Max 30 amp extension cord length - Fiberglass RV


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Old 02-06-2019, 06:49 PM   #1
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Max 30 amp extension cord length

Hello,


What is the max length for 30 amp extension cord with full RV electrical functionality (AC, lights, water pump, etc)?


I am not an electrician so any pointers, guidelines, warnings, restrictions, it depends, etc are greatly appreciated.


Thank you
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Old 02-06-2019, 07:21 PM   #2
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Mine is 25'. It's a removable power cord. I also have another one the same length as there was at least one spot I stayed in last year where the cord barely reached and that was after moving the trailer a bit closer (but couldn't get too much closer due to the trees). Of course, I've not needed it (purchased after that trip), but it's there in case I do need it.


I just looked and Amazon also sells a 50' 30 amp extension cord, as well as a 30'. Normally you shouldn't need that much at campsites. Wish they had 10 footers as sometimes that's all you need!
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Old 02-06-2019, 07:41 PM   #3
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It depends how how much voltage drop you're prepared to accept. How much you'll accept might depend on how much voltage drop the A/C will accept before doing damage to the motor (brown out).

Voltage drop is a function of amperage the line is carrying, the wire Gauge (i.e. it's resistance) and the round trip length (some formulas account for this so the details are in the footnotes.) Legit 30A plugs usually have 10 ga wire.

If I'm not running the A/C I will tolerate 50 feet of 30A cord. But that's me. I don't think I've ever run the A/C with 50 feet out there. But I'm not telling anybody else what to do.

There are charts laying out voltage drops everywhere on the web or you can do the calculation yourself. The math's not hard.

My AC voltage monitor (Blue Sea #8247) low voltage warning starts beeping at 105VAC (default). So that's their idea of "kinda low".

FWIW, I've seen really low voltages into the trailer from full-up campgrounds or even at the end of the loop from distribution panels so you don't even need the cord voltage drop to see problems so why stack self induced problems on top of campsite problems.
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Old 02-07-2019, 08:24 AM   #4
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Originally Posted by Steve L. View Post
It depends how how much voltage drop you're prepared to accept. How much you'll accept might depend on how much voltage drop the A/C will accept before doing damage to the motor (brown out).

Voltage drop is a function of amperage the line is carrying, the wire Gauge (i.e. it's resistance) and the round trip length (some formulas account for this so the details are in the footnotes.) Legit 30A plugs usually have 10 ga wire.

If I'm not running the A/C I will tolerate 50 feet of 30A cord. But that's me. I don't think I've ever run the A/C with 50 feet out there. But I'm not telling anybody else what to do.

There are charts laying out voltage drops everywhere on the web or you can do the calculation yourself. The math's not hard.

My AC voltage monitor (Blue Sea #8247) low voltage warning starts beeping at 105VAC (default). So that's their idea of "kinda low".

FWIW, I've seen really low voltages into the trailer from full-up campgrounds or even at the end of the loop from distribution panels so you don't even need the cord voltage drop to see problems so why stack self induced problems on top of campsite problems.
Here is a simple voltage drop calculator: https://www.calculator.net/voltage-d...s=30&x=40&y=12.

Resistance is a term that describes the degree to which a conductor opposes the flow of current. Over a long run of cable, this can cause the cord itself to act as a small load on your power source, like a tiny lightbulb.

The resistance of the wire has to do with the material, gauge, condition, and temperature. Since we aren't supercooling our extenstion cords, the effect of temperature is negligible--it's not even present in this calculator. It's almost certain that your extension cord has a copper conductor. Aluminum wiring was popular in houses in from the mid-60s through early-70s. I doubt you will see that in any extension cord. If you do, do yourself a favor and take it to the scrap yard. Gold and silver I believe are mostly used in electronics. That only leaves the gauge (diameter) of the wire as a factor for you to consider.

For a 30A run, you will never go small than 10 AWG, regardless of the length. (The gauge system is counterintuitive--smaller gauge=larger diameter.) So if you see a cable that says "Rated for 30A" but the gauge is listed as "12 AWG," you do not want it. The ratings are designed based on the amount of heat generated as a given amount of current passes through a given gauge. The danger is that some yokel rigs up a couple 15A-30A adapters and uses a regular extension cord to run his/her camper and ends up causing a fire.

You can mess around with that calculator to find the maximum length you are comfortable with at a given gauge, depending on what your electrical management system can handle. You will select "Copper" as the material, "AC 3-phase" as the phase, "30" as the load current, and unless otherwise specified by the cord manufacturer, "single set of conductors." The "Voltage at end" is what you are concerned with. Receiving 110V from the source, you could have a 100 ft, 8-gauge cord and still be just above 105V at the trailer.

Ideally you will get 120V coming from the campground connection, but as Steve pointed out, that's not always the case. Even my household wiring does not always provide 120V. Assuming 115V at the post may even be optimistic. I did a little searching and some campers reported voltages dropping as low as 100V. I'd be interested to hear what "typical" values have been experienced.

I read where someone suggested using a 50A-30A adapter to improve voltage, if that's an option at the campground. (Note: unlike the yokel I mentioned before, don't do this with another 30A-50A adapter at the trailer end if you happen to have a trailer that takes 50A.) That may be because the 50A circuit will necessarily have a larger gauge wire, so voltage drop from the "hub" somewhere in the park to your campsite will be lower. I tend to think that a campground which struggles to provide clean power at 30A will not have a 50A circuit available. Of course its possible they added the 50A circuit as a sort of retrofit and didn't go to the trouble or cost of updating their 30A wiring.
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Old 02-07-2019, 08:52 AM   #5
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Thank you Steve and Justus. Terrific information.

To provide additional context, I would like to run 30 amp power connection to a trailer which would sit in our backyard approx 100 feet (as the crow flies) from our basic home breaker box. (The only reason I mention the breaker box is because I assume, possibly incorrectly, that a connection for the trailer would originate at that location.)

There are also standard household outlets at that location but I don't know if there is a safe way to use those outlets or a new 30 amp type connection is definitely needed. (I am also planning to talk to local electricians but thought this forum would be a good place to start based on the knowledge and experience here.)

In summary, I'm trying to understand the safest option for providing power to run all electrical trailer components (AC, lights, water pump, etc) without risk of fire or damage to the trailer.

Again, your knowledge is appreciated.
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Old 02-07-2019, 09:33 AM   #6
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There are also standard household outlets at that location but I don't know if there is a safe way to use those outlets or a new 30 amp type connection is definitely needed. (I am also planning to talk to local electricians but thought this forum would be a good place to start based on the knowledge and experience here.)
Unless you are just wanting to trickle charge the batteries to keep them topped off, you will want a dedicated 30A connection. Conceivably you could use a 15 or 20A circuit with an adapter if you know you'll stay under that amperage--if you go over (on the entire circuit) you'll pop the breaker. So think about any lights and appliances in the house drawing from that circuit and subtract those amps from the circuit rating--you might be down in single digits, which means you'd need to keep the trailer load in single digits.

The connection will begin at your breaker panel or at your meter, depending on your home's capacity and your desired design.

Typical homes will have 100A service. 200A service I would say is less common, but it's out there--I know when looking at houses, sellers touted 200A service as a desirable upgrade. You could have something in between as well. If you have less than 100A service, you're going to need an update; I would be very surprised if this is the case, as any home with A/C is going to have at least 100A. If you have 100A and a small home, you might be able to get away with just adding a new breaker, or (depending on location and if you have an existing 30A circuit), running a cable from an existing 30A outlet. You'd have to swap out with the existing appliance anytime you wanted to power the camper, but that might be an OK short term solution that beats spending thousands of dollars to upgrade.

Once you know what service your house has, you can find out if you have amps to spare. You can determine your maximum installed amperage capacity by adding up the values of all the breakers currently installed. If you subtract this from the service from the meter and you have 30A or more to spare, you are in great shape. Your typical amperage draw will depend on what appliances and lights you run. An electrician can help you out with that. An electrician can also tell you if your breaker box can handle an additional 30A breaker. Not only do you need to consider physical space, but the rating of the bus bar(s) (the "power strip" of the breaker panel that your breakers connect to).

If you want the 30A circuit to come off an outbuilding, or you just want it separated from the house, you can have a separate breaker panel installed that comes right off the meter. Another reason to do a separate panel would be to avoid replacing your entire breaker box; you would do this by installing a subpanel. This would be fed off of it from a feeder breaker; that means you'd only pursue this course if the bus bar on your main panel had sufficient capacity, but you were physically out of space or you wanted to have several circuits in an outbuilding (for instance lights in the detached garage in addition to the 30A circuit).
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Old 02-07-2019, 10:22 AM   #7
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My garage is about 50 feet from the breaker box. The previous owner just ran underground 12 ga out there. My trailer is nearer the garage than the house and I don't run the A/C while plugged into the garage. When I recently had the fuse box replaced with breakers to support adding central air to the house it looks like they just paired 20a breakers with 12 ga wires.

My Casita is a 16' and it came from the factory with a window AC built into the bottom of the front closet, so it's basically only a 15A unit in order to work with most older house receptacles. Newer construction frequently comes with 20a receptacles (presumably wired with 12 ga minimum) but not my house built in the late 50's.

Water pumps, lights etc. are all 12v so they're running off the converter/battery. It's max charge is something like 45ADC which translates to about 4.7 amps (with inefficiencies) at 115VAC so I would squeak under the 20a breaker but it doesn't address the voltage loss. And I wouldn't open the garage door at the same time, or run the compressor, or sneeze for that matter. Anyhoo, I choose not to run the trailer AC off the garage circuit.

I periodically think about having a properly wired service box put out there (along with tapping into the sewer service line that runs back in the middle of the yard for a pump out "station", two different topics obviously). But I'm all talk at the moment.
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Old 02-08-2019, 09:23 AM   #8
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Thank you both for the thorough information.
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Old 02-08-2019, 02:18 PM   #9
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Once you know what service your house has, you can find out if you have amps to spare. You can determine your maximum installed amperage capacity by adding up the values of all the breakers currently installed. If you subtract this from the service from the meter and you have 30A or more to spare, you are in great shape.
The power companies supply homes with 240 volt service, which comes by way of two 120 volt feeds. I am guessing that what you mean by “what service you have” you mean the amperage of the main breaker. Most new homes would have 200 amp entries given wells and modern appliances. However, if you add up the amperage of all the individual breakers controlling various circuits, the sum will always exceed the amperage of the main breaker given that clothes dryer and electric range circuits together will be approximately 100 amps. The assumption is that not all of these high amp items will be running at any one time. If space allows in the breaker box to add a 30 amp circuit, there really is no need to determine capacity. The worst thing that will happen is the main breaker may trip if all the circuits IN USE exceed its carrying capacity. I have added several circuits in most every house I have owned, including a 30 amp RV outlet, and I have NEVER had the main breaker trip, even with the dryer and electric oven running and what ever else might be on. And voltage drops from 120 to 114 on a 20 amp circuit will not harm the RV’s AC, if it even drops that much. Voltage drop is a normal occurance on many circuits.
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Old 02-08-2019, 03:49 PM   #10
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The power companies supply homes with 240 volt service, which comes by way of two 120 volt feeds. I am guessing that what you mean by “what service you have” you mean the amperage of the main breaker. Most new homes would have 200 amp entries given wells and modern appliances. However, if you add up the amperage of all the individual breakers controlling various circuits, the sum will always exceed the amperage of the main breaker given that clothes dryer and electric range circuits together will be approximately 100 amps. The assumption is that not all of these high amp items will be running at any one time. If space allows in the breaker box to add a 30 amp circuit, there really is no need to determine capacity. The worst thing that will happen is the main breaker may trip if all the circuits IN USE exceed its carrying capacity. I have added several circuits in most every house I have owned, including a 30 amp RV outlet, and I have NEVER had the main breaker trip, even with the dryer and electric oven running and what ever else might be on. And voltage drops from 120 to 114 on a 20 amp circuit will not harm the RV’s AC, if it even drops that much. Voltage drop is a normal occurance on many circuits.
I thought it was fairly obvious that I was referring to amps when my whole spiel was about amps, but we often see what we want. You make a good point about the breaker amps exceeding actual capacity; I was envisioning my breaker panel last night thinking "even if those are all 15A I'm way over 100." In our last two houses there was no room to spare in the breaker panel. I'm thinking now, why not run off of a dedicated 20A circuit? If the breaker trips, it trips. I looked up a Coleman Cub 9200 and it can run on 2000W, which you'd exceed with 20A. You're never going to overload your shore power cable if the breaker is the "weak link."
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Old 02-08-2019, 04:13 PM   #11
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The power companies supply homes with 240 volt service, which comes by way of two 120 volt feeds. I am guessing that what you mean by “what service you have” you mean the amperage of the main breaker. Most new homes would have 200 amp entries given wells and modern appliances. However, if you add up the amperage of all the individual breakers controlling various circuits, the sum will always exceed the amperage of the main breaker given that clothes dryer and electric range circuits together will be approximately 100 amps. The assumption is that not all of these high amp items will be running at any one time. If space allows in the breaker box to add a 30 amp circuit, there really is no need to determine capacity. The worst thing that will happen is the main breaker may trip if all the circuits IN USE exceed its carrying capacity. I have added several circuits in most every house I have owned, including a 30 amp RV outlet, and I have NEVER had the main breaker trip, even with the dryer and electric oven running and what ever else might be on. And voltage drops from 120 to 114 on a 20 amp circuit will not harm the RV’s AC, if it even drops that much. Voltage drop is a normal occurance on many circuits.
Residential electrical service calculations are shown in Chap 9 of the NEC
Adding up the sum of the amperages of the branch circuits is not one of them
Residential services are derated based on the fact that most loads are not continuos and are not all running at the same time .
Many times homes have 200 amp services to provide adequate Branch circuit breaker space not because there is 200 amps of load .
If your home drew 100 amps continually at 240 VAC most people could not afford to pay their electric bill .
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Old 02-08-2019, 06:55 PM   #12
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If your home drew 100 amps continually at 240 VAC most people could not afford to pay their electric bill .
Yup. You don’t have to tell me that Steve. Before moving to FL I lived in NH in a house that when I first bought it, was heated with electric baseboards. Talk about an electric bill! And that was nowhere near 100 amps of continuous draw. Within a year it was heated with a combination of wood and heating oil.
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Old 02-13-2019, 11:46 AM   #13
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Your average RV equipped with a single A/C, will draw generally less than 20 Amps with A/C operating, as long as you're NOT running electric water heater, Microwave, or such. I use a 20 Amp breaker, 20 Amp GFCI, and 12 Ga wire to power my 30 Amp RV outlet located outside rear corner of garage (Legal, because all components are protected at or below rated current). I also have a 20 Amp T-slot outlet on outside diagonal corner, on separate circuit, in case I have visitor needing power for another RV, as long as they have a 30 Amp RV connection, and follow rules above.
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Old 02-13-2019, 02:04 PM   #14
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Quite a few of us have converted their trailer to a 15 amp power inlet, doing away with the heavy 30 amp cord. I did this within a year of buying my trailer new in August 2000. I run anything I want in my camper (Including a roof mounted air condition or heater) on about 65 feet of 16 gauge orange extension cord in the backyard. The air conditioner (and the rest of the trailer) has been used for 18 seasons with no apparent ill effects.

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