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Old 05-15-2023, 10:53 AM   #1
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Maximum Power Cord Length

Asking for a buddy. The trailer is only a small 14' Jayco with 30 amp service. The power post has a 50 amp receptacle and it's about 120 feet away.

I see they make 100' extension cords. With the trailer's existing cord, it "might" reach. Or, It might require adding a second extension.

My question is what is the max length of an extension cord that can safely be used? I realize that there might be a reduction in amps, but careful usage of AC, MW, etc. should not be a problem.

Neither the power pole of the RV can be moved. This is a permanent site.
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Old 05-15-2023, 12:24 PM   #2
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The voltage drop is the bigger consideration when using a long extension cord. Generally speaking you would want to keep the drop to less than 5% overall (3% preferred but 5% overall at full load would be acceptable). Which means you (what I would do) is to purchase an 8 awg 3 conductor soow cable and add my own ends to it and only use the length needed, plus a few extra feet perhaps to move it when mowing or etc.. Perhaps a cord similar to this one on Ebay.

https://www.ebay.com/itm/26478525263...Bk9SR67P64mEYg

In addition, you can do your own calculations using a tool like this which should be already filled in with what I would suspect to be somewhat accurate for what you stated. 125' 8awg, 30 amps. Anything smaller than 8awg (ie 10 awg as most 30a extension cords are) would result in more voltage drop and if you like you can modify the calc to show 10awg.

In any case, there really isn't a max length other than maintaining less than a 5% drop, which is calculated based on cable length, cable size (awg, gauge), amperage (max load current), and the voltage. The example in the calc shows 4.88% voltage drop at 30 amps (I suspect the average continuous amp load would be closer to ~20 amps for most 30a rv's which puts you closer to the 3% voltage drop) ~CA

https://www.calculator.net/voltage-d...y=30&ctype=nec
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Old 05-21-2023, 08:38 AM   #3
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Thanks for the analysis. The link is valuable to have.

I looked at specs for 50 amp extension cords and they list 120/240 volts. The receptacle we will be using is a 4-prong 50 amp. The end of the 50 amp extension cord will have to have a 50/30 adapter to enable plugging in the existing cord on the trailer.

Will that end up delivering 120 v to the camper? If you have a trailer that actually uses a 50 amp cable, is there an item in the RV that actually uses 240?
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Old 05-21-2023, 08:53 AM   #4
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Can you run new hard wire from the old box to the new box by your camper location and add a new plug box? Then use the current cable you have and no extension.
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Old 05-21-2023, 08:58 AM   #5
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Can you run new hard wire from the old box to the new box by your camper location and add a new plug box? Then use the current cable you have and no extension.
No. There are 2 campers on the property. The box is right next to one of them. The second camper has no box. We don't intend to dig a trench and install a second box.
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Old 05-21-2023, 09:00 AM   #6
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Copy. Just a thought.
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Old 05-21-2023, 09:14 AM   #7
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Yes, a 50a extension cord is usually 6awg and can carry 30 amps without an issue at the length you indicated, although it could be an issue with a 50a RV that pulls a high load as with a high load (amps) and with such a long length you could encounter a larger voltage drop (depends on the load). With a 50a cable to a 30a RV, you are only using 3 of the 4 conductors, in any case 120v is not an issue and very few RV's (generally specialty busses) ever use 240v. In either case, I would have a digital voltage meter plugged into an outlet in the RV and monitor the voltage and make sure the load doesn't drag the voltage down lower than ~108v for any extended length of time.

One item that I likely missed earlier and should point out, using an extension cord of any size should only be used for temporary use, if the site is a permanent site (as I missed earlier) you should take the time to have a trench dug and bury the cable underground and use a UF (underground feeder) cable designed for such purpose. If you rent a trencher and do this, you could make a couple trenches one for water and another for power and perhaps another for the drain if you were so inclined. ~CA
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Old 05-21-2023, 09:21 AM   #8
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No. There are 2 campers on the property. The box is right next to one of them. The second camper has no box. We don't intend to dig a trench and install a second box.
I didn't see this when I posted my last comment. This information could change the scenario as the size of the cable to the first RV is likely not sufficient to support 2 RV's. In order to provide further details, it would be helpful to know how long the cable run is to the first camper, the size (awg) of that cable and the load the first RV may pull (which can be estimated). For example if it is 50a, and the RV has the A/C(s) running then the load may already be too much to support another camper ~120' away. In this case, the load of the two RV's is being pulled from the same cable that the first RV is using. ~CA
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Old 05-21-2023, 10:59 AM   #9
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OK, some further clarification...

The power post has 30 and 50 amp receptacles. There is 60 amps going to the post.

Both trailers have 30 amp wiring. Trailer #1 is next to the post and uses the standard onboard 20 foot power cable plugged into the 30 amp receptacle. Trailer #2 is the one far away that needs an extension cord.

Both are weekend use only, and we always take the cables in after we leave. They are not plugged in unless in use. So, yes, the cables are only temporary.

It is unlikely that both trailers would be in use at the same time. The only time they might would be in winter, so no AC in use - only furnaces.

The reason for inquiring is to be sure it could safely be done. I'm merely trying to provide some electric to a neighbor, rather than him needing to bring in his own service from the power company, which would be prohibitively expensive. Since I have an extra 50 amp receptacle it will cost me nothing.
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Old 05-22-2023, 09:48 AM   #10
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Originally Posted by gypsmjim View Post
OK, some further clarification...

The power post has 30 and 50 amp receptacles. There is 60 amps going to the post.

Both trailers have 30 amp wiring. Trailer #1 is next to the post and uses the standard onboard 20 foot power cable plugged into the 30 amp receptacle. Trailer #2 is the one far away that needs an extension cord.

Both are weekend use only, and we always take the cables in after we leave. They are not plugged in unless in use. So, yes, the cables are only temporary.

It is unlikely that both trailers would be in use at the same time. The only time they might would be in winter, so no AC in use - only furnaces.

The reason for inquiring is to be sure it could safely be done. I'm merely trying to provide some electric to a neighbor, rather than him needing to bring in his own service from the power company, which would be prohibitively expensive. Since I have an extra 50 amp receptacle it will cost me nothing.
Safety is certainly an important consideration. I don't see any issues with your plan as it is as safe as any long running extension cord could be. I would consider making sure the connections are out of the weather. Safety is also considered by most to include the safety of the electrical appliances in use, for that concern I would monitor the voltage in both RV's and make sure the voltage doesn't drop too low (~108 or less). If you find that the voltage does drop too much or the breakers trip, then there may be options you could do to help with that such as making sure each RV is on its own 120v leg that is coming into the pedestal. ~CA
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Old 05-22-2023, 03:45 PM   #11
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I'm assuming the neighbor is on his own property hence no desire to add a second utility pedestal.

One thought, if you ever decided to put in a covered deck would it be between those two points? Having your neighbor cover the expense of adding a pedestal may be a win for both of you. You'd have another electrical connection point and your neighbor saves on electrical costs.
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Old 05-27-2023, 12:01 PM   #12
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If you're going to run 125' #10 wire will handle 30amp, 120 V. for 50 amp 240 V you will need #6 4 conductor wire.

These sizes will minimize your voltage drop when running 125' from the power pole.

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Old 06-05-2023, 03:13 PM   #13
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Southwire calculator comes up with #8 awg min. #10 even with only 20 amps loaded is only good to about 75 feet before you have more than 3% voltage drop. Use a 50 amp cord as stated above and you will be golden on a 30 amp trailer. That cord will be #6 and voltage drop won't be a problem. If you attempt to use a #10 cord at that length, you will almost certainly have too much voltage drop and risk damaging your ac or appliances.
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