Watts (not Charles) are Watts. Really, really long - Fiberglass RV


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Old 02-09-2003, 12:40 PM   #1
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Watts (not Charles) are Watts. Really, really long

I'm going to offer up something that I may regret and others may care less about :zz but I'm interested in learning something and those with more specialized knowledge can help me. I need a sanity check :o from those in the know.

After wandering around the web, here's what I think I know:

The watt is an electrical unit of work done. Watts are watts. (No comments about how much work Charles does :wink .) 1500 watts DC is the same work as 1500 watts AC. Watts = amps x volts. A 12 volt DC device can do the same work as a 120 volt AC device only the 12 volt DC device needs more current to do the same work. If watts = amps x volts (it does) then amps = watts / volts. A 1500 watt heater needs 12.5 amps AC (1500 watts/120 volts AC=2.5 amps). The same heater will need 125 amps of 12 DC power (1500 watts/12 volts DC=125 amps).

It's probably no coincidence that the maximum power for most home electric heaters is 1500 watts. It draws 12.5 amps and most home wall sockets are on 15 amp circuit breakers/fuses. We like round numbers. A 2000 watt heater would require 16.7 amps, thus blowing a 15 amp fuse.

The smaller the wire, the greater the resistance (ohms) to flow (amps) it has. Also, the greater the flow, the greater the resistance. As an analogy, try blowing softly and then as hard as you can on a small bar straw. The higher the pressure the greater the resistance. Do the same on a normal straw. Less resistance at the higher pressures than the bar straw. No! Duh! as a young woman I know used to say. Anyhoo, it's similar for electrical wiring. (For the purists it's not EXACTLY the same thing because with air through straws we're actually dealing with the Bernoulli effect, Reynolds numbers, laminar vs. turbulent flow etc.)

The voltage drop through a wire can be calculated. It's a function of the amps you're forcing through the wire, the wire's length and the wire's inherent resistance. I had some trouble getting consistent numbers for resistance in a wire, but the relative numbers still work. Using some ball park numbers and going back to the 1500 watt heater on a 50 foot extension cord I get a voltage drop of 2.0 volt AC drop for a 12 ga wire, 3.2 volt AC drop for a 14 ga wire and 5.1 volt AC drop for a 12 ga wire. Resistive loads like the heater element usually aren't fussy about the voltage, while inductive loads like the heater's fan motor are less forgiving.

So what? Well, while everything may still work, you're stressing the motor more with a 16 gauge extension than with a 14 ga or heavier extension. Further, the more current you try to pull through a wire, the hotter the wire gets (No! Duh!) Sometimes you can get wire failure as the insulation melts and the conductors touch. And don't forget start up loads (amps). Start up loads can double, perhaps triple the current requested. While a fuse may have a delay action to tolerate this, the wire will seem to 'bind up'; under this current draw, causing further stress on a motor. An air conditioner starting up can really spike the inrush current.

Hmmm. Watts are watts. This put me to thinking about generators. What if I added up all the watt demands I would see at one time and then chose a generator with equal to or greater watt capacity. At that point in time I was introduced to something called the Power Factor. Something only a trained electrical engineer can properly explain. It seems to boil down to the fact that whenever you convert power, especially between DC and AC, there are inefficiencies that get introduced. It turns out that the square wave AC voltage that most generators put out is considerably less efficient than true sine wave voltage (some Hondas and Yamahas). For converting between DC and AC and using a square wave generator, you need to increase the requirement by about 20%. You can apparently disregard or minimize this if you use a pure sine wave generator. As an example, I have a 45 amp DC converter in the Casita. 45 amps times 14 volts (I believe it charges at 14 volts and to be conservative lets pretend the whole 45 amps is at 14 volts DC) equals 630 watts (45x14=630). However, converting from the generator's square wave 120 volt AC to 14 volt DC needs to be bumped up 20% (630x1.2=756 watts). So assuming a Coleman square wave generator, 756 of its watts can go to the converter/charger, while 630 watts of a Honda EU pure sine wave generator goes to the converter/charger.

To the 756 watts dedicated to the converter I'd have to add the pure AC loads, like a cube heater or an air conditioner. If my air conditioner used 15 amps at 120 volts AC, that would add 1800 watts (15 x 120=1800). So, if I had every DC load running simultaneously such that the converter/charger was generating 45 amps and I had the air conditioner running also, I'd need 2556 watts. Probably a 3000 watt square wave generator called for. Of course, if I'm only running a trickle charge on the batteries (13 volts DC at less than 1 amp) from the converter/charger, I could would probably need only 12 watts or so, and a 2000 watt square wave generator would suffice. Buy a pure sine wave generator and I'd have even more cushion with a 2000 watt generator. Lastly, if I can suffer through without the air conditioner, a 1000 watt generator would be sufficient.

So, how does this all sound? Am I on the right track? I apologize if this is intuitive to most others but I wanted to verbalize it to get some feedback.
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Old 02-09-2003, 12:43 PM   #2
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What:wak
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Old 02-09-2003, 12:58 PM   #3
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Too Much Wattage From Too Dim a Bulb

I know, I know. Please put me out of my misery. Kill the thread.

And why don't my apostrophes always come out?
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Old 02-09-2003, 12:59 PM   #4
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Ches

Not 'what.' Watt!;)
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Old 02-09-2003, 01:02 PM   #5
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Apostrophes

I'm not really sure why they don't work. I cannot use quotes, so use two apostrophes instead (I have AOL).

Michael will probably be able to help you with that.

Kill the thread? Why? Someone (probably Morgan) will come along and know exactly what you said!;)
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Old 02-09-2003, 01:05 PM   #6
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I think life is getting too complicated.:) I just might go back to a kerosine lantern and a block of ice.;)
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Old 02-09-2003, 01:17 PM   #7
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watts = whats vis-a-vis watts ?

Scary.....I followed all that !:r

Guess I'll pass on the 40kw Onan diesel and look for a 2kw or 3kw :loltu


Old adage......you get what you pay for.
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Old 02-09-2003, 01:25 PM   #8
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Now, seeeeee.......?

Steve....aren't you glad you left it there? Noel, you did understand that, didn't you? You're not just pulling my leg, right? :o

Someday, even I might understand it...................NOT!

That's why we have all you smart people here to help us!
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Old 02-09-2003, 02:01 PM   #9
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Yes Steve,

I only see one error. You meant to say 5.1acv drop in 50ft of 16 gage wire. That is why I stated in another thread that a you can use a 16 gage extension cord for a 1500 watt heater. (120volts - 5.1volts x 13.1amps = 1505watts) As you can see it leaves a safety margin of 2amps. But remember house current can drop down close to 110volts at the receptacle so the heavy gage wire is best to insure the safety margin. This is why it is always best to have a 12 or better yet 10gage extension when going more than a few feet.

On the other thread they were using a 25ft cord, so there shouldn't be a problem.


Be safe and use the extra heavy extension cord.
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Old 02-09-2003, 03:25 PM   #10
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Leave it to me to answer the important question (ha!)

RE quotation marks.

Steve, This software doesn't parse smart quotes correctly (it displays the command/code instead; even if you get it looking right once, if someone subsequently ''quotes'' your message, the smart quotes will convert to the command/code. Ugh. Not pretty).

Things are much more predictable if you use straight (stupid?) quotes, although as Suz said, several of us have found we can't even get away with straight double quotation marks - instead, we use two single straight quotes to represent a double.

I edited your thesis on watts and amps; all I did was fix the quotes. Hope this is okay.

:sunny
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Old 02-09-2003, 04:01 PM   #11
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Steve -- to answer your question, yes, you are on the right track. Two other considerations: A square wave current is what we might refer to as "dirty" power. It is fine for many applications, but can cause some devices to run poorly, or can even damage some things, such as computers. If you run a laptop which has a little plug-in power supply, you'll be fine. If you try to run an actual PC, or other computerized devices (DVD player, PlayStation, some microwaves), you could experience trouble. Probably not, but you might.

Also, if you push a generator close to it's limits, even a true sinewave unit, you will get a "clipped" wave. Picture a sine wave with the peaks and valleys squared off. Similar to a square wave, but not quite. This can produce numerous problems. So, allow yourself sufficient "headroom" on your generator, or limit what devices you will use when running the generator.

Don't forget, as well that the smallest and larger generators get noisier, and become less pleasant to have around. May not be allowed in many campgrounds.

Back to the extension cords. Having been a caretaker at a California RV park in a previous life, I can vouch that most campground electrical supplies are not "up to spec". The voltage drop that occurs in an extension cord also applies to the wire that is buried in the ground. To build a campground correctly, they'd have to install a transformer or two on every loop of the campground, and then "spider" the connections out, keeping them all under 200 feet. I've seen some campgrounds that have done this, but I've seen many more that haven't. Couple that with the fact that many campgrounds are in the boonies, and at the end of the utility company supply line, and the voltage at the power box on your campsite may only be 100-106 volts. Now, add a cheap 16g extension cord, and you are asking for it. That 16g cord is ok for your weedwhacker, but not for much else. Get a 12g cord, and you will never regret it. If you run air conditioning and a microwave simultaneously, you might want to spring for a 10g cord.

In a different previous life, I built privacy fences. I had a large air compressor that ran on 110v (not the best idea), and I often had to put in on 200 feet of extension cord. I had two 100' 12g cords to use. It did ok that way. But, if I accidentally hooked it up to a 100' 14g cord (even only one), the compressor wouldn't even start. It would bog down and blow the breaker. That's the startup load Steve mentioned. Again, don't cheap on your extension cord.
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Old 02-09-2003, 06:34 PM   #12
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If I figure this all out, can I have college credit?
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Old 02-10-2003, 06:12 AM   #13
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Posting Help

Ron, You're right, I fat-fingered the wire size. In the example I used, a 16 ga wire would have a 5.1v drop.

Mary F., Thanks indeed for the apostrophe help. Sometimes I write my notes out in Word and then cut and paste into the note. I wonder if that's were the apostrophes are lost in the translation.
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Old 02-10-2003, 06:42 AM   #14
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Steve

Steve ... good information! I appreciate you posting it! Electricity is something we all use ... but most don't even begin to understand.

And you are right, while Watts are a measurement of work done ... many people have tried to measure the amount of work I do ... and come up empty handed!
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