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240v Extension Cord Question

Rocky_H

Well-Known Member
Feb 19, 2015
7,332
9,029
Boise, ID
10ga is clearly suitable for carrying 30 amperes according to NEC ampacity charts
"clearly" you are not understanding what those charts say. NEC ratings for current versus wire size are only for intermittent non-continuous loads. NEC is very clear that it is forbidden to run 30A continuous on 10 gauge wire. This is a specific difference between the electrical codes in the U.S. versus the code standards in Europe. In European code, they actually do specify the wire gauge ratings based on continuous 100% rated current usage for a circuit rating. But that is not so in the U.S. NEC.
The 30A rating is for "non-continuous" 30A, as the heat may/will accumulate at the 30A level, but not at the 24A.
This.
 
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ewoodrick

Well-Known Member
Apr 13, 2018
5,285
4,282
Buford, GA
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.

Why would you think that the 80% derating for continuous load exists?

Also, as I believe that I worded, on a "perfect" cord, it may not be a big problem, but since nothing is perfect...

And by the way, at what distances are you reading the specs?
 

electrongeek

Metrology Fanboy
Nov 1, 2019
69
75
Maine
"clearly" you are not understanding what those charts say. NEC ratings for current versus wire size are only for intermittent non-continuous loads. NEC is very clear that it is forbidden to run 30A continuous on 10 gauge wire. This is a specific difference between the electrical codes in the U.S. versus the code standards in Europe. In European code, they actually do specify the wire gauge ratings based on continuous 100% rated current usage for a circuit rating. But that is not so in the U.S. NEC.

This.

Please refer me to the relevant NEC language that states the standard NEC ampacity charts are for non-continuous loads. The NEC ampacity charts I have access to do not reference this limitation. In fact, by definition these charts appear to be for continuous current carrying capacity, as the definition of the word itself can be found in Art. 100 of the NEC:

Ampacity: The current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

Allowed current for a conductor can be derated for other circumstances, e.g. for more than 3 conductors in a raceway, but that isn't the case for an extension cable out in the open. And unlike what some above have implied, temperature of a current carrying wire does not keep climbing without limit - it will reach a steady state - one that the NEC ampacity charts take into account.

Now would it be prudent to charge at a lower rate than one's extension cable is rated for? Sure. You may not know the condition of the circuit proximal to your extension cord, so play it safe and don't push the limit. But at 30 amperes, the 10ga extension cord is not going to be the source of your house fire, unless you do something stupid and coil it up in a box, or allow it to become damaged. If damaged, it is still a hazard at much lower currents.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,332
9,029
Boise, ID
Please refer me to the relevant NEC language that states the standard NEC ampacity charts are for non-continuous loads. The NEC ampacity charts I have access to do not reference this limitation. In fact, by definition these charts appear to be for continuous current carrying capacity, as the definition of the word itself can be found in Art. 100 of the NEC:

Ampacity: The current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
My goodness. Sure--I will give you the NEC code section. It is 210.19(A)(1) that specifies this 125% oversizing of the conductors for continuous loads. Here is the wording of it:

"210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Conductors shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b).

(a) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

(b) The minimum branch-circuit conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors."


It is section 220-10 that has the mirror wording of this same provision of the 125% oversizing for selecting the overcurrent protection device (circuit breaker). The breaker and conductor size both must be 125% of the continuous load. In the case you are talking about, 24A continuous draw must have 125% of that rated for the breaker and the wire size = 30A. So that is why you can use the 10 gauge wire, for a 30A rated circuit, but it cannot use more than 24A draw continuously.

If you were going to have a 30A continuous draw, the breaker and wire must be 125% sized to 37.5, which needs to be rounded up to 40A rated breaker and wire, which cannot be 10 gauge for the 60 or 75 degrees C categories. It would have to go up to 8 gauge wire.

Here are plenty of articles that talk about this.

https://www.ecmweb.com/content/article/20885567/sizing-continuously-loaded-conductors-made-simple

125% Continuous Rule - Panelboards

NEC - when to use Continuous Load rating (X125%)

Continuous Loads Decoded | Explanation of the 125%

210.19(A)(1) Conductors Minimum Ampacity and Size.

Sizing Conductors, Part III
 
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electrongeek

Metrology Fanboy
Nov 1, 2019
69
75
Maine
My goodness. Sure--I will give you the NEC code section. It is 210.19(A)(1) that specifies this 125% oversizing of the conductors for continuous loads. Here is the wording of it:

"210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Conductors shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b).

(a) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

(b) The minimum branch-circuit conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors."


It is section 220-10 that has the mirror wording of this same provision of the 125% oversizing for selecting the overcurrent protection device (circuit breaker). The breaker and conductor size both must be 125% of the continuous load. In the case you are talking about, 24A continuous draw must have 125% of that rated for the breaker and the wire size = 30A. So that is why you can use the 10 gauge wire, for a 30A rated circuit, but it cannot use more than 24A draw continuously.

If you were going to have a 30A continuous draw, the breaker and wire must be 125% sized to 37.5, which needs to be rounded up to 40A rated breaker and wire, which cannot be 10 gauge for the 60 or 75 degrees C categories. It would have to go up to 8 gauge wire.

Here are plenty of articles that talk about this.

https://www.ecmweb.com/content/article/20885567/sizing-continuously-loaded-conductors-made-simple

125% Continuous Rule - Panelboards

NEC - when to use Continuous Load rating (X125%)

Continuous Loads Decoded | Explanation of the 125%

210.19(A)(1) Conductors Minimum Ampacity and Size.

Sizing Conductors, Part III

OK, got it.Should not use that 10ga extension cord at 86 deg F for more than 3 hours at 30 amperes, although these codes are intended for branch circuit wiring and overcurrent devices. I don't think NEC addresses extension cords, but UL says 30 amperes via a 10ga extension cord is OK, though I can see if queried they might decide to back-pedal on that rating for continuous duty.
Wire Ampacity Chart with Rated Amperage - StayOnline Ampacity Chart
 

ewoodrick

Well-Known Member
Apr 13, 2018
5,285
4,282
Buford, GA
OK, got it.Should not use that 10ga extension cord at 86 deg F for more than 3 hours at 30 amperes, although these codes are intended for branch circuit wiring and overcurrent devices. I don't think NEC addresses extension cords, but UL says 30 amperes via a 10ga extension cord is OK, though I can see if queried they might decide to back-pedal on that rating for continuous duty.
Wire Ampacity Chart with Rated Amperage - StayOnline Ampacity Chart

Remember, that extension cords have different connection methods, i.e. plug, that must be taken into consideration. A 50A plug has bigger blades than a 30A. Also have to make sure that the manufacturer does a great job of bonding the wires to the plug elements.
And don't forget that even around 70F, a cord laying in sunlight can get much warmer with no current.
 
Will be installing a nema 14-50 outlet into my 1964 house. currently tryin to decide on two options:

1. run direct from panel to garage (~25' of 6/3)
2. run to washer & dryer location (~10' of 6/3), then extension cable to tesla (50' amzn.to/2TqPp3X )

(for visualization- green is electrical panel, orange is washer & dryer outlet, and pink is potential tesla connection point)

garage etc.png


The reason I'm considering option #2 is for future-proofing. It would give options for a higher powered washer & dryer... whether that be for me or a future owner (though maybe they have an EV and would've wanted it in the garage? ugh). Also, it gives me an RV extension cable to take with me on the road.

What I'm wondering is... (A) what % of washer & dryers require 14-50?, and (B) will the amount of charge I can / should pull decrease with the extension cable?

Anything else I'm not considering?
 
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electrongeek

Metrology Fanboy
Nov 1, 2019
69
75
Maine
Dryers are not typically 50 amp devices, so I would not bother trying to locate a 14-50 there. They are almost always 30 amp devices and are equipped with a 14-30 plug, which won't fit a 14-50 outlet anyway.

Just run directly from the panel to the garage. Also consider installing a 6-50 in the garage. An EV does not need the neutral that a 14-50 supplies, just the two hot wires and a ground. The 6-50 would eliminate the need for one fat wire going from panel to outlet. Another advantage would be that you could use a 3 wire 6-50 extension cord, which will be lighter and easier to handle than a 14-50 extension with its addittional neutral conductor. If you really think you might want to plug an RV into this outlet, and not just an EV or welder, then yes a 14-50 would be best.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
7,332
9,029
Boise, ID
Well, I haven't heard of someone trying to future proof for a higher power dryer, but at least your questions are easy.

What I'm wondering is... (A) what % of washer & dryers require 14-50?,
None. Dryers have been 30A outlets for the last several decades, and it's not like the process of drying water out of fabric is going to have big technological advances in the future.

(B) will the amount of charge I can / should pull decrease with the extension cable?
Not really to any noticeable difference. These kinds of lengths of 10 or 25 or 30 feet are not very long, and it will probably show up as a 1 or 2 volt drop. It's not going to be anything important. Charging speed scales with power, which is volts times amps. So if you're using 240V at 32A (7,680 Watts) or 238V at 32A (7,616 Watts), that's about a 1% difference.
 
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Thanks for the responses y'all.

Just run directly from the panel to the garage. Also consider installing a 6-50 in the garage. An EV does not need the neutral that a 14-50 supplies, just the two hot wires and a ground. The 6-50 would eliminate the need for one fat wire going from panel to outlet. Another advantage would be that you could use a 3 wire 6-50 extension cord, which will be lighter and easier to handle than a 14-50 extension with its additional neutral conductor. If you really think you might want to plug an RV into this outlet, and not just an EV or welder, then yes a 14-50 would be best.

What about other tools that might go in a garage... like a cabinet table saw? I don't think I'd be buying one requiring that sort of amperage, but it would be nice to have wire in the wall capable of handing anything. If there's a number of other things (including an RV) that could make use of the 14-50, I think maybe it outweighs the benefits of saving one length of wire? Is there a big cost savings, or something else I don't understand?
 

billxx

New Member
Mar 20, 2020
2
0
Toronto
I think it is not a good idea to plug a lower gauge extension cord (the 30A you have) into a higher amp outlet (50A). The only thing that would make it safe is if the breaker at the main box is 30A on that line. Gauge sizes should always be equal to or greater than the compatible amperage, and greater in gauge size for longer extension cords.

Here is a website where you can do all your calculations for Volts (incl Voltage drops), Amps, AWG and length of ext cord:

Website: Wire Size Calculator

Here are some calculations from the site:

240V 50amp extension cord allowing no more than 1V drop
Up to 23 ft maximum with 6 AWG

240V 30amp extension cord allowing no more than 1V drop
Up to 15 ft maximum with 10 AWG
 

billxx

New Member
Mar 20, 2020
2
0
Toronto
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.

Hi. New here. Quick comment on voltage drops. With a 25 ft cord and AWG 10 you will get a 1.7% voltage drop over the length of your cord. Any longer cord you would be better to go to 8 AWG if you are concerned about too much drop, although it's still not dangerous unless you sense the cord is heating up on you, which I doubt. The maximum voltage drop allowed in 5%. Cheers.
 

MD-2000

Member
May 1, 2019
617
419
Winnipeg
AFAIK - electrician building code states a continuous load must be no more than 80% of the breaker rating (and the wire is usually sized to the breaker rating).

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
Also AFAIK I read somewhere that the LR Model 3 will charge at a max of 40A and the SR+ at max 32A (240V) based on the capabilities of charger electronics inside the vehicle.
However, the mobile charger for the Model 3 is limited to 32A regardless of the car capabilities.
The 14-50 end for the mobile charger will draw 32A 240V. Other ends are similarly limited based on the plug rating - ie. a 30 will draw 24A. the regular home wall socket, 15A, will draw 12A 120V. And so on. The plug piece you put on the mobile charger will tell the charger what to limit current draw to.
The wall chargers will do up to 100A (I think) for Model X or S which can handle that. My Gen2 has a dial inside to set max amps - for my car M3 LR AWD, on a 50A breaker, set to 40A. The Gen3 according to the manual is set by wifi.
 

thefrog1394

Member
Dec 15, 2019
70
65
Ohio
For the folks worried about plugging a lower gauge extension cord into an outlet rated for higher current, don't forget people do this every day on 15 and 20 amp circuits. Your typical extension cord (even a "heavy duty" 3 prong outdoor rated one) is only 16 gauge and rated for 13 amps (or 10 amps continuous after 80% derating).

Pulling 32 or 40 amps continuous through a 10 gauge extension cord is definitely dangerous though, so I'd only feel comfortable recommending this setup if the mobile connector adapter is a 14-30 or 10-30 adapter which will automatically limit current draw to 24 amps.
 

thefrog1394

Member
Dec 15, 2019
70
65
Ohio
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.
I "standardized" on 6-50 welder plugs/cords. Readily available and affordable extension cords on amazon, and you aren't paying for and hauling around the unneeded neutral wire. Although the one I bought was advertised as 50 amp but really an 8 gauge listed at 40 amp on the cord itself which is still plenty for the 32amp mobile connector. Adapters from 10-30/50 and 14-30/50 are available too. Only downside is I have to software limit to 24 amps in the car on 30 amp circuits which I don't love considering I will mostly be plugging into 30 amp dryer outlets.

If I were to do it again, I might make my own cable with something like this this with standard 14-50 ends on it. I don't believe the mobile connector needs neither neutral nor ground to operate, so might as well make the cord as slim and flexible as possible. And then I could buy a 14-30 mobile connector adapter and knock the neutral pin off to give me a "hardware locked" 24 amp setup.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,332
9,029
Boise, ID
If I were to do it again, I might make my own cable with something like this this with standard 14-50 ends on it. I don't believe the mobile connector needs neither neutral nor ground to operate,
That wouldn't work. It doesn't use neutral, but it absolutely, definitely DOES do an active check that there is a real ground, and it will just sit there with a blinking red light and not work if that isn't there.
And then I could buy a 14-30 mobile connector adapter and knock the neutral pin off to give me a "hardware locked" 24 amp setup.
I did that with mine. My extension is a full retail 14-50 cable, so it has the 14-50 receptacle on the end. So I did use a hacksaw to cut the neutral pin off my Tesla 14-30 adapter to plug into it, so it will signal the 24A, so that when I use an adapter on the far end to plug into a 10-30 or 14-30, I don't have to remember to manually set the current in the car.
 

davewill

Active Member
Feb 5, 2014
1,833
2,088
San Diego, CA, US
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.
It does until the day it doesn't. Plenty of people have been surprised when their car suddently decides to NOT follow the amperage that had been previously set. It's fine for casual use, but to depend on it to keep your house safe day in and out....no way.
 
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