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# Voltage Drop and other Math

Discussion in 'Lighting and Electrics' started by zac850, Apr 25, 2006.

1. ### zac850Active Member

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Location:
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Hi all,
I was wondering if I could get some help with a math problem that is facing me.

My school is having a massive outdoor concert, and since my job is dependent on electricity, I want to get some out there. I am getting a 60Amp breaker box, but I need more then 60 amps.

So I am going to be running power cables out from outlets of my two theater spaces that I am near. Distance from the outlets to the stage is about 250 feet. I just bought three 250 foot cables, 14-3 AWG and some edison plugs to wire on them. I know 12 gauge cable would be better, but the budget wasn't there, and I ran the idea by an industry professional who said it should be fine.

However, just for my mental rest, in practice, am I running into any problems with running 15 to 20 amps through this cable?

Also, what kind of voltage drop would I be getting? I'm in the US, so its around 120 volts at the outlet.

Thanks,
Zac

2. ### kingfisher1Member

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Resistance is equally to p, the resistivity of the wire times teh quantity Length of the wire in square meters times the cross section area

3. ### kingfisher1Member

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14 gauge wire has a resistance of about 2.5 omhs per 1000ft, or some wher about 1/4 of and ohm for a 250 foot run.

4. ### kingfisher1Member

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i just realized this doesn't say much about votage drop now does it?
can we applie the lovely Ohm's law to these lovely ohms?
R=V/I

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Location:
Franklin, TN
resistivity, that'll be on the vocab quiz friday... lol

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Using Ohm's Law, V=IR, so the voltage drop = 20*0.25 = 5 volts. Now we must consider that the electricity has to flow up the cable and back again, so in fact you have a voltage drop of 10 volts. You are looking at loosing 200 watts of power in each cable to heat caused by resistance.

I am concerned about the statement "I am getting a 60Amp breaker box, but I need more then 60 amps." If you are overdrawing your breakers, then you are likely to have one of two results, a] breaker tripping or b] fire. Fire being caused by overheating.

I don't know what the regulations are over in the US, but here in Australia, you cannot have an extension lead longer than 30 metres (100'), irrelevant of the cable gauge, at least with single phase, three phase has different regulations. So the 250' extension here would be illegal. To comply with legislation, you would have to put two joins in the cable, I would think that less safe. What I am saying is that your NEC may have something to say about maximum length of an extension cable.

7. ### fosstechMember

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Tacoma, WA USA
Over 60 amps on 3 14/3 cables? :shock:

And why do you need a breaker box if you're running just three separate circuits out of the main building? Those circuits are already breakered for probably 20A already.

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I was more concerned about the running more than 20 amps through a 20 amp connector and breaker. You would need to ensure that they are plugged into three different circuits and that on the breaker panel they are fed from, that the breakers are not next to each other. This being because a breaker can handle less overload when the one next to it is also being overloaded. Think heat buildup.

9. ### MayhemSenior Team EmeritusPremium Member

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Relax guys – he said that in addition to the 60A breaker box, he intends on running 3 additional circuits from an adjacent building. Providing he monitors the draw on each lead he should have no problems. The one thing that needs to be considered in that as the voltage drops, the current draw will increase to compensate.

A 600W lamp on a 120V supply will draw 5A but the same lamp on a 100V supply will pull 6A.

A general rule of thumb is to load your circuits to 80% to account for fluctuations in the power supply.

Remember that 120V power supply will vary in actual voltage output. I know that our 240V supply is often up as high is 250V (have seen it exceed this level) and as low as 220V.

I would also stick a meter into each lead to visually see what you have at your disposal.

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I see that I misread the situation. I agree that a meter into the lead would be the best idea, but that would only work if you meter it whilst fully loaded. Given that the current will alter, so too will the voltage drop. The voltage available that you measure unloaded will likely change when you meter with the full load applied.

The voltage fluctuations are generally caused by changes in loading to the power grid. So at times when people are using a lot of power for cooking, heating or cooling. So unfortunately if you are having this in the early evening, then you will probably be getting a lower voltage from the outlet to start with.

11. ### zac850Active Member

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Location:
New York
Yes, thank you Mayhem. I am getting a 60 Amp box brought out and in addition need more power.

I have been playing with Lightwrite for a while, patching dimmers and making sure I keep individual runs below around 17 amps, just in case.

So its possible that I will have voltage drop of 20 volts.

Thanks guys,
Zac

12. ### CURLSMember

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In addition to all this on a sidenote a really great program that can teach you a lot whether you are a sound guy or a lighting guy is LD Calculator Lite. It's relatively easy to find on the interet and a free download. Download it and I am sure you will fall in love with it!

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My calculations would indicate that it will be 5 volts each way for a total of 10 volts

14. ### tenor_singerMember

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Is this a high school event?

Who is doing the wiring of the 60-amp breaker?

15. ### zac850Active Member

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Location:
New York
Oh, I run a mac and I forgot that I have LD Calculator Lite in Virtural PC. Forgot about that!

Wiring will be done by an electrician, a lighting designer/rigger/electrican who designs the lighting for clubs all around the world. Shortly, I trust his wiring.

16. ### tenor_singerMember

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Location:
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I just wanted to be sure that your high school wasn't making you do the wiring.

As my drama students would say... I am fanatically obsessive about high school students' safety (considering that I was almost killed in high school working with electricity because the school wouldn't step in to fix things).

BTW... I visited your site the other day. Great work. I think you'll do an amazing job in college .

17. ### kingfisher1Member

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just as an update were running and extra run for lights to stay on the safe side, and maybe even to more if we can scrowng up the cable. hopeflly we won't be stuck patch edison-pin converters tryihng to get some extra five feet

18. ### AVGuyAndyMember

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You should be getting about 30v of voltage drop, which is unnaceptable.

20amps over 250 feet is alot to ask out of 14awg. I would go up to 10awg. With 10 you would only have a 12v drop.

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14ga cable is only rated for 15A, even if you ignore the voltage drop.
/mike

20. ### kingfisher1Member

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we ended up spliting up the runns to reduce volt drop, so we ended up only pulling about 11 amps per run

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