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How to test standard 110/120 V outlet?

tameware

Member
Nov 11, 2019
5
2
Keystone, CO
I park in a communal garage with a dozen or so standard outlets. I can charge my Model 3 successfully at two of them - I get about 4 miles of range per hour. For the others, the car complains that charging is not possible. I've measured the voltage for the working and non-working outlets - both are roughly 115 VAC per my VOM. I've purchased a $5 outlet tester and it reports that all the outlets are wired correctly. What else should I look at? I'd like to be able to report a specific issue to management rather than "Won't charge my car." They have granted permission to use their power, but otherwise are mostly hands-off. I don't mind purchasing a more sophisticated testing device if it's likely to tell me something useful.
 

gavine

Petrol Head turned EV Enthusiast
Apr 1, 2014
2,577
2,112
Philadelphia, PA
I'm no electrician (or an expert) but it could have something to do with voltage-drop. Perhaps there are other loads on that circuit that are drawing amps? Are all 12 of the outlets on separate circuits? I'm guessing not.

Have you tried dropping the charging amps down just to see if it'll then allow charging? If so, then it's likely voltage drop. If not, then there must be something in the circuit that Tesla just doesn't like.
 
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srs5694

Active Member
Jan 15, 2019
1,024
1,148
Woonsocket, RI
Does it refuse to charge from the moment you try to charge, or does it begin charging and then stop and complain? If the latter, it could be that the Mobile Connector is detecting a fault such as an excessive drop in voltage as the car tries to draw more amps. (The exact wording of the error message may provide a clue, too, so take note of that.) If it's refusing to charge at all, then that suggests a grounding fault or something similar. (Again, the exact wording of the error message may provide useful information.) It's possible that Tesla could look at the logs and tell you something more specific, so you might try calling them for help.
 

Rocky_H

Well-Known Member
Feb 19, 2015
5,990
6,909
Boise, ID
My two initial thoughts would be disconnected ground or the hot and neutral wires being swapped, but even a cheap basic outlet tester would flag either of those two issues.
 

gavine

Petrol Head turned EV Enthusiast
Apr 1, 2014
2,577
2,112
Philadelphia, PA
Are you using an extension cord? If so, you'll need a 12 gauge (expensive) one.

I actually use an extension cord because the outlets in our parking structure are upside down (ground on top) and the UMC is not designed to plug-in that way. The cord I use is only 1ft long. I'm not sure if it's 12 guage or not, but it is rated at 15A. It definitely gets warm to the touch and I have had the car drop the amps down after charging for a long time. There was a message that mentioned something about an extension cord being detected.
 

Rocky_H

Well-Known Member
Feb 19, 2015
5,990
6,909
Boise, ID
I actually use an extension cord because the outlets in our parking structure are upside down (ground on top) and the UMC is not designed to plug-in that way. The cord I use is only 1ft long. I'm not sure if it's 12 guage or not, but it is rated at 15A. It definitely gets warm to the touch and I have had the car drop the amps down after charging for a long time. There was a message that mentioned something about an extension cord being detected.
If you don't know, you really should check, and you probably should be turning the amps down on that if you can feel it's warm already, and the car is detecting a lot of voltage drop or weak connection. That's already not a good sign. It would give that about 1% chance your little extension uses 12 gauge wire. It's probably (shudder) 16 gauge, which isn't good for running 12A continuous for many hours at a time. When I first looked at this when I got my car, I checked the two "beefy" looking "heavy duty" extension cords I had in my garage, thinking they would be sufficient--nope--both 16 gauge. So I ordered a pretty stiff 12 gauge cord from Amazon. (bright yellow with LED lit ends)

Obviously you do what you need to to get plugged in, but I would probably turn down the amps to 9 or 10 if I were using that little 1 ft extension that you say is getting hot.
 

M3BlueGeorgia

Active Member
Dec 10, 2018
1,342
1,068
Atlanta, GA
I actually use an extension cord because the outlets in our parking structure are upside down (ground on top) and the UMC is not designed to plug-in that way. The cord I use is only 1ft long. I'm not sure if it's 12 guage or not, but it is rated at 15A. It definitely gets warm to the touch and I have had the car drop the amps down after charging for a long time. There was a message that mentioned something about an extension cord being detected.
Oh yes, you need a heavier duty extension cord.

However, be wary of meaningless descriptions like "heavy duty".
Specifically get a 12 gauge extension cord. It won't get warm and the car won't downgrade the amps.
 
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gavine

Petrol Head turned EV Enthusiast
Apr 1, 2014
2,577
2,112
Philadelphia, PA
Oh yes, you need a heavier duty extension cord.

However, be wary of meaningless descriptions like "heavy duty".
Specifically get a 12 gauge extension cord. It won't get warm and the car won't downgrade the amps.

Thanks guys. I checked the cord and it doesn’t have a legible label on it so I ordered one of these 12awg 20A cords (pictured below). The garage outlets are NEMA 5-20 so it’s compatible. I probably should but the Tesla NEMA 5-20 adapter for my UMC but the 12A charging that I do now is plenty fast for the daily commute.

upload_2019-11-12_22-26-46.png
 

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Apr 20, 2017
154
56
Arizona Territory
Thanks guys. I checked the cord and it doesn’t have a legible label on it so I ordered one of these 12awg 20A cords (pictured below). The garage outlets are NEMA 5-20 so it’s compatible. I probably should but the Tesla NEMA 5-20 adapter for my UMC but the 12A charging that I do now is plenty fast for the daily commute.

View attachment 476392

There's lots of concerns with adapters, EVSE, and cords on Amaz○n. That being said, there's a guy, Rush, with TucsonEV and Tony, with QuickChargePower.com that could probably make something quality and reasonable for you.

Case in point: the swell, quality stubby extension cord in the photo specifically says 15A on it!

Now, why and when would it be okay to send 16 Amps continuously through something rated for 15 Amps? Exactly! Never!

Avoid using an extension cord whenever possible, period. When you do, know your ratings and ideally, dial back the amperage levels via the car.

Side note: After years charging on 120V I decided to check the temperature of the junction box and conduit. Long story short, there was stranded 12V automotive wire to the junction box and receptacle I had been plugged into. Fortunately, a dear friend had leftover Romex and I rewired it all and replaced the receptacles at both boxes.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
5,990
6,909
Boise, ID
There's lots of concerns with adapters, EVSE, and cords on Amaz○n. That being said, there's a guy, Rush, with TucsonEV and Tony, with QuickChargePower.com that could probably make something quality and reasonable for you.
Unnecessary. It's just a stubby extension, and if you know the wire gauge, you know the capabilities.

Case in point: the swell, quality stubby extension cord in the photo specifically says 15A on it!

Now, why and when would it be okay to send 16 Amps continuously through something rated for 15 Amps? Exactly! Never!
You are mixing this up. I couldn't tell you why it has that 15A text molded onto it. That is an error. It is using 12 gauge wire, and it is using a 5-20 plug and receptacle.
Ampacity Charts - Cerrowire
So it is not "something rated for 15 Amps". It is properly rated for 20 Amps. So yes, running 16A continuously through that would be proper.
I guess you could question if they were stamping that "15A" text on there as a CYA measure, can you really trust what gauge of wire is in it? That's a level of suspicion I wouldn't go to.
 
Apr 20, 2017
154
56
Arizona Territory
Unnecessary. It's just a stubby extension, and if you know the wire gauge, you know the capabilities.


You are mixing this up. I couldn't tell you why it has that 15A text molded onto it. That is an error. It is using 12 gauge wire, and it is using a 5-20 plug and receptacle.
Ampacity Charts - Cerrowire
So it is not "something rated for 15 Amps". It is properly rated for 20 Amps. So yes, running 16A continuously through that would be proper.
I guess you could question if they were stamping that "15A" text on there as a CYA measure, can you really trust what gauge of wire is in it? That's a level of suspicion I wouldn't go to.
So... we will assume that the 15A marking is a typo and that there's actually 12/3 wire in there and running 16A continuously is "fine."

Bet it isn't UL rated, that it is made in China and has no manufacturer associated with it. Oh, and if there is a problem, good luck with having insurance cover it.

Again, as per Tesla, it's best to avoid using an extension cord. Alternatively, do whatever you want.

And look, there's a circle on the plug which looks like it might be a UL rating. Unfortunately, it isn't and might just be a Chinese symbol of some kind.

Also, from my own personal experience, I bought a US made 12/3 short cord for charging an electric motorcycle and even that got uncomfortably warm.
 
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doghousePVD

My grandfather’s car
Dec 3, 2018
583
468
New England, USA
Thanks guys. I checked the cord and it doesn’t have a legible label on it so I ordered one of these 12awg 20A cords (pictured below). The garage outlets are NEMA 5-20 so it’s compatible. I probably should but the Tesla NEMA 5-20 adapter for my UMC but the 12A charging that I do now is plenty fast for the daily commute.

View attachment 476392

That picture shows a cable too small to be 12 awg wire.
 
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doghousePVD

My grandfather’s car
Dec 3, 2018
583
468
New England, USA
Unnecessary. It's just a stubby extension, and if you know the wire gauge, you know the capabilities.


You are mixing this up. I couldn't tell you why it has that 15A text molded onto it. That is an error. It is using 12 gauge wire, and it is using a 5-20 plug and receptacle.
Ampacity Charts - Cerrowire
So it is not "something rated for 15 Amps". It is properly rated for 20 Amps. So yes, running 16A continuously through that would be proper.
I guess you could question if they were stamping that "15A" text on there as a CYA measure, can you really trust what gauge of wire is in it? That's a level of suspicion I wouldn't go to.

That plug is a 5-15 which is only rated for 15A. A 5-20 plug does not have parallel blades. Good 5-15s can handle 20A; cheap ones definitely can’t. I had a cheap one that melted and caught fire.
 
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gavine

Petrol Head turned EV Enthusiast
Apr 1, 2014
2,577
2,112
Philadelphia, PA
I can’t believe I didn’t notice that it was stamped with 15A on it. I’m not going to use it with a 5-20 adapter so hopefully it’s better with the 5-15 adapter vs. what I’m currently using. I’ll,report back when I get it and let you all know what it’s like.
 

gavine

Petrol Head turned EV Enthusiast
Apr 1, 2014
2,577
2,112
Philadelphia, PA
I reached-out to the seller to ask about this. Here's the response that I got. I can't wait to see what I actually receive -->

Hi buyer

this is 12AWG SJTW cord, it is 20amp male to 20amp female T blade power cord, it works in 125V 20Amp, the picture of the female with 15Amp mark is not correct, the item you received with 20A mark on the female side.

it is our person edit the picture use 15A receptacle,


sorry for this

Lynn

 

ajdelange

Active Member
Dec 10, 2018
1,077
540
Virginia/Quebec
I park in a communal garage with a dozen or so standard outlets. I can charge my Model 3 successfully at two of them - I get about 4 miles of range per hour. For the others, the car complains that charging is not possible. I've measured the voltage for the working and non-working outlets - both are roughly 115 VAC per my VOM.

When charging is initiated the car checks the impedance between the charger and the source (transformer) and if it is high assumes that something is wrong and won't allow charging in order to reduce fire risk. My guess is that this is what is happening here i,e. the two outlets that work have lower impedance to the source than the ones that don't. You can verify this with a voltmeter and a load such as a 1 kW portable heater. Measure the voltage at the outlet with no load and then do it with the load on. The impedance is the ratio of the voltage drop to the current drawn. A 1 kW heater draws about 8 amps so if the voltage drop is 1 volt the impedance is 1/8 Ω. The actual numbers aren't that important. If the working outlets show a 1 V drop under load and the non working ones 3 V then my thesis is correct and the cause is found.

The trick here is to come up with some way of safely measuring the voltage with the load connected. I leave you to come up with a method.
 

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