Charging Voltage vs Amp Question

[hkninja]

Hello all,

I discovered something interesting today regarding my wall connector charging rate and wonder if anyone had the same or similar situation. I have the new Tesla Wall Connector installed in my garage with a 40amp breaker. So, my LR RWD Model 3 generally get a charging rate at about 7kW with 240V/32A. Because my electrical box is only on the other side of the garage wall (wire is probably 1.5ft) from the connector, oven my Model 3 reads 243V/32A charging rate.

So, like everyone, I got bored with the pandemic and decided to upgrade to a 50amp breaker so I can charge a little bit faster than the mobile charger (to justify spending $500 for a wall connector ). So, I replaced my breaker today and reconfigured my WC from 40Amp to 50Amp. Nothing else changed. My Model 3 now can charge at 40A @ approximately 9 ~ 10kW rate. However, I noticed that my charging voltage dropped to approximately 235~236V instead of the 240V. While I did not expect a huge jump on the charging rate, I'd expect the voltage to stay the same. So I multi-metered tested the breaker and it is at 240V. Anyone know why the charging voltage dropped with the amp is up while the charging rate has improved 2 - 3kW points?

 

[jjrandorin]

I dont have an answer to your question, but I do have another question. You "switched the breaker" but did you also replace the wire, or, ensure that the wire is the right gauge?

 

[hkninja]

I dont have an answer to your question, but I do have another question. You "switched the breaker" but did you also replace the wire, or, ensure that the wire is the right gauge?

Thanks for asking. I did ensure the wires will work with the 50amp breaker. Mine is wired with 8 gauge copper wires. It works fine with that short of distance (wire temp did not change after 1hr of charging.) I also found an interesting website that helps calculate voltage drop.
Voltage Drop Calculator

I also tried to manually lower the charging amp to 32A and that did not bring the voltage back up to 240V.

Based on Tesla's doc, I am very close to the expecting charging rate of 9.6kW. Maybe that is by design? :/
Wall Connector

 

[UncleCreepy]

While it is perfectly normal for the voltage to drop as you increase the amperage, a 7-8 V drop by going from 32 A to 40 A seems excessive.

What is the voltage if you dial the amperage back to 32 A? Does it go back to 243 V?

In any case, I'd check the connections for hot spots. Something doesn't seem right.

Also, what jjrandorin said: check the wires and make sure they are rated for the higher current.

 

[jjrandorin]

Thanks for asking. I did ensure the wires will work with the 50amp breaker. Mine is wired with 8 gauge copper wires. It works fine with that short of distance (wire temp did not change after 1hr of charging.) I also found an interesting website that helps calculate voltage drop.
Voltage Drop Calculator

I also tried to manually lower the charging amp to 32A and that did not bring the voltage back up to 240V.

Based on Tesla's doc, I am very close to the expecting charging rate of 9.6kW. Maybe that is by design? :/
Wall Connector

Since its a short run from what you say, maybe changing the wire for a the next thicker gauge wire might help (meaning going to 6 gauge)? I am not anywhere near being an expert on this stuff ( although there are several here that are), but my gut feeling is the issue is your cable, at the higher charging amperage.

 

[Sophias_dad]

You might have a 'marginal' connection either from the breaker to the wire, or the wire to the HPWC. Since its a very short run, I assume its in conduit(because 8AWG isn't rated to carry 50 amps(or 40 continuous) as NMB wire).

Voltage coming in off the street can vary quite a bit across a day or even by the minute, based on loads outside or inside your home. This means you almost have to read the voltage at the car at the same time you are looking at the voltage on the breaker/feed to make a real comparison.

 

[camalaio]

I was pretty much going to say what @Sophias_dad did.

My own voltage at the house, without heavy loads active (charging, dryer, stove, etc.), will change by something like +-8V. This isn't just my place, it's the entire valley that's like this.

You almost did this already by checking again at 32A, but try the following:

• Charge at 40A, wait a few seconds, check voltage
• Charge at 6A, wait a few seconds, check voltage

If there isn't a large difference between those two, your setup is probably fine? And it's just line fluctuation for other reasons?

 

[Sophias_dad]

Fun fact, I was monitoring my line voltage for a few weeks, it ranged from 216 to 245 volts, with occasional forays down to 210. I called the electric company and they came out to check things out. They pulled my meter and attached a big intermittent switched load device(presumably so they could measure voltage drops).

They didn't admit anything was wrong, but I explained the voltages I was seeing... Within a week they were out at the pole on a Saturday morning at 7am, changing the transformer that feeds my house and two others with a 25Kva model. Sadly, while I looked to see what the old transformer was rated before it was pulled, it was not clearly visible from the ground.

I was somewhat impressed, since that transformer and the crew to change it probably cost the company half a year's electric bill from my house. Since the swap, I still occasionally see lower than I'd like voltages, but they don't last long. If I notice its drifting low I can run around and intentionally push it unacceptably low by cranking up my usage, but even then it seems something notices and jacks it up pretty quickly.

Before the swap, and the reason I started monitoring at all, was that working from home lets me hear when my UPS starts protecting stuff from brownout conditions at around 107V, and there were days it would do so every hour or two.

 

[tilt]

As stated earlier, unless it is THHN in conduit, you shouldn't be running 50A (40A continuous) on 8 AWG wire.

Voltages varies a lot depending on the season, time of day, and the load you are placing on your wiring. I'll see 124v during the winter all the time. During the summer when it is hot and everyone is running their AC I'll see 118v. It may start the day at 122v in the morning during the summer, but when it heats up it will be 118v. So a single measurement isn't going to tell you much.

 

[pb2000]

If you have a ≥150A panel, change the wire out to #4 (Cu or Al will work) and so long as the wall connector is hard wired, you can go with a 60A breaker and get the full 48A of charging.

 

[Lloyd]

Wall connectors call for Copper Wire Only ! No AL.

 

[pb2000]

Wall connectors call for Copper Wire Only ! No AL.

Good catch. A bit of an odd choice considering how much more expensive copper is, but I suppose it is a PITA to deal with aluminum connections.

 

[user212_nr]

So, like everyone, I got bored with the pandemic and decided to upgrade to a 50amp breaker so I can charge a little bit faster than the mobile charger (to justify spending $500 for a wall connector ).

While I did not expect a huge jump on the charging rate, I'd expect the voltage to stay the same. So I multi-metered tested the breaker and it is at 240V. Anyone know why the charging voltage dropped with the amp is up while the charging rate has improved 2 - 3kW points?

Circuit breakers are installed to protect the wire from overheating. If your electrician installed a 40A breaker, then that means that he determined the wire to be sufficient for 40A, but not 50A. It could also mean that he determined your home not to have sufficient power.

Given that you are asking these questions, your knowledge of electricity is minimal. Its fine to DIY electrical work, but you need to have someone knowledgeable review your plans before your do the work.

Now you need to go back and determine whether or not what you did was safe.

 

[Brock-WI]

I would third or fourth the #6 or better wire size with a 50 amp breaker.

But what else I would do is see what the voltage is somewhere else in the house at the same time. So if the Tesla is reporting 235 what is it at the main panel? Is that sagging a bit as well, if it is 237/238 then the wire to the car is likely fine, if it is sitting at 235 as well, then it is the feed to the panel. Is this in a sub panel or main panel? If the main panels is 243 and the Tesla has it at 235 then something is wrong between the panel and car.

 

[Rocky_H]

I did ensure the wires will work with the 50amp breaker. Mine is wired with 8 gauge copper wires.

That's not a full answer. That's why this is so problematic when people just throw out wire gauge numbers without specifying what the wire type is. This could be wrong or right depending on what you are actually using.

As stated earlier, unless it is THHN in conduit, you shouldn't be running 50A (40A continuous) on 8 AWG wire.

Since its a very short run, I assume its in conduit(because 8AWG isn't rated to carry 50 amps(or 40 continuous) as NMB wire).

This. Look at the row for 8 gauge on this table.
Ampacity Charts - Cerrowire
See how the first column for NM-B says it's only rated for 40A? And then the next column for THWN says it's 50A?
NM-B is Romex--it's that bundled multi-wire cable in a rubber sheath that is run inside insulated walls. If that's what you are using, you violated electric code, because that is too thin for a 50A circuit and getting hot and obviously dropping the voltage.
The second one is for conduit with individual wires in it, which dissipate heat better, so they can have a higher rating. 8 gauge would be barely OK for it, but probably would still have a noticeable few volts drop.

 

[Sophias_dad]

Another potential failure point, as yet fairly unmentioned, is the possibility that your main breaker or the breaker going to the HPWC is starting to go bad or have a bad connection to the bus bars. If it were the main breaker, you'd probably see some evidence of it in the house like flickering lights. OTOH, if its an almost-good connection/breaker the fluctuations might not be noticeable.

When I installed my HPWC(breaker in a subpanel next to the main), I happened to have the cover off the main to upgrade the subpanel feed. Noticed an intermittent faint sizzling(not buzzing, but sizzling) coming out of the main which could be fixed temporarily with a light tap. I flipped all the breakers off, turned the main off/on a few times, and turned the rest of the breakers back on. The sizzling still returned, and I had an electrician come out and swap the main breaker, which has not sizzled since.