Extenion cord rated to 10 amps, AC draws 9.5 Amps...

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Postby some seeing eye » Wed Aug 18, 2010 9:07 am

[quote]I don't see why gauge is the end all be all measurement of extension cables.[/quote]

It is because of Ohm's law. Many people don't believe in physics, but last I checked it is still active on the playa. It does interact with insulation thermals as many commentators point out. If you would like to overrule physics with someone's sales pitch, that's up to you.

Over 100 feet the voltage drop difference between #18 and #12 at 10A is not huge - 6V vs 1V. If you are not starting at the generator with your 100 feet at 120V you could have a problem. Many loads draw more amps if operated at low voltage. So your 10A could become 13A at 90V stressing your extension cord and some loads can be damaged by undervoltage/ over current.

But the thermals on the playa are your bugaboo - that is why everyone is suggesting reasonable sizing of #12. I wouldn't buy a black cable.

And from the Captain:
[quote]OK here's a little real-word Burning Man info for you. In '01 I was at the BRC Post Office, and ran A/C through some long 12-gauge extension cord. The ends got so hot they DID melt down. [/quote]

That was before the grid used step down transformers to maintain 120 at the spider boxes. Entirely expected that would happen.
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Postby Lassen Forge » Wed Aug 18, 2010 9:48 am

Thank you SSE for the explanation. (Kept me from posting it!!)
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Postby Talynt » Wed Aug 18, 2010 12:01 pm

ok, then, so lets say I have a magic generator that puts out a perfect sine wave of pure AC power at 120v, and say 10000 watts of power. Basically, lets say I'm plugging this into a household electrical system.

If I understand it right, the voltage drop over 100 ft (that voltage has got to be going somewhere, meaning heat due to resistance in the copper, right?) is going to be negligible with a 10 OR 12 gauge wire, correct? So lets forget we're running an air conditioner. If we're running say, 10 amps of lights then there shouldn't be a problem with a 12 gauge 15 amp cable, correct?

The only real issue then is if the compressor draws way more than 9.5 amps when it kicks on? I know with motors there can be huge draws to start (and of course GENERATED power while it stops spinning), but from what I'm gathering half the people say the desert heat won't be an issue, while half keep warning me it will.

Basically, this is useful information and it goes way past paying .50/ $1 a foot for extension cord. I definitely don't want to waste money if I don't have to, but I'm one of those guys who likes to know exactly what's going on. Thus my conclusion so far is this, and correct me if I'm wrong: If the max draw of the air conditioner/lights/vibrating bed is less than what its rated for, then there's no problem choosing 12 gauge vs 10. The voltage drop over 100 feet is negligible, and the max capacity, resistance, and voltage drop of the cable are unaffected by the desert heat.

so the real and only question is whats the max draw of my ac when it clicks on? Does anyone have a kill-a-watt I can borrow?
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Postby some seeing eye » Wed Aug 18, 2010 12:39 pm

The developed countries' electrical code is over designed. But that's good.

The voltage drop with 10 Amps for 100' of #12 is 1.5V, for #10 it's .99V. #10 has about 60% more copper in it than #12. There are many more #12 extension cables made. Those two factors drive price. In other words, #10 is a fine cord but #12 will easily meet your spec for current, and it has many uses around the house.

You can calculate it yourself with V=IR. You can look up R for any wire gauge by unit length - search American Wire Gauge.

I'm not sure if thinking about voltage drop "going somewhere" is he best analogy. Voltage is not a thing, it is a measurement. It's electrons that go - at roughly the speed of light - the current.

If you want to be truly dangerous, get an inexpensive meter. Even clamp on amp meters are cheap now. Be careful how you handle the voltage probes.
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Postby Lassen Forge » Wed Aug 18, 2010 1:07 pm

Talynt, check your PM box. Thanks, bb


(PS - Voltage Drop Calculator:
http://www.nooutage.com/vdrop.htm )
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Postby gyre » Wed Aug 18, 2010 1:38 pm

some seeing eye wrote:
The voltage drop with 10 Amps for 100' of #12 is 1.5V, for #10 it's .99V. #10 has about 60% more copper in it than #12. There are many more #12 extension cables made. Those two factors drive price. In other words, #10 is a fine cord but #12 will easily meet your spec for current, and it has many uses around the house.
Are you talking about the voltage drop with a nominal current or the full load?

Talynt, the thing is that an AC is an inductive load, in this case meaning it reacts to voltage and current restrictions.
Unless you have something hotsy totsy it will try to draw to handle whatever heat load is on it.
That is exactly why hvac guys say to leave the ac on, to keep that load down.

Heating elements are resistive loads.
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Postby MikeVDS » Wed Aug 18, 2010 3:25 pm

The developed countries' electrical code is over designed. But that's good.


I disagree with this. Sometimes if you just meet minimum code it's on the line of poor work. Code just makes sure that for a huge general category of things something is good enough. In many cases it's overkill because of the general nature of code but in many cases I would never do things to minimum code requirements in my house because it's not good enough to not trip breakers, or have enough load for a typical family. They group things together. There is no 11 guage wire or 11.753 guage wire to fit exact needs. Under the most extreme allowed situation for most guage wire you'll see you are bordering failure. All it takes is a slightly loose connection or weak breaker and you start having problems.

[qoute]The voltage drop with 10 Amps for 100' of #12 is 1.5V, for #10 it's .99V. #10 has about 60% more copper in it than #12. There are many more #12 extension cables made. Those two factors drive price. In other words, #10 is a fine cord but #12 will easily meet your spec for current, and it has many uses around the house.

You can calculate it yourself with V=IR. You can look up R for any wire gauge by unit length - search American Wire Gauge.[/quote]

As Gyre said, you're neglecting that initial load, plus with that voltage drop, you pull higher amperage, which increase voltage drop some more. Also how many more amps will his unit draw under your assumed 1.5 voltage drop? Also you're assuming your resistance to match some table which is probably average. That is a dangerous assumption that his cord will match that. You're right that if he takes an ammeter and runs some tests he should be ok. That is probably what I would do if AC were not critical for the Burn. All the theory in the world is nothing compared to actually testing your set-up.
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Postby some seeing eye » Wed Aug 18, 2010 6:29 pm

Thanks Bay Bridge Sue for the handy voltage drop calculator - I've bookmarked it!

At a relatively small 10 Amps, 120V, #12 wire and 100 feet, the startup surge impact on the cable for an inductive load, like a motor, will not be significant. For a large motor with large currents it's usually the breaker that is the critical link, not instantaneously overheating the wire. By the way, old school fuses are not a bad thing.

Note the voltage drops I gave are each way - without a neutral ground to earth at the load and the genie, the distance is doubled - still no problem with #12, but something to think about with #18.
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Postby MikeVDS » Wed Aug 18, 2010 9:03 pm

It's not that the wire will instantaneously overheat with inrush, it's that the compressor may overload if the voltage is too low. With a high inrush amp draw the voltage drop is likely going to be more than 1.5v, there is a chance the compressor won't start, especially if it cycles off then on and tries to start under load. If it's a decent unit it will just overload, if it's a cheap one with no overload it may just try to run pulling around LRA which is probably closer to 40 amps, which would hopefully trip the breaker on his geny before it melted anything. This is why it's good to test, test, test or just go overkill in the first place.
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Postby Captain Goddammit » Thu Aug 19, 2010 6:48 am

100 feet of 12-gauge cord running appliances like A/C in a desert = overheat. It's not theory, I've done it.
And yes, an A/C draws a lot more than it's "rated" current every time the compressor starts up.

I'll never do it again, but you should get the 12-gauge, it will marginally and unsafely work... Burning Man isn't safe already.

I've looked into buying wire and plugs separately and it seems more cost-effective to just get a premade cord, oddly enough, unless you want to buy a ridiculous amount of wire.
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Postby ygmir » Thu Aug 19, 2010 6:50 am

With some shopping, and due diligence, I've found 10 ga 50' chords pretty reasonably
Of course, when found, I bought 10............a lifetime supply, as such.
Yard sales and CL are great places, as well.
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