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I'm installing a Tesla charger, but I also want to install a NEMA 14-50 outlet so other EV owners can charge in that spot as well, but as it's one spot, both won't be used at the same time. I don't want to have to run two independent lines when only one will be in use at a time. Is there something I could use that would take one line input, and output two separate lines with a toggle switch determining which one is active? And even if so, would such a thing be within code?

  • What size breaker and wire is required for a Tesla home charger? Do Tesla chargers come with a NEMA 14-50 plug or are they hard wired? – Jim Stewart Dec 28 '18 at 17:05
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    When the chart says "55A @60C", you are allowed to round up to 60. 6 is my breakover point for switching for aluminum, so I would use 6Cu or 4Al depending on cost. You don't get any brownie points for using copper, the fear associated with aluminum is totally inapplicable to both modern wire and large wire at these feeder sizes. – Harper - Reinstate Monica Dec 28 '18 at 18:18
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    Indeed, and Tesla themselves recommend 80% for people who charge up daily. I have a Model 3, which by far has their most stable batteries, the 2170s. At 80% You can expect around 5% degradation at 50k miles and 10% at 150k miles. With available data, it's projected 20% degradation at around 500k miles. Warranty will replace the battery at 30% (that is, 70% remaining). Charging to 90% will have an impact, but I find the projected losses acceptable. – directedition Dec 28 '18 at 19:03
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    If I recall a news story correctly, Tesla flashed a software update to everyone in Florida right before the hurricane, telling their chargers to top the battery to 100% and allowing cars to discharge much deeper than is normally allowed under terms of sale, i.e. If you didn't pay for an extended range battery they enabled that DoD anyway. – Harper - Reinstate Monica Dec 28 '18 at 19:24
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    @JimStewart You can set the amperage on the Tesla charger, but that requires dissembling the unit (exposing hot wires, meaning you'll want to switch off the breaker). So in any practical sense you would set it once and leave it, which largely defeats the purpose of installing a Level 2 charger. – directedition Dec 31 '18 at 1:50
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To do it with a switch, you would need a DPDT switch with one throw wired to the charger, the other wired to the receptacle. The DPDT switch would need to be rated for the ampacity of the circuit, which means it's going to be BIG. For example, this GE switch would work:

GE TC35322

It might be easier to just install the receptacle, and cord and plug connect the charger.

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You are not allowed to put multiple outlets on a 50A/60A circuit. You can on a 30A, and that would've been simple, but that will slow charge.

However, your Tesla charger wants 60A and you cannot put a 50A receptacle on a 60A circuit.

enter image description here

Really, the best way to do this is a subpanel. But wait.

The keystone is a $30 interlock kit made by Siemens for its panels. This replaces the expensive, huge Frankenstein switch. The interlock kit straps between two 2-pole breakers, making it so only one can be on at a time. This is intended for generator interlocks, where the breakers are back-fed, but it's fine to normal-feed the breakers too. So your cost is

  • $30 interlock kit
  • $40 8-space, main-lug (no breaker on the sub**) panel
  • $18 two 40-60A breakers, $9 each
  • $5 ground bar

I also got you 4 extra spaces in the panel to use as you please.

You feed the lugs normally, you don't backfeed. The two breakers on the interlock are

  • a 60A breaker going to the Hardwired Tesla charger, full rate unthrottled
  • a 50A breaker going to the NEMA 14-50, which is the legal/right way to do that

And honestly, I do not see a reason you can't simply forego the interlock and allow both outlets to be hot at the same time. The breaker in the main panel protects the wiring to the subpanel and the subpanel itself (which is rated 125A), and the individual breakers protect the wiring to the equipment. If you try to charge two cars at once without rate throttling, yeah, you'll trip the breaker downstairs.

The feed from the main panel to this subpanel can be any size of wire you please, as long as the breaker in the main panel protects the wire. At these sizes, there are no brownie points for using copper*, so consider aluminum wire/cable. For 60A feeder, use #6Cu or #4Al. For 100A feeder, #3Cu or #1Al at which point you definitely don't need the interlock. (You're allowed to oversubscribe a panel and this won't matter).

* Aluminum feeder does not have the problem that 1970s branch circuits had. The metal conducts 12x better per dollar. This is why it is widely used on feeder, where the mineral value becomes a significant part of cable cost.

** because it's not needed in subpanels in the same building, a breaker is needed in the main panel to feed/protect the cable run. Getting the subpanel main to pop before the feed breaker doesn't work, without deep-diving into SCADA tech.

  • Just curious, not familiar with US electrical code: Would wiring an extra 50A breaker in series with the 50A socket on the 60A circuit make it OK? – rackandboneman Dec 28 '18 at 19:24
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    I'd imagine not wanting to risk tripping the house breaker by trying to charge 2 cars at once would be the motivation for wanting an interlock to only power on charger at a time. That goal should still be possible without an interlock though, just use a smaller sub-panel so that the subpanel's main breaker trips if both chargers go live simultaneously. – Dan Neely Dec 28 '18 at 20:04
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    @DanNeely doesn't work, because you still need to protect the wire to the subpanel with a ~60A breaker in the main panel. It is impossible to "sequence" breakers, i.e. Get the more convenient one to trip first. In an outbuilding for instance, Murphy's Law requires the breaker in the other building to trip anytime it is raining. – Harper - Reinstate Monica Dec 28 '18 at 20:14
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    @DanNeely -- selective coordination of circuit breakers is a tricky business at best that is generally unsupported for light-duty panelboard breakers. – ThreePhaseEel Dec 28 '18 at 22:12
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    Since EV owners are likely to have guests with EVs and some EV chargers use a NEMA 6-50 whereas others evidently use a NEMA 14-50, the subpanel approach of @Harper would allow having a TESLA charger hardwired for the resident and would allow installation of both a NEMA 6-50R and a NEMA 14-50R to accommodate any EV charger. Although I gather there are adaptors to allow a NEMA 6-50P to connect to a NEMA 14-50R so all that is needed is a 14-50R. – Jim Stewart Dec 31 '18 at 12:21

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