L1 UMC charging question

[skyline385]

We just moved into an apartment with a garage and the construction here isn’t new exactly and as such, the garage outlets don’t have a dedicated receptacle. Because of this, when i try to charge at 12A, the voltage drops from 122V to 109V which isn’t safe for extended usage per my understanding. I checked the UMC as well as the plug and neither was warm.

Anyways, to get around this, i am charging it at 8A where the voltage hovers between 113-114V. I don’t have a lot of daily commute so even this trickle charge would work for me if I can keep it plugged in for extended periods. But my worry is whether it’s safe to do so. I already know that the circuit in the garage isn’t the greatest considering the voltage drop at max current draw but is it safe to use at 8A where the voltage drop will be limited and between 113-114V?

 

[Cosmacelf]

A large voltage drop isn’t dangerous per se. It just means they probably used the “normal” 14 gauge wire for a 15A circuit, but didn’t take into account the long run. So if the breaker doesn’t trip, 12A should work and be safe.

Edit, you’ll be charging really, really slowly at 8A.

 

[skyline385]

A large voltage drop isn’t dangerous per se. It just means they probably used the “normal” 14 gauge wire for a 15A circuit, but didn’t take into account the long run. So if the breaker doesn’t trip, 12A should work and be safe.

Edit, you’ll be charging really, really slowly at 8A.

I should clarify that the voltage drop happens only after i start charging. I get 122V at the garage outlet when it’s not being used but as soon as i start charging, the voltage drops depending on the current draw. Would you still say it’s safe to use at 12A? I am also slightly concerned about the heat generation in the wires inside the walls if i were to use 12A for extended periods with the voltage dropping at 109V.

 

[Cosmacelf]

I should clarify that the voltage drop happens only after i start charging. I get 122V at the garage outlet when it’s not being used but as soon as i start charging, the voltage drops depending on the current draw. Would you still say it’s safe to use at 12A? I am also slightly concerned about the heat generation in the wires inside the walls if i were to use 12A for extended periods with the voltage dropping at 109V.

Yes, there is heat generation, but it is spread over the entire length of the wire, so no concerns. The only concern is the breaker tripping, but as long as it doesn’t, you should be good.

 

[Eric33432]

I should clarify that the voltage drop happens only after i start charging. I get 122V at the garage outlet when it’s not being used but as soon as i start charging, the voltage drops depending on the current draw. Would you still say it’s safe to use at 12A? I am also slightly concerned about the heat generation in the wires inside the walls if i were to use 12A for extended periods with the voltage dropping at 109V.

The voltage drop is caused by resistance. At 12 amps, you are getting a 13 volt drop across the circuit which is right at a 10% voltage drop which, again, is caused by resistance.

The question is: is this resistance from the length of the wire, or is it caused by something else such as a bad connection somewhere in the circuit?

My Southwire voltage drop app says the wire would have to be ~190 feet long to get a 10% voltage drop when 12 amps is carried on #14 wire. Based on the distance of the garage from your electrical panel, does that seem reasonable? If so, it is likely safe, because as mentioned by Cosmeself the heat being dissipated is spread over the length of the wire.

BTW, the amount of heat dissipated is 156 watts.

If your distance of the circuit to the garage is substantially shorter than 190 feet, then there is something else causing the voltage drop and it would likely be unsafe to run 12 amps (or even 8 amps) continuously on this circuit. With that kind of power being dissipated in, say a bad or loose connection, that bad or loose connection will very likely heat up very hot and eventually burn up. Just think about the amount of heat produced by a 100 watt incandescent light bulb for comparison.

If the voltage drop is caused by the length of the circuit, and you are worried the low voltage will damage the charging equipment you your Tessie, that is not a concern. The car will be fine with 109 volts.

 

[rrolsbe]

We just moved into an apartment with a garage and the construction here isn’t new exactly and as such, the garage outlets don’t have a dedicated receptacle. Because of this, when i try to charge at 12A, the voltage drops from 122V to 109V which isn’t safe for extended usage per my understanding. I checked the UMC as well as the plug and neither was warm.

Anyways, to get around this, i am charging it at 8A where the voltage hovers between 113-114V. I don’t have a lot of daily commute so even this trickle charge would work for me if I can keep it plugged in for extended periods. But my worry is whether it’s safe to do so. I already know that the circuit in the garage isn’t the greatest considering the voltage drop at max current draw but is it safe to use at 8A where the voltage drop will be limited and between 113-114V?

Just to verify ALL the voltage drop is on the wiring between the service panel and outlet being used to charge your vehicle, do the following. Check the (open circuit voltage) before starting a charging session with your EVSE then go to another unloaded 120V outlet on a different circuit and check the voltage (the unloaded/open circuit voltages should be the same) Note: this voltage could also be read from inside the service panel but should not be necessary. Start charging your vehicle. Now check the voltage on the other unloaded circuit. If that voltage is lower than the initial open circuit voltage reading, then there is a voltage drop at your service panel end. Take that panel voltage and subtract the loaded voltage at your EVSE outlet. This will be the actual wire voltage drop. When charging at 40A/240V at my home, the voltage drops by around 4V at the service panel end. Appears the service side wiring, transformer or something upstream might be less than optimal coming into my panel.

 

[skyline385]

The voltage drop is caused by resistance. At 12 amps, you are getting a 13 volt drop across the circuit which is right at a 10% voltage drop which, again, is caused by resistance.

The question is: is this resistance from the length of the wire, or is it caused by something else such as a bad connection somewhere in the circuit?

My Southwire voltage drop app says the wire would have to be ~190 feet long to get a 10% voltage drop when 12 amps is carried on #14 wire. Based on the distance of the garage from your electrical panel, does that seem reasonable? If so, it is likely safe, because as mentioned by Cosmeself the heat being dissipated is spread over the length of the wire.

BTW, the amount of heat dissipated is 156 watts.

If your distance of the circuit to the garage is substantially shorter than 190 feet, then there is something else causing the voltage drop and it would likely be unsafe to run 12 amps (or even 8 amps) continuously on this circuit. With that kind of power being dissipated in, say a bad or loose connection, that bad or loose connection will very likely heat up very hot and eventually burn up. Just think about the amount of heat produced by a 100 watt incandescent light bulb for comparison.

If the voltage drop is caused by the length of the circuit, and you are worried the low voltage will damage the charging equipment you your Tessie, that is not a concern. The car will be fine with 109 volts.

View attachment 1012733

View attachment 1012732

Thanks for the detailed post. I can't really tell exactly how much the distance to the garage receptacle from the breaker would be but the shortest run to it would be about 75-100 ft not 190 ft for sure. However, the garage outlets are not on a dedicated receptacle and are shared by other outlets in the apartment. There are two 15A breakers for all the outlets in the apartments so I cant tell which outlets are on the same circuit as the garage and which ones aren't. But, assuming half of the ones are on the same circuit and the wires loop between the receptacles before getting to the garage, would that be a fair assumption of where the 190 ft length is coming from?

Also, I downloaded your app and checked for 8A which results in a 9V drop typically from 122V so around 7% which matches what the app is showing for the same distance on 14 Guage for 190 ft. So, atleast the losses are linear which makes me think there is a long loop of wires in the wall equivalent to 190 ft? Let me know what you think about this.

 

[skyline385]

The voltage drop is caused by resistance. At 12 amps, you are getting a 13 volt drop across the circuit which is right at a 10% voltage drop which, again, is caused by resistance.

The question is: is this resistance from the length of the wire, or is it caused by something else such as a bad connection somewhere in the circuit?

My Southwire voltage drop app says the wire would have to be ~190 feet long to get a 10% voltage drop when 12 amps is carried on #14 wire. Based on the distance of the garage from your electrical panel, does that seem reasonable? If so, it is likely safe, because as mentioned by Cosmeself the heat being dissipated is spread over the length of the wire.

BTW, the amount of heat dissipated is 156 watts.

If your distance of the circuit to the garage is substantially shorter than 190 feet, then there is something else causing the voltage drop and it would likely be unsafe to run 12 amps (or even 8 amps) continuously on this circuit. With that kind of power being dissipated in, say a bad or loose connection, that bad or loose connection will very likely heat up very hot and eventually burn up. Just think about the amount of heat produced by a 100 watt incandescent light bulb for comparison.

If the voltage drop is caused by the length of the circuit, and you are worried the low voltage will damage the charging equipment you your Tessie, that is not a concern. The car will be fine with 109 volts.

Just to verify ALL the voltage drop is on the wiring between the service panel and outlet being used to charge your vehicle, do the following. Check the (open circuit voltage) before starting a charging session with your EVSE then go to another unloaded 120V outlet on a different circuit and check the voltage (the unloaded/open circuit voltages should be the same) Note: this voltage could also be read from inside the service panel but should not be necessary. Start charging your vehicle. Now check the voltage on the other unloaded circuit. If that voltage is lower than the initial open circuit voltage reading, then there is a voltage drop at your service panel end. Take that panel voltage and subtract the loaded voltage at your EVSE outlet. This will be the actual wire voltage drop. When charging at 40A/240V at my home, the voltage drops by around 4V at the service panel end. Appears the service side wiring, transformer or something upstream might be less than optimal coming into my panel.

I went around the house measuring voltages with the charger turned on and there is a drop between the receptacles all the way to the garage. The first receptacle measured 120.8V so I don't think there is a service issue but you can see how the outlets kept on dropping voltages as I got to the garage outlet which measured at 113.8V on 8A draw. There was also one outlet mounted in the ceiling in the garage which I couldn't get to and probably would measure between 114-116V since the rest of the outlets measured are in the living room above the garage.

 

[Eric33432]

Thanks for the detailed post. I can't really tell exactly how much the distance to the garage receptacle from the breaker would be but the shortest run to it would be about 75-100 ft not 190 ft for sure. However, the garage outlets are not on a dedicated receptacle and are shared by other outlets in the apartment. There are two 15A breakers for all the outlets in the apartments so I cant tell which outlets are on the same circuit as the garage and which ones aren't. But, assuming half of the ones are on the same circuit and the wires loop between the receptacles before getting to the garage, would that be a fair assumption of where the 190 ft length is coming from?

Also, I downloaded your app and checked for 8A which results in a 9V drop typically from 122V so around 7% which matches what the app is showing for the same distance on 14 Guage for 190 ft. So, atleast the losses are linear which makes me think there is a long loop of wires in the wall equivalent to 190 ft? Let me know what you think about this.

Glad I could help you.

Can you post a photo of the breaker panel breakers and the circuit directory? If the apartment has a kitchen, I would think there would be more than two circuits for *all* the outlets unless you are excluding the kitchen counter outlets in your count, but if there are only two circuit breakers for all the outlets (or all the outlets not counting the kitchen outlets) I would suspect the wiring would run from the breaker panel point to point from one outlet to the next, etc. You could turn off one or the other of the breakers and check to see which one shuts off the garage outlet, then check the remaining outlets to see which ones are on the garage circuit, which you should know. You might also get an idea of how the circuit runs and be able to come up with a rough idea of the distance. You also mention that the shortest distance is 75 to 100 feet from the breaker panel which seems to be pretty far unless the garage detached or otherwise further from the living space than would be normal, so if you add up the distance between all the outlets before the circuit ends up in the garage you might get a very rough idea of how long the circuit is.

(I have never seen a 15 amp circuit run any where close to 190 feet in any kind of normal residential situation.)

If you do charge at 12 amps, or even 8 amps, you need to know what else is on the circuit so you do not use anything on that circuit that has a substantial draw, which would be almost anything other than a couple of lamps with LED bulbs. For example, I would not want to plug in a vacuum cleaner on that circuit whilst the car is charging. And since the charging is so slow, you will need to let the car charge all the time it is parked in the garage, so you can't say you won't be using other devices while the car is charging.

Hope this makes sense, and good luck!

 

[Eric33432]

I went around the house measuring voltages with the charger turned on and there is a drop between the receptacles all the way to the garage. The first receptacle measured 120.8V so I don't think there is a service issue but you can see how the outlets kept on dropping voltages as I got to the garage outlet which measured at 113.8V on 8A draw. There was also one outlet mounted in the ceiling in the garage which I couldn't get to and probably would measure between 114-116V since the rest of the outlets measured are in the living room above the garage.

Your photos show from 1 to 1.5 volts difference between some of the outlets. Southwire's calculator says a 1 volt drop (at 8 amps / 122 volts) on #14 wire is 28.45 feet of cable. Most outlets are much closer than this, and I suspect your's are too.

Because a circuit in a small apartment would likely never be 190 feet in total length, I suspect you have a combination of voltage drop due to wire length and the connections at the various outlets. It is possible the outlets were wired with the backstab method:

This is usually OK, but an EV is a continuous load, on for many hours at a time, so if a loose connection is heating up, over time such loose connection can get very hot. Even if the outlets are not wired with the backstab method, if they are old and worn they could be causing some voltage drop.

If this was me, I would identify all the outlets that are involved in this circuit, remove them from the wall, and inspect them. I would rewire any outlets that were backstabbed so the wire is under the screw (as shown in the above video), and if it were me, whilst I was at it I would replace all the older outlets, as they are not expensive. This could be a nice Saturday afternoon project for someone like me, but not everyone is me, and some people are not cut out to do stuff like this.

This is a short video that explains how to replace an outlet:

What I write above is likely, but not the only possible problem. There could be splices in the wires, where they are connected with wire nuts, that could also be causing this problem, but that is less likely.

Good luck and let us know what you end up doing.

 

[skyline385]

Your photos show from 1 to 1.5 volts difference between some of the outlets. Southwire's calculator says a 1 volt drop (at 8 amps / 122 volts) on #14 wire is 28.45 feet of cable. Most outlets are much closer than this, and I suspect your's are too.

Because a circuit in a small apartment would likely never be 190 feet in total length, I suspect you have a combination of voltage drop due to wire length and the connections at the various outlets. It is possible the outlets were wired with the backstab method:

This is usually OK, but an EV is a continuous load, on for many hours at a time, so if a loose connection is heating up, over time such loose connection can get very hot. Even if the outlets are not wired with the backstab method, if they are old and worn they could be causing some voltage drop.

If this was me, I would identify all the outlets that are involved in this circuit, remove them from the wall, and inspect them. I would rewire any outlets that were backstabbed so the wire is under the screw (as shown in the above video), and if it were me, whilst I was at it I would replace all the older outlets, as they are not expensive. This could be a nice Saturday afternoon project for someone like me, but not everyone is me, and some people are not cut out to do stuff like this.

This is a short video that explains how to replace an outlet:

What I write above is likely, but not the only possible problem. There could be splices in the wires, where they are connected with wire nuts, that could also be causing this problem, but that is less likely.

Good luck and let us know what you end up doing.

Thanks again for the detailed response, learn a lot about wiring today.

Unfortunately, this is a rental apartment which we plan to stay at only for 11 months so I doubt I will be able to replace all the outlets or have an electrician do it. Going back to the voltage drops, I can count atleast 7 dual plug outlets from the breaker to the garage outlet making a loop around the living room and dining area before going downstairs so I agree with you that the wiring length combined with the resistance at the outlets themselves and possible use of backstab method is all adding up for the voltage drop seen. Do you think it is still unsafe to let the car charge at 8A/113V when there is no other load on the circuit? If there is a heat build up wouldn't the breakers trip first?

 

[Eric33432]

Do you think it is still unsafe to let the car charge at 8A/113V when there is no other load on the circuit? If there is a heat build up wouldn't the breakers trip first?

No, if your Tessie is drawing 8 amps, the breaker will not trip. It will not be aware of a loose connection heating up. The breaker is only aware of the current through the breaker, it does not have any way to look out on the circuit to detect a loose connection that is heating up, unless the loose connection is where the wire connects to the breaker itself (it is not psychic!).

If you feel confident in your ability, even if you are only going to be there for 11 months I would replace the 7 duplex outlets. It is not a very big job. For me this would be a simple 2 or 3 hour project, but I understand if you do not feel like this is something you would want to do.

If you do not want to do that you can ask your landlord but very likely your landlord will not take kindly if you ask him/her as most landlords do not like to do things to fix problems they do not perceive to exist.

Not being able to personally examine the situation, I would not feel comfortable advising you it is OK. I feel pretty anonymous here on teslamotorclub.com but I would not want to be sued if I was discovered to say this is OK and something bad happened.

But you can do whatever you feel is OK.

BTW, I am not a licensed electrician, but I have a master of science degree in Electrical Engineering.

 

[skyline385]

No, if your Tessie is drawing 8 amps, the breaker will not trip. It will not be aware of a loose connection heating up. The breaker is only aware of the current through the breaker, it does not have any way to look out on the circuit to detect a loose connection that is heating up, unless the loose connection is where the wire connects to the breaker itself (it is not psychic!).

If you feel confident in your ability, even if you are only going to be there for 11 months I would replace the 7 duplex outlets. It is not a very big job. For me this would be a simple 2 or 3 hour project, but I understand if you do not feel like this is something you would want to do.

If you do not want to do that you can ask your landlord but very likely your landlord will not take kindly if you ask him/her as most landlords do not like to do things to fix problems they do not perceive to exist.

Not being able to personally examine the situation, I would not feel comfortable advising you it is OK. I feel pretty anonymous here on teslamotorclub.com but I would not want to be sued if I was discovered to say this is OK and something bad happened.

But you can do whatever you feel is OK.

BTW, I am not a licensed electrician, but I have a master of science degree in Electrical Engineering.

So after some thinking on the matter, I am probably going to replace the first outlet in the circuit (the one where I read 120.8V under load) with a commercial outlet while wiring it properly this time and then use a 30ft 10/3 extension cord to the garage. I think this is the easiest solution and will give me power from right near the breaker without running through the loop in the living room. I will lose the temperature sensing from the UMC but I figure with using a commercial receptacle, those risks will be minimized and I will monitor the temps for the next few days. Since you know so much about this stuff, do you have any recommendations on the receptacle I should get.

 

[Eric33432]

Probably not good for many reasons, but it is probably better electrically than your alternative. But not ideal because who wants an extension cord running through the house, you will have to leave the door to the garage cracked, etc. I'm sure others here can add to the list.

Let us know what you end up doing. In the mean time, if you can post a photo of your panel and the circuit directory for the panel, we may be able to come up with some additional thoughts for you.

Leviton 15 Amp Commercial Grade Tamper Resistant Single Outlet, White R52-T5015-0WS - The Home Depot

It is easy to comply with the 2008 NEC mandate for TR Receptacles in all new residential housing. This requirement resulted from a Consumer Product Safety Commission study that reported alleged injuries

www.homedepot.com

Southwire 25 ft. 10/3 SJTOW AgriPro Farm/Workshop Heavy-Duty Extension Cord 64817501 - The Home Depot

AgriPro extension cords are constructed from heavy-duty copper wire and have oil resistant vinyl jackets to protect from chemicals and moisture, making it suitable for the rugged outdoors. These all-weather

www.homedepot.com

 

[skyline385]

Probably not good for many reasons, but it is probably better electrically than your alternative. But not ideal because who wants an extension cord running through the house, you will have to leave the door to the garage cracked, etc. I'm sure others here can add to the list.

Let us know what you end up doing. In the mean time, if you can post a photo of your panel and the circuit directory for the panel, we may be able to come up with some additional thoughts for you.

Leviton 15 Amp Commercial Grade Tamper Resistant Single Outlet, White R52-T5015-0WS - The Home Depot

It is easy to comply with the 2008 NEC mandate for TR Receptacles in all new residential housing. This requirement resulted from a Consumer Product Safety Commission study that reported alleged injuries

www.homedepot.com

Southwire 25 ft. 10/3 SJTOW AgriPro Farm/Workshop Heavy-Duty Extension Cord 64817501 - The Home Depot

AgriPro extension cords are constructed from heavy-duty copper wire and have oil resistant vinyl jackets to protect from chemicals and moisture, making it suitable for the rugged outdoors. These all-weather

www.homedepot.com

Here are photos of the panel, there are only two 15A breakers for lighting & outlets.

 

[Eric33432]

Well, I am glad to see that your kitchen, microwave, dishwasher, etc., are not also on those circuits. And there is a bathroom circuit and another one for bedroom. So not as bad as it sounded to me. And it looks like a fairly new panel, probably was recently replaced or is not original?

It looks like besides the receptacles these 2 15 amp circuits are also providing juice to all the lights! If you have just a normal amount of 120 volt devices and lamps, plus built in lighting, you will very likely overload the 15 amp circuit you are using to charge your Tessie.

Also, keep in mind that the overhead whilst charging is about 250 watts, maybe more, just to run the Tessie's electronics, so if you are charging at 8 amps, only about 5 amps, or about 0.6 kW are going into the battery. You will recover maybe 2 miles of range for each hour you charge. So very slow, and the car has to stay on all the time wasting electricity and wearing out pumps, etc.

Have you considered using your dryer outlet? There are many threads on this web site discussing that. Where is the dryer in relation your garage? You can use a one-in two-out splitter or better, a device like this:

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www.amazon.com

There are many other versions of this device. If you get one, be sure to get one with the correct plug for your existing dryer plug (NEMA 10-30 or 14-30).

This is still not ideal, unless your dryer is in the garage, but it will be much better than what has been previously discussed.

 

[skyline385]

Well, I am glad to see that your kitchen, microwave, dishwasher, etc., are not also on those circuits. And there is a bathroom circuit and another one for bedroom. So not as bad as it sounded to me. And it looks like a fairly new panel, probably was recently replaced or is not original?

It looks like besides the receptacles these 2 15 amp circuits are also providing juice to all the lights! If you have just a normal amount of 120 volt devices and lamps, plus built in lighting, you will very likely overload the 15 amp circuit you are using to charge your Tessie.

Also, keep in mind that the overhead whilst charging is about 250 watts, maybe more, just to run the Tessie's electronics, so if you are charging at 8 amps, only about 5 amps, or about 0.6 kW are going into the battery. You will recover maybe 2 miles of range for each hour you charge. So very slow, and the car has to stay on all the time wasting electricity and wearing out pumps, etc.

Have you considered using your dryer outlet? There are many threads on this web site discussing that. Where is the dryer in relation your garage? You can use a one-in two-out splitter or better, a device like this:

Loading…

www.amazon.com

There are many other versions of this device. If you get one, be sure to get one with the correct plug for your existing dryer plug (NEMA 10-30 or 14-30).

This is still not ideal, unless your dryer is in the garage, but it will be much better than what has been previously discussed.

The dryer unfortunately is not in the garage and is on the floor above. It would have been my first option otherwise. A coworker of mine also uses the splitter plug at the dryer for his Tesla.

Also, i just attached the 30ft 10/3 extension to the first outlet and now the voltage drop is within spec even at 12A. I will make sure to not power on any high wattage equipment while it’s charging and am also going to monitor the receptacle for heat. But this is just a temporary solution, need to figure out something which is more permanent.

 

[Cosmacelf]

If the outlet is near the panel, then simply adding a new 240V outlet near the panel with its own breaker would be an easy solution.

 

[tga]

Check the (open circuit voltage) before starting a charging session with your EVSE then go to another unloaded 120V outlet on a different circuit and check the voltage (the unloaded/open circuit voltages should be the same)

This only works if you're measuring on the same 120V leg/phase as the charging load. The reading on the other leg shouldn't change.

It is possible the outlets were wired with the backstab method:

Backstabbing is a newbie (or lazy pro) hack, IMHO. If I were writing the NEC, it would be banned.

 

[rrolsbe]

This only works if you're measuring on the same 120V leg/phase as the charging load. The reading on the other leg shouldn't change.


Backstabbing is a newbie (or lazy pro) hack, IMHO. If I were writing the NEC, it would be banned.

True, I did my voltage measurements on two 240V home run circuits.