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Suddenly can't charge above 24 amps?

Hi Everyone,

I've got a NEMA 14-50 outlet in my garage and have been charging my month old Model S with no issues from day 1. However, yesterday I noticed that the breaker had popped at my house and ever since I switched it back on my Tesla will only charge at 24 amps max. In the car settings I can change the setting to 40 amps, but moments after plugging in the cord it drops down to 24 amp. Any ideas what's going on?
 

gregd

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Dec 31, 2014
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Is the voltage reported by the car still correct (normally 240v +/- a volt or two)? I'd check to see if the breaker is physically getting warm. It might be that the contacts aren't quite seated right, which could be "fixed" by cycling it manually once or twice. Or, the breaker could be faulty.
 
Do you have a 50 amp breaker on that circuit? What is its value? What is the wiring capacity?

Some 14-50 installations are actually wired as if they only supply 40 amps, not 50. Depending on code application, it was acceptable to use a 40 amp 8 gauge copper circuit and a 14-50 receptacle. Correct use is 6 gauge copper and a 50 amp breaker. Since charging is considered a continuous load you are only supposed to draw 80% of the breaker capacity which is then 40 amps. If you have a 40 amp circuit your continuous load capacity is 32 amps.

Your scenario (blown breaker and car drawing 40 amps) would be unlikely with a 50 amp circuit

My understanding... the car uses location to guide itself on capacity of the circuit. If you had an event during a session (i.e. the blown circuit breaker), the car will reduce and remember the current setting for that location. I don't know how you reset this other than several successful sessions. Maybe a UI or car reset or a power down/restart?
 

BerTX

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May 2, 2014
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Seems likely that you have a bad connection or a bad breaker. My guess is a bad connection. You might try changing the amps before plugging in, then plug in and quickly go watch the voltage drop.

But you have a problem in the circuit and you should get that fixed.

Edit: regrettably, electricians do not always use new breakers
 
I had a similar situation. I found a loose connection at the terminal block in the HPWC where the main power lines connected. If you have a loose connection anywhere in the circuit from the breaker box to the HPWC, the car will sense the voltage drop and throttle back your current.
 
The thing is it’s showing 239 volts (just checked)... so that’s the right voltage it’s just not pulling more than 24 amps.
 

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BerTX

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May 2, 2014
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The thing is it’s showing 239 volts (just checked)... so that’s the right voltage it’s just not pulling more than 24 amps.
As the system is loaded, the voltage will drop. The car sees the voltage drop and reacts by lowering the amperage until the voltage drop is acceptable to it. That's why I recommended disconnecting the car and raising the amps again. Then when you plug it in, the voltage will drop. The voltage dropping indicates that it can't handle that much current without a problem, so it reduces the amps. The voltage reading is OK because it reset the current.

Think of it as a garden hose with a partial kink in it. You can still spray a long distance with the kink in it, but you have to put your finger over the end to reduce the amount of water leaving the hose to get the pressure up. The car is reducing the current to get the voltage up.
 
if what everyone above says is correct and I’ve got a bad breaker or have some issue with the way the electrician ran the outlet then I would guess that the car always displays the maximum number of amps you can pull with the current connection. I’ve only owned the car for a month and have only charged at home so maybe someone else on here can verify that.

Since it’s showing 24 amps maybe some part of the breaker failed and it’s behaving like a 30 amp breaker (80% of 30 is 24).
 

gregd

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Dec 31, 2014
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The only other possibility that I can think of (besides a loose wire or bad breaker) is that your UMC (presuming that's what you are using) has a problem. Is there another 14-50 nearby, e.g. neighbor or campground, where you can try charging, to see if the problem repeats? It might be sensing the pigtail incorrectly. If cycling the breaker and re-plugging into the 14-50 don't help, maybe detach and reattach the pigtail to clean the contacts?

If there is a loose wire somewhere, it should be getting noticeably warm, even at "only" 24 amps.

Oh, and per the other thread, wiggle the connector going into the car. :)
 
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Reactions: GSP
Why doesn't the car say 24/40A?
This is the right question. I've had my car reduce current. It would show 30/40. The only reason the car would show 24/24 is if the UMC adapter is for a 30A receptacle. Assuming you still are using the 14/50 adapter, it is faulty and signaling the UMC that it is a 30A adapter.

Bring your car and UMC with all its adapters to a service center. They'll test it and replace it under warranty if I'm right.
 

GSP

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Dec 28, 2007
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My Model S displayed 24/24 A when it reduced current on a 30 A EVSE a few weeks ago. It initially displayed 30/30 for a minute or so.

In that case, it was happening at multiple locations, and the car also would not supercharge. The service center replaced the master on-vehicle charger to resolve the issue.

@Theflash95 - I would try charging at a different 14-50 outlet, as suggested above. If you have the same problem, then contact Tesla. If not, then have your electrician check your breaker and wiring.

Good Luck,

GSP
 

BerTX

Active Member
Supporting Member
May 2, 2014
3,508
3,663
Texas/Washington
if what everyone above says is correct and I’ve got a bad breaker or have some issue with the way the electrician ran the outlet then I would guess that the car always displays the maximum number of amps you can pull with the current connection. I’ve only owned the car for a month and have only charged at home so maybe someone else on here can verify that.

Since it’s showing 24 amps maybe some part of the breaker failed and it’s behaving like a 30 amp breaker (80% of 30 is 24).
This is the right question. I've had my car reduce current. It would show 30/40. The only reason the car would show 24/24 is if the UMC adapter is for a 30A receptacle. Assuming you still are using the 14/50 adapter, it is faulty and signaling the UMC that it is a 30A adapter.

Bring your car and UMC with all its adapters to a service center. They'll test it and replace it under warranty if I'm right.
I think the confusion here is the difference in the display when one manually lowers the amperage as opposed to when the car reduces the amperage.When the car reduces it, it displays the available maximum as its determined maximum. (24/24).

I might go along with the idea of he defective reading of the adapter resistor if the car had not ever charged at 40 amps, but the OP says it did. Resistors don't often fail, especially not something just providing a signal .
 
I admit a failed resistor is not likely.
Higher resistance is required for a lower current signal. I remember because I would have paralleled the resistor to use my 14/50 with a 30A outlet if that would have worked.

It really needs to be tested on another receptacle that is known to be in working order, I still suggest the nearest service center. It'll most likely be either be something wrong with the UMC/adapter, or something in the car's charging system. Either way Tesla would take care of it.
 

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