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Old 01-23-2009, 11:57 PM   #1
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Wire Sizing


I have tried to use the online wire sizing calculators to determine what I am going to need and found that almost every calculator gives a different answer.

So I am asking for some help. I need to set up a panel for my RV 300 feet from the main panel. I need a 110 volt 30 amp RV plug and a 110 volt 20 amp utility. I can go direct burial or PVC.
Would I make two runs (1-30 amp and 1-20 amp) or should I make one 50 amp run?
What size wire depending on the run?
Would there be a better way to use 220 and setup a sub panel?

I appreciate the help and expertise.

Thanks,
Ron

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Old 01-24-2009, 02:31 AM   #2
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Originally Posted by shastaron View Post
I have tried to use the online wire sizing calculators to determine what I am going to need and found that almost every calculator gives a different answer.

So I am asking for some help. I need to set up a panel for my RV 300 feet from the main panel. I need a 110 volt 30 amp RV plug and a 110 volt 20 amp utility. I can go direct burial or PVC.
Would I make two runs (1-30 amp and 1-20 amp) or should I make one 50 amp run?
What size wire depending on the run?
Would there be a better way to use 220 and setup a sub panel?

I appreciate the help and expertise.

Thanks,
Ron
Wow, 300 feet is quite a run for a 120 V circuit! You would be better off running 240 V, because it will help alleviate the voltage drop. Unfortunately, you will still end up using quite a large expensive wire. For a 50 A subpanel, you will need to use #2 copper in order to keep the voltage drop below the recommended 3%.

1/0 aluminum quadplex URD direct burial would also do the trick and may be cheaper. I'll leave it to others to detail the burial depths and fun stuff

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Old 01-24-2009, 11:58 AM   #3
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Originally Posted by shastaron View Post
almost every calculator gives a different answer.

Different calculated answers might have been from using hard-drawn or annealed conductors, different conductor temps. and different amb. temp. assumptions.

300 feet from the main panel. I need a 110 volt 30 amp RV plug and a 110 volt 20 amp utility. I can go direct burial or PVC.
Would I make two runs (1-30 amp and 1-20 amp) or should I make one 50 amp run?
What size wire depending on the run?
Would there be a better way to use 220 and setup a sub panel?
You'd think cable pricing would directly follow the price/pound for copper or alum., but supply vs. demand makes for some interesting pricing strategies.

Last edited by Yoyizit; 01-24-2009 at 01:42 PM.
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Old 01-25-2009, 05:16 AM   #4
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Wire Sizing


Would 50A sub panel be 25 Amp per leg? If so isn't #2 overkill?
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Old 01-25-2009, 06:03 AM   #5
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Originally Posted by TazinCR View Post
Would 50A sub panel be 25 Amp per leg? If so isn't #2 overkill?
A 50A 240v panel is 2 120V legs at 50A each.

Jamie
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Old 01-25-2009, 08:35 AM   #6
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You'd think cable pricing would directly follow the price/pound for copper or alum., but supply vs. demand makes for some interesting pricing strategies.
Hasn't it? At least the cost of smaller cable (12/2, etc) is way down from 6mos ago.

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Old 01-25-2009, 08:58 AM   #7
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Wire Sizing


use a 30 amp sub service 240/120 one leg will give you your 30 for the RV and the other your 20 a receptical, use a subpanel to individually breaker each circuit if you follow to the rule you will need a #4copper conductor to go 300 feet at a 2.15% drop, however a #6 will only cause a 3.31% loss which is trivial, and most inspectors will pass it. but ask "your inspector" first, it is not a constant load at 30 amps, its only needed to start the AC,, my own rv I can start the AC while plugged into a 20 amp receptical with the fridge running on 120 without tripping a breaker. without the AC on just the fridge and lights mine draws under 8 amps
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Old 01-25-2009, 11:16 AM   #8
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without the AC on just the fridge and lights mine draws under 8 amps
Depending on your duty cycle (less than 50%, maybe less than 10%) you can run charging current through the 300' as long as you have at least 13.3vdc at the receiving end, then use an inverter on your batteries. Or a variation of this with step-up, step-down xformers and lower transmitted current.

I don't know how low energy storage need has to go before this
http://en.wikipedia.org/wiki/Flywheel_energy_storage
becomes feasible. Batteries only give back 2/3 rds of the charge you put in.
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Old 01-25-2009, 01:36 PM   #9
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however a #6 will only cause a 3.31% loss which is trivial, and most inspectors will pass it. but ask "your inspector" first,
You let your inspectors enforce a FPN? Jeez......

Personally, I'd run a 30 amp 240v feeder in 1 1/4 pvc consisting of either 4-#6 Cu or 4-#4 AL USE. Puts you a touch over 3% when I did my calcs at 25 amps.
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Old 01-25-2009, 03:58 PM   #10
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For 3% drop at 240v, 300' (600' loop distance)
I
in
Amps
\/

4A ...#14 AWG, $160 for 500'
8A ...#12
16A.. #8
32A.. #6, $180 for 400'?

Seems like no Economy Of Scale for wire prices. . .
If you can store energy in the RV, your peak power can be higher than these amps x 240v.
The average house pulls 4A all the time.

Last edited by Yoyizit; 01-25-2009 at 05:47 PM.
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Old 01-25-2009, 05:41 PM   #11
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I'd run copper #2's (...ok...maybe #4's) in 2" PVC conduit to a 240v sub panel. Your next RV might have a bigger AC.
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Old 01-25-2009, 11:02 PM   #12
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Originally Posted by TazinCR View Post
Would 50A sub panel be 25 Amp per leg? If so isn't #2 overkill?
Voltage drop.

I agree with using #2 copper.

Last edited by Magnettica; 01-25-2009 at 11:04 PM.
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Old 01-26-2009, 12:06 PM   #13
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Originally Posted by jamiedolan View Post
A 50A 240v panel is 2 120V legs at 50A each.

Jamie
No it is 50 Amps total.
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Old 01-26-2009, 12:11 PM   #14
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Wire Sizing


Did you get enough answers?

I would start by going to the local electric supply house to check the price of various wires. Aluminum will be cheaper than copper. I usually find I can buy a 2-2-2-4 URD cheaper than smaller sizes. Install a sub panel at the RV sized for wire you buy. Install breakers in that panel for the circuits you need. One circuit on one leg and the other circuit on the other leg. The voltage drop will depend on how much the actual load is, not on the size of the service you run.
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Old 01-26-2009, 12:39 PM   #15
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No it is 50 Amps total.
?Explain please

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