Acceptable voltage drop at 120V/16A?

[chriswood]

I'm not in a position to add a 240V outlet that I can charge from, but my outdoor outlet is 20A, which means I can use a NEMA5-20 cable. I use a long (90') 10/3 outdoor extension cord to reach my car's location, which I've been doing with my existing Prius Prime drawing 12A without issue. That said, there is a bit of a voltage drop - about 7V from my tests (121.5 normal voltage to 114.5V under 12A load last time I tested). Everything I've found so far tells me this isn't enough of a drop to cause the car to reduce its current, so I'm optimistic this will work, without being able to test until the car arrives, of course.

That said, I'd much rather charge at 16A if at all possible, with the NEMA 5-20 adapter. Calculating the expected voltage drop under a 16A load, I'm arriving at (7 / 12) = .583 ohms, and then (16A x .583) = 9.328 volts, resulting in ~112V under load. Will this still be within range for the MY, or will this be too much of a drop to allow for 16A current? And if so, will it drop to 12A, or to a different amperage?

 

[jcanoe]

This does not answer your question but would you answer one or two questions?

How many miles per day do you drive typically Monday through Friday? What about on the weekend?

 

[Johnny Vector]

As best I can tell, ANSI C84.1 says the voltage at the utilization point is allowed to be as low as 110 V, so the mobile connector should operate normally with 112 V incoming. Of course if your voltage sags due to power company issues, you might go lower. Worst case the car will stop charging more often than you'd like, and you'll have to get a larger gauge cord.

BTW, 10 AWG copper is almost exactly 1 mΩ/ft at room temperature, so 90 feet times 2 conductors means you should be seeing 0.180 Ω. If you're really seeing .583 Ω, either it's not actually 10 AWG copper or you have a lot of resistance in the connections. If it's the latter, you should probably replace the cord before it gets worse and causes a fire. Check the ends of your cord to make sure they don't look damaged.

 

[LoudMusic]

I've been charging on a marine 30A circuit at 24A for the past two months. For a long time it was fine drawing full 24A, sagging to about 114V. But for some reason for a few weeks I saw it was dropping below 108V and the car would automagically change to 18A in order to get the volts back up. The automatic reduction is 3/4, or 75%, of the prior set limit. The default set limit is 80% of the adapter plug type's designation.

Two nights ago it started charging at 24A without significant sagging again. Because this is a public area I'm assuming there was something else drawing massive power on the bigger circuit previously.

I've had a poke around the internet and I'm not finding any hard numbers on when the trigger is to reduce amps. But my guess is, 8 to 10 volts. So a 120v circuit dropping to 112v or a 240v circuit dropping to 232v. But LOTS of people talking about. Some sag is perfectly acceptable and expected. Significant sag is an indication of overloading the circuit. If it's supposed to be 15A, and it's sagging a lot drawing 12A, something might be wrong like bad wire terminations on the wall plug, or at the fusebox, or a bad fuse, or the wrong cable was used. Anything that might introduce significant electrical resistance.

 

[chriswood]

This does not answer your question but would you answer one or two questions?

How many miles per day do you drive typically Monday through Friday? What about on the weekend?

I did this math before buying to make sure I'd be OK without a L2 at home

Going by my current car's odometer and length of ownership, I've been averaging 14.6 miles per day for the past few years. That said, I'm working from home at the moment; once I start commuting again I'll be adding 60 miles per week, for an average of about 23 miles per day, which is what I'd expect to be able to charge overnight from 120/12. If I can get 16A, I expect that I'll be able to run errands after work and still have a full charge the next morning.

On top of that, my grocery store has a L2 charger (I'd expect I'll be plugging in there whenever I shop if I'm not near-full), and I live about 2 miles away from a Supercharger if I *really* need to add range quickly. As such I'm not worried about the L1 not meeting my daily needs even at 12A - just not having to spend as much time/money on a public L2 or SC.

 

[LoudMusic]

I did this math before buying to make sure I'd be OK without a L2 at home

Going by my current car's odometer and length of ownership, I've been averaging 14.6 miles per day for the past few years. That said, I'm working from home at the moment; once I start commuting again I'll be adding 60 miles per week, for an average of about 23 miles per day, which is what I'd expect to be able to charge overnight from 120/12. If I can get 16A, I expect that I'll be able to run errands after work and still have a full charge the next morning.

On top of that, my grocery store has a L2 charger (I'd expect I'll be plugging in there whenever I shop if I'm not near-full), and I live about 2 miles away from a Supercharger if I *really* need to add range quickly. As such I'm not worried about the L1 not meeting my daily needs even at 12A - just not having to spend as much time/money on a public L2 or SC.

Is the L2 at the grocery store free? I've seen quite a few that are and I'd really like to see that become the norm.

 

[jcanoe]

I have extensive experience charging using Level 1 at 8A and 12A to charge my 2017 Chevy Volt. (The Volt would only charge at Level 1 at 8A or 12A. The Volt had no option for charging at 120V at 16A.)

120V and 8A was fine for up to ~30 miles of driving per day. (10 hours charging)

120V and 12A was good for up to ~40 miles of driving per day. (10 hours charging)

At 8A the power plug and charging cable barely became warm. At the 12A setting the power plug would become noticeably warm as did the charging cable. (I never used an extension cable when charging.)

Currently I am able to use a free Level 2 charging station to charge my Model Y. I can charge at ~200V and 30A (~6kW) and add approximately 8% charge to the battery per hour of charging. I charge for about an hour to an hour and a half, most days. If you have free Level 2 charging available that is close to your home then you may find that to be the best option. (I have not charged my Model Y at home in over 6 months.) I currently drive less than 15 miles per day.

 

[Rocky_H]

@chriswood I would say that @LoudMusic has about the most complete answer on this. It's safe enough to just try to find a level that will work. The car will try using 16A on a 5-20, but if the voltage sags too much, the car won't like it and will reduce the current to three fourths of it (12A). If it does that, you can try again and probably find some level that is more than 12, but less than 16 at which it can run continuously without causing the car to reduce it.

 

[chriswood]

Following up on this post after taking delivery. My estimates were pretty much dead on - the car reports 112V (a ~9V drop under 16A load) and is able to sustain that amperage, for a charge rate of 6 miles per hour - more than enough for my daily needs. Bought an infrared thermometer, and pointed it at every junction point I could - wall plug, extension cable end, plug end, even the breaker switch, ... the junction points measured around 15°F above ambient, which doesn't seem abnormal to me.

Obviously, I'll be checking these every so often to look for signs of damage, but so far, so good.

 

[guydot]

Using a 12awg 80’ extension cord using NEMA 5-15 120v, the car reported 113v and would not go above 9amp. Not sure if a 10awg cord would help get to 12amp. Can anybody recommend a specific brand for 75-80ft know to reach the full 12amp?

 

[LoudMusic]

Using a 12awg 80’ extension cord using NEMA 5-15 120v, the car reported 113v and would not go above 9amp. Not sure if a 10awg cord would help get to 12amp. Can anybody recommend a specific brand for 75-80ft know to reach the full 12amp?

Use of extension cords is strongly discouraged. It's also large and expensive.

https://www.amazon.com/Extension-Cord-SJTW-Lighted-LifeSupplyUSA/dp/B08SQ547BG/ref=sr_1_26?dchild=1&keywords=10+awg+extension+cord&qid=1632938310&sr=8-26

 

[chriswood]

Using a 12awg 80’ extension cord using NEMA 5-15 120v, the car reported 113v and would not go above 9amp. Not sure if a 10awg cord would help get to 12amp. Can anybody recommend a specific brand for 75-80ft know to reach the full 12amp?

Before you buy a 10AWG cable, I'd suggest checking your idle voltage levels first. I use a Kill-A-Watt to easily see the voltage drop in real time, but a standard multimeter will work as well. If you're substantially below 120V idle, then you may want to check the wiring between your breaker and the receptacle.

If you want to do the math comparing 12AWG to 10AWG, know that the conductor resistance is 1Ω/1000ft for a 10AWG, and 1.5Ω for 12. As such, a 90' extension cable's resistance can be calculated (remember the current path is bidirectional!).
10AWG: (80 / 1000) * 2 = .16Ω
12AWG: ((80 / 1000) * 2) * 1.5 = .24Ω

From here, you can calculate the voltage drop at 12A:
10AWG: .16Ω * 12A = 1.92V
12AWG: .24Ω * 12A = 2.88V

Given you're reading 113V with a 12AWG cord, you probably have less than 120V at the outlet (absent other points of resistance, probably only 116V), which should warrant some investigating into your house wiring. If it is 120 but drops down to 113 under load, then you should probably look at replacing your outlet or extension cord (try the outlet first, it's cheaper).

Back to the specific question, here are the 12AWG and 10AWG cables I used, first drawing 12A with the 12AWG cord, and then switching to NEMA5-20 outlet/plugs with the 10AWG cable, yielding 16A of current (don't do this if your outlet isn't on a 20 amp breaker):
10AWG: https://www.amazon.com/gp/product/B08SQ547BG/
12AWG: https://www.amazon.com/gp/product/B004QXUCDM/

 

[LoudMusic]

Before you buy a 10AWG cable, I'd suggest checking your idle voltage levels first. I use a Kill-A-Watt to easily see the voltage drop in real time, but a standard multimeter will work as well. If you're substantially below 120V idle, then you may want to check the wiring between your breaker and the receptacle.

If you want to do the math comparing 12AWG to 10AWG, know that the conductor resistance is 1Ω/1000ft for a 10AWG, and 1.5Ω for 12. As such, a 90' extension cable's resistance can be calculated (remember the current path is bidirectional!).
10AWG: (80 / 1000) * 2 = .16Ω
12AWG: ((80 / 1000) * 2) * 1.5 = .24Ω

From here, you can calculate the voltage drop at 12A:
10AWG: .16Ω * 12A = 1.92V
12AWG: .24Ω * 12A = 2.88V

Given you're reading 113V with a 12AWG cord, you probably have less than 120V at the outlet (absent other points of resistance, probably only 116V), which should warrant some investigating into your house wiring. If it is 120 but drops down to 113 under load, then you should probably look at replacing your outlet or extension cord (try the outlet first, it's cheaper).

Back to the specific question, here are the 12AWG and 10AWG cables I used, first drawing 12A with the 12AWG cord, and then switching to NEMA5-20 outlet/plugs with the 10AWG cable, yielding 16A of current (don't do this if your outlet isn't on a 20 amp breaker):
10AWG: https://www.amazon.com/gp/product/B08SQ547BG/
12AWG: https://www.amazon.com/gp/product/B004QXUCDM/

The outlet is a really good point - there are some absolute crap outlets on the market. And I've seen some really bad wiring termination and attachment on them as well.

 

[guydot]

Before you buy a 10AWG cable, I'd suggest checking your idle voltage levels first. I use a Kill-A-Watt to easily see the voltage drop in real time, but a standard multimeter will work as well. If you're substantially below 120V idle, then you may want to check the wiring between your breaker and the receptacle.

If you want to do the math comparing 12AWG to 10AWG, know that the conductor resistance is 1Ω/1000ft for a 10AWG, and 1.5Ω for 12. As such, a 90' extension cable's resistance can be calculated (remember the current path is bidirectional!).
10AWG: (80 / 1000) * 2 = .16Ω
12AWG: ((80 / 1000) * 2) * 1.5 = .24Ω

From here, you can calculate the voltage drop at 12A:
10AWG: .16Ω * 12A = 1.92V
12AWG: .24Ω * 12A = 2.88V

Given you're reading 113V with a 12AWG cord, you probably have less than 120V at the outlet (absent other points of resistance, probably only 116V), which should warrant some investigating into your house wiring. If it is 120 but drops down to 113 under load, then you should probably look at replacing your outlet or extension cord (try the outlet first, it's cheaper).

Back to the specific question, here are the 12AWG and 10AWG cables I used, first drawing 12A with the 12AWG cord, and then switching to NEMA5-20 outlet/plugs with the 10AWG cable, yielding 16A of current (don't do this if your outlet isn't on a 20 amp breaker):
10AWG: https://www.amazon.com/gp/product/B08SQ547BG/
12AWG: https://www.amazon.com/gp/product/B004QXUCDM/

Thank you so much for the detailed answer. The voltage at the outlet (measured by my multimeter) is 121.9-122. I am not sure that there’s any significance to this fact but it is a CFGI outlet.

When I tried a 16 gauge extension cord on the same outlet I got a slightly lower voltage (about 110, measured by the car) but the same 9 amp charge.

My 12awg extension cable is made by Woods and the label says that it’s distributed by Southwire. It might be the same as what you use….

 

[Tronguy]

Um. To all you guys thinking about using 120 VAC to charge your Model Y's..
So, you get, depending upon whether you're running to a 15A or 20A socket somewhere between 4.5 to 6 MoCpH (Miles of Charge per Hour). And, during the late spring, summer, and early fall, that's what you get.
However, here's the warning: Let the temperatures get well below freezing and the car is going to want to warm the battery up before or during charging. If it's 15F out there, your charge rate will drop to 1 or 2 MoCpH, which Will Not Be Fun.
This tends not to be a problem with 220 VAC because, with roughly 7 to 10 kW present, there's no problem getting the battery warm and keeping it that way. The 1.44 kW at 12A @ 120 VAC.. On well below freezing days, the heat radiates away from the battery fast enough to keep it from getting up to temperature.
To those of you with heated garages: Not as much a trouble.
A year or so ago on a forum that's dead now a bunch of us were advising some fellow in North Dakota. He was renting a room in a house up that way and did have access to 120 VAC, but no Superchargers within 100 miles or something. We strongly suggested that he talk to his landlord about getting a 220 VAC socket put on the house. Luckily, the landlord was amenable. Last I heard, our guy, a science teacher at the local high school, was wowing the students and locals with his Model 3.

 

[guydot]

For me it’s not a permanent solution. I have a charging solution at home but I want to be able to take a reliable extension cord for trips.