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Old 03-28-2009, 01:10 PM   #1
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RV Power Cord Extension Voltage Drop

Hereís the scenario, you back in to your site and find the sweet spot, door lines
up with the pad, an opening in the trees for the satellite dish and youíre level.
Then you pull out your 25 foot built in cable and itís anywhere from 2 to 5 feet short of the pedestal.
You could just back up but donít want to do that. You do have a 25-foot extension but what about line loss?
What Iím thinking of doing is to make up an 8-foot extension just to have on hand if needed.
Now for my question, would the line loss difference between an 8-foot or 25 foot extension be significant?
I do have a piece of 3/10 30 amp. cable with a male end on it, so all Iíll need is a female end.

Greg
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Old 03-28-2009, 01:23 PM   #2
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I haven't found that to be a problem with my 25' extension cord.

The 8 footer would be more convenient, though.
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Old 03-28-2009, 05:21 PM   #3
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Dirk and I are the same. I have an extension cord, mounted to a SurgeGuard, which stays in the electrical bay. This means I am always using the extension cord. I've never had a bit of trouble.
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Old 03-28-2009, 05:30 PM   #4
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I to have never had a problem using my 25 ft ext, and I seem to need it often. But the idea of having a 8 ft does sound good because you are correct one seldom needs more than 5 or 6 ft. Just may have to make one of those up.
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Old 03-28-2009, 05:40 PM   #5
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Oldpa

Voltage drop off the pedestal in whith a hundred foot cord the voltage drop would be none. As long as the cords has good ends ( tight connections ) and the cord is the same gauge as the trailer cord 30 or 50 amp. That is as long as the voltage is up to snuff ( 120 volts ) at the pedestal but less volts more power loss.

Hope this helps Wayne
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Old 03-28-2009, 06:28 PM   #6
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I bought the 15ft 50A extension cord from CW when it was on sake for $99.00 so I could also have our portable surge guard locked in the storage area. We have not seen any power reduction.
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Old 03-29-2009, 03:27 PM   #7
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The 8 feet of 10 gauge wire will have a maximum 0.629 volt drop when you are drawing 30 amps. That's not very significant.

25 feet of 10 gauge would have a 1.967 volt drop at 30 amps. Again that's not very much even when added to the voltage drop across the existing cable on your RV.

As I recall, appliances are designed to operate at 120 vac plus or minus 10% or 132 vac to 108 vac.

Unless the pedestal voltage is low you should be okay with either 8 feet or 25 feet of extra 10 gauge cord .
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Old 03-29-2009, 06:21 PM   #8
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Hi Ho: Voltage drop in extension cords follows Ohm's Law which is simply that the voltage (Volts) is the current (amps) times resistance (ohms). For example if you use 10 AWG copper wire, resistance is 0.00118 ohms/ft. If you have a 30 amp load and a 25 foot cable, then the voltage drop is 0.00118 times 30 times 25 or 0.89 volts. If you add another 25 foot extension cord you get another 0.89 volt drop. A total voltage drop of about 2 volts is normally not a problem unless the voltage is too low to begin with.

If you want to play around with different wire sizes and load currents, here is a simple web site: http://www.stealth316.com/2-wire-resistance.htm

Another concern for RVers is that the connectors become corroded or don't make good contact with the wires in the cables. When you disconnect pay attention that the plug is not hot, especially when using A/C or other large loads.

Good luck and happy miles, Dirk
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Old 03-29-2009, 06:38 PM   #9
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Quote:
Originally Posted by Dirk Ostermiller View Post
Another concern for RVers is that the connectors become corroded or don't make good contact with the wires in the cables.
Now that is one of the real problems out there!!! This same corrosion/bad-contact problem also extend to the condition of the receiving prongs in the pedestal. We've all seen burned up pedestals and plugs or the black residue on a pedestal that's been fixed (hopefully) all over the place!
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Old 03-29-2009, 08:07 PM   #10
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We were at a cmpground in Carlock, IL a few years ago. when I pplugged in at the brand new 50A pedistal, I found the voltage to be under 108 volts with only one AC running.

I talked to the owner and he told me with a big, proud smile that his buddy on the local police force was an expert and they had rewired the park from a new big transformer.

He took me over to the transformer (over 300' from my site) which certainly looked to be big enough and said it had cost him a bundle to run the #10 ALUMINUM wires to each double site.

Can you imagine? 100A for 2 RVs? On a 300' #10 run? Nothing could disuade him from his position.

OH, the low voltage took out my AC control board.
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Old 03-29-2009, 09:30 PM   #11
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Can you imagine? 100A for 2 RVs? On a 300' #10 run? Nothing could disuade him from his position.
Don't you just love that type of DIYer? A double whammy eh? 1) that they did it and 2) nothing could convince him of the utter error/saftey hazard!!!!

So, the NEC rates #10 AL in 3wire configuration at 25amp at 25degrees C. They further rate it as fusing at 247amp. So either these guys are basically running a 300ft fuse for your protection (how nice of them LOL) or you are getting a line loss at 240v at ONLY 25amps of nearly 30V so each 120v leg is delivering only (240-30=210/2) or 105v at 25a/leg!!! And they are thinking that 2 * 50amp power hogs will plug into the same #10 line? Run Toto, run!!!

Oh, I get it now, that CG wired by those DIYers is really multi-tasking - they are in a snowy area and they use the wire heat to melt the drievway snow right?
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Old 03-29-2009, 09:42 PM   #12
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Great reply Dirk! Excellent advice.
Hamguy, this is a solid reason to always have a line voltage monitor plugged in where it is easily seen_and frequently. One model costs <$20, very cheap considering the cost of A/C repair.
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Old 03-29-2009, 11:17 PM   #13
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Quote:
Originally Posted by Dirk Ostermiller View Post
Hi Ho: Voltage drop in extension cords follows Ohm's Law which is simply that the voltage (Volts) is the current (amps) times resistance (ohms). For example if you use 10 AWG copper wire, resistance is 0.00118 ohms/ft. If you have a 30 amp load and a 25 foot cable, then the voltage drop is 0.00118 times 30 times 25 or 0.89 volts. If you add another 25 foot extension cord you get another 0.89 volt drop. A total voltage drop of about 2 volts is normally not a problem unless the voltage is too low to begin with.

If you want to play around with different wire sizes and load currents, here is a simple web site: http://www.stealth316.com/2-wire-resistance.htm
Sorry to say, but in taking a closer look there is a slight, but common, oversight in the calculation given above and of that used on the particular web site listed.

Ohms law as used in both these cases applies to a single wire, not a paired wire. Hence, in a paired wire situation (120vAC) the entire circuit length is twice the extension cord length (the hot PLUS the neutral return). Therefore, for a 25ft cord, the circuit length is actually 50ft. This means the voltage loss for each 25foot #10 copper extension cord operating loaded at 30amps is 1.8v (double the 0.89v above). Round that up to about 2v per cord used and that loss is from the voltage at the pedestal (which is the correct voltage to plug into Ohm's law as used here as well).

This is a common oversight and is frequently used incorrectly in many DIY web site calculators where they use all the correct NEC tables (as the steath316.com site does) but make the incorrect assumption about total circuit length in the examples (like the stealth316.com site does). If you use a web calculator you must verify exactly what assumption they use to calculate wire length. Seldom do the non-industry web sites identify their wire length assumption in their calculator which means you are likely to get erroneous results. All the industry web sites are clear about this assumption. Of course, you can always just use Ohm’s law yourself with the correct circuit/wire length as indicated above and the correct starting voltage. Also note that lower voltage (like 12v landscape lights and other 12v car/RV calculations) will have different wire factors than those for higher voltages. Losses at low voltage are significantly higher than at higher voltages.
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Old 03-30-2009, 09:14 AM   #14
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RVDude is correct about the length. Also there should be some derating for temperature increase of the cable. In addition the ohms per foot tables out there are usually for solid cable while the cables used are stranded and some de-rating should be applied for the "lay" of the cable. That is since the cable is not solid there will be a smaller actual cross section for the stranded cable than for the solid thus a higher oms per foot.

I wrote a program some time back that takes these things into account as well as makes it clear that the total run length must be used.
If anyone is interested it can be downloaded at http://www.rvforum.net/index.php?opt...=289&Itemid=63
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