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14-50 Extension Cord use (Long Run)

DDrawer

Member
Mar 25, 2019
656
584
PA
I know extension cords are not recommended in the manual, but I also see a lot of threads and videos saying it's fine as long as you get the right gauge cord.

My situation is this: My breaker box is at the front of my house, its a 50-60 foot run from the breaker to the back of my house. Then, from the back of my house the car is parked on a drive way at the end of my back yard another 50 feet away. So total distance from breaker to car is about 110 feet.

My plan is to have a 60 amp breaker installed, 6 gauge wire run to the back of the house, 14-50 outdoor outlet installed on the back of the house. Then run a 50 foot 6 gauge 14-50 extension cord from the outlet to my car area. Tesla mobile connector connected to the extension cord.

Will this be okay safety wise? Should I expect voltage loss? I plan to protect the ends of the extension cord with some sort of weather protection, and make sure they are elevated about 2 feet off the ground at all times.

To me this seems no different than having the electrician trench the yard, lay conduit, and continue to run the 6 gauge wire from the breaker box out to an outlet near my car area.
 

jdr93

Supporting Member
Jun 5, 2017
135
67
wooded vermont
be safe. perhaps ask an electrician for their code-wise suggestions. trenching a conduit to a real weather proof charge station or planting a pole near the car and running a real weather capable wire overhead to a real weatherproof charge station on the pole. trenching by hand isn't impossible (once the frost is out of the ground) and 60 feet isn't that far. be safe
 

DDrawer

Member
Mar 25, 2019
656
584
PA
It's going to cost thousands to have the yard surveyed and trenched by the city if the electrician does it. This is a rental property. If I can safely use an extension cord, I would much rather do that. The way I see it, what is the difference between having 1 110 foot length of 6gauge wire run from the breaker, or 1 60 foot length from the breaker and another 50 foot 6 gauge length of extension cord run from the outlet. Same gauge wire, same distance, same current.
 

Kirby64

Member
Jun 28, 2018
485
486
Austin, TX
I'm sure others will chime in about extension cords being bad yada yada, but one thing I will point out: DO NOT use a 60 amp breaker backing a NEMA 14-50 outlet. That's a strong code violation and definitely a fire risk. A 14-50 outlet needs a 50 amp breaker or a 40 amp breaker. No other options are allowed.
 

DDrawer

Member
Mar 25, 2019
656
584
PA
I'm sure others will chime in about extension cords being bad yada yada, but one thing I will point out: DO NOT use a 60 amp breaker backing a NEMA 14-50 outlet. That's a strong code violation and definitely a fire risk. A 14-50 outlet needs a 50 amp breaker or a 40 amp breaker. No other options are allowed.

Thank you, I was going off what the electrician quoted me for the breaker install. I will let him know to install a 50amp rather than the 60amp he quoted.
 

johnmodely

Member
Jan 14, 2020
260
189
NY
I know extension cords are not recommended in the manual, but I also see a lot of threads and videos saying it's fine as long as you get the right gauge cord.

My situation is this: My breaker box is at the front of my house, its a 50-60 foot run from the breaker to the back of my house. Then, from the back of my house the car is parked on a drive way at the end of my back yard another 50 feet away. So total distance from breaker to car is about 110 feet.

My plan is to have a 60 amp breaker installed, 6 gauge wire run to the back of the house, 14-50 outdoor outlet installed on the back of the house. Then run a 50 foot 6 gauge 14-50 extension cord from the outlet to my car area. Tesla mobile connector connected to the extension cord.

Will this be okay safety wise? Should I expect voltage loss? I plan to protect the ends of the extension cord with some sort of weather protection, and make sure they are elevated about 2 feet off the ground at all times.

To me this seems no different than having the electrician trench the yard, lay conduit, and continue to run the 6 gauge wire from the breaker box out to an outlet near my car area.

Nothing wrong with doing this, your just going to have higher resistive loses between the longer cord and extra plug contacts. Im assuming your outside plug will be weather protected, but better be extra safe where you'll plug the mobile connector. If you leave it on the ground and/or it gets wet with 240v you'll be dead if you touch it the wrong day.
 

DDrawer

Member
Mar 25, 2019
656
584
PA
Nothing wrong with doing this, your just going to have higher resistive loses between the longer cord and extra plug contacts. Im assuming your outside plug will be weather protected, but better be extra safe where you'll plug the mobile connector. If you leave it on the ground and/or it gets wet with 240v you'll be dead if you touch it the wrong day.

The resistive loss is what I'm wondering about. Will that be significant? Will my charge rate be impacted noticeably? As for protecting the part where the mobile connector plugs in, I plan to some how secure about 2 foot length of the extension cord to a post of some sort so that the plug is off the ground and then protect it from the weather with some form of umbrella or enclosure. I want to easily be able to unplug my connector and keep it in the car, but also be protected. Something like this.
ChargeSetup.jpg
 
  • Funny
Reactions: deonb

johnmodely

Member
Jan 14, 2020
260
189
NY
The resistive loss is what I'm wondering about. Will that be significant? Will my charge rate be impacted noticeably? As for protecting the part where the mobile connector plugs in, I plan to some how secure about 2 foot length of the extension cord to a post of some sort so that the plug is off the ground and then protect it from the weather with some form of umbrella or enclosure. I want to easily be able to unplug my connector and keep it in the car, but also be protected. Something like this.
View attachment 522905


Youll lose about 200 watts from a 50ft 6 gauge cord pulling the full 40Amps, and obviously another 200 from the wire from your house to your breaker.

Your charge rates won't be affectived because the mobile charger will pull the full amps at the charger (ie if your charger is pulling 8kw, it will really be pulling 8.4kw at the breaker). This is one of the many reasons you de-rate breakers.
 

mspohr

Well-Known Member
Jul 27, 2014
9,216
10,742
California
I'd recommend keeping the 6 gauge wire but lower your charge rate to 32 or 24 amps. That's what I do and never a problem getting a full charge overnight.
You'll have less resistive loss.
You do need to be aware that you will have a 240v extension cord lying in the open. You might want to consider running it in some flexible conduit for extra protection.
 

DDrawer

Member
Mar 25, 2019
656
584
PA
Youll lose about 200 watts from a 50ft 6 gauge cord pulling the full 40Amps, and obviously another 200 from the wire from your house to your breaker.

Your charge rates won't be affectived because the mobile charger will pull the full amps at the charger (ie if your charger is pulling 8kw, it will really be pulling 8.4kw at the breaker). This is one of the many reasons you de-rate breakers.
aaah I see, I didn't realize that is how it worked. So basically, I get the same charge rate, but it's just less efficient because my electric meter still says I used X amount of electricity, but not all of that X actually makes it into the battery.

I'd recommend keeping the 6 gauge wire but lower your charge rate to 32 or 24 amps. That's what I do and never a problem getting a full charge overnight.
You'll have less resistive loss.
You do need to be aware that you will have a 240v extension cord lying in the open. You might want to consider running it in some flexible conduit for extra protection.

Good suggestion, I will lower the charge rate a tick or two as well. I was planning on burying the extension cord (maybe running it through some buried PVC or something) after I made sure it wasn't getting hot or anything.
 

Kirby64

Member
Jun 28, 2018
485
486
Austin, TX
aaah I see, I didn't realize that is how it worked. So basically, I get the same charge rate, but it's just less efficient because my electric meter still says I used X amount of electricity, but not all of that X actually makes it into the battery.

No, @johnmodely is wrong. You'll get a slower charge rate because the extra power will get wasted in resistive losses in the cable (heat). What that means is that the voltage at the charger (the input to the Tesla UMC) will be lower under full load. If you have 240V at the breaker panel, it might only be 235V at the UMC.

It won't be significant though. Assuming 6 gauge wire, here's your calculation: voltage drop = I * R. Resisitivity of 6 gauge wire would be 0.043 ohms for 110ft of 6 gauge wire. Since you've got 2 conductors, it's double that effectively. That means you're looking at ~3.44V of drop for 40 amp charging. So, power loss in the wire itself should be closer to ~137W if you're charging at a full 40 amps. If you're only charging at 32A (UMC max), then the power loss drops to 88W. Either way, it doesn't matter, and totally safe. So at 32A, you'd be going from 7.68kW to 7.60kW. Basically no difference in charging rate.

Not sure where @johnmodely is getting his figure of 400W power loss in 110ft of 6 gauge wire. That's way too high.
 

DDrawer

Member
Mar 25, 2019
656
584
PA
Good to know. Thank you for the help guys. That is going to be my plan then. Electrician is coming out Saturday to do the install!
 

Kirby64

Member
Jun 28, 2018
485
486
Austin, TX
@Kirby64 from real world applications designing power circuit boards for a living. Not everything in physics follows simple equations ;)

Please, do tell how you get a 3x increase in power loss compared to calculated numbers? With copper, things actually DO follow simple equations. The only thing I didn't account for is any temperature rise and any contact resistance, but 20C temperature vs. 65C temperature doesn't actually make that much difference... your resistance per 110ft goes from 0.043 ohms to 0.051 ohms. So power loss goes up to a whopping 163W at 40A. Anything beyond 65C is unsafe, and also there's NO way the temperature rise from self-heating would even get you close to that (unless you've got the thing coiled up... or it's already 50C outside).

Unless you want to argue that that you've got 250W of contact resistance (which, probably means those contacts are on fire or have melted), your numbers are way, way off.

This isn't some complicated power converter board that has inefficiencies. It's literally copper wire and contacts. There's no point for inefficiencies besides ohmic losses.
 

mspohr

Well-Known Member
Jul 27, 2014
9,216
10,742
California
Thank you JP, that is the one I had added to my amazon wish list in the 50' version. Good to hear from someone that it is quality.
If you're going to use conduit, it would be cheaper to just use two strands of 6 gauge copper plus a 10 gauge ground.
 

Webeevdrivers

Active Member
Jan 2, 2017
2,248
3,956
Canada
For what it’s worth RVers do this. We do it with a 25 foot cord all the time. Use a proper 50 foot RV 50 amp 240 volt outdoor ext cord built for exterior UV conditions. Available at any RV dealer. Not cheap.
 

DDrawer

Member
Mar 25, 2019
656
584
PA
If you're going to use conduit, it would be cheaper to just use two strands of 6 gauge copper plus a 10 gauge ground.

Not planning to use actual conduit. I just want to bury the extension cord a little bit so it isn't in the lawn when it is getting mowed.
 

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