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how does charger not blow circuit breaker?

rhumbliner

Member
Sep 24, 2015
673
814
Las Vegas
so I'm curious, if I don't need a hpwc to charge my tesla -- I can use a 240v/50a outlet, or a 240v/30a outlet, or even a 120v/15a outlet -- how do I not blow the circuit breaker?

if I plug an appliance (like an arc welder, say) that draws a continuous 40 amps at 240 volts into a 240v/30a outlet I will pop that circuit breaker in a few seconds. but I can plug a tesla into just about anything (with the proper adaptor) and it seems to draw only the current that the circuit can safely provide. how does it do this? must the mobile power cord be configured somehow? surely those small adapters don't contain the electronics to limit the current draw?

I'm hoping someone can explain this to me.

tia
 

djp

Model 3 Performance
Aug 28, 2011
1,120
60
Toronto, Canada
surely those small adapters don't contain the electronics to limit the current draw?

In fact, yes they do. The adapters each have a different resistor which identifies them to the Universal Mobile Connector. The UMC then signals the car how much current is available from that adapter, and the on-board chargers configure themselves to draw no more than that current. Pretty ingenious.
 

green1

Active Member
Mar 25, 2014
4,548
1,121
Calgary, Alberta, Canada
So, question, if you had a 14-50 outlet, but for some reason it was on a 30A breaker, could you add some form of a resistor somewhere to tell the UMC to dial it down? or do you really need the right plug and adapter?

(eg, many people have come up with charging adapters to let the UMC plug in to just about anything, usually they put a 14-50 outlet on their adapter and manually set max current in the car, but it feels like this could be automated by adding a specific value resistor somewhere?
 

davewill

Active Member
Feb 5, 2014
1,825
1,975
San Diego, CA, US
... usually they put a 14-50 outlet on their adapter and manually set max current in the car, but it feels like this could be automated by adding a specific value resistor somewhere?
Unfortunately, the resistor is molded inside the adapter, so you can't modify it without literally tearing the adapter apart. Someone could design and build a replacement 14-50 adapter with a switch on it to select amperage, but its not a garage type build.
 

green1

Active Member
Mar 25, 2014
4,548
1,121
Calgary, Alberta, Canada
My thought was more along the lines of an external resistor that would therfore modify the resistance overall seen by the UMC now of course this only works if you're looking for a lower resistance than the stock, but I don't know if that's the case.
 

reddy

Member
Jan 26, 2013
916
1,239
Amarillo, TX
The charge screen allows you to set the amps for that location.

So you can set 26 amps for a 30 amp breaker , or 34 amps for a 40 amp breaker.

Your car will remember the setting at that GPS location , so you only need to do it once.
 

green1

Active Member
Mar 25, 2014
4,548
1,121
Calgary, Alberta, Canada
The charge screen allows you to set the amps for that location.

So you can set 26 amps for a 30 amp breaker , or 34 amps for a 40 amp breaker.

Your car will remember the setting at that GPS location , so you only need to do it once.
I am completely aware of that. But that is not at all what i was suggesting. It should be possible to automate an adapter using this system so that you don't risk forgetting to set the limit at a new location using the adapter that you carry with you.
The only real caveats I see are that the resistance you add has to pass through the tesla supplied adapter, and it has to total a lower resistance than the stock value
 

Cosmacelf

Well-Known Member
Mar 6, 2013
8,573
20,574
San Diego
There is no easy way to modify the resistor value in a Tesla adapter. Each adapter has two sides - one side plugs into the wall receptacle and uses two current carrying contacts and one ground. The other side has five contacts, two current carrying contacts, one ground, and then two small signal contacts. It is this side that plugs into the UMC. To modify the resistor value, you'd have to intercept the two signal pins, and that part of the adapter is a very unique custom plug, so good luck!
 

djp

Model 3 Performance
Aug 28, 2011
1,120
60
Toronto, Canada
My thought was more along the lines of an external resistor that would therfore modify the resistance overall seen by the UMC now of course this only works if you're looking for a lower resistance than the stock, but I don't know if that's the case.

The resistor is mounted internally in the adapter, between the small pin and ground. There's nothing you can do externally to change the resistance the UMC sees. You'd need to open up the adapter, which isn't easy given that it's a single molded piece of plastic.
 

reddy

Member
Jan 26, 2013
916
1,239
Amarillo, TX
I am completely aware of that. But that is not at all what i was suggesting. It should be possible to automate an adapter using this system so that you don't risk forgetting to set the limit at a new location using the adapter that you carry with you.
The only real caveats I see are that the resistance you add has to pass through the tesla supplied adapter, and it has to total a lower resistance than the stock value

ok, I get it.

you want to create a method to change a setting on the adapter, rather than change a setting that already exists and does the same exact function on the car.

If you have lots of extra time on your hands and like messing with 240v, go for it.

I charge at a lot of places that are not meant for EV. RV parks, welder sockets, clothes dryer sockets, etc.

Each is unique, particularly RV parks, as a 50 amp service may blow at 36 amps. IMO having software
selectable amps that is sticky by location, like we already have, would be preferred. I don't think you have experienced the nuances of 240v charging 'in the wild'.
 

Cosmacelf

Well-Known Member
Mar 6, 2013
8,573
20,574
San Diego
Yeah, charging in the wild, you find some pretty wild things. Once I plugged into a NEMA 14-50 and found that power was disconnected after about about 30 minutes. So I traced the circuit back to the breaker panel. A bunch of 50 amp and 30 amp breakers, and the 30 amp one was flipped. Oops. So they had a NEMA 14-50 connected to a 30 amp breaker.

Now I know when charging in the wild, check out the breaker box before charging, and feel the breakers as you start charging. If you find a 30 amp breaker getting warm, dial it back :)
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
If you have a 14-50 on a 30 amp breaker you should change the outlet to a NEMA 10-30 and just use that adapter from tesla.

This is illegal. No 10-30's may be installed since NEC 1996, you may only replace existing receptacles with 10-30's. Now, is it safe? Yes. But there may be liability and/or insurance implications.

- - - Updated - - -

that's a really bad idea.

It's actually not that much of a bad idea, and frequently you might find a 6-50 receptacle on a 30A or 40A OCPD, even sometimes on #12 or #10 wire -- this is legal for arc welders. As long as the 14-50 is the only receptacle on the circuit, it can be on any breaker size, provided that you don't plug in an appliance that is rated for more than the breaker. From a Code perspective, the Tesla UMC with its 14-50 adapter requires a 50A conductor and breaker, because its installer-configured nameplate rating with that adapter requires a 50A circuit (regardless of whether you can manually adjust the current). If there are multiple 14-50's on the circuit, it must be a 40A or 50A breaker.
 

rhumbliner

Member
Sep 24, 2015
673
814
Las Vegas
It's actually not that much of a bad idea,

let me say up front that I'm not an electrical engineer nor a licensed electrician, but it's seems to me that when someone sees a 14-50 outlet they would naturally assume that this is a 50-amp circuit, i.e., 50-amp breaker with wire beefy enough to safely handle the load, like #6 wire. isn't placing a 14-50 receptacle on a 30-amp circuit a dangerous form of false advertising?
 

Cottonwood

Roadster#433, Model S#S37
Feb 27, 2009
5,088
166
Colorado
let me say up front that I'm not an electrical engineer nor a licensed electrician, but it's seems to me that when someone sees a 14-50 outlet they would naturally assume that this is a 50-amp circuit, i.e., 50-amp breaker with wire beefy enough to safely handle the load, like #6 wire. isn't placing a 14-50 receptacle on a 30-amp circuit a dangerous form of false advertising?

I've encountered 10-50's with 40 Amp breakers and 14-30's with 20 Amp breakers when living off the land in my Teslas. Whenever my charging suddenly stops and there is no Voltage on the outlet, I go searching for the breaker box. More often than not, it's a breaker smaller than the outlet would indicate.

Using a 40 Amp breaker with smaller wire on a 10-50, feeding a kitchen electric range, was a common way to save money. The 20 Amp breaker feeding the 14-30 was for a high efficiency European Dryer than only drew 16 Amps. In those cases, I just dialed the car's current to 32 and 16 Amps respectively. The charging was a little slower, but worked.
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
let me say up front that I'm not an electrical engineer nor a licensed electrician, but it's seems to me that when someone sees a 14-50 outlet they would naturally assume that this is a 50-amp circuit, i.e., 50-amp breaker with wire beefy enough to safely handle the load, like #6 wire. isn't placing a 14-50 receptacle on a 30-amp circuit a dangerous form of false advertising?

Not necessarily. Making the assumption that it's a 50A circuit is the dangerous part. Many ranges have a 14-50 receptacle with a 40A breaker. Arc welders are permitted to have undersized wires *and* breakers.

Now, with that said, yes - if you're going to install a receptacle, then you should probably invest in conductors and OCPD for the full 50A, but you have to pay attention to the nameplate capacity of the appliance that will use it. For example, there are welders that have a 6-50 plug but state a maximum breaker size of 40A - you may not use a 50A breaker on them.
 

Brass Guy

Active Member
Jan 5, 2014
1,155
943
Holbrook, MA
Some time ago I was considering if there was a way to do this. (Alter the adapter's current signal.)
I don't remember where I found this, and am unsure of its accuracy:
40 amps - 9.08k ohms
24 amps - 33.16k ohms
16 amps - 75k ohms
12 amps - 140k ohms

So the problem is that while I could easily add a resistor in parallel without wrecking my 14-50 adapter, that would reduce the resistance. You need to increase the resistance in order to specify a lower charge current.
You could put 160k ohm resistance parallel to the 140k of the 5-15 and use it in a 20A outlet. I've never tried that. I hope I never have to rely on 120V charging.
 

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