Wire size
what size wire do I need to run 500ft I am running it to a 30amp 5th Wheel trailer

Can you tell us what Voltage? What's the size of the load that you expect to connect?
 pete 
Assuming it's a 30 amp 'travel trailer' type of plug, it's 120 volt. The absolute minimum size would be #10. Considering the length, I'd run either #8, or #6. If you have an A/C unit, and plan to use it, go with the #6. #8's would probably start it ok, but the #6's will dim the lights alot less.
Rob 
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The original poster needs to figure out exactly how much his travel trailer will really pull. If you want close to 30 amps, you're going to need ~1/0 copper or 3/0 aluminum minimum. If you think it'll pull less, you may be able to drop a few sizes. Either way, your into it big money. 
Will the trailer be a permanant fixture? If so consider talking to the electric company about putting in a new meter. It's worth a shot and with my power company it was free run a new line from the pole to my new workshop. I could save you big big bucks. good luck.

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I agree!!! (you could go to 5% or maybe even 7% but have the whole thing checked out first by a reliable licensed electrician)  pete 
Boy, did I ever goof!! I thought I read TWO hundred feet, not 500.
A #6 at 200' will give about 10 volts drop at 30 amps. 30 amps will never be drawn for any length of time, but 15 amps is reasonable. That would drop 5 volts, reasonable for a travel trailer. I guess I just couldn't get 500' through my head. At 500', a #2 CU would be about the smallest I'd consider installing. A #1 or #1/0 would be even better. Especially if you're running an A/C unit. 500' is a LONG way to push 120 volts. Rob 
#6 copper stranded is good for 30 amps at 120v for a distance of 500 ft.
I'm not sure where the 7 amps came from? Cmils= 2 x K x I x D / VD 
I don't know about the USA but calculating the voltage drop for a given cable length & current, is not a simple process in Australia.
We are required to use AS 3008 (an Australian Standard of about 100 pages), which covers the installation of cables for alternating voltages up to and including 0.6/1 kV—Typical Australian installation conditions. From many of the calculations that I have done, there is a noticeable difference in cable size for a given installation/load according to; 1. the installation method (ie buried, unenclosed in air etc), 2. the type of cable (type of conductor, number of cores, insulation type etc), 3. available fault current (OCPD's may need to be changed to minimise cost). The standard also recommends a maximum allowable voltage drop of no more than 5% of the measured supply voltage. It would be impossible for me to "safely" answer the OP's question without knowing how the OP wishes to install this cable. On the other hand & if the OP is not sure of how he/she wishes the cable to be installed, I would calculate using a couple of different types of cables as well as a couple of different installation methods. 
Better yet, take the cost of materials and labor and buy a portable generator. Or if possible, move the trailer closer to power source.

this what i came up
120 v 30 amp [ i always figure with the travel trailer always loaded up near the max espcally that is true with larger travel trailer when they have 2 roof mounted A/C running full speed ] at 152.4 metre [ 500 feet ] 2.9% voltage drop wire size 0 cu [ i dont think anyone make that size so you have to go next size larger ] 1/0 you will need to make a pigtail to termated at the breaker and the travel trailer repectale box. Merci,Marc 
Around here, voltage drop is figured based on simple resistance. One must remember that a twowire circuit (120 or 240) is measured out and back. So the distance must be doubled.
If I'm figuring voltage drop for a hardwired load, I'll use the fullload current, plus if it's a motor, allow for start surge. If it's a receptacle, I use 80% of the rating of the receptacle. Even if the load on the recptacle is known, there's no gaurantee something else won't be plugged in to it. The code recommends not more than 3% voltage drop on branch circuits. This is a recommendation, not a requirement. Since this only allows 3.6 volts drop, I personally think it's a bit tight. A 20 amp circuit using #12's will drop that much at just over 50'. Are there any such circuits at your house? There sure are at mine! I usually figure 5% for motors, and 7 or 8% for anything else. Almost all electrical equipment is designed to operate properly on a 10% voltage variation. I also don't put lights and recptacles on the same circuit. That way when you run your hair dryer, the lights don't dim. If the 3% were a requirement, we'd be running #10's and #8's to most stuff in any building. This is one area where common sense and basic experience needs to be employed. Rob 
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I've just finished a calculation for a 750 Watt single phase 220 volt motor that is 170 metres away from the main supply. The motor FLA is 5.4 Amps. I allowed for the 10% voltage drop & further estimated that if the correct cable size was used, it would be within a 5% voltage drop at FLA. My final result was to use 10mm squared cable (copper, PVC/PVC 70 degrees Celsius, buried direct at no less than 0.5 metres). At a pinch, 6mm squared cable could be used. I'm sorry but you'll have to convert these figures to Imperial if you wish to understand this. 
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ok for other guys to figure out the wire size here it is 10MM2 = little bigger than #8 awg 6MM2 = little bigger than #10 awg Merci,Marc 2.5mm2= simair to 14 4.0mm2 = simiaiur to 12 
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