# Wire Gauge for 30A RV Outlet (long span)



## jayne1986

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!


----------



## oleguy74

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


----------



## boman47k

#8? Is that because of the distance?


----------



## oleguy74

yes.#10 would do but if you are concerned just run #8


----------



## a7ecorsair

Does the travel trailer have A/C that you plan to run?


----------



## rjniles

#10 at 30 amps is fine for 80 feet.


----------



## jadbad2004

*which is better*

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.


----------



## Speedy Petey

jadbad2004 said:


> 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.




jadbad2004 said:


> 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.





jadbad2004 said:


> 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. 





jadbad2004 said:


> 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.


----------



## jadbad2004

Speedy Petey said:


> 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.


----------



## Speedy Petey

jadbad2004 said:


> 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. 






jadbad2004 said:


> 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??






jadbad2004 said:


> 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.


----------



## frenchelectrican

Jadbad.,

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

Merci.
Marc


----------



## jadbad2004

Speedy Petey said:


> 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.


----------



## Stubbie

jadbad2004 said:


> 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.


----------



## Stubbie

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.


----------



## jadbad2004

I stand corrected, it is a fine print note,it is my local AHJ who requires less than 5% VD.


----------



## jadbad2004

Stubbie said:


> 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.


 That is correct,the 24 inches which may be 36 inches is refering to nipples which you do not need to adjust apacity for bundling, is the conduit going to only contain 3 wires? That wasnt mentioned. If there are more than 3 current carring conductors there are additional corrections in addition to temp corections. And depending on average abient temp (source would be determined by state code usually) which i am not familiar with summer weather in that state but 140 F is very low for my area in an atic.


----------



## Stubbie

jadbad2004 said:


> That is correct,the 24 inches which may be 36 inches is refering to nipples which you do not need to adjust apacity for bundling, is the conduit going to only contain 3 wires? That wasnt mentioned. If there are more than 3 current carring conductors there are additional corrections in addition to temp corections. And depending on average abient temp (source would be determined by state code usually) which i am not familiar with summer weather in that state but 140 F is very low for my area in an atic.



I guess your an engineer but you guys always make things 10 times more complicated than it needs to be. Your assuming a 30 amp load and the circuit may or may not be carrying 30 amps. And certainly not continuous load. The op is talking about a 120/240 individual branch circuit to an RV 30 amp outlet. Numbers of wires involved is rather obvious. There is not going to be more than three current carrying wires !!!

Please consider the OP's original question for advice on this RV branch circuit. All this dispute over how to interpret code or trying to give lessons on what the code provides for is not answering the OP's questions. Much of what you have been discussing will not apply to this thread. Ask the right questions and let the op give you the information you need to apply code compliance.


----------



## Saturday Cowboy

i want to know why the OP is not running a 240v circuit to his RV.


----------



## rjniles

Me thinks we are over-thinking the problem. I installed a 30 amp, 120 volt (not 120/240) circuit for my small RV in my side yard. Prior to that I ran the RV with a 25 foot heavy duty extension cord from a 20 amp receptacle in my garage. My point being that the loads in the RV are relatively small with the exception of the air conditioner (and it is less that 20 amps). Unless the OP were to run the AC, use a microwave and perk coffee all at the same time, the load will be well under 30 amps and voltage drop on less than 100 feet of #10 wire will not be an issue.


----------



## oleguy74

Saturday Cowboy said:


> i want to know why the OP is not running a 240v circuit to his RV.


 mostly rv's that require a 30 amp are only 120 volt.


----------



## a7ecorsair

NEMA TT–30 (TT stands for Travel Trailer) is a 30 A, 125 V recreational vehicle standard (hot-neutral-ground) which may also be known as RV 30. It is frequently (and sometimes disastrously) confused for a NEMA 10–30. The RV receptacle is common in nearly all RV parks in the United States and Canada, and all but the largest RVs manufactured since the 1970s use this plug. The hot and neutral blades are angled at 45° from vertical and 90 degrees to each other, similar to NEMA 10 devices. The plug is slightly smaller than a NEMA 10 but larger than ordinary 5–15 plugs. The ground pin, however, is round, like those on straight-blade NEMA grounding devices. Referring to the diagram, the orientation is the same as the NEMA 5 plug and socket, with the receptacle neutral on the lower right. Due to the appearance of this plug, many people assume that it is to be wired for 240 V, but this is a 120 V device. Adapters exist with the TT–30 plug on one side and a 5–15 or 5–20 socket on the other side. When the cord is detachable from the RV an L5–30 is usually used on the RV end of the cord.


----------



## kbsparky

Stubbie said:


> ...The op is talking about a 120/240 individual branch circuit to an RV 30 amp outlet.....


Not quite. There is no such thing as a 120/240 30 Amp RV outlet. 



> I'm installing 30A, 120V service for my travel trailer...


RV outlets are either:

30 Amp @ 120 Volts

OR

50 Amp @ 120/240 Volts.


----------



## kbsparky

In any case, I would install this with some #10 wire, fed from a single pole 30 Amp breaker. The distance involved should not present much of a problem, even at 120 Volts.


----------



## iamrfixit

I ran 10 gauge 100 feet from the panel through a large pole building to a 30 amp RV receptacle and connected it to a 30 amp single pole breaker. The RV would run for days at the campground with no problem of any kind, so I knew the RV itself was fine.

When connected to the new outlet 100 wire feet away from the panel; the breakers in the RV would heat up and trip repeatedly when running the AC and the refrigerator at the same time. After upgrading to 8 gauge it worked fine. The 30 amp breaker in the panel never did trip and the wire did not heat up. Apparently there was enough voltage drop with 10 gauge at 100 feet to cause this. 

If I were you, at 80 feet I would certainly go with 8 gauge the first time, either in conduit all the way or NM (romex) cable all the way and skip all the extra connections.


----------



## Stubbie

kbsparky said:


> Not quite. There is no such thing as a 120/240 30 Amp RV outlet.
> 
> 
> RV outlets are either:
> 
> 30 Amp @ 120 Volts
> 
> OR
> 
> 50 Amp @ 120/240 Volts.


Quite correct that was certainly an error of thinking on my part. 

Aside from the fact the op stated 120 volts in his 1st post I would at least have a 50/50 chance of being correct.

Since I'm on a roll here and rarely ever being wrong ... (I only make mistakes .. this is different than being wrong) , .. I'm providing an opportunity for anyone to invest in the stock market via *ME*. If interested send your hard earned money to 

Stubbie
PO BOX "Sure Thing " 30
Ottawa, Kansas 66065-3254

Please do not send any coin just large bills,, fives will be accepted reluctantly.


----------

