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Old 02-19-2011, 03:16 PM   #1
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Wire Gauge for 30A RV Outlet (long span)


I'm installing 30A, 120V service for my travel trailer so I can plug it in at home. I need to know the correct gauge wire for this installation, based on the specs below, to ensure voltage loss will not damage any of my RV components will run on full load:

1) installing a 30A, single-pole breaker in my breaker box

2) running THHN wire from breaker box through conduit into attic (approximately 10')

3) splicing THHN to Romex inside a junction box in the attic and running Romex across span of attic (approximately 60')

4) splicing back to THHN from Romex inside another junction and then down the other side of the house through conduit to a TT-30R receptacle mounted on a post (approximately 10')

I'm getting all sorts of conflicting info on wire gauge...#6, #8, #10...which should I use? Thanks in advance for your help!

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Old 02-19-2011, 03:37 PM   #2
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Wire Gauge for 30A RV Outlet (long span)


run #8 in conduit all the way.wast of time spliceing.

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Old 02-19-2011, 03:56 PM   #3
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Wire Gauge for 30A RV Outlet (long span)


#8? Is that because of the distance?
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Old 02-19-2011, 04:40 PM   #4
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Wire Gauge for 30A RV Outlet (long span)


yes.#10 would do but if you are concerned just run #8
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Old 02-19-2011, 05:35 PM   #5
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Wire Gauge for 30A RV Outlet (long span)


Does the travel trailer have A/C that you plan to run?
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Old 02-19-2011, 05:49 PM   #6
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Wire Gauge for 30A RV Outlet (long span)


#10 at 30 amps is fine for 80 feet.
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Old 02-19-2011, 07:06 PM   #7
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Wire Gauge for 30A RV Outlet (long span)


10 awg will handle 30 amps but, that is the maximum that the NEC allows. Since it is going through your attic there could be adjustments for temperature corrections to the rating of the wire. To be on the safe side you should run 8 awg wire. Just remember this, you cant use wire to large but if you go to small you can have extreme problems. And besides that the larger wire the less resistance and the cheaper it is to run. Few people realize that for electricity is consumed traveling through wire and how much is consumed is determined by the resistance of the wire (wire size). This is a small amount but it all adds up! Also for distance you do not need corrections for anything under 100 feet.
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Old 02-19-2011, 07:19 PM   #8
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Wire Gauge for 30A RV Outlet (long span)


Quote:
Originally Posted by jadbad2004 View Post
10 awg will handle 30 amps but, that is the maximum that the NEC allows.
Well, sort of. There are times where this rule of thumb does not apply, but a DIY forum is not the place for that.


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Originally Posted by jadbad2004 View Post
Since it is going through your attic there could be adjustments for temperature corrections to the rating of the wire.
Well, considering that 30A is very much "underkill" due to 240.4(D), even after quite a bit of adjustment the ampacity of #10 would still be 30A.



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Originally Posted by jadbad2004 View Post
And besides that the larger wire the less resistance and the cheaper it is to run. Few people realize that for electricity is consumed traveling through wire and how much is consumed is determined by the resistance of the wire (wire size). This is a small amount but it all adds up!
Yeah, it adds up to pennies. The amount of efficiency in oversizing wire is negligible in almost any residential application.



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Also for distance you do not need corrections for anything under 100 feet.
Where do you find this rule? I don't think you will since it is not true at all. Do the math for 24A, @120v, for the 80' in the OP's example. Voltage drop at 120v is quite severe. VD at 120v is roughly four times greater than at 240v.
Considering that the OP's installation could very well see 20+ amps on a regular basis upsizing the wire is a good idea.
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Old 02-19-2011, 09:50 PM   #9
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Wire Gauge for 30A RV Outlet (long span)


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Originally Posted by Speedy Petey View Post
Well, sort of. There are times where this rule of thumb does not apply, but a DIY forum is not the place for that.



Well, considering that 30A is very much "underkill" due to 240.4(D), even after quite a bit of adjustment the ampacity of #10 would still be 30A.




Yeah, it adds up to pennies. The amount of efficiency in oversizing wire is negligible in almost any residential application.



Where do you find this rule? I don't think you will since it is not true at all. Do the math for 24A, @120v, for the 80' in the OP's example. Voltage drop at 120v is quite severe. VD at 120v is roughly four times greater than at 240v.
Considering that the OP's installation could very well see 20+ amps on a regular basis upsizing the wire is a good idea.
The NEC state that voltage drop can be no more than 5 % of the source. the engineering formula for finding voltage drop is 2 x K x D x I then divide by the cm of the wire and this will give you the voltage drop. soooooo 2x 12.9 x 80 feet x 30 amps divided by the circular mills of 10 awg wire 10383 = 5.96 volts. 5% of 120 volts is 6 volts. Therefore 10 awg wire is acceptable but barely. That is why I recomended going with 8 wire. Just so you know, you dont do you calculations based on anticipated circuit use but rather the rating of the intended circiut. You are correct in saying you will not find the 100 foot rule in the NEC it is an engineering certainty! Do the math and you will see. From an engineering standpoint voltage has nothing to do with voltage drop it is calculated with the rseistance ofn the wire, the distance, the amperage,and the circular mills of the wire. However, there is always more than one way to calculate anything; in college this was and still is the formula preferred for accuracy. Oh and temp has worlds to do with wire rating refer to NEC 310.15 for adjustment factors, a short run (anything over 24 inches) in any temp. other than what wire was tested at requires adjustment! With that said , unless his attic is air conditioned adjustments must be made according to the NEC.
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Old 02-19-2011, 10:08 PM   #10
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Wire Gauge for 30A RV Outlet (long span)


Quote:
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The NEC state that voltage drop can be no more than 5 % of the source.
It absolutely with 100% certainty does NOT.
It is suggested. It is NOT a code requirement.




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Just so you know, you dont do you calculations based on anticipated circuit use but rather the rating of the intended circiut.
You really think this??




Quote:
Originally Posted by jadbad2004 View Post
Oh and temp has worlds to do with wire rating refer to NEC 310.15 for adjustment factors, a short run (anything over 24 inches) in any temp. other than what wire was tested at requires adjustment! With that said , unless his attic is air conditioned adjustments must be made according to the NEC.
You misread what I said. Go back and re-read it.

Last edited by Speedy Petey; 02-19-2011 at 10:28 PM.
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Old 02-19-2011, 10:12 PM   #11
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Wire Gauge for 30A RV Outlet (long span)


Jadbad.,

Can you show us the NEC art where it say what the max % voltage drop it say ??

Merci.
Marc
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Old 02-19-2011, 10:38 PM   #12
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It absolutely with 100% certainty does NOT.
It is suggested. It is NOT a code requirement.




You really think this??



You misread what I said. Go back and re-read it.
The code says it! it is in black and white! 5 % is not a suggestion it is a reqiurement, i dont have my code book with me but when i return to my office i will look it up and post where you can see it for yourself.
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Old 02-19-2011, 11:16 PM   #13
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Wire Gauge for 30A RV Outlet (long span)


Quote:
Originally Posted by jadbad2004 View Post
The code says it! it is in black and white! 5 % is not a suggestion it is a reqiurement, i dont have my code book with me but when i return to my office i will look it up and post where you can see it for yourself.
The NEC does not require 5% or less voltage drop. Rather the NEC addresses voltage drop as an FPN (fine print note) in article 210.19(A)(1) exception 2 FPN #4... this is for branch circuits and refers you to 215.2 (A)(3) for voltage drop in feeders. The total voltage drop to the farthest outlet of the feeder and branch circuit is recommended to not exceed 5%. Branch circuits are recommended to not exceed 3% at the farthest outlet as an operational efficiency for correct performance.

Though not required it is a very good idea to adhere to those standards when ever possible.
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Old 02-19-2011, 11:26 PM   #14
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Wire Gauge for 30A RV Outlet (long span)


In this situation we are talking about any adjustments would begin at 90 C insulation for thhn and nmb cable. For #10 copper that would be 40 amps so an attic ambient temp of 132 to 140 F would be required to reduce the ampacity of the wire to less than 30 amps. The 24 inch rule is talking about bundling not distance minimums.
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Old 02-19-2011, 11:30 PM   #15
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I stand corrected, it is a fine print note,it is my local AHJ who requires less than 5% VD.

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