I see posts pop up once in a while with people asking about voltage drop when their charger is pulling full amperage (be it 32A for Mobile connector or 40A for wall connector).
Main thing to know is that voltage drop is expected and often comes from 2 sources.
1. Grid feed to your house
2. Wiring between the breaker panel and your car (it is important here to include the 20ft of mobile connector, 10 AWG and also contributes to voltage drop).
House Feed From the Grid:
a. Grid voltage may fluctuate throughout the day depending on grid load by a few volts. So comparing day to day charge voltage may not make sense in some cases.
b. Even if entirety of your house's wiring is copper it does not mean that the feed from the grid is copper. It very much could be aluminum, which has higher resistance. It could be under sized if its old and the house was upgraded to 200A service at some point (changing that wire is typically thousands of dollars, tens of thousands of its buried).
Wiring between the breaker and the car:
a. When using cable length for calculations use the distance between the breaker panel and your outlet; however do consider that the charger adds to the cable length (Mobile connector has 20ft of 10AWG cable which can contribute to up to 0.57% drop on its own (32A))
b. Use calculator like this to figure out expected voltage drop Voltage Drop Calculator | Southwire
Investigating:
Use multimeter to test voltage at the breaker panel. First measure the voltage before charging. For example for me this is 236 V. Then start charging and measure once the charger is pulling full amperage (32A for me). I saw 233.5V at the panel. This means I have 2.5V sag due to the conductor coming from the grid/transformer before it enters my house. Now look at what voltage the car reports. For me its reporting 232V. This means I have 1.5V sag in my wiring (from the breaker panel to the car).
Now its time to figure out if this is within acceptable range:
Example:
While idling, the car pulls 0A/236V (this number can be measured at the receptacle with no loads), 6AWG copper conductor is used for a 12 foot run to the outlet. Plug in conductor size (6), length (12ft), voltage (234) and amperage (32A) into the Voltage Drop Calculator | Southwire calculator and it tells me to expect up to 0.14% drop. Easy way to translate into expected voltage is to do 1 - 0.0014 = 0.9986 and multiply that by voltage after removing grid feed sag. 0.0986 * (236 - 1.5) = 233.17 V. So while charging thats the lowest voltage I should expect to see when pulling full 32A. BUT, this math is for the run from the breaker panel up to the receptacle. Remember that there is 20ft of mobile connector 10AWG wire (which can add up to another 0.57% drop), so 1 - 0.0057 = 0.9943, then 0.9943 * 233.17 = 231.84 V
This should be the worst case scenario, meaning in reality voltage reported by the car should be somewhere between 231.84v and 233.5v. When I look at what my car is reporting, its reporting 32A/232V.
So my voltage drop is within the expected range.
Hope this is helpful. And remember, electricity is dangerous, if you dont feel comfortable with any of this ask a qualified professional for help.
Main thing to know is that voltage drop is expected and often comes from 2 sources.
1. Grid feed to your house
2. Wiring between the breaker panel and your car (it is important here to include the 20ft of mobile connector, 10 AWG and also contributes to voltage drop).
House Feed From the Grid:
a. Grid voltage may fluctuate throughout the day depending on grid load by a few volts. So comparing day to day charge voltage may not make sense in some cases.
b. Even if entirety of your house's wiring is copper it does not mean that the feed from the grid is copper. It very much could be aluminum, which has higher resistance. It could be under sized if its old and the house was upgraded to 200A service at some point (changing that wire is typically thousands of dollars, tens of thousands of its buried).
Wiring between the breaker and the car:
a. When using cable length for calculations use the distance between the breaker panel and your outlet; however do consider that the charger adds to the cable length (Mobile connector has 20ft of 10AWG cable which can contribute to up to 0.57% drop on its own (32A))
b. Use calculator like this to figure out expected voltage drop Voltage Drop Calculator | Southwire
Investigating:
Use multimeter to test voltage at the breaker panel. First measure the voltage before charging. For example for me this is 236 V. Then start charging and measure once the charger is pulling full amperage (32A for me). I saw 233.5V at the panel. This means I have 2.5V sag due to the conductor coming from the grid/transformer before it enters my house. Now look at what voltage the car reports. For me its reporting 232V. This means I have 1.5V sag in my wiring (from the breaker panel to the car).
Now its time to figure out if this is within acceptable range:
Example:
While idling, the car pulls 0A/236V (this number can be measured at the receptacle with no loads), 6AWG copper conductor is used for a 12 foot run to the outlet. Plug in conductor size (6), length (12ft), voltage (234) and amperage (32A) into the Voltage Drop Calculator | Southwire calculator and it tells me to expect up to 0.14% drop. Easy way to translate into expected voltage is to do 1 - 0.0014 = 0.9986 and multiply that by voltage after removing grid feed sag. 0.0986 * (236 - 1.5) = 233.17 V. So while charging thats the lowest voltage I should expect to see when pulling full 32A. BUT, this math is for the run from the breaker panel up to the receptacle. Remember that there is 20ft of mobile connector 10AWG wire (which can add up to another 0.57% drop), so 1 - 0.0057 = 0.9943, then 0.9943 * 233.17 = 231.84 V
This should be the worst case scenario, meaning in reality voltage reported by the car should be somewhere between 231.84v and 233.5v. When I look at what my car is reporting, its reporting 32A/232V.
So my voltage drop is within the expected range.
Hope this is helpful. And remember, electricity is dangerous, if you dont feel comfortable with any of this ask a qualified professional for help.
