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240v Extension Cord Question

RubinRA

Member
Jan 4, 2020
32
10
Fuquay-Varina, North Carolina
I bought a heavy 15-foot extension cord to connect my Model 3 (via a Gen 2 mobile connector with a NEMA 10-30R adapter) to my in-laws' old NEMA 10-30R basement dryer plug. It works fine to recharge my car. Here's my question: If I get an adapter like this one (http://www.shorturl.at/ruT39), am I risking electrical disaster using the 30-amp extension cord with my 50-amp home NEMA 14-50 outlet? Trying to avoid burning down the house or damaging my new Model 3!
 

SSedan

Active Member
Jul 24, 2017
2,948
2,589
Greenville Wisconsin
I didn't cut and paste the link but long as the car is seeing the 30amp adapter it won't pull over 24amps.
Not right to plug a 30amp cord into a 50amp outlet.

This isn't "right" and I don't think anyone would recommend it but so long as the car is seeing the 30amp Tesla adapter the risk seems minimal.

This is presuming the linked adapter is going between the outlet and extension cord and the UMC will be using the Tesla 30amp adapter.

With an extension cord I try to dial the amps down a little too.
 

GalacticHero

M3-AWD+
Feb 7, 2019
91
45
Denver
A very quick web search shows 10-30 extension cords are 10 gauge and 14-50 are 6 gauge. So, my wild ass guess is your cable is not up to it. You might be able to get away with it short term by manually turning the current down on your charge screen in the car. Also, Tesla specifies that extension cords not be used with the Mobile Charger; so, if you do burn down your house you may be SOL on insurance since NEC requires you to follow manufacturer's recommendations. This should be taken as wild speculation, not legal advise. You're not likely to damage your M3 (unless you burn a house down around it.)
 
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electrongeek

Metrology Fanboy
Nov 1, 2019
69
74
Maine
It all depends on whether you feel capable of managing your charge rates properly as dependent on the circuits you are attached to. If that is confusing, don’t use extension cords and only use Tesla adapters.

For myself, I took a 25 foot 10 gauge extension cord, cut the 10-30 plug and receptacle off and replaced them with a 14-50 plug and receptacle. I also removed the neutral blade from the plug so it would fit either 14-50 range or 14-30 dryer receptacles (EVs ignore the neutral). I bought a 10-30P to 14-50R adapter to adapt the extension to 10-30R dryer outlets. I labeled the cord “For EV Use Only” and “24 Amp Maximum”. I carry this for travel charging at homes of my family. One does have to take personal responsibility for being certain the vehicle charge rate is set not to exceed 24 Amps. For the record, I don’t advocate others do this, but it works well for me.
 
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Petrlol

Member
Oct 16, 2018
405
550
Ohio
Extension cords are generally frowned upon.

Modifying one to have plugs that are higher amperage than the cord was originally designed for and with an insufficient gauge is a whole another level.

I understand it's marked and such but eh....

That said, I've used this cord many times at a relative's house. It stays ice cold pulling 32amps and it's 6ga.

https://smile.amazon.com/dp/B07FY691WQ/ref=cm_sw_r_other_apa_i_JEmqEbCCK4CJ2
 

ewoodrick

Well-Known Member
Apr 13, 2018
5,285
4,270
Buford, GA
I bought a heavy 15-foot extension cord to connect my Model 3 (via a Gen 2 mobile connector with a NEMA 10-30R adapter) to my in-laws' old NEMA 10-30R basement dryer plug. It works fine to recharge my car. Here's my question: If I get an adapter like this one (ShortURL - URL Shortener), am I risking electrical disaster using the 30-amp extension cord with my 50-amp home NEMA 14-50 outlet? Trying to avoid burning down the house or damaging my new Model 3!

Yes.

If you have to extend a NEMA 14-50, why not get a NEMA 14-50 extension cord? Readily available, as they are what larger motorhomes use.
 

doghousePVD

My grandfather’s car
Dec 3, 2018
653
600
New England, USA
The car remembers these amperage set to by gps location.

For example my home charger (internally set to 72 A) charges only at 64 A because I set it to that. The electrician only put In an 80 A breaker.

My weekend house (also set to 72A) charges at 72A. The amps change without any manual intervention.
 

user212_nr

Active Member
Aug 26, 2019
1,407
874
US
I bought a heavy 15-foot extension cord to connect my Model 3 (via a Gen 2 mobile connector with a NEMA 10-30R adapter) to my in-laws' old NEMA 10-30R basement dryer plug. It works fine to recharge my car. Here's my question: If I get an adapter like this one (ShortURL - URL Shortener), am I risking electrical disaster using the 30-amp extension cord with my 50-amp home NEMA 14-50 outlet? Trying to avoid burning down the house or damaging my new Model 3!

Your link is not working for me, but I assume that you are talking about 14-50 to 10-30 adapter.

The NEMA 14-50 is protected by a 50A circuit breaker and this has two protections:
1) Short circuit protection
2) Protection of the wire for drawing over 50A

If you plug in a 30A extension cord, you will have protection #1 short circuit, as short circuit current is going to be very high. The protection #2 is going to be not-applicable to your 30A extension cord because 50A will damage the cord.

That being said, you still have other protections for #2:
1) Using the 10-30 adapter, it will not intentionally ever draw more than 24A
2) The mobile connector and/or the car have a few other sanity checks

Since this is your house where you will be charging every day, this is a bad idea. If you were doing this at your in-laws house and didn't want to buy the expensive cord for a few days, that would make sense.
 
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kkillebrew

Banned
Jun 23, 2019
401
237
austin, tx
I bought a heavy 15-foot extension cord to connect my Model 3 (via a Gen 2 mobile connector with a NEMA 10-30R adapter) to my in-laws' old NEMA 10-30R basement dryer plug. It works fine to recharge my car. Here's my question: If I get an adapter like this one (http://www.shorturl.at/ruT39), am I risking electrical disaster using the 30-amp extension cord with my 50-amp home NEMA 14-50 outlet? Trying to avoid burning down the house or damaging my new Model 3!

If you turn down the maximum draw to 24 amps AND the cord never gets beyond warm to the touch it is fine. So you can do this safely its just not a good idea in general... I would say in a pinch but definitely not as a regular means of charging. You can safely build or buy a 50 amp NEMA XX-XX extension cord to take full advantage of the mobile charger - you need a 40 amp breaker. I do this at a seasonal residence with zero problems and I bought all the parts at the local Menards in Mason City, Iowa.

BUT as everyone will tell you in varying degrees, do be careful and monitor your first few charges.
 

user212_nr

Active Member
Aug 26, 2019
1,407
874
US
If you turn down the maximum draw to 24 amps AND the cord never gets beyond warm to the touch it is fine. So you can do this safely its just not a good idea in general... I would say in a pinch but definitely not as a regular means of charging. You can safely build or buy a 50 amp NEMA XX-XX extension cord to take full advantage of the mobile charger - you need a 40 amp breaker. I do this at a seasonal residence with zero problems and I bought all the parts at the local Menards in Mason City, Iowa.

BUT as everyone will tell you in varying degrees, do be careful and monitor your first few charges.

He's using a 10-30 plug. There's no need to dial down the amps. It is set at 24 already.
 
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user212_nr

Active Member
Aug 26, 2019
1,407
874
US
The OP stated he had been using 10-30 at his relatives and was considering using an adapter to plug his 10-30 extension into a 14-50 plug at his house.

Correct. So the adapter is on the M side of the extension cord, but the F side is still a 10-30, and the Tesla UMC connects there.

In this case "adapter" refers to a generic plug adapter, the kind that existed before Tesla. It is implied that he is using the 10-30 Tesla adapter to plug into the 30A extension cord.
 

electrongeek

Metrology Fanboy
Nov 1, 2019
69
74
Maine
Extension cords are generally frowned upon.

Modifying one to have plugs that are higher amperage than the cord was originally designed for and with an insufficient gauge is a whole another level.

I understand it's marked and such but eh....

That said, I've used this cord many times at a relative's house. It stays ice cold pulling 32amps and it's 6ga.

https://smile.amazon.com/dp/B07FY691WQ/ref=cm_sw_r_other_apa_i_JEmqEbCCK4CJ2

If limited to charging the Model 3, the most the mobile charger could try to pull through my 8 lb / 10ga cord is 32 amperes, where 10ga wire is rated for 30 amperes. That isn't exactly going to burn down the house. The most likely source of trouble would be if one were trying to pull 32 amperes from a 10-30 outlet, but then the breaker would likely trip. Naturally its far better not to be testing the limits of the circuit breaker.

The cord you link weighs 37 pounds - not great for traveling, but leaving at a fixed location it makes good sense.
 

Petrlol

Member
Oct 16, 2018
405
550
Ohio
If limited to charging the Model 3, the most the mobile charger could try to pull through my 8 lb / 10ga cord is 32 amperes, where 10ga wire is rated for 30 amperes. That isn't exactly going to burn down the house. The most likely source of trouble would be if one were trying to pull 32 amperes from a 10-30 outlet, but then the breaker would likely trip. Naturally its far better not to be testing the limits of the circuit breaker.

The cord you link weighs 37 pounds - not great for traveling, but leaving at a fixed location it makes good sense.

True, I do leave it in the sub-trunk all the time, I'm sure it's hurting efficiency to some degree. I do have my own set of various 14-50 to X adapters which require me to dial down the amps. I just like that I know there is no way I'm going to pull more than the cord can handle.
 

kkillebrew

Banned
Jun 23, 2019
401
237
austin, tx
What is funny here is that the 14-50 shown previously and 6-50 Tesla mobile adapter plugs state 30A 250V yet both charge at 32A
 

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ewoodrick

Well-Known Member
Apr 13, 2018
5,285
4,270
Buford, GA
If limited to charging the Model 3, the most the mobile charger could try to pull through my 8 lb / 10ga cord is 32 amperes, where 10ga wire is rated for 30 amperes. That isn't exactly going to burn down the house. The most likely source of trouble would be if one were trying to pull 32 amperes from a 10-30 outlet, but then the breaker would likely trip. Naturally its far better not to be testing the limits of the circuit breaker.

The cord you link weighs 37 pounds - not great for traveling, but leaving at a fixed location it makes good sense.

This is indeed the reason why houses burn down. If you pulled 30A from a cord rated at 30A for many hours, there is going to be some significant heat build up. If there is any wear, tear, or manufacturing issues, there might be some smoldering or melting start to occur.

That's why there's a 80% maximum on circuits with continuous loads.

If I slap you once, you can probably take it, twice, the same. But if I slapped you once a second for 6 hours, that's another story.
 
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electrongeek

Metrology Fanboy
Nov 1, 2019
69
74
Maine
This is indeed the reason why houses burn down. If you pulled 30A from a cord rated at 30A for many hours, there is going to be some significant heat build up. If there is any wear, tear, or manufacturing issues, there might be some smoldering or melting start to occur.

That's why there's a 80% maximum on circuits with continuous loads.

If I slap you once, you can probably take it, twice, the same. But if I slapped you once a second for 6 hours, that's another story.

Interesting. Can you supply some data on 30 amp rated wiring causing house fires when pulling 30 amps continuously, or is this just a supposition? 10ga is clearly suitable for carrying 30 amperes according to NEC ampacity charts although the wire can reach a warm, but safe for thermoplastic, 60 deg C. I would hope that any user of an extension cord would be aware enough that damage to such cord - or even to your mobile connector cord (wonder what gauge is used in this?) could cause a hazard, but then these days that assumption may be too great a leap for the masses.
 
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user212_nr

Active Member
Aug 26, 2019
1,407
874
US
Interesting. Can you supply some data on 30 amp rated wiring causing house fires when pulling 30 amps continuously, or is this just a supposition? 10ga is clearly suitable for carrying 30 amperes according to NEC ampacity charts although the wire can reach a warm, but safe for thermoplastic, 60 deg C. I would hope that any user of an extension cord would be aware enough that damage to such cord - or even to your mobile connector cord (wonder what gauge is used in this?) could cause a hazard, but then these days that assumption may be too great a leap for the masses.

It is no doubt a general statement about people overloading undersized extension cords and using adapters without understanding what effects they have.

If you want to know about the 30A over 10 AWG continuous specifically, I suppose you will need to try it with appropriate safety precautions. The heat will build up over time, whether it eventually catches fire is harder to tell. The damage may be not only over the 12 hours of charging, but the cord may build up damage over time. Even if only 1% of houses are damaged, that 1 percent is enough to justify the cord.

The safety margin (unknown what it is), exists for a reason and it is not worth testing to save a few $.

The 30A rating is for "non-continuous" 30A, as the heat may/will accumulate at the 30A level, but not at the 24A. If you have only a slight accumulation, and you are charging potentially to infinity, then at some point you will have a breaking point.
 
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