RV Electricity – Q&A on installing a pedestal at home

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From the RV Travel reader comments

Q: I want to install a 30-amp box hookup on a stand in my RV concrete slab in my back yard. I hear of several methods, but some don’t sound so safe. What is the proper and safe way? I know it has to be totally separate with its own breaker and the wire needs to be at least 6-12 inches underground, and that wire is expensive. I’d like it to be in a PVC pipe so if there is digging at least the pipe will be visible. Yes, I realize I need an electrician, but not all electricians deal with RVs. Would like your input.

A: According to the 2011 National Electrical Code NFPA-70, any non-protected wire over 20 amperes must be buried at least 18 inches below grade. And since plastic (non-metallic) conduit does not protect the wiring from penetration due to a shovel or whatever, it still needs to be 18″ deep for a 30-amp circuit, unless it’s buried under a concrete slab, in which case the depth can be 12″. However, if you use metallic conduit then your wiring may only need to be 6″ below grade. These cover depths can vary from state to state, county to county, and municipality to municipality, so you’ll need to check with your local electrical inspector (aka the AHJ for Authority Having Jurisdiction) before you start digging the trench.

BTW: I just dug an 18″ deep trench for my dad and rented a small Ditch Witch for the job. His 125 ft. trench took me less than 90 minutes to dig, so that’s under 1 minute per foot of trench, which I thought was amazing. And always remember to check with your local utilities such as Miss Utilities a least a few days before you plan to dig your trench. That trencher would easily slice though any previously buried electrical wires, and you certainly don’t want to do that. It’s not only dangerous, the utility can fine you and require you to pay for the repair.

Also, many residential electricians will take one look at a TT-30 receptacle and assume it’s an early 30-amp dryer outlet and has to be wired for 240-volts, and that’s simply not the case. If you look carefully at the front of the receptacle you’ll see that it’s rated for a maximum of 30A-125V (30 amperes at 125 volts). Can’t these residential electricians read?

So your 30-amp RV outlet is indeed a 120-volt electrical service which only needs a single-pole circuit breaker to power it. Take a look at the graphic on the right for proper wiring. NEVER let any electrician convince you that a 30-amp outlet for an RV is a 240-volt/2-pole service, as plugging into it will generally destroy your RV’s electrical system in a few seconds. And yes, the electrician may blame you for his mistake and ask that you get your own insurance company to come up with the $10,000 it will take to fix your RV’s electrical system. No kidding.

As far as how heavy the wire needs to be, in most instances you can get away with 10-gauge direct burial wire for a 30-amp service. I always run it in schedule 80 non-metallic conduit, even if I’m going 18″ deep. But your local AHJ might allow thinner-wall Schedule 40 to be used instead. In any case, if there’s any significant length of run (100 feet or more) it’s much better to use the next heavier 8-gauge wire in order to reduce the voltage drop under load. Your air conditioner will thank you for it later.

 

Email me at mike (at) noshockzone.org with your questions.

Click here to return to RV Electricity Newsletter Issue 14.

Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

 

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alvin.e

I just recently did something similar. I do have a 20/30/50 RV outlet box installed. About 120 feet from my shop and 200 feet from the MAIN breaker panel.
In your case, since it is only a 30 A circuit, you will need a 30A single pole breaker, and depending on the entire distance from your MAIN panel (the first panel in your house) to your new RV outlet, you might be able to use #10 AWG wire (3-conductor, THW or THHW, THHN or similar or the direct burial type (check with NEC 300.5 – see below). Your electrician should be able to figure out what wire size will be needed. You don’t want to be in a pickle if your voltage drop is too much and your A/C unit doesn’t start or gets damaged from an under voltage situation. For my own “eyeball calculation”, when the length is under 75 feet, a #10 AWG wire would be OK. Any longer, I usually do a real calculation on what wire size is needed and take a voltage drop of about 5 % into account. Again, this is how I would approach the situation.

I installed my cables in Sched. 80 PVC conduit. Mainly because it is very rocky ground, easy to bend to follow contours, 90 deg/45 deg/22 deg elbows are easy to work with (PVC glue) and you only need a hack saw or other saw to cut the needed length. I also have no vehicle or even foot traffic on that wire run. Rigid Metallic Conduit or Intermediate Metallic Conduit are allowed. EMT (Electrical Metallic Tubing), also know as “Thin Wall Conduit” is not permitted. I personally have seen EMT corrode and crumble after 10 years.

You might have the electrician look at the NEC code (National Electrical Code), Article 300 (wiring methods), Chapter 300.5 (underground installations), Table 300.5. There are several wiring methods and burial debts listed according to your specific situation. (under driveway, residential parking, open ground, etc.). An electrician should have access to the most recent NEC Code Book – I only have one from 2011 – but the electrician should also know in case there are specific state / county regulations.

Depending on where you reside (like one of the summer heat areas), your electrician might want to look at Table 310.16. For instance, if you live in Arizona and would like to run your A/C unit on a 123 F day, you might have to upsize the wire accordingly. It all comes down to a proper load calculation.

I hope that helps.
alvin.e

alcomechanic

In talking about installing the trench, a call to 811 is important! This will protect you physically and legally. Without a call to “Digger’s Hotline” you are responsible for the repair and possibly the monetary loss of service while the service is down, as well as the fine for not calling “811”. Not only that, but you might not live through the accident.