NEMA 1-15 120V charging without ground

hiroshiy said:

1) use UMC, NEMA 5-15 adapter and 3-prong-to-2-prong adapter (so ground pin open and not connected).

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i know for sure this does not work. I've tried it myself with that adapter. you'll get the red ring because theres no ground. I had to find different outlets that were actually grounded.

yobigd20 said:

i know for sure this does not work. I've tried it myself with that adapter. you'll get the red ring because theres no ground. I had to find different outlets that were actually grounded.

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How about bridging ground to Neutral? The Model S UMC should be unable to tell the difference.

deonb said:

How about bridging ground to Neutral? The Model S UMC should be unable to tell the difference.

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I did not attempt this. Old house. And I'm not an electrician so I wasn't going to mess with it (plus its a lease, not my house..don't want to burn the house down, haha)

yobigd20 said:

I did not attempt this. Old house. And I'm not an electrician so I wasn't going to mess with it (plus its a lease, not my house..don't want to burn the house down, haha)

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That wont burn down your house - that's not the purpose of the ground wire. Instead, you'll just electrocute yourself .

A house with 1-15s might even have aluminum or cloth covered wires. It also might have fuses instead of breakers. Unless you can access the panel directly and make proper accommodations, I wouldn't risk it.

If you're going to tie ground to neutral, I would bump it down to 5A.

Tying the NEMA 5 ground to the NEMA1 neutral will indeed make it work in most cases, however it's extremely dangerous unless you know for sure that the receptacle is polarized correctly. If the receptacle is wired backwards - as many were prior to polarized receptacles, required starting in 1948 - you'll then leave equipment ground at ~120V relative to earth. That's bad.

Don't those two-prong outlets usually have that ground screw on the lower half?

Chris TX said:

A house with 1-15s might even have aluminum or cloth covered wires. It also might have fuses instead of breakers. Unless you can access the panel directly and make proper accommodations, I wouldn't risk it.

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Code introduced grounded receptacle requirements for new construction in 1962, and has required any NEMA1's to be replaced with NEMA5 since 1974, so you shouldn't find 1970's aluminum wiring behind a NEMA 1- but a 2-prong outlet is a reasonable indicator of knob-and-tube wiring, which will be taxed at 12A.

(Note that today's aluminum wiring isn't bad at all - it acts much like copper and works just fine.)

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Koryin said:

Don't those two-prong outlets usually have that ground screw on the lower half?

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The faceplate has a screw - and adapters typically had a wire that attached to the receptacle cover plate screw which was usually grounded (required since 1960's). But the box was required to be grounded, and sometimes the wiring wasn't there.

Hmm, that's interesting. In the Tesla Model S section of EVSEAdapters.com, they used to have a device they called the "cheater plug", which you could use on ungrounded outlets to fake the ground for the Tesla UMC. It is gone from that page now.

FlasherZ said:

Tying the NEMA 5 ground to the NEMA1 neutral will indeed make it work in most cases, however it's extremely dangerous unless you know for sure that the receptacle is polarized correctly. If the receptacle is wired backwards - as many were prior to polarized receptacles, required starting in 1948 - you'll then leave equipment ground at ~120V relative to earth. That's bad.

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Very important to test these first!

The original part of my house was built around 1950 with an addition from about 1959, so it mostly had 2 prong polarized outlets. One of the first things I did was to test each one for proper polarity. Found several wired backwards. Even in the few places where there were 3 prong outlets, a couple had open grounds, etc. (No GFI.) I corrected things as best I could. No K&T. Just that cloth covered "Romex" that had 2 conductors with no separate ground wire. A few boxes had a separate ground wire tied to a cold water pipe. All gone now, after we rebuilt and expanded the house a few years ago. (Still have a Zinsco sub-panel in the garage which will need to disappear if I decide to use a branch off it for EV charging. Have a modern 14-50 off the main panel I will be able to use in the meantime.)

For a polarity tester, I used one of the yellow testers available at any hardware store. Plugged it into a polarized 3->2 adapter. Then tied the dangling green ground wire from the adapter to a nearby cold water pipe via a length of stranded green wire. Used the same setup for the room my daughter rented in college, also in an older home, and found one outlet wired backwards there too!

I recommend making a ground adapter with a "Y" consisting of two 100k 1/2w resistors, such that the bottom of the "Y" (2 resistor common) goes to ground and the top of the "Y" resistors go to neutral and hot (line) respectively. This puts the floating ground in the middle, which is safer than trying it one way or the other. This will also enable charging on generators and inverters as long as they are stable enough. This circuit will safely spoof the ground verification in the UMC, but will not introduce a serious shock hazard should current flow from a fault, and will put the floating somewhere in the middle, which is usually about 60V in the US or 50V in Japan. This is probably what's inside the "Prius adapter".

You can get a male 1-15 plug and a 5-15 socket and wire them together with the resistors inside one end. I also made one out of an old surge protector cube, leaving the surge protection MOV intact so it does double-duty.

hiroshiy said:

Great idea! So this "ground fake" adapter should work with 100V or 200V...

So today I tried to use a cheater plug (no ground connection) with Prius PHV charging adapter for 100V, and my Tesla charged from 2P 100V wall outlet successfully. It started charging at 92V 12A (wow), and a few minutes later Tesla lowered the current to 9A, then the voltage somewhat recovered to 95V. 0.855kW or 3km per hour.

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So you're looking at around 100 hours to charge an 85! Ouch!

Yes, the "Y" configuration will also enable groundless charging on 200V in Japan. (208/240v here)

Ingineer said:

I recommend making a ground adapter with a "Y" consisting of two 100k 1/2w resistors, such that the bottom of the "Y" (2 resistor common) goes to ground and the top of the "Y" resistors go to neutral and hot (line) respectively. This puts the floating ground in the middle, which is safer than trying it one way or the other. This will also enable charging on generators and inverters as long as they are stable enough. This circuit will safely spoof the ground verification in the UMC, but will not introduce a serious shock hazard should current flow from a fault, and will put the floating somewhere in the middle, which is usually about 60V in the US or 50V in Japan. This is probably what's inside the "Prius adapter".

You can get a male 1-15 plug and a 5-15 socket and wire them together with the resistors inside one end. I also made one out of an old surge protector cube, leaving the surge protection MOV intact so it does double-duty.

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Hi Ingineer!
This sounded like a good idea to have in my box of adapters so I made one (using 120K resistors since that's what I had laying around) but it doesn't seem to work. I get a red light on the UMC so no go... do you think I should try again with 100k resistors? Doesn't seem like that would make much difference.

mspohr said:

Hi Ingineer!
This sounded like a good idea to have in my box of adapters so I made one (using 120K resistors since that's what I had laying around) but it doesn't seem to work. I get a red light on the UMC so no go... do you think I should try again with 100k resistors? Doesn't seem like that would make much difference.

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I just tested my current UMC. On 240v, even 130k is fine, but on 120v it needed to be around 64k before it enabled. When I use this, it's always for 240v, so I never tested on 120v. You could try paralleling two 120k's each. That would give you 60k, which should work. Note that with this low value, the resistors could be called on to dissipate up to about 1/4w total on each side, so it's a good idea to use 1/2w units. (also for the voltage safety, since larger resistors have better spacing) Also, in the event of a ground fault, up to 2ma of current could flow. Not enough to really be dangerous, but you'd definitely be able to feel it if you were part of the circuit!

Ingineer said:

I just tested my current UMC. On 240v, even 130k is fine, but on 120v it needed to be around 64k before it enabled. When I use this, it's always for 240v, so I never tested on 120v. You could try paralleling two 120k's each. That would give you 60k, which should work. Note that with this low value, the resistors could be called on to dissipate up to about 1/4w total on each side, so it's a good idea to use 1/2w units. (also for the voltage safety, since larger resistors have better spacing) Also, in the event of a ground fault, up to 2ma of current could flow. Not enough to really be dangerous, but you'd definitely be able to feel it if you were part of the circuit!

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Thanks for checking on this for me.
I have a very limited supply of 1/2w resistors but was able to cobble together a 68k-62k Y and this still gave me a red light. I then tried 47k to the white wire and I would expect this would work when connected properly but gave me the red light both orientations. I'm thinking I may have a later version of the UMC (part number ends in 99-B) where they have tightened up the ground check.
Where do you find 240v on a 1-15 socket?

You cannot connect it to only the white wire! If you want this to work and be safe, it has to have the "Y"; TWO resistors of equal value, 60k is what I recommend, one going from AC Neutral to the EVSE ground pin, and one going from AC Hot to the EVSE ground pin. The balance is important and also insures the body of the car doesn't become a shock hazard.

You can also put two 1/4w resistors in series at half the value of ohms on each to end up with a 1/2w. Or put two 1/4w's in parallel at twice the value.