Wire size on 30 amp RV cables

[Grandpa_Joe]

Do the commercial 30 amp RV cables you purchase from Camping World, Walmart, etc use #10 gage wire or larger? I have 2 of these, both 25 feet long and noticed a 10 volt drop when running the heat pump on my AC unit with the fan on low speed. Hot water heater was off, no other heavy loads.I do have a surge protector which will disconnect if voltage gets too low.

[JT]

Thanks for the voltage drop calculations and site link; very useful
information!

[gotsmart]

RoyB wrote:
...

When we are using our setup in the cold weather we try to use the 20AMP PEDESTAL service at the camp grounds using a second HD Extension cord running into the trailer to just run our portable electric heaters. This 20AMP service doesn't pull power from the same leg the 30A source is using to feed the trailer.
....

Be mindful of 20 amp outlets on 30/20 pedestals - especially in older parks. Unless you know the park in question and how the sites are wired, that 20 amp outlet may not have its own path to the bus bar.

If you're at an older "30 amp only" park, don't be so sure about the 'separate leg' thing. I was on a crew that pulled the 20 amp outlets from pedestals in an older park - because the 20 amp outlet and its 20 amp breaker were daisy-chained off of the 30 amp socket.

Don't get me wrong. There is a convenience factor to having the 20 amp outlet, but when folks started heavily using the 20 amp outlet for supplemental power on pedestals that I described above they were robbing power from their own coaches. It tripped breakers, caused phone calls to the office to complain about the power, etc.

Since no amount of explaining can fix "stupid", the solution was to rip out the 20 amp outlets and install cover plates over the opening for the breaker and the outlet.

[Dusty_R]

To begin with most people do not realize that circuits are not supposed to be loaded full time more than 80%.

20 amp circuit 20x80%=16 amp
30 amp " 30x80%=24 amp.


Dusty

[BruceMc]

I'd look at the connections - after a few minutes, feel the connector bodies - are any warm/hot? You have a high resistance connection. If they are hot, the prongs and/or receptacles are not clean and/or tight. The hotter they get, the higher resistance, the hotter they get until things start to melt.

Like GordonThree mentioned - never use extension cords for moderate to heavy loads while coiled up. I have a friend who needed a 6' extension for a load within the cord's rating, so he used his tightly wound 50' cord after unwrapping what he needed. Yep, he melted the entire mess into a useless tangle of plastic & copper in short order.

[tenbear]

GordonThree, thanks for the explanation.

There is an easier way though. Here is a link.

[GordonThree]

For future reference, the math behind tenbear's post 🙂

Voltage drop formula: V=IR

10ga wire is 3.4 ohms per kilometer, 100 feet (50 feet * 2 conductors) is (rounded up) 31 meters

31/1000 = 0.031 * 3.4 = 0.11 ohms for the 100 foot of conductor (rounded up).

30a * 0.11 ohms = 3.3 volts ...

a drop of more than 3v from one end of the cable to the other indicates it's overloaded, damaged, or it's not really using 10 gauge conductors...

12 gauge; 5.4 ohms per kilometer = 0.031 * 5.4 = 0.17 * 30 = 5.1 volts
14 gauge; 8.54 ohms per kilo = 0.031 * 8.54 = 0.27 * 30 = 8.1 volts

the scary part:

P=V*I ... 3.3 * 30 = 99 watts ... good reason to not coil up shore power cords, 100 watts of heat is no small matter.

[tenbear]

50 feet of #10 AWG 2 conductor cord should have a voltage drop of 3v with a current of 30A. Your 10v drop is somewhere else, the connectors or wiring to the pedestal. Or maybe your current is greater than 30A.

[Johno02]

Probably wiring to the pedestal. The wiring run to the pedestal from wherever is probably a lot longer than your cords, and may have several others pulling power from it also. Also, like others said, check and clean blades on you plugs. Also, check you plugs and cord ends for overheating. With that much voltage drop, something is getting hot somewhere. Also, check voltage at the pedestal. If the drop is there, it is a park problem.

[Ozlander]

Yeah, the question would be ,What is the voltage at the pedestal under load.
I'll bet it's not 120.

[Bumpyroad]

does a RV heat pump have an electric heat backup like household ones do, or does the gas furnace kick in. could it be the electric heat that is increasing the draw since I would assume that the heat pump would draw the same as the AC does normally:
bumpy

[2oldman]

Lots of RV parks have power systems that are not up to snuff for large loads.. kinda like their wifi.

[Grandpa_Joe]

Thanks Guys,
The voltage with no load is 120, drops down to around 115 when the unit starts up and as heat is being produced, it drops to around 111 over a short period of time then stays there. When we park closer to the pedestal, I only use 1 cord and did not notice this happening. leaving for the outer banks this week so I will try with the one cord and see what the drop is. Thanks for all the feedback.

[JT]

If you're using *both* cords between the power supply and your rig, a 10 volt drop over a run of 50' is not unusual.

[GordonThree]

Grandpa Joe wrote:
Do the commercial 30 amp RV cables you purchase from Camping World, Walmart, etc use #10 gage wire or larger? I have 2 of these, both 25 feet long and noticed a 10 volt drop when running the heat pump on my AC unit with the fan on low speed. Hot water heater was off, no other heavy loads.I do have a surge protector which will disconnect if voltage gets too low.

I suspect the outlet. Worn / loose contacts are probably to blame, more so that the wire in the cord. Repeat your test using a 50a to 30a adapter, as the 50a contacts are a lot beefier and generally used less.