nema 14-50 and setting chargerate at 20A

nema 14-50 and setting chargerate at 20A

Hi -

I have a weird problem - charging my car using a nema 14-50 at 40A works perfectly fine, zero problems.

Charging it at 10A (with the nema 14-50) works just fine as well (in the UI change it from 40->10) but if I try 20A and plug the car in, the light turns red and the the little black chargecable box's LED also turns red (instead of green lights).

The only way to fix it, is by turning the breaker off/on. Pulling the cable out and plugging it back in doesn't seem to work. Very strange.

Anyone else had this problem? Not like I really need to be able to do 20A but it's a curious thing.

jat | 14 September 2013

That sounds pretty weird - can you try a different UMC?

tes-s | 15 September 2013

Or a different 14-50 outlet somewhere else with your UMC?

ye | 15 September 2013

When you say that pulling the cable out and plugging it back in doesn't help, do you mean pulling it out of the car or pulling it out of the wall? I'm sure that the UMC can't tell the difference between being unplugged from the wall and having the circuit breaker turned off.

unclegeek | 15 September 2013

+1 ye

lnxcar | 15 September 2013

ye : unplugged wallside and carside both. I did try with a different umc and the same thing happened.

in fact, when it turned red 2 days ago, I unplugged the umc, plugged in my spare and it instantly turned red as well (not plugged into car at that point) and then I reset the circuit breaker and it worked again.

I do not have another wall plug to try. I should ask electrician it's very odd.

tes-s | 15 September 2013

Could be a bad breaker, and the UMC does not like the power it is seeing. As @ye said, pulling the cable out of the wall and flipping the breaker "should" look pretty much the same to the UMC.

You could try plugging into another 14-50 and see if you have any issues, but my money is on the breaker. It is an inexpensive part, and if it was supplied and installed recently by an electrician, theyt may replace it at no charge.

ye | 15 September 2013

Hmm. I guess I shouldn't have been so sure...

I agree with tes-s. It does sound like something's wrong with the circuit breaker, although I don't know exactly what sort of failure would cause these symptoms.

If that's correct, it's just a coincidence that the problem happened when you set the car for 20 amps. Did you try that more than once?

lnxcar | 15 September 2013

yes at least 3 or 4 times over the last few months. 10A and 40A been 100% reliable.

ye | 15 September 2013

Ok, so 10 amps and 40 amps always worked. What about 20 amps? Did it fail all three or four times you tried it, or only once?

DonS | 16 September 2013

- If the voltage is low, a message appears on the console, which you did not mention so i assume that is not the trouble.
- The UMC does not know anything about the breaker, and the UMC lights are still on, so it is not the breaker.

I think it is more likely ground noise. The UMC puts a small current trough the ground to verify there is a ground connection. Most likely either the ground is corroded or there is other equipment injecting noise onto that ground.

bsmytti | 16 September 2013

you guys realize a breaker is just a switch with a thermal and a magnetic device that will open under overcurrent or shortcircuit conditions, the real question is why would you try to charge a batter pack the size our cars have with 10 or 20 amps ??? there you will use the same amount of power just how long it takes, there are some possible heat losses but that is minimal, sounds to me like the processor is seeing a square peg in a round hole for lack of a better term.

ye | 17 September 2013

How could cycling the circuit breaker fix a corroded or noisy ground? The circuit breaker controls the two hot legs. The neutral and the ground are always connected.

Thomas N. | 17 September 2013

The heat losses are not minimal.

tes-s | 17 September 2013

The observed symptoms do not make sense - cannot be explained by anything other than gremlins.

Therefore, the appropriate way forward is to try changing components. My guess was circuit breaker, but I could be wrong.

One other question - does the UMS give any indication of what is wrong, or just a "fault" light? Does the fault light blink a code of some sort to indicate what type of fault it sensed?

My next step would be to try the car and UMC at another 14-50 outlet. Most Tesla service centers and sales locations have them. If that works, then we can be sure it is something with the 14-50 circuit at the house. If it does not, then the service center can diagnose and fix it - either the car or UMC.

Once you are sure it is the 14-50, then you can let an electrician fix it.

DonS | 17 September 2013

Ground noise is the technical name for gremlins. A hard reset reset via the breaker will reset the GFI protection circuit on the UMC. Momentarily unplugging the UMC would achieve the same result.

ye | 18 September 2013

But he said that he did unplug the UMC, and plugged in his spare UMC instead, and that one immediately lit up red too. Then he cycled the circuit breaker, and that fixed the problem.

ye | 18 September 2013

The UMC does blink its red light to indicate what's wrong. Here's the manual:

pylt | 18 September 2013

bsmytti wrote (SEPTEMBER 16, 2013):
you guys realize a breaker is just a switch with a thermal and a magnetic device that will open under overcurrent or shortcircuit conditions, the real question is why would you try to charge a batter pack the size our cars have with 10 or 20 amps ??? there you will use the same amount of power just how long it takes, there are some possible heat losses but that is minimal, sounds to me like the processor is seeing a square peg in a round hole for lack of a better term.


Frankly, we should all be charging at the maximum voltage possible, AND the MINIMUM AMPS.


Because from my 1 year of EE classes I learned that line loss is a function of AMPS, not voltage. Here's some info on the calculation:

But here's the take home:

"This presents a fundamental axiom of electrical theory: line loss is greatly decreased by an increase in voltage."

In other words, AMPS are bad, volts are good.

In our experience, dropping our amps to 20 from 40 results in a far cooler power cord, cooler NEMA 14-50 plug outlet, and less wasted energy. We may even drop it to 15 or 10 amps to see how that works for us. Realistically, it should have no impact--we drive home and plug the car in overnight. Adding a few hours to the charge time is of no consequence.

In the interest of saving even more electricity, I recommend you try this out as well.

tes-s | 18 September 2013

Maybe the original poster could tell us what the UMC sees as the problem by idenfifying the blinks.

reitmanr | 18 September 2013

I have no idea why 20A produces a fault. Hope the problem is resolved and we all hear what the heck is going on.

I got excited about the idea of saving energy by charging at 20 A vs 40 A. Then I realized we are trying to save energy, which means that at lower currents we run longer, and the energy nets out the same, IVT, kwh. Am I missing something? Hmmmm.

DTsea | 18 September 2013

Yes @reitmanr you are missing something.

Heat wasted is proportionate to Current squared * resistance.


Case 1- 240V, 40A- 9.6kW at the wall, for one hour.
Case 2- 240V, 20A- 4.8kW at the wall, for TWO hours.

The BATTERY will see less than the wall power because of heat loss in the cables and internal wiring (exposed to the higher current). That loss will be FOUR TIMES as much at 40A as at 20A.

So, the battery- having pulled 9.6 kWh in both cases- should be at a higher charge state (less waste) with the lower current. How MUCH the absolute loss is depends on the resistance of the system though- if low, the difference is trivial. If high, then not trivial.

Hope this helps.

Bighorn | 18 September 2013

The instantaneous heat loss will be 4 fold higher at 40 amps, but twice as high for the full charge because of duration.

LMB | 18 September 2013

(lmb spouse) @pylt We have a 125 foot run of #6 from the panel to the 14-50 socket in the detached garage. I find that the charge time is somewhat less than twice as long when charging at 20 amps, because line losses are several volts less.

However, Tesla's charge calculator shows more kilowatt-hours used when charging at 24 amps (e.g. 30 amp dryer socket) than at 40 amps (14-50 socket). In fact, the calculator shows substantially more kW-hr when charging from a 120 volt socket. This probably indicates that there is a constant load, perhaps from the battery cooling, in addition to the basic charger load.

We've compromised by charging at 24 amps, as this keeps the UMC much cooler and cuts wasted energy by close to 40% (wasted power is proportional to 24/40 squared, but charge time is longer by around 40/24).

TeslaWeasel | 19 September 2013

Another thing to consider in the 'time vs amps' decision is the possible impact of multiple high current draws all starting up and or running simultaneously during the night. If Teslas become as popular as we all hope, having 10% of homes in a low density neighborhood all drawing max amps simultaneously could start impacting infrastructure and the timing to upgrade it. Dropping the current to 20 or 25, along with changing the default delayed start charge time (to avoid current surge at midnight) helps keep the voltage from dipping (maintaining efficiency), lowers heat losses and staves off infrastructure upgrades.

Add to that the trend (at least here in CA) of switching to Smart Meters and tiered rates depending on time of use causing people to push more of their high current appliance usage (dishwasher, washer, dryer, pool pump etc) to off peak hours.

I changed our charge current to 20 and it has not impacted us except to reduce heat losses (that cable got quite hot when drawing 40 amps).

I realize 10% of homes having plug-in EVs is a crazy number I pulled from the ether, but:

1. I'm already planting the seeds with spouse about adding another Tesla

2. There are several homes in this neighborhood with 2 and 3 plug-in EVs

3. My average MBT (Miles Between Teslas) lately has been around 3-5 and dropping. [NOTE: The MBT is calculated using sightings of Teslas on the road and not in owner driveways. The correct metric to use when counting just Teslas in their driveway is TPA (Teslas Per Acre).]

lnxcar | 19 September 2013

I will 'reproduce' and check the red light flashes on the mobile connector.

Brian H | 20 September 2013

The cable should not be hot at 40. Heat comes from one thing only: resistance. Where is it?