Voltage Drop and other Math

[zac850]

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

[kingfisher1]

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

[kingfisher1]

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.

[kingfisher1]

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

[Radman]

resistivity, that'll be on the vocab quiz friday... lol

[Chris15]

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.

[fosstech]

Over 60 amps on 3 14/3 cables?

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.

[Chris15]

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.

[Mayhem]

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.

[Chris15]

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.