Max extension cord length for 120V 20A home use?

[TxCowboy]

My RV is packed at home currently and, from time to time, we use it as a guest room.

We currently have it hooked to home shore power, 120V, 20 amp service (an external patio plug) via a 75 foot long, 12 AWG copper insulated extension cord (from Home Depot). That extension cord is plugged into a 30 amp adapter which is then plugged into a 50 amp adapter when is then plugged into the standard 20 foot 50 amp power cable (the one that came with my Greystone RV).

A friend of mine, a HVAC guy, is telling me that this setup is going to kill my RV electrical system, particularly the air conditioner. He basically said that, even though the RV is getting the full 120 Vac, the distance (probably a total of 100 feet) with an extension cord is going to drop the amps down to where it will damage the AC and other alternating current appliances.

Short of cutting open the extension cord to test it, is there a way (mathematically?) to compute the amount of amps that are actually reaching the RV?

Help!!

 

[danemayer]

Here's a link to a voltage drop calculator.

Part of the problem is that your house voltage may not be delivering a consistent 120V. If your house voltage drops to 112V, the additional loss on the long wire may drop your voltage to the coach enough to damage motors.

 

[TxCowboy]

That was my initial thought but, nope, volts are good. Checked them a multiple points along the way including several outlets within the RV. Virtually no drop.

 

[danemayer]

If there's no appreciable voltage drop, I think you're ok.

 

[jimtoo]

Turn your A/C on and recheck your voltage at one of the RV wall outlets.

 

[TxCowboy]

OK, will do that tomorrow and post the results here.

So, it's NOT an amperage thing, it's all about the volts?

 

[stevenssr]

It's all related. As voltage drops, current goes up. Same principle that happens when you try to use electric front end jacks with a low battery. In that case the voltage drops, current goes up and pops the fuse.

Scott

 

[danemayer]

OK, will do that tomorrow and post the results here.

So, it's NOT an amperage thing, it's all about the volts?

I'm not an electrical engineer, but I think amperage is an expression of watts (power required) divided by voltage. If the line resistance causes voltage to drop, but power required stays the same, amps goes up, and heat with it. Eventually you trip the house breaker.

 

[TxCowboy]

Scott & Dan, good comments and that makes sense to me (and I'm not an engineer either).

As I said in a previous post, I tested the outlets in the RV and they were at 120 volts but I didn't test it with the AC on. What voltage should I see with the AC on?

Also, since I'm testing, what else should be running in addition to the (single) AC? The radio and TV? What else? (Obviously, not the microwave.)

 

[danemayer]

The power converter is always running in the background to supply 12V to the coach and to charge the battery. It can at times consume around 12 amps. I would think A/C would be enough given the background load from the converter.

 

[TxCowboy]

The AC uses 13.4 amps (per the spec sheet). If the power convertor uses 12 amps, that is already over the 20 amp limit for the available shore power.

 

[SeattleLion]

Scott & Dan, good comments and that makes sense to me (and I'm not an engineer either).

As I said in a previous post, I tested the outlets in the RV and they were at 120 volts but I didn't test it with the AC on. What voltage should I see with the AC on?

Also, since I'm testing, what else should be running in addition to the (single) AC? The radio and TV? What else? (Obviously, not the microwave.)

Volts times amps = watts. If the voltage drops (you can only observe that under load), the amps go up. If you want to minimize voltage drop, use a 10 ga. Extension cord. We hav a 100ft one to our trailer.

 

[porthole]

If you want your guests to be comfortable, which I presume you do since you are in Texas and are most likely using using the AC, it would be cheaper now to just run your power correctly then the cost of repairs later.

At the minimum I would consider a 10 gauge cord with no adapters. Buy the wire then buy the approbate ends, a 20 amp for the house end and a RV 50 amp for the trailer end and make your own extension cord.

Your 12 gauge is barely adequate. Each adapter you use adds resistance and potential for conductor issues due to heat and corrosion.

And if you plan on doing this long term, bite the bullet and run a 50 amp service from your house panel to a pedestal where you keep the RV. Aside from the labor involved (18" trench), that might end up being less expensive.

 

[evolvingpowercat]

Your worst case voltage drop thru the 75 foot 12 AWG cord at full 20 amp load is 10 volts AC. You might lose a few more volts in the house wiring leading up to the 20 amp outlet, and in your shore cable.

Buy a Kill-A-Watt Electricity Monitor for $ 22 and you can plug into an outlet in the RV and watch the voltage in your Greystone when the AC starts up. If it stays at or above 110 VAC you should be fine. If you see it drop below 110 volts while the AC starts then I would beef up your power feed. However, going from 12 to 10 extension cord will only reduce your extension cord full 20 amp load voltage drop by 4 volts from 10 volts AC to 6 volts AC so its not a huge difference.

 

[evolvingpowercat]

I have watched my 50 amp 3 stage converter on the kill-a-watt when plugging my camper into shore power. It initially draws 600 watts AC ( 5 amps ) for a short time then once it senses the charge state of the batteries it will stay at that "fast charge" level only if the batteries are below about 80% charged and stay there until the batteries are about 80% charged. Then it will drop to a "slow charge" level, then finally a "float charge" level.

If I recall the "slow charge" used around 200 watts AC, and once it drops to the float charge level its down to about 50 watts.

If you AC-DC converter is bigger than 50 amps maximum output then this would all scale up accordingly.

 

[TxCowboy]

Thanks for all of the great posts above. I'll answer those in a bit.

Here's the volt numbers that were asked for previously.

Patio outlet (home source) 121.7
RV with all AC sources off 121.2
AC only 119.8
AC w/ TV 119.3
AC w/ radio 119.8
AC w/ TV and radio 119.8
AC w/ TV, radio, small fan 118.9

Looks like the extension cord cost a .5 volt drop over its 75 foot length. Plus the small fan by itself took almost 1 volt.

So, engineer friends, please provide your keen wisdom for I am a turnip. :angel:

 

[TxCowboy]

At the minimum I would consider a 10 gauge cord with no adapters. Buy the wire then buy the approbate ends, a 20 amp for the house end and a RV 50 amp for the trailer end and make your own extension cord.

I have a friend who is an HVAC guy and he recommends this as well. 10 gauge is priced at about $105 locally plus the appropriate connectors. Much less expensive than dropping a separate 50 amp service (which is still not out of the question). I'll probably drag him to the store when it is time to pick up these supplies.

Clearly, I don't want to damage anything in either my home or the RV, particularly since I'm moving this unit to the coast next Spring but I did want it available to me when the weather is nice over this summer. Putting in a 50 amp is an option here but an expensive one.

 

[porthole]

Your worst case voltage drop thru the 75 foot 12 AWG cord at full 20 amp load is 10 volts AC. You might lose a few more volts in the house wiring leading up to the 20 amp outlet, and in your shore cable.

Buy a Kill-A-Watt Electricity Monitor for $ 22 and you can plug into an outlet in the RV and watch the voltage in your Greystone when the AC starts up. If it stays at or above 110 VAC you should be fine. If you see it drop below 110 volts while the AC starts then I would beef up your power feed. However, going from 12 to 10 extension cord will only reduce your extension cord full 20 amp load voltage drop by 4 volts from 10 volts AC to 6 volts AC so its not a huge difference.

Your leaving out some of the other variables. He is using multiple adapters. The longer the power is used the warmer the wire gets and the higher the resistance goes.

 

[HornedToad]

And if you plan on doing this long term, bite the bullet and run a 50 amp service from your house panel to a pedestal where you keep the RV. Aside from the labor involved (18" trench), that might end up being less expensive.

$200 materials, $0 labor (favor from electrician), no trench, on side of garage about 20 ft from panel.

 

[evolvingpowercat]

Thanks for all of the great posts above. I'll answer those in a bit.

Here's the volt numbers that were asked for previously.

Patio outlet (home source) 121.7
RV with all AC sources off 121.2
AC only 119.8
AC w/ TV 119.3
AC w/ radio 119.8
AC w/ TV and radio 119.8
AC w/ TV, radio, small fan 118.9

Looks like the extension cord cost a .5 volt drop over its 50 foot length. Plus the small fan by itself took almost 1 volt.

So, engineer friends, please provide your keen wisdom for I am a turnip. :angel:

Voltage drop is directly related to current, the loads you list above draw almost NO current so that's why you are seeing little drop. If you turn on the AC too and measure AC at an outlet in the camper again you will see the voltage drop many volts, if the draw after startup while running is 10 amps you will see a drop of at least 5 volts and perhaps a lot more depending on the drop thru your multiple adapters, shore cable, house wiring. Once again if it drops below 110 with AC on you will need to change something. If you are going to have a TT with 50 amp shore cable as occasional guest quarters for the long run, I think you would be wise to have at least a 30 Amp RV outlet installed where you could plug in directly to there with your RV shore cable and one 50 to 30 adapter. Better yet, do the 50 amp RV outlet if you have 2 AC and want to run them both at the same time. The 30 A would be cheaper to install it only takes one space in the breaker box and 8/2 with Ground cabling. The 50 A takes 2 spaces in the breaker box and 6/3 with Ground cabling.

Other consideration - to add 50 A RV outlet if your current electrical service is not 200 Amp feed and panel but only 100 Amp, chances are the Electrician is going to tell you need to upgrade to 200 Amp service from 100 Amp Service in meet codes and good practice. This will cost well above $ 1000.