getting it right!)

[Gravity]

Originally Posted by LX3

Not sure when a capital E started getting used for voltage. Maybe it's a transatlantic thing (I'm in the UK). Or, as I said before, maybe I'm a bit old-school and I've lost touch!

Paul

I've seen E... It's actually a bit more old school, representing Electromotive Force IIRC... Not improper, but not as commonly used as V. I think over here we just use P for power.