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Extension cord (gauge) for charging M-Y

rick245

Member
Aug 26, 2020
74
8
NJ
I would need an extension cord to charge my MY for a few months. what is the best gauge recommended for a 50' feet extension cord? 12/3 or 10/3? Both are rated for 15amps.
 

Wshowell

Member
Jan 1, 2015
507
353
Arroyo Grande Ca
The thicker the better to reduce resistance heat, voltage drop, unnecessary tripping of circuits etc. When we visit our vacation house up in the Sierras I use a 75 foot 10/3 to charge one of our cars and a 100 foot 10/3 to charge the other. I upgraded the outlet to a Hospital Grade one and the other one (separate circuit) to a twist lock 110 20 amp hospital grade outlet. So far so good and I can charge our Y at 5 MPH and our S at 4 MPH. That gives us plenty of range overnight for day tripping and hopefully enough to hit the Ski slopes and back. My real concern is what happens when it snows and the temp is below freezing and all that power just goes to heat the battery. We just bought it 2 months ago. We may have to supercharge before we head up the mountain and plug in just to maintain our charge but not gain any miles. I figure I can always add a nema 14-50 to the bourse if and spend a few hundred for a 240V extension cord. The cords I have were construction grade and I was fortunate enough to find them on Craigslist. They would be about $185 each.
32AE9F98-4EC2-4EE8-A664-029FC1C85B77.jpeg
 

brkaus

Well-Known Member
Jul 8, 2014
7,742
6,275
Austin, TX
Sounds like 10/3 is a better way to go since I need to use it for another ~5-6 months.
I would for your particular use.

the 12/3 is going to be lighter, so better for traveling. It will also see a bit more voltage drop, so the car might complain esp if the circuit is already in the edge.
 

jcanoe

Active Member
Oct 2, 2020
2,177
2,153
Maryland
I would need an extension cord to charge my MY for a few months. what is the best gauge recommended for a 50' feet extension cord? 12/3 or 10/3? Both are rated for 15amps.
Are you planning to charge at 8 amps or 12 amps? Either the 10 gauge or the 12 gauge would work. I would be more concerned with the wall receptacle/extension cord interconnection overheating or the interconnect with the extension cord and the Mobile Connector power plug. They sell waterproof housings for connecting extension cords. The Mobile Connector is not rated NEMA Level 3 or 4, i.e. it is not water resistant. You definitely want to keep the Mobile Connector off of the ground and protect it from the weather.
 

rick245

Member
Aug 26, 2020
74
8
NJ
Are you planning to charge at 8 amps or 12 amps? Either the 10 gauge or the 12 gauge would work. I would be more concerned with the wall receptacle/extension cord interconnection overheating or the interconnect with the extension cord and the Mobile Connector power plug. They sell waterproof housings for connecting extension cords. The Mobile Connector is not rated NEMA Level 3 or 4, i.e. it is not water resistant. You definitely want to keep the Mobile Connector off of the ground and protect it from the weather.
I have a gfti outdoor outlet, hoping this should help. Good to know to protect mobile charger/extension cord interconnect. I will avoid using during rain/snowy days but will get something to cover.
 

rick245

Member
Aug 26, 2020
74
8
NJ
Are you planning to charge at 8 amps or 12 amps? Either the 10 gauge or the 12 gauge would work. I would be more concerned with the wall receptacle/extension cord interconnection overheating or the interconnect with the extension cord and the Mobile Connector power plug. They sell waterproof housings for connecting extension cords. The Mobile Connector is not rated NEMA Level 3 or 4, i.e. it is not water resistant. You definitely want to keep the Mobile Connector off of the ground and protect it from the weather.
I would prefer to have 12 amps that can get me 5mi/hr.
 

brkaus

Well-Known Member
Jul 8, 2014
7,742
6,275
Austin, TX
What's the price difference between 10 & 12? Do you plan on carrying the cord with you frequently when traveling?

There are boxes like this - Bluelounge Cable Box, Black-CB-01-BL - The Home Depot available that can protect the box. Be sure to check the size, this was just a quick web search.

Or, any plastic box would work fine if you need to use it in bad weather. tt should be fine in the occasional light rain.
 

jcanoe

Active Member
Oct 2, 2020
2,177
2,153
Maryland
I would prefer to have 12 amps that can get me 5mi/hr.
Have you tried to charge at 120V/12 amps? Perhaps you can gain 5 miles per hour of charging at 12 amps if your driving efficiency is very good. For the rest of us 3 miles per hour at 8 amps and 4 miles per hour at 12 amps is realistic.
 

Snow Drift

[Off-Road Assist] Activated
Feb 10, 2016
1,968
1,497
Long Island
Have you tried to charge at 120V/12 amps? Perhaps you can gain 5 miles per hour of charging at 12 amps if your driving efficiency is very good. For the rest of us 3 miles per hour at 8 amps and 4 miles per hour at 12 amps is realistic.
I got 5mi/hr on the screen of my M3 via a 120v outlet plugged into a 12/3 extension cord plugged into the UMC for a year, without issue.
 

jcanoe

Active Member
Oct 2, 2020
2,177
2,153
Maryland
I got 5mi/hr on the screen of my M3 via a 120v outlet plugged into a 12/3 extension cord plugged into the UMC for a year, without issue.
That is a really good number. The Model 3 has higher efficiency than the Model Y. I expect 4 miles per hour of charging at 120V/12A for the Model Y.
 

rick245

Member
Aug 26, 2020
74
8
NJ
To be safe, I am going with 50' 10/3 extension cord. Since the winter is starting, I would appreciate that extra 1mi/hr.
 

jcanoe

Active Member
Oct 2, 2020
2,177
2,153
Maryland
To be safe, I am going with 50' 10/3 extension cord. Since the winter is starting, I would appreciate that extra 1mi/hr.
Re-reading what Wshowell posted earlier "I use a 75 foot 10/3 to charge one of our cars and a 100 foot 10/3 to charge the other. I upgraded the outlet to a Hospital Grade one and the other one (separate circuit) to a twist lock 110 20 amp hospital grade outlet. So far so good and I can charge our Y at 5 MPH and our S at 4 MPH" there are 2 separate 120V circuits in use. The 15 amp circuit is capable of a maximum of 120V/12A for 1.44kW; this is used to charge the Model S and gain ~4 miles per hour of charging. The second circuit is rated for 20 amps and is capable of a maximum 120V/16A for 1.92kW. Using the 20 amp circuit the Model Y is able to gain ~5 miles per hour of charging. At 120V charging efficiency is ~80 to 85% versus ~90% efficiency at 240V.

On a standard 120V/15A circuit you can only safely charge at the higher (80% of the circuit rating) 12 amp rate if there is nothing else connected, powered by the same circuit. On a 120V/20A circuit the maximum (80% of the circuit rating) is 16 amps.

One way to double the power, double the charging rate and improve charging efficiency is to have an electrician re-purpose, rewire an existing 120V circuit to be 240V circuit. This can only be done if there is just the one outlet/receptacle on the circuit.
 
Last edited:

rick245

Member
Aug 26, 2020
74
8
NJ
Re-reading what Wshowell posted earlier "I use a 75 foot 10/3 to charge one of our cars and a 100 foot 10/3 to charge the other. I upgraded the outlet to a Hospital Grade one and the other one (separate circuit) to a twist lock 110 20 amp hospital grade outlet. So far so good and I can charge our Y at 5 MPH and our S at 4 MPH" there are 2 separate 120V circuits in use. The 15 amp circuit is capable of a maximum of 120V/12A for 1.44kW; this is used to charge the Model S and gain ~4 miles per hour of charging. The second circuit is rated for 20 amps and is capable of a maximum 120V/16A for 1.92kW. Using the 20 amp circuit the Model Y is able to gain ~5 miles per hour of charging. At 120V charging efficiency is ~80 to 85% versus ~90% efficiency at 240V.

On a standard 120V/15A circuit you can only safely charge at the higher (80% of the circuit rating) 12 amp rate if there is nothing else connected, powered by the same circuit. On a 120V/20A circuit the maximum (80% of the circuit rating) is 16 amps.

One way to double the power, double the charging rate and improve charging efficiency is to have an electrician re-purpose, rewire an existing 120V circuit to be 240V circuit. This can only be done if there is just the one outlet/receptacle on the circuit.
This sounds like a good option to amplify power. However, I live in an apartment now and don't have luxury to mess around with the circuit panel and outlets. This is something, I can do it myself being if it was my own house. Will be moving soon so it is a temporary inconvenience.
 

Johnny Vector

Member
Jun 21, 2020
177
243
Maryland
It's easy to figure out what the difference is. At 50 feet, let's calculate the difference between 10 and 12 gauge.

10 AWG is 1 mΩ/ft
12 AWG is 1.5 mΩ/ft

Okay, so 12 gauge has 50% more resistance! Sounds like a lot, doesn't it? (Spoiler: no, it's not)

We've got a 50 foot cord; multiply by two because the current flows in both directions. So, 100 feet of wire added to the circuit. The resistance is thus

10 AWG: 100 mΩ
12 AWG: 150 mΩ

The maximum charge rate you can set is 12 A.

E = I * R, so the voltage drop due to the cable is...

10 AWG: 1.2 V
12 AWG: 1.8 V

So the 10 AWG wire gets you an extra 0.6 V to the car. Or in power, it's 0.6 * 12 = 7.2 W

You're getting ~1400 W in either case, so you just have to ask whether spending 30% more and having a 30% heavier cable to haul around is worth it for that extra 7 watts.

I submit that it's not worth it.
 
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