Low Charging Rate vs. 40A expected

[urbanscribe]

howdy

have looked hard for this on the forum can't find - so apologies if i missed it

just took delivery of a P90D - very very happy

have NEMA 14-50 plug installed

1. my understanding from electrician is that it is a 50A circuit
2. app shows 40A
3. chaging rate - first several days - is around 28A resulting in 19 miles/hours (much less than 29 target)
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questions

A. is it normal that app shows 40A on a 50A circuit. circuit breaker in box clearly shows 50A
B. why 28? any ideas why if the App shows 40A available it is drawing at 28A only.
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see screenshot

 

[Polly Wog]

@urbanscribe, circuits are derated 20%, so 40 amps is what a 50 amp circuit actually can deliver continuously. FlasherZ has a home charging FAQ that you need to checkout. Also, look at the "14-50 vs HPWC dropping 25%" thread under Model S: Battery & Charging.

 

[David99]

A 50 Amp breaker can take up to 50 Amp before it trips but I think per code a continuous load can only be 80% of the max. So 40 Amp it is. If the car adjusts it down to 28, it means the car has detected something that it thinks could be bad wiring or something similar and plays it save by reducing the current. Or you just have it accidentally adjusted down by hand. See if you can adjust it up/

 

[FlasherZ]

have NEMA 14-50 plug installed
1. my understanding from electrician is that it is a 50A circuit
2. app shows 40A

See the FAQ linked in my signature (below); or, if you're using Tapatalk, it's in the Charging & Infrastructure -> North America forum as a sticky.

I'm going to quickly offer a technical correction those who posted above... circuits aren't derated 20%, and continuous loads aren't always limited to 80% of the max (sometimes they're lower due to other, true derating factors). Instead, the right way to think about this is that for continuous loads like EV charging, where power is drawn constantly and doesn't cycle intermittently (think electric oven or stove elements that cycle several times a minute), the NEC says that the electrical infrastructure supporting it (conductors, breakers, and connectors/receptacles) must be rated at 125% of the load offered. Tesla selected a 40A charging load because when you apply the 125% rating requirement, it reaches 50A, and there is prevalence of 50A-rated equipment out there in RV/range outlets.

3. chaging rate - first several days - is around 28A resulting in 19 miles/hours (much less than 29 target)

This is most likely due to someone manually adjusting the current on the touchscreen. If it showed 30/40A, it would likely be due to the car sensing voltage fluctuations (see the FAQ for more details), but at 28A it's likely that someone adjusted the charging rate down. Try going to the charging screen (touch the battery icon), and ensure that the charging rate is set to the highest possible (80A), before plugging in.

40 amps is the maximum charging load that can be provided on a circuit rated for 50 amps.

 

[tstafford]

Also - once the car thinks it can only get 28 amps (or whatever) that's all it will ever try at that location until you manually push it back up.

Very early on (day one or two) that I owned my car it dropped to 30 amps (I was worried) but I pushed it back up to 40 and it's been fine ever since. I suspect there was just a spike in load on the overall service to my house or the area or whatever and the car detected the drop. Hopefully you'll experience the same.

 

[Polly Wog]

@FlasherZ, thank you for correcting me. Still, I at least tried to nudge the OP in your direction smile, knowing that you know more about these things than most of the rest of us.