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Two EVs. How I automated charging with a transfer switch.

Jsvette56

Member
Feb 25, 2013
86
53
Santa Clara
Last week I took delivery of my new red Tesla at the factory. My wife already has a Leaf and uses our J1772 AVinc charger to top it off every time she takes a trip, so it's basically plugged in all the time when the car is in the garage requiring coordination as to who was going to get the one plug.

To avoid arguments about whose car did or did not get charged up I added a separate NEMA 14-50 outlet so I could plug in the Tesla and because I only have power (50A circuit) to charge one of the EVs at a time I needed a method to transfer power to one or the other cars on some prearranged schedule. To solve this I went to the local electric supply store and bought two 50A contactors, and an electric timer control with NO and NC contacts. I wired the timer control to the contactors so that one or the other of the charger circuits are powered up. Then I set the timer so that the LEAF charger is on 20 hours a day as the Leaf has only a 3.3KW charger and slow and set the Tesla to be charged in the middle of the night.

It seems to work pretty well and there is a manual switch to power one or the other up separately.
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FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
Battery powered mechanical timer? I don't see a safety ground going to that plastic timer box, nor a neutral...
 

Jsvette56

Member
Feb 25, 2013
86
53
Santa Clara
As I'm not an electrician I always look for advice on these kinds of projects, especially where safety is concerned. The timer had no internal ground connection and everything inside was plastic so I am not sure how I would ground it.
 

mckemie

Member
Mar 27, 2013
160
46
Dale, Texas
Unless you have a new Leaf, it will pull only 12-14 amps. Using only 40 amps of your 50 amp circuit, you would have 26-28 amps left to charge your Tesla concurrently with the Leaf. I have noted that my Tesla seems to remember the last used charger setting for each location. At one of my locations, a J1772, I limit current to about 20 amps to minimize voltage drop. So, I don't have to remember to set the charging current each time I start a charge at that location. At other J1772 locations, it starts at 30 amps.
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
Unless you have a new Leaf, it will pull only 12-14 amps. Using only 40 amps of your 50 amp circuit, you would have 26-28 amps left to charge your Tesla concurrently with the Leaf. I have noted that my Tesla seems to remember the last used charger setting for each location. At one of my locations, a J1772, I limit current to about 20 amps to minimize voltage drop. So, I don't have to remember to set the charging current each time I start a charge at that location. At other J1772 locations, it starts at 30 amps.

By NEC rule you may not do this. Continuous loads, when added together, must operate over conductors and overcurrent protection devices that are rated at 125% of that load. This means that you are not permitted to "use up" the "leftover" 10 amps that the Model S doesn't use with another continuous load. You may use intermittent loads, but the Leaf doesn't qualify as one.

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As I'm not an electrician I always look for advice on these kinds of projects, especially where safety is concerned. The timer had no internal ground connection and everything inside was plastic so I am not sure how I would ground it.

No worries... many timers do expect a ground connection, and many of them require a neutral to operate (as they're 120V).
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
He was talking about using the leftover 10 (50-40) but the leftover 28 (40-12). Not sure of that's ok, just pointing out for clarification.

Oh yeah. I passed math once, I think.

My apologies for my inability to subtract.
 

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