A/C question

Tree · Sep 20, 2002

This is not directly related to flashlights, so moderators feel free to move this to The Cafe if necessary.

I wanted to get a better understanding on how electrical current works. So I figured I'd ask the experts.

First I'll give you the scenario that started this. A few nights ago I did a show (I am a soundguy/audio engineer) at a departments store in a mall and the only power I could get was 1 Edison jack about 100' away. I have a voltage meter on the spike protector that I use so I was monitoring this throughout the night. Now I have a pretty heavy load on the lone Edison cable ( 7 monitors, 2 FOH speakers, 1 sub, all self powered) and noticed the meter was dropping down below 99 volts sometimes when the band would get loud.

Now the question: is there any direct relation between the voltage dropping and the amps being used? Maybe some kind of formula to tell when the voltage drops this much you are using this many amps? Also, I know I can meter the amp usage on a hot leg of a 3 phase system, but how do you do it on a single 3 wire Edison cable? This would help me so I can make sure I am not going to trip the breaker in the middle of a show.

Thanks in advance for any replies.

bmsmith · Sep 20, 2002

Hi Tree,

99 Volts is scarily low. Any idea how many Amps you are drawing total? Perhaps the labels on the equipment tell you the power requirements?

Check this web site out: http://www.ifigure.com/engineer/electric/electric.htm

Specifically, check out this voltage drop calculator: http://www.electrician.com/vd_calculator.html

- Brian

Tree · Sep 20, 2002

Awesome!! That calculator is exactly what I was looking for. Now to figure out how to put that on my PDA.

Tree · Sep 20, 2002

Actually the speakers I was using can safely go down to 80 volts, it's the mixer and drive rack I was worried about.

Unfortunately the amp requirements on the speakers vary greatly depending on how loud they are used.

Albany Tom · Sep 21, 2002

Keep in mind, to figure the current from the voltage drop, you need the voltage across the extension cord. If you measure just the voltage at the outlet end with the load, and subtract that from the voltage at the plug end without the load, you're measuring the drop of the extention cord, plus all the wiring back to the transformer on the pole. (more or less) More accurate, simpler way is to wire an ammeter in your strip.

The fun part is that you won't know if the outlet your using also has a something else on it, like an air conditioner, and could be ready to trip anyway. One trick I've seen used by a band was to carry a small 1000W ceramic heater, and run that for about 15 minutes before setting up their stuff. Better to have the heater trip the breaker than your gear in the middle of a wedding reception.

luxO · Sep 21, 2002

" is there any direct relation between the voltage dropping and the amps being used?"

Yes! as dictated by Ohms law:
V = I X R...the voltage dropped across the cable is equal to the product of current through the cable and the resistance of the cable.

"I know I can meter the amp usage on a hot leg of a 3 phase system, but how do you do it on a single 3 wire Edison cable"

This worries me a little, there are 3 hot legs in a 3 phase system and one neutral...which are you monitoring?
On a single phase system, if you have an inductive ammeter, just clamp it around the hot wire, as there is only one.
For a non inductive ammeter, you need to insert the meter in series with the load to measure the current drawn.

jtivat · Sep 21, 2002

Tree, also remember that many of the power conditioners or power strips with voltage meters in them are not very accurate and need to be calibrated on a regular basis. We use the Furman 20 AMP PRO Series in all of are racks and I have noticed they will read 5 to 10 volts difference on the same power source with no load. Also for our smaller Amp racks they have a 120V version with and Amp meter which is very nice for making sure you do not trip a breaker.

Tree · Sep 21, 2002

One trick I've seen used by a band was to carry a small 1000W ceramic heater, and run that for about 15 minutes before setting up their stuff

That's a good idea, we usually have dozens of 525 watt lights at a show I could just plug in 4 of those for a while and see what happens.

This worries me a little, there are 3 hot legs in a 3 phase system and one neutral...which are you monitoring?

In our 3 phase 110 volt system there are 5 wires 3 hots a neutral and a ground. I measure the amps across each of the 3 hot legs one at a time.

On a single phase system, if you have an inductive ammeter, just clamp it around the hot wire, as there is only one.
For a non inductive ammeter, you need to insert the meter in series with the load to measure the current drawn.

By inductive ammeter you mean the clamp that measures amps on a multimeter? Could you explain more about the non inductive ammeter, this seems to be more of what I am looking for. Thanks

also remember that many of the power conditioners or power strips with voltage meters in them are not very accurate

I know, I also had my multimeter plugged into a quad box as a backup and it was ranging from 103 to 109 volts throughout the night.

Also for our smaller Amp racks they have a 120V version with and Amp meter which is very nice for making sure you do not trip a breaker.

That sounds like just what the doctor ordered!! Do you know the model number?

Tree · Sep 21, 2002

Oh and I had another question. We have a voltage conditioner that keeps the voltage at 120 no matter what (although I did not have this on the show I talked about). How does this relate to amp usage? It would seem that if the equipment is seeing 120 volts it would be able to work properly therefore using more amps.

Tree · Sep 21, 2002

Also for our smaller Amp racks they have a 120V version with and Amp meter which is very nice for making sure you do not trip a breaker.

The PM-8 on this page seems like what you are talking about. Is there a way to calibrate it so you know it's reading accurately?

**edit** It says it can be easily calibrated on the site.

Tree · Sep 21, 2002

Just to make sure I have this right. As I understand it...

3 phase = 3 separate hot legs, neutral, ground

single phase = 2 separate hot legs, neutral, ground

Is there a similar term for a regular Edison 3 wire (1 hot, neutral, ground) service?

MrAl · Sep 21, 2002

Hello,

If you can afford a power line regulator
they make them too, although not real cheap.

--Al

jtivat · Sep 21, 2002

Originally posted by Tree:
Just to make sure I have this right. As I understand it...

3 phase = 3 separate hot legs, neutral, ground

single phase = 2 separate hot legs, neutral, ground

Is there a similar term for a regular Edison 3 wire (1 hot, neutral, ground) service?

Yes you have this right. I don't have a trem for it just a regular 110v Edison

jtivat · Sep 21, 2002

Originally posted by Tree:
<blockquote>quote:<hr /> Also for our smaller Amp racks they have a 120V version with and Amp meter which is very nice for making sure you do not trip a breaker.

That sounds like just what the doctor ordered!! Do you know the model number?[/QB]<hr /></blockquote>I am at a show in Chicago right now with two Amp racks back stage we break at noon and I will get the model number off the units.

jtivat · Sep 21, 2002

We use the PM-PRO it is the 20Amp version.

Tree · Sep 21, 2002

If you can afford a power line regulator
they make them too, although not real cheap

We have a Tripp Lite LCR2400 in a soundsystem we keep on a riverboat casino that used to use a generator. We've been contemplating buying more.

We use the PM-PRO it is the 20Amp version

Thanks!!

Is there a way to build an inexpensive ammeter into a short Edison cable? I could have one of these on every Edison power source to monitor activity.

luxO · Sep 21, 2002

Originally posted by Tree:
Just to make sure I have this right. As I understand it...

3 phase = 3 separate hot legs, neutral, ground

single phase = 2 separate hot legs, neutral, ground

Is there a similar term for a regular Edison 3 wire (1 hot, neutral, ground) service?

not quite right.

Single phase supply has 1 hot, 1 neutral and, of course, ground.

a 2 phase supply has 2 hots, 1 neutral and ground.
With each hot being 180 degrees delayed from each other.You'll measure 240 av between phases and 120 V between each phase and neutral.

A 3 phase supply has 3 hots, 1 neutral and ground.
Each hot is 120 degrees delayed or advanced with respect to each other.
You'll measure 208 volts between phases and 120 v between phase and neutral.

Tree · Sep 21, 2002

That makes more sense to me, but I've never heard of 2 phase.

You could put a panel mount ammeter in a box. An example is:

http://www.allelectronics.com/cgi-bin/category.cgi?category=385&item=PMA-15A&type=store

So I would run the hot leg through this? Sounds like a cheap solution, I could just build a small case for it.

Oh, and thanks to everyone for the replies, I'm learning a lot.

luxO · Sep 21, 2002

"but I've never heard of 2 phase"

Every house in the US and Canada is fed with a 2 phase supply.

"So I would run the hot leg through this? Sounds like a cheap solution, I could just build a small case for it."

No, maybe not. This meter may require a shunt, you cannot pass 15 Amps through the meter itself.
Something to check first.

Tree · Sep 21, 2002

I guess what I always called single phase is actually 2 phase. It never made sense to me that (what I thought was) single phase had 2 hots.

Any info on how to wire the ammeter? Not too much info on the web site, suppose I could shoot them an email.

A/C question

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