wiring for metal shed and 50 amp rv plug

Old.DanOctober 8, 2011

Hello, I'm new here and need your help. I have given what you might view as too much detail, but I've seen you try to answer questions such as "I want to rewire my ________, so what do I need?" and don't want to fall off into that barrel.

I am running underground (in conduit 18" deep) electrical service to a metal tractor shed - length of run = 172' from the 200 AMP main breaker box in the house to a smaller, maybe 100 AMP (1 of my questions) main breaker subpanel in the shed. I am setting up to power a 50 AMP RV trailer just outside this shed, a couple of small fluorescent light circuits and 3-4 20 AMP outlet circuits (largest appliance is a small 12 AMP pancake compressor).

I currently use a 30 AMP trailer but I don't want to redo all this when I step up to a 50 AMP trailer. To quote http://www.dasplace.net/RVWiring/wiring.html: "In the case of most 50 AMP RV's, 250 volts is still supplied to the coach just like your home electrical service but typically one side of the line is used for all the 125 volt appliances except the rear AC and the other side is used for the rear AC only." This feature on 50 AMP RV trailers has been considered to different ends by 3 different people with recommendations for 2 AWG, 4 AWG and even 6 AWG conductors from house to shed with a mix of "3 equal sized conductors for HOT, HOT, NEUTRAL" to "2 equal size for HOT, HOT and a much smaller for NEUTRAL". My understanding is that, since the trailer will not have any 240V appliances to "balance" the alternating current on the HOTs, an equal size NEUTRAL must be used for most frequent use of one HOT leg and the occasional use of the second HOT leg to operate the 2nd A/C unit.

Before I ask my questions, I'll point out that I live in the country and can't get anyone to care what I do out here... not the closest city, not the county, not any permit-issuing entity, not any inspection department.

1. What size conductors do you recommend for these three underground runs? I already bought 2 runs of 4 AWG stranded CU wire for the two HOTs based on my first recommendation before I got nervous about the light NEUTRAL wire this person recommended. The code on these runs is 'ENCORE WIRE CORP 4 AWG (21.2mm2) TYPE MTW OR THHN OR THWN-2 OR GR II SUN-RES VW-1 600V (UL) OR AWM OR C(UL) TYPE T90 NYLON OR TWN 75 FTI 30/AUG/11'. If they won't work, I'll attempt to trade them for what will.

2. My plan is to use a 100 AMP breaker in my indoor house panel to service the inside-the-shed panel with 1-2 breakers for lights, 3-4 20 AMP breakers for outlet circuits and a 50 AMP breaker to power the 50 AMP RV receptacle on a 20' run from the shed panel. What size wire would you recommend for the 20' run inside the shed from the subpanel in the shed to the 50 AMP RV plug (which is an outdoor plug and will be mounted outside to service the trailer nearby)? BTW, I have a 'dog bone' adapter to reduce the 14-50R 50 AMP receptacle to 30 AMPS for the current trailer.

3. What size panel do you recommend for a subpanel inside the metal tractor shed to hold breakers for the RV trailer and the lights and outlets? I am looking at a 12-Circuit 12-Space 100 AMP Main Breaker Load Center, but I have a 250 AMP panel left over from a temp. power pole from when we built the house so I could save that $$$, but it's overkill and its guts are upside down... plus the 250 AMP breaker would not provide protection - merely an OFF switch.

4. I plan to ground the subpanel in the shed instead of running a GROUND from the house. I will separate (unbond) the GROUND bar from the NEUTRAL bar in this panel. The shed is made of 2" square tubing resting on gravel with 14 3' rebar "spikes" driven through the base plate tubes into the ground. Is that sufficient for my panel ground or should I drive down an 8' copper clad bar?

Well, I'm tired of thinking and typing so here's my commendation for any of you who read past my first question.

Thanks in advance for your time and attention.


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Ron Natalie

1. The wire you have selected is fine, but you do need to run a neutral and a ground.

2. Depends what kind of conductor you are running. If more THHN/THWN-2, then you can get by with #8, otherwise #6.

3. Well it sounds like 12 spaces will be fine here.

4. You are REQUIRED to do both. You must run the ground from the house PLUS you must install a ground system (2 rods typically) at this structure.
The rebar is most likely NOT sufficient grounding. You should however connect your ground rods to the metal structure.

    Bookmark   October 9, 2011 at 8:12AM
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I don't believe code would allow the #4 on a 100A breaker, so with that said, you can use the #4 cu wire as it is adequate for the loads you have. I would run a full size neutral, but in no case should you use anything less than a #6. Considering the length of the feeder, I would use a 70 amp breaker in the house panel. Don't worry about it being too small, it won't be.

    Bookmark   October 9, 2011 at 1:11PM
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Great! Thanks for your patience in reading all that detail. Thanks for your responses.

I will run a full size (4 AWG) NEUTRAL. What size GROUND should I run in the house to shed panel conduit... a green insulated 6 AWG stranded copper maybe?

I will use a 70 AMP breaker in my house panel. Another person gave the same recommendation. Should I make the associated step-down for the main breaker in the shed panel from 100 AMPs to 70 AMPs or leave that as described?

Regarding "install a ground system (2 rods typically)": My house was built in 2005 with one 8' ground rod wired into the main house panel by the electrician. Electric co-op employees stood on that single bar when they connected my meter to the power pole. Is the "2 rod" ground system new since 2005 or did everyone involved in energizing my house leave me with something that is dangerous, illegal, bad?

Thanks again.

    Bookmark   October 10, 2011 at 2:49PM
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Ron Natalie

You can use a single rod when you can show that it is sufficient, but that's usually beyond the capability of even knowledgeable amateurs.

No reason to change the "main" in the panel. It's not playing any protective roll other than providing the disconnect for the structure.

    Bookmark   October 10, 2011 at 3:10PM
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Roger, that.
Do you have a size/type recommendation for this house-to-shed GROUND wire to be run with the 2 HOTs and the NEUTRAL? Would a 'green-insulated 6 AWG stranded copper' conductor be suitable?

Thanks again.

    Bookmark   October 10, 2011 at 4:16PM
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a #8 green cu ground would be sufficient, but if you want to increase it to a #6 cu, that would be fine.

    Bookmark   October 10, 2011 at 5:51PM
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