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I have a Camper that needs a 30 amp service. Its about 125' away from the panel in the house. I'm looking into using aluminum, but I'm struggling to find the right information.

I see 6-6-6 URD and it seems perfect for what I'm looking at. Cheap, can handle 50 amps. Since I'm only doing 30 amp service, seems like a win.

But everything I'm reading is suggesting 2-2-4 URD. Isn't this overkill?

Any suggestions on how to cheaply run this would be great.

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  • What are you planning to put out there for RV equipment? A pedestal, or is there a structure there you're putting a RV box on? – ThreePhaseEel Aug 24 '20 at 23:02
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URD cable is a hot mess

URD cable is made with the bad AA-1350 alloy wire, infamous for the aluminum-wiring scandal of the 80s and 90s. (this alloy is banned anywhere else, see NEC 310.106(B)).

That's because URD cable is made to be on the power company's side of the electric meter - and power companies play by different rules (because their wires are all outdoors and not buried in walls of buildings). Note that you're not running a service (pole to meter) ... you're running a branch circuit (panel to socket).

That dangerous AA-1350 metal will be a nightmare to get the terminations correct so you don't get a repeat of the 80s house fires.

Cheap doesn't mean stupid (much the opposite)

Cheapness is a grand thing. But it does not mean we are morons. We still think ahead and are smart about being cheap now to save money later.

I'm thinking hard on two scenarios: #1 wanting to run other loads besides the camper, out there today. Using power tools to work on stuff. Having a picnic area with power for grill etc. #2 a future RV that will take a 50A/240V receptacle. #3 fitting a subpanel out here if needs increase.

So I recommend you throw one more #6 Al wire into the trench.

For now, you share the neutral with the other circuit, and bring it out to a common 120V GFCI outlet right next to the TT30 trailer circuit. Now you can just plug in anything you want and you don't need the trailer there as a 3000 pound "adapter". Back at the panel you use a 20A breaker for that circuit and a 30A breaker for the TT30 trailer, and handle-tie the two breakers.

In the happy future you get a big RV, you just change the recep to a NEMA 14-50. Just like that and you're done. Well change the breaker also, but that's $10. Or you could install a trailer power stand which provides all of them: 14-50, TT30 and 120V GFCI at the same time.

All you need to do to enable that is carry a 4th wire... so 6-6-6-6 for instance. 33% more for the wire but that's a small part of total project cost.

Conductor size is just fine though

Generally I don't even bother checking the voltage drop calculators unless a run is over 160' (or 80' if it's 120V).

Normally you use #10 copper (or #8 aluminum) for a 30A circuit. Given the distances we need 1 wire size bump, and that puts us at #6 aluminum, so that's just right.

Thinking about that larger future RV at 50 amps... #8 copper or #6 aluminum is the minimum size allowed for 50A if the wire is allowed to run a 75C (so not UF-B or NM-B). Since it's 240V we're not far enough to need a wire size bump, so we're all set there too. See how easy that was?

Cable/Wire Type

I see where people might recommend 2-2-4-6 since URD is such a hot mess. That's because when you get away from URD cable, #6 cables get hard to find in an all-in-one cable -- for instance, in MH feeder, the smallest made is 2-2-4-6 which is overkill and won't be fun to attach to a 30A breaker. You'd need some fairly hefty Polaris connectors on both ends to adapt it down to some #10 copper for that final connection.

As it happens, USE-2/RHH/RHW-2 is available in individual #6 wires, quite cheaply. (it's more or less MH feeder, but separated). These individual wires can be directly buried. But they can't be directly run inside the house on rafters and in walls - inside the house they need to be in conduit. Or you could stop at a junction box and transition to a different cable type for inside the house.

The only snag I see with USE-2 individual wires is officially, the neutral is supposed to be white and the ground is supposed to be green since these are #6 or below. However if that size of wire simply isn't sold in colors, the inspector is sure to let you off the hook.

Buy your wires locally at an electrical supply house. Big-box stores overcharge you, and Internet sales companies build a lot of shipping/handling cost into their prices. Electrical gear is too low-value and heavy for mail-order to work all that well.

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  • My god, your responses are a thing of beauty. Literally made an account, hoping you would reply. Thanks a ton, this has made my day. – Trevor Aug 24 '20 at 19:27
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A voltage drop calculator shows the voltage drop for 24A (the maximum continuous load on a 30A circuit) at 125 feet would be just 3.1%, which is within reason (and remember the drop will be less when the load is less). So #6 wire should be fine.

Running a 50A circuit (40A max load possible) on it at that distance would be a little iffy. Voltage drop on the #6 wire would be 5.2%; total voltage drop at the appliances when you factor in the house wiring, RV cable, and RV wiring would be a bit more.

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I don't know exactly what you're reading, but one issue is wire color identification. The code has rules about how wires are to be marked and what sizes of wires are allowed to be re-identified by phase taping.

Typically the Equipment Grounding Conductor (EGC) has to be green or bare unless #4 or larger (NEC 250.119). The typical Underground Residential Distribution cable is intended for "services", and are marked by the manufacturer as two ungrounded conductors and a striped or ribbed neutral (actually called the grounded circuit conductor), with no EGC. #6 is too small to be identified by tape or other permanent marking as an EGC.

Your best bet is to use #10 UF or conduit and THWN copper wire. At 80% load your voltage drop on 120v would be right around 5%, which is barely within NEC recommendations.

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  • A 30 amp RV receptacle is 120v only 1 hot required. – Ed Beal Aug 24 '20 at 13:54
  • @EdBeal I didn't specify voltage, because marking requirement for the EGC doesn't change for 120v or 240v, the #6 URD either way still typically doesn't have a wire you can re-identify as an EGC. – NoSparksPlease Aug 24 '20 at 14:08
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I use south wire’s voltage drop calculator even if your RV could draw 30 amps #4 wire would be large enough and have a 2.86% voltage drop on a 120v line. You could even use #8 aluminum but the voltage drop would be 7.09%.

Code recommends 3% & 5% as maximum voltage drops and those are at 80% of the circuit capacity so if you use a value of 24 amps as the max load Your voltage drop will be 5.67% with #8 aluminum.

on a small load like this I would suggest changing to #10 copper at 24 amps 5.25% voltage drop

So you really don’t need anything larger than 8al if under the NEC but larger wire is ok but not a larger breaker (note a 30 amp breaker may not take #6 depending on the brand and I don’t think any brand would take #2.

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